II/IV B.Tech.

– Mechanical Engineering

CASTING
Name of the subject : PRODUCTION TECHNOLOGY

UNIT-1: Metal casting: part - 1

INTRODUCTION

Casting is a process in which molten metal flows by gravity or other force into a mold where it
solidifies in the shape of the mold cavity. The term casting is also applied to the part that is made
by this process.

Casting includes both the casting of ingots and the casting of shapes. The term ingot is usually
associated with the primary metals industries; it describes a large casting that is simple in shape
and intended for subsequent reshaping by processes such as rolling or forging.

Shape casting involves the production of more complex geometries that are much closer to the
final desired shape of the part or product.

Advantages:

A variety of shape casting methods are available, thus making it one of the most versatile of all
manufacturing processes. Among its capabilities and advantages are the following:

1. Casting can be used to create complex part geometries, including both external and

internal shapes.

2. Some casting processes are capable of producing parts to net shape. No further
manufacturing operations are required to achieve the required geometry and dimensions of
the parts. Other casting processes are near net shape, for which some additional shape
processing is required (usually machining) in order to achieve accurate dimensions and
details.
3. Casting can be used to produce very large parts. Castings weighing more than 200 tons can
been made.
4. The casting process can be performed on any metal that can be heated to the liquid state.
5. Some casting methods are quite suited to mass production.

There are also disadvantages associated with casting. These include limitations on mechanical
properties, porosity, poor dimensional accuracy and surface finish for some casting processes,
safety hazards to humans when processing hot molten metals, and environmental problems.

Parts made by casting processes range in size from small components weighing only a few grams
up to very large products weighing tons. The list of parts (Applications) includes dental crowns,
jewelry, statues, wood-burning stoves, engine blocks and heads for automotive vehicles, machine
frames, railway wheels, frying pans, pipes, and pump housings.

All varieties of metals can be cast, ferrous and nonferrous. Casting can also be used on other
materials such as polymers and ceramics.

Fig. 1.1 Classification of Casting Processes

SAND CASTING PROCESS.

Definition: Sand casting, also known as sand-mold casting, consists of pouring molten
metal into a sand mold, allowing the metal to solidify, and then breaking up the mold to remove
the casting. The casting must then be cleaned and inspected, and heat treatment is sometimes
required to improve metallurgical properties.

Sand casting is the most widely used casting process, accounting for a significant majority of the
total tonnage cast. Nearly all casting alloys can be sand cast like metals with high melting
temperatures, such as steels, nickels, and titanium. Its versatility permits the casting of parts
ranging in size from small to very large and in production quantities from one to millions.
 Fig.1.2 Terminology used in sand moulding.

Important casting terms (Refer Fig.1.2)

Flask: A metal or wood frame, without fixed top or bottom, in which the mould is formed.
Depending upon the position of the flask in the moulding structure, it is referred to by various
names such as drag – lower moulding flask, cope – upper moulding flask, cheek – intermediate
moulding flask used in three piece moulding.
Pattern: It is the replica of the final object to be made. The mould cavity is made with the help
of pattern.
Parting line: This is the dividing line between the two moulding flasks that makes up the mould.
Moulding sand: Sand, which binds strongly without losing its permeability to air or gases. It is a
mixture of silica sand, clay, and moisture in appropriate proportions.
Facing sand: The small amount of carbonaceous material sprinkled on the inner surface of the
mould cavity to give a better surface finish to the castings.
Core: A separate part of the mould, made of sand and generally baked, which is used to create
openings and various shaped cavities in the castings.
Pouring basin: A small funnel shaped cavity at the top of the mould into which the molten
metal is poured.
Sprue: The passage through which the molten metal, from the pouring basin, reaches the mould
cavity. In many cases it controls the flow of metal into the mould.
Runner: The channel through which the molten metal is carried from the sprue to the gate.
Gate: A channel through which the molten metal enters the mould cavity.
Chaplets: Chaplets are used to support the cores inside the mould cavity to take care of its own
weight and overcome the metallostatic force.
Riser: A column of molten metal placed in the mould to feed the castings as it shrinks and
solidifies. Also known as “feed head”.
Vent: Small opening in the mould to facilitate escape of air and gases.
 The production sequence of a sand casting is outlined in Figure 2.

Fig.1.3 Steps in the production sequence in sand casting.

The steps include not only the casting operation but also pattern making and mold making.
The cavity in the sand mold is formed by packing sand around a pattern (an approximate
duplicate of the part to be cast), and then removing the pattern by separating the mold into two
halves. The mold also contains the gating and riser system. In addition, if the casting is to have
internal surfaces (e.g., hollow parts or parts with holes), a core must be included in the mold.
Since the mold is sacrificed to remove the casting, a new sand mold must be made for each part
that is produced.

Advantages

• Molten material can flow into very small sections so that intricate shapes can be made by this
process. As a result, many other operations, such as machining, forging, and welding, can be
minimized.

• Possible to cast practically any material: ferrous or non-ferrous.

• The necessary tools required for casting moulds are very simple and inexpensive. As a result,
for production of a small lot, it is the ideal process.

• There are certain parts (like turbine blades) made from metals and alloys that can only be
processed this way. Turbine blades: Fully casting + last machining.

• Size and weight of the product is not a limitation for the casting process.

Limitations

• Dimensional accuracy and surface finish of the castings made by sand casting processes are a
limitation to this technique.

• Many new casting processes have been developed which can take into consideration the
aspects of dimensional accuracy and surface finish. Some of these processes are die casting
process, investment casting process, vacuum-sealed moulding process, and shell moulding
process.

• Metal casting is a labour intensive process.

• Automation is expensive.

Steps in making sand castings

The six basic steps in making sand castings are, (i) Pattern making, (ii) Core making, (iii)
Moulding, (iv) Melting and pouring, (v) Cleaning.

Pattern making - Pattern: Replica of the part to be cast and is used to prepare the mould cavity.
It is the physical model of the casting used to make the mould. Pattern is made of wood plastic or
metal.

The mould is made by packing some readily formed aggregate material, such as moulding sand,
surrounding the pattern. When the pattern is withdrawn, its imprint provides the mould cavity.
This cavity is filled with metal to become the casting.

If the casting is to be hollow, additional patterns called ‘cores’, are used to form these cavities.

Core making

Cores are placed into a mould cavity to form the interior surfaces of castings. Thus the void
space is filled with molten metal and eventually becomes the casting.

Moulding

Moulding is nothing but the mould preparation activities for receiving molten metal.

Moulding usually involves:

(i) preparing the consolidated sand mould around a pattern held within a supporting metal
frame,
(ii) Removing the pattern to leave the mould cavity with cores.

Mould cavity is the primary cavity.

The mould cavity contains the liquid metal and it acts as a negative of the desired product.

The mould also contains secondary cavities for pouring and channeling the liquid material in to
the primary cavity and will act a reservoir, if required.

Melting and Pouring

The preparation of molten metal for casting is referred to simply as melting. The molten metal is
transferred to the pouring area where the moulds are filled.

Cleaning

Cleaning involves removal of sand, scale, and excess metal from the casting. Burned-on sand
and scale are removed to improve the surface appearance of the casting. Excess metal, in the
form of fins, wires, parting line fins, and gates, is removed. Inspection of the casting for defects
and general quality is performed.

Making a simple sand mould

1) The drag flask is placed on the board

2) Dry facing sand is sprinkled over the board

3) Drag half of the pattern is located on the mould board. Dry facing sand will provide a non-
sticky layer.

4) Molding sand is then poured in to cover the pattern with the fingers and then the drag is filled
completely

5) Sand is then tightly packed in the drag by means of hand rammers. Peen hammers (used first
close to drag pattern) and butt hammers (used for surface ramming) are used.

6) The ramming must be proper i.e. it must neither be too hard or soft. Too soft ramming will
generate weak mould and imprint of the pattern will not be good. Too hard ramming will not
allow gases/air to escape and hence bubbles are created in casting resulting in defects called
‘blows’. Moreover, the making of runners and gates will be difficult.

7) After the ramming is finished, the excess sand is leveled/removed with a straight bar known as
strike rod.

8) Vent holes are made in the drag to the full depth of the flask as well as to the pattern to
facilitate the removal of gases during pouring and solidification. Done by vent rod.

9) The finished drag flask is now made upside down exposing the pattern.

10) Cope half of the pattern is then placed on the drag pattern using locating pins. The cope flask
is also located with the help of pins. The dry parting sand is sprinkled all over the drag surface
and on the pattern.

11) A sprue pin for making the sprue passage is located at some distance from the pattern edge.
Riser pin is placed at an appropriate place.

12) Filling, ramming and venting of the cope is done in the same manner.
13) The sprue and riser are removed and a pouring basin is made at the top to pour the liquid
metal.

14) Pattern from the cope and drag is removed.

15) Runners and gates are made by cutting the parting surface with a gate cutter. A gate cutter is
a piece of sheet metal bent to the desired radius.

16) The core for making a central hole is now placed into the mould cavity in the drag. Rests in
core prints.

17) Mould is now assembled and ready for pouring.

13) The sprue and riser are removed and a pouring basin is made at the top to pour the liquid
metal.

14) Pattern from the cope and drag is removed.

15) Runners and gates are made by cutting the parting surface with a gate cutter. A gate cutter is
a piece of sheet metal bent to the desired radius.
16) The core for making a central hole is now placed into the mould cavity in the drag. Rests in
core prints.

17) Mould is now assembled and ready for pouring. Pour

PATTERNS AND CORES

A pattern is the replica of the final object to be produced by casting with some
modifications in shape and dimensions. The modifications include pattern allowances core prints
and handling aids for moving the casting.
Sand casting requires a pattern - a full-sized model of the part, enlarged to account for
shrinkage and machining allowances in the final casting. Materials used to make patterns
include wood, plastics, and metals. Wood is a common pattern material because it is easily
shaped. Its disadvantages are that it tends to warp, and it is abraded by the sand being compacted
around it, thus limiting the number of times it can be reused. Metal patterns are more expensive
to make, but they last much longer. Plastics represent a compromise between wood and metal.
Selection of the appropriate pattern material depends to a large extent on the total quantity of
castings to be made.
Types of pattern
There are various types of patterns, as illustrated in Figure 3.
a) Solid or single piece pattern: The simplest is made of one piece, called a solid pattern-same
geometry as the casting, adjusted in size for shrinkage and machining. Although it is the easiest
pattern to fabricate, it is not the easiest to use in making the sand mold. Determining the location
of the parting line for a solid pattern can be a problem, and incorporating the gating system and
sprue depend on the judgment and skill of the foundry worker. So, solid patterns are generally
limited to very low production quantities.

Fig.1.4 Types of patterns used in sand casting: (a) solid pattern, (b) split pattern, (c)
match-plate pattern, and (d) cope-and-drag pattern.
b) Split patterns: It consist of two pieces, dividing the part along a plane coinciding with the
parting line of the mold. Split patterns are appropriate for complex part geometries and moderate
production quantities. The parting line of the mold is predetermined by the two pattern halves,
rather than by operator judgment.
c) match-plate patterns: In match-plate patterns, the two pieces of the split pattern are attached
to opposite sides of a wood or metal plate. Holes in the plate allow the top and bottom (cope and
drag) sections of the mold to be aligned accurately.
d) Cope-and-drag patterns: These are similar to match-plate patterns except that split pattern
halves are attached to separate plates, so that the cope and drag sections of the mold can be
fabricated independently, instead of using the same tooling for both. Part (d) of the figure 2
includes the gating and riser system in the cope-and-drag patterns. It is also called gated pattern.
For higher production quantities, match-plate patterns or cope-and-drag patterns are used.
Skeleton Pattern: This consists of frame of wood representing the interior and exterior forms.
Strickles (like strike off bars) are used to remove excess sand which is purposely rammed with
extra thickness than required for desired mold surfaces.
CORE: Patterns define the external shape of the cast part. If the casting is to have internal
surfaces, a core is required. A core is a full-scale model of the interior surfaces (holes or pockets)
of the part. It is inserted into the mold cavity prior to pouring, so that the molten metal will flow
and solidify between the mold cavity and the core to form the casting’s external and internal
surfaces. The core is usually made of core sand (different from moulding sand), compacted into
the desired shape. As with the pattern, the actual size of the core must include allowances for
shrinkage and machining.
Chaplets: These are the metallic supports used to hold and position the core in the mold cavity
during pouring. Chaplets are made of a metal with a higher melting temperature than the casting
metal. For example, steel chaplets would be used for cast iron castings. On pouring and
solidification, the chaplets become bonded into the casting. A possible arrangement of a core in a
mold using chaplets is shown in Figure 3. The portion of the chaplet protruding from the casting
is subsequently cut off.
Fig.1.5 a) Split piece, b) Follow board, Match plate, d) Loose piece, e) Sweep, f) Skeleton
patern.

g) Gated pattern for making eight small patterns.

h) Skeleton Pattern
 Fig.1.6 (a) Core held in place in the mold cavity by chaplets, (b) possible chaplet design, and
(c) Casting with internal cavity.

Pattern allowances

The pattern and the part to be made are not same. They differ in the following aspects.

1. A pattern is always made larger than the final part to be made. The excess dimension is known
as Pattern allowance. Pattern allowance => shrinkage allowance, machining allowance

2. Shrinkage allowance: will take care of contractions of a casting which occurs as the metal
cools to room temperature.

Liquid Shrinkage: Reduction in volume when the metal changes from liquid state to solid state.
Riser which feed the liquid metal to the casting is provided in the mould to compensate for this.

Solid Shrinkage: Reduction in volume caused when metal looses temperature in solid state.
Shrinkage allowance is provided on the patterns to account for this. Shrink rule is used to
compensate solid shrinkage depending on the material contraction rate.

Cast iron: One foot (=12 inches) on the 1/8-in-per-foot shrink rule actually measures 12-1/8
inches. So, 4 inch will be 4-1/24 inch for considering shrinkage allowance.

The shrinkage allowance depends on the coefficient of thermal expansion of the material (α). A
simple relation indicates that higher the value of α, more is the shrinkage allowance.

For a dimension ‘l’, shrinkage allowance is αl (Tf –T0). Here Tf is the freezing temperature and
T0 is the room temperature.

Patterns are made by using shrink rules which take into account the shrinkage allowance (1’ will
be 1’ 3/16’’ in a shrink rule for brass)
 Fig.1.7 Various allowances incorporated into a casting pattern

Machining allowance: will take care of the extra material that will be removed to obtain a
finished product. In this the rough surface in the cast product will be removed. The machining
allowance depends on the size of the casting, material properties, material distortion, finishing
accuracy and machining method. For internal surfaces, the allowances should be negative.

Draft allowance: All the surfaces parallel to the direction in which the pattern will be removed
are tapered slightly inward to facilitate safe removal of the pattern. This is called ‘draft
allowance’. General usage: External surfaces; Internal surfaces, holes, pockets.

Fig. 1.8 Draft allowance

Core and core print: - Cores are used to make holes, recesses etc. in castings - So where coring
is required, provision should be made to support the core inside the mould cavity. Core prints are
used to serve this purpose. The core print is an added projection on the pattern and it forms a seat
in the mould on which the sand core rests during pouring of the mould.The core print must be
ofadequate size and shape so that it can support the weight of the core during the casting
operation.
 Fig.1.9 Core print and Core positioning.
Distortion allowance (camber)

Vertical edges will be curved or distorted. This is prevented by shaped pattern converge slightly
(inward) so that the casting after distortion will have its sides vertical - The distortion in casting
may occur due to internal stresses. These internal stresses are caused on account of unequal
cooling of different sections of the casting and hindered contraction. Prevention: - providing
sufficient machining allowance to cover the distortion affect - Providing suitable allowance on
the pattern, called camber or distortion allowance (inverse reflection).

Fig.1.10 Effect of distortion

Pattern materials

For most expendable mould casting techniques a pattern is required.

Pattern material depends on number of castings to be made, metal being cast, process being used,
size and shape, dimensional precision required
Wood is Cheap, easily machined but prone to warping, swelling (moisture), unstable, wears.
Used for small runs. Different woods for pattern making are white pine, sugar pine. The wood
should be straight grain, light, easy to work, little tendency to develop crack and warp. More
durable wood is Mahogany.

Metal - more expensive but stable, accurate, durable. Typically aluminium, cast iron or steel.
Either cast then machined or machined directly (e.g. NC-machining). Large runs and elevated
pressure and/or temperature moulding process. When metal pattern are cast from the wooden
master pattern, double shrinkage allowance must be provided on the wooden master pattern.
Metal such as cast iron or aluminium are used for making metallic patterns.
Plastic - Epoxy and Polyurethane. More common now. Easy preparation, stable and durable
relative to wood. Cast & machined, easily repaired, can be reinforced/backed.

Expendable (single-use) patterns:

Wax - used for investment casting. Wax formulated for melting point, viscosity, ash residue etc.
Melted out (mostly) before casting

EPS - Expanded PolyStyrene. Pre-expanded beads blown into mould, heated (steam) to
completely fill mould and bond beads. Pattern is burnt out by molten metal. Carbon film
possible.

Different ways for making a casting mold

Case i)

Cope

Flat back pattern can be used for this. In this after moulding, the mold cavity is either in the drag
side or in cope side or in both. The hole is formed by the molding sand. The outside edge around
the flat back is the parting line and it is the starting place for draft. This is the simplest and
easiest method.

Case ii)
Using a dry sand core to obtain the core and this is split pattern. The axis of the hole (and core
print) is vertical in first case. The second case is same as first, except that the hole axis is
horizontal.

Gating design

A good gating design should ensure proper distribution of molten metal without
excessive temperature loss, turbulence, gas entrapping and slags. If the molten metal is poured
very slowly, since time taken to fill the mould cavity will become longer, solidification will start
even before the mould is completely filled. This can be restricted by using super-heated metal,
but in this case solubility will be a problem. If the molten metal is poured very faster, it can
erode the mould cavity. So gating design is important and it depends on the metal and molten
metal composition. For example, aluminium can get oxidized easily. Gating design is classified
mainly into two (modified: three) types: Vertical gating, bottom gating, horizontal gating.

Fig. 1.11 Advantage of tapered sprue

Vertical gating: the liquid metal is poured vertically, directly to fill the mould with atmospheric
pressure at the base end.

Bottom gating: molten metal is poured from top, but filled from bottom to top. This minimizes
oxidation and splashing while pouring.

Horizontal gating is a modification of bottom gating, in which some horizontal portions are
added for good distribution of molten metal and to avoid turbulence.
Analysis of pouring and filling up mould Top

gating:

For analysis we use energy balance equation like Bernoulli‟s equation

Assuming p1 = p3 and level at 1 is maintained constant, so v1 = 0; frictional losses are neglected.

The energy balance between point 1 and 3 gives,

Here v3 can be referred as velocity at the sprue base or say gate, vg

Continuity equation: Volumetric flow rate, Q = A1v1 = A3v3

Above two equations, V3 and Q, say that sprue should be tapered.

As the metal flows into the sprue opening, it increases in velocity and hence the cross-sectional
area of the channel must be reduced Otherwise, as the velocity of the flowing molten metal
increases toward the base of the sprue, air can be aspirated into the liquid and taken into the
mould cavity. To prevent this condition, the sprue is designed with a taper, so that the volume
flow rate, Q = Av remains the same at the top and bottom of the sprue.

The mould filling time is given by

Ag = cross-sectional area of gate; V = volume of mould

Note: This is the minimum time required to fill the mould cavity. Since the analysis ignores
friction losses and possible constriction of flow in the gating system; the mould filling time will
be longer than what is given by the above equation.

(b) Bottom gating

Apply Bernoulli‟s eqn. between points 1 and 3 and between 3 and 4,

Assuming in the mould the height moves up by “dh” in a time “dt”; Am and Ag are mould area
and gate area, then

Combining above two eqns., we get

Aspiration effect

Aspiration effect: entering of gases from baking of organic compounds present in the mould
into the molten metal stream. This will produce porous castings. Pressure anywhere in the
liquid stream should not become negative.

Ideal and actual profiles of sprue

Approximating tapered spure using choke mechanism

(a)Choke core, (b) Runner choke In many high production casting systems, tapered sprue will
not be provided. Instead it is compensated by having chokes at the end of sprue or runner.
Sudden change in flow direction

A sharp change in flow direction is avoided by designing the mould to fit vena-contracta.

Preventing impurities and turbulence in casting

The items provided in the gating system to avoid impurities and turbulence are:

Pouring basin: This reduces the eroding force of the liquid metal poured from furnace. This also
maintains a constant pouring head. Experience shows that pouring basin depth of 2.5 times the
sprue entrance diameter is enough for smooth metal flow.
Delay screen/Strainer core: A delay screen is a small piece of perforated screen placed on top
of the sprue. This screen actually melts because of the heat from the metal and this delays the
entrance of metal into the sprue, maintaining the pouring basin head. This also removes dross in
the molten metal. Strainer core is a ceramic coated screen with many small holes and used for
same purpose.

Splash core: provided at the end of the sprue length which reduces the eroding force of the
liquid metal

Skim bob: this traps lighter and heavier impurities in the horizontal flow

Gating ratios

Gating ratio: sprue area : runner area : gate area

Non-pressurized: has choke at the bottom of the sprue base, has total runner area and gate areas
higher than the sprue area. No pressure is present in the system and hence no turbulence. But
chances of air aspiration is possible. Suitable for Al and Mg alloys.

In this, Gating ratio = 1 : 4 : 4

Pressurized: Here gate area is smallest, thus maintaining the back pressure throughout the gating
system. This backpressure generates turbulence and thereby minimizes the air aspiration even
when straight sprue is used. This is not good for light alloys, but good for ferrous castings. In
this, Gating ratio = 1 : 2 : 1.

Gating ratios used in practice

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Bri‹ss

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 Module-I
Introduction

Improved civilization is due to improved quality of products, proper selection of design as well as
manufacturing process from raw materials to finished goods.

Classification of manufacturing processes:-

(a) Casting
(b) Forming
(c) Fabrication
(d) Material Removal

Metal Casting Process:-

Metal Casting is one of the oldest materials shaping methods known. Casting means pouring molten
metal into a mould with a cavity of the shape to be made, and allowing it to solidify. When solidified,
the desired metal object is taken out from the mould either by breaking the mould or taking the mould
apart. The solidified object is called the casting. The process is also called foundry.

Advantages:-

 Any intricate shape can be produced.

 Possible to cast both ferrous and non ferrous materials
 Tools are very simple and expensive
 Useful for small lot production
 Weight reduction in design
 No directional property

Limitations:-

 Accuracy and surface finish are not very good for final application
 Difficult to remove defects due to presence of moisture

Application:-

Cylindrical bocks, wheels, housings, pipes, bells, pistons, piston rings, machine tool beds etc.

Casting terms:-

Flask- It holds the sand mould intact. It is made up of wood for temporary application and metal for
long term use.

Drag- Lower moulding flask

Cope – Upper moulding flask

Cheek – Intermediate moulding flask used in three piece moulding.

Pattern - Replica of final object to be made with some modifications. Mould cavity is made with the
help of pattern.
Parting line – Dividing line between two moulding flasks.

Bottom board – Board used to start mould making (wood)

Facing sand - Small amount of carboneous material sprinkled on the inner surface of the mould
cavity to give better surface finish to casting.

Moulding sand – Freshly prepared refractory material used for making the mould cavity. (Mixture of
silica, clay & moisture)

Backing sand – used and burnt sand

Core – Used for making hollow cavities in the casting

Pouring basin – Funnel shaped cavity on the top of the mould into which molten metal is poured

Sprue – Passage from pouring basin to the mould cavity. It controls the flow of molten metal into the
mould.

Figure 1 Cross-section of a sand mould ready for pouring

Runner – Passage ways in the parting plane through which molten metal flow is regulated before they
reach the mould cavity

Gate – Actual entry point through which molten metal enters the mould cavity

Chaplet – Used to support the core to take of its own weight to overcome the metallostatic force.

Chill – Metallic objects to increase cooling rate of casting

Riser – Reservoir of molten metal in the casting so that hot metal can flow back into the mould cavity
when there is a reduction in volume of metal due to solidification.

Sand mould making procedure:-

The first step in making mould is to place the pattern on the moulding board. The drag is placed on the
board (figure 2 (a)). Dry facing sand is sprinkled over the board and pattern to provide a non sticky
layer. Moulding sand is then riddled in to cover the pattern with the fingers; then the drag is
completely filled. The sand is then firmly packed in the drag by means of hand rammers. The
ramming must be proper i.e. it must neither be too hard or soft. After the ramming is over, the excess
sand is levelled off with a straight bar known as a strike rod. With the help of vent rod, vent holes are
made in the drag to the full depth of the flask as well as to the pattern to facilitate the removal of gases
during pouring and solidification. The finished drag flask is now rolled over to the bottom board
exposing the pattern. Cope half of the pattern is then placed over the drag pattern with the help of
locating pins. The cope flask on the drag is located aligning again with the help of pins (Figure 2
(b)). The dry parting sand is sprinkled all over the drag and on the pattern. A sprue pin for making the
sprue passage is located at a small distance from the pattern. Also, riser pin, if required, is placed at an
appropriate place. The operation of filling, ramming and venting of the cope proceed in the same
manner as performed in the drag. The sprue and riser pins are removed first and a pouring basin is
scooped out at the top to pour the liquid metal. Then pattern from the cope and drag is removed and
facing sand in the form of paste is applied all over the mould cavity and runners which would give the
finished casting a good surface finish. The mould is now assembled. The mould now is ready for
pouring (Figure 2 (c)).

Figure 2 (a) Figure 2 (b)

Figure 2 (c)
Pattern:-

Replica of the object to be made by the casting process with some modifications.

Modifications-

(a) Addition of pattern allowance

(b) Provision of core prints
(c) Elimination of fine details
(A) Pattern Allowances-

Pattern dimensions are different from final dimension of casting.

Shrinkage

All metals shrink when cooling except bismuth. This may be due to inter atomic vibrations amplified
by increase in temperature.

Two types:-

Liquid shrinkage – Reduction in volume when metal changes from liquid to solid at solidus
temperature.

Solid shrinkage – Reduction in volume when a metal loses temperature in solid state.

Actual value of shrinkage depends on

 Composition of alloy cast

 Mould materials used
 Mould design
 Complexity of pattern
 Component size

Metallic pattern casting – double shrinkage

Finish/ machining allowance –

Extra material provided which is to be machined or cleaned for good surface finish and dimensional
accuracy. It depends on

 type of casting material

 dimensions
 finishing required
 complexity of surface details

Range – 2 to 20 mm

To reduce the machining allowance, the entire casting should be kept in the drag flask such that
dimensional variation and other defects due to parting plane can be reduced.
Draft –

Vertical faces of the pattern are to be made tapered to reduce the chances of damage to the mould
cavity. It varies with the complexity of the job. Inner details require more allowance than outer. This
allowance is more for hand moulding than machine moulding.

Shake allowance –

This is a negative allowance. Applied to those dimensions which are parallel to parting plane.

Distortion allowance –

Metals just solidified are very weak, which may be distorted. This allowance is given to the weaker
sections like long flat portion, U & V sections, complicated casing, thin & long sections connected to
thick sections. This is a trial and error method.

(B) Core prints:-

Core prints are required for casting where coring is required.

(C) Elimination of fine details:-

Type of details to be eliminated depends on

 Required accuracy
 Capability of the chosen casting process
 Moulding method employed

Pattern Materials:-

Usual materials – wood, metals & plastics

Wood-
Adv:- Disadv:-
Easy availability Moisture absorption
Low weight Distortion
Easily shaped Dimensional change
Cheap
Care to be taken – seasoning
Example – Pine, Teak, Deodar
Others – plywood boards and particle boards
Reason – Availability in various thicknesses
Higher strength
No need for seasoning
Use – Used for flat type and no three dimensional contour shapes
Large scale casting
Choice of pattern materials depends on
 Size of casting
 No. of castings to be made from pattern
 Dimensional accuracy required
Metals:-
Advantages- Durability
Smooth surface finish
Light weight
Easily worked
Corrosion resistant
Use - For large scale casting production
Closer dimensional tolerance

Plastics :-
Advantages - Low weight
Easier formability
Smooth surface, durability
Do not absorb moisture
Dimensionally stable
Can be easily cleaned
Example – Cold setting epoxy resin with filler
Polyurethane foam – Light weight
Easily formed into any shape
Used for light duty work for small no of casting
For conventional casting
For single casting
Plastics have low ash content and it can be burned inside the mould.

TYPES OF PATTERNS:-

Various types of patterns depends on - Complexity of the job

- No of castings required
- Moulding procedure adopted
(a) Single piece pattern – Inexpensive and simplest one
Single piece
Simple job
Useful for Small scale production
Pattern will be entirely in the drag
One surface is flat and at the parting line
Used for very small scale production (Figure 3)
(b) Split or two piece pattern – Used for intricate casting
Split along the parting line
Used where depth of job is too high
Aligned with dowel pins fitted to cope
(c) Gated pattern – Gating and runner system are integrated with the pattern
Improves productivity
(d) Cope and drag pattern - Similar to split pattern
For cope and drag, separately attached gating system to metal plate
Heavy and inconvenient for handling
Useful for Continuous production
(e) Match plate pattern – Similar to cope and drag patterns with gating and risering system
mounted on a single matching plate
Pattern and match plate are made up of metal (Al)
Useful for small casting with high dimensional accuracy
Suitable for large scale production
Gating system is attached to the match plate
Expensive
(f) Loose piece pattern – Withdrawing of the pattern from the mould is difficult
Useful for highly skilled job
Expensive
(g) Follow board pattern – Used for structurally weak portions
Bottom board is modified as follow board
(h) Sweep pattern – Useful for axi-symmetrical and prismatic shape
Suitable for large scale production
(i) Skeleton pattern – Stripes of wood are used for building final pattern
Suitable for large casting

MOULDING MATERIALS

Different types of moulding materials are

-moulding sand
-system sand (backing sand )
-rebonded sand
-facing sand
-parting sand
-core sand

Choice of moulding materials depends on processing properties.

Properties_-

1) Refractoriness- Ability to withstand high temperature of molten metal so that it does not cause
fusion
Refractory materials - silica, zirconia, alumina
2) Green strength- Moulding sand containing moisture is known as green sand. The strength of
the green sand is known as green strength.
3) Dry strength- When moisture is completely expelled from the moulding sand, it is known as
dry sand and the strength of the sand is the dry strength.
4) Hot strength- After moisture elimination, the sand is exposed to higher temperature of molten
material. Strength of sand to hold the shape of mould cavity at this higher temperature is
known as hot strength.
5) Permeability – Moulding sand is porous, so it escapes gases through it. This gas evolution
capability of moulding sand is known as permeability.

Other properties include collapsibility, reusable, good thermal conductivity etc.

MOULDING SAND COMPOSITION-

Main ingredients of moulding sand are silica grain (SiO2), Clay (binder) and moisture (to activate
clay and provide plasticity)

(a) Silica sand- this is the major portion of the moulding sand. About 96% of this sand is silica grain.
Rests are oxides (Al2O3), sodium (Na2O +K2O) and magnesium oxide (MgO +CaO). Main source of
silica sand is river sand (with /without washing). Fusion point of sand is 14500C for cast iron and
15500C for steels. Grain size varies from micrometer to millimetre. The shape of the grains may be
round, angular, sub angular or very angular.

(b) Zircon sand- The main composition is zirconium silicate (ZrSiO2).

Composition- ZrO2- 66.25%

SiO2-30.96%
Al2O3-1.92%
Fe2O3-0.74%
Other - oxides
It is very expensive. In India, it is available at quilon beach, kerela. The fusion point of the sand is
2400oC.

Advantage - High thermal conductivity

High chilling power
High density
Requires very small amount of binder (3%)
Use - Precision steel casting
Precision investment casting
(c) Chromite sand – The sand is crushed from the chrome ore. The fusion point of the sand is 18000C.
It requires very small amount of binder (3%).

Composition- Cr2O3- 44%

Fe2O3 -28%
SiO2 -2.5%
CaO -0.5%
Al2O3 +MgO -25%
Use – heavy steel castings
Austenitic manganese steel castings
(d) Olivine sand- This sand composed of the minerals of fosterite (Mg2SiO4) and fayalite (Fe2SiO4).
It is versatile in nature.

CLAY :–

Clay is a binding agent mixed to the moulding sand to provide strength. Popular types of clay used are
kaolinite or fire clay (Al2O3.2 SiO2.2H2O) and Bentonite (Al2O3.4 SiO2.H2O nH2O). Kaolinite has a
melting point from 1750 to 17870C where as Bentonite has a melting temperature range of 1250 to
13000C. Bentonite clay absorbs more water and has increased bonding power. To reduce
refractoriness, extra mixtures like lime, alkalis and other oxides are added.
Bentonite is further of two types. (a) Western bentonite and (b) southern bentonite
Western bentonite – It is rich with sodium ion
It has better swelling properties
When it mixes with sand, the volume increases 10 to 20 times.
High dry strength, so lower risk of erosion
Better tolerance of variation in water content
Low green strength
High resistance to burn out
Southern Bentonite - It is rich with calcium ion
It has low dry strength and high green strength
Its properties can be improved by treating it with soda ash (sodium carbonate)
Water:- Used to activate the clay
Generally 2 to 8% of water is required

Other materials added:- Cereal binder – (2%) – to increase the strength

Pitch (by product of coke) – (3%) – to improve hot strength
Saw dust (2% ) – To increase permeability

Testing sand properties:-

Sample preparation can be done by mixing various ingredients like sand, clay and moisture.

During mixing, the lump present in sand should be broken up properly. The clay should be uniformly
enveloped and the moisture should be uniformly distributed.

The equipment used for preparation of moulding sand is known as Mueller. This is of two types.

(i) Batch Mueller- Consists of one/two wheels and equal no. of blades connected to a single
driving source. The wheels are large and heavy.
(ii) Continuous Mueller- In this type, there are two bowls with wheel and ploughs. The
mixture is fed through hopper in one bowl. After muelled, it is moved to another bowl.
This type of Mueller is suitable for large scale production.

Moisture content:-

1st method - 50g of moulding sand sample is dried at 1050C to 1100C for 2hrs. The sample is then
weighed.

Wt. diff * 2= % of moisture content

2nd method - Moisture teller can be used for measuring moisture content.

The Sand is dried suspending sample on fine metallic screen allowing hot air to flow through
sample. This method takes less time in comparison to the previous one.

3rd method - A measured amount of calcium carbide along with moulding sand in a separate cap is
kept in the moisture teller. Both should not come in contact with each other. Apparatus should be
shaken vigorously such that the following reaction takes place.

CaC2 + 2H2O – C2H2 + Ca(OH)2

The acetylene coming out will be collected in space above the sand raising the pressure. A pressure
gauge connected to the apparatus would give directly the amount of acetylene generated, which is
proportional to the moisture present.

Clay content:-

A 50g of sand sample is dried at 1050C to 1100C and is taken in a 1lt. glass flask. 475ml distilled
water and 25ml of a 1% solution of caustic soda (NaOH 25g/l) is added to it. The sample is
thoroughly stirred (5 mins). The sample is then diluted with fresh water upto 150 mm mark and then
left undisturbed for 10mins to settle. The sand settles at bottom and the clay floats. 125mm of this
water is siphoned off and again topped to the same level. The process is repeated till water above the
sand becomes clear. Then the sand is removed and dried by heating. The difference in weight
multiplied by 2 will give the clay % of sand.

Sand grain size:-

For sand grain size measurement, the moulding sand sample should be free from moisture and
clay. The dried clay free sand grains are place on the top sieve of sieve shaker (gradually decreasing
mesh size). The sieves are shaken continuously for 15 mins. After this the sieves are taken apart and
the sand over each sieve is weighed. The amount retained on each sieve is multiplied by the respective
weightage factor, summed up and then divided by the total mass f the sample which gives the grain
fineness number.

GFN= ƩMi fi/ Ʃfi

Mi= multiplying factor for the ith sieve

Fi= amount of sand retained on the ith sieve

Permeability:-

Rate of flow of air passing through a standard specimen under a standard pressure is known as
permeability number.

P=VH/pAT

V= volume of air= 2000cm3

H= height of sand specimen= 5.08cm

P= air pressure, 980Pa (10g/cm2)

A= cross sectional area of sand specimen= 20.268 cm2

T= time in min. for the complete air to pass through

Inserting the above standard values in the expression we get, P= 501.28/ P.T

Permeability test is conducted for two types of sands

(a) Green permeability – permeability of green sand

(b) Dry permeability – permeability of the moulding sand dried at 1050C to 1100C to remove the
moisture completely

Strength:-

Measurement of strength of moulding sand is carried out on the universal sand- strength testing M/C.
The strength can be measured in compression, shear & tension. The types of sand that can be tested
are green, dry, core sands.

Green compressive strength:-

Stress required to rupture the sand specimen under compressive loading refers to the green
compressive strength. It is generally in the range of 30 to 160KPa.

Green shear strength:-

The stress required to shear the specimen along the axis is represented as green shear strength. The
range is 10 to 50 KPa.

Dry strength:-

The test is carried out with a standard specimen dried between 105 to 110°C for 2 hours. The range
found is from 140 to 1800KPa.

Mould hardness:-

A spring loaded steel ball (0.9kg) is indented into standard sand specimen prepared. If no penetration
occurs, then the hardness will be 100. And when it sinks completely, the hardness will be 0 indicating
a very soft mould.

Moulding sand properties:-

The properties of moulding sand depends upon the variables like –

 sand grain shape and size

 Clay types and amount
 moisture content
 method of preparing sand mould

Sand grains:-

The grain shape could be round or angular. Angular sand grains require high amount of binder,
where as round sand grains have low permeability.

Similarly the grain size could be of coarse or fine. Coarse grains have more void space which
increases the permeability. Fine grains have low permeability, but provide better surface finish to the
casting produced. The higher the grain size of the sand, higher will be the refractoriness.

Clay and water:-

Optimum amount water is used for a clay content to obtain maximum green strength. During sand
preparation, clay is uniformly coated around sand grains. Water reacts with the clay to form a linkage
of silica - water – clay- water- silica throughout the moulding sand. Amount of water required
depends on the type and amount of clay present. Additional water increases the plasticity and dry
strength, but decreases the green strength. There is a maximum limit of green compression strength.
This type of sand is known as clay saturated sand and used for cast iron and heavy non ferrous metal
casting. This type of sand reduces some of the casting defects like erosion, sand expansion, cuts &
washes. These sands have green compression strength in a range of 100 to 250 KPa.

CORES:-

Cores are used for making cavities and hollow portions. These are made up of sand and are used in
permanent moulds. Core are surrounded by molten metal and therefore subjected to thermal and
mechanical conditions. So the core should be stronger than the moulding sand.

Desired characteristics of a core:-

(1) Dry strength- It should be able to resist the metal pressure acting on it.
(2) Green strength- It should be strong enough to retain its shape.
(3) Refractoriness- Core material should have higher refractoriness.
(4) Permeability- Core materials should have high permeability.
(5) Collapsibility- (ability to decrease in size). It is likely to provide resistance against shrinkage.
(6) Friability- Ability to crumble
(7) Smoothness- good finish to the casting
(8) Low gas emission- minimum

Core sand:-

The core sand should contain grains, binders and additives.

Sand- The silica sand without clay is used as a core sand material. Coarse silica is used in steel
foundries where as fine silica is used for cast iron and non ferrous alloys.

Binders:- The normal binders used are organic in nature, because this will burnt away by the heat of
molten metal and make the core collapsible during cooling. The binders generally used are linseed oil,
core oil, resins, dextrin, molasses etc. Core oils are the mixture of linseed, soy, fish, petroleum oils
and coal tar.

Types of cores:-

Two types:-

(a) Green sand core:- This is obtained by the pattern itself during moulding. Green sand
has low strength, so is not suitable for deep holes.
(b) Dry sand core:- This is made with special core sands in separate core box, baked &
placed in mould. Different types of dry sand cores are
-Unbalanced core -cover core -drop core
-balanced core -vertical core

Core prints:-

Core prints are used to position the core securely and correctly in mould cavity. It should take care of
the weight of the core and upward metallostatic pressure of molten metal.
 Gating System For Casting

Gating system:- It refers to all those elements connected with the flow of molten metal from ladle to
mould cavity.
Various elements:-

 pouring basin
 sprue
 sprue base well
 Runner runner extension
 Ingate
 Riser

Figure 4 Typical gating system

Requirements for defect free casting :-

 Mould should be filled in smallest time

 Metal should flow without turbulence
 Unwanted material should not enter into the cavity
 Atmospheric air should be prevented
 Proper thermal gradient should be maintained
 No gating or mould erosion should take place
 Enough metal should be there inside the mould cavity
 Economical
 Casting yield should be maximized
Elements

Pouring basin:- The molten metal is entered into the pouring basin, which acts as a reservoir from
which it moves into the sprue. The pouring basin stops the slag from entering into the mould cavity by
the help of skimmer or skim core. It holds the slag and dirt which floats on top and only allows the
clean metal. It should be always full during pouring and one wall should be inclined 450 to the
horizontal.
Function:-This will reduce the momentum of liquid flowing into mould
Design:- Pouring basin should be deep enough. Entrance into sprue be a smooth radius of 25mm.
pouring basin depth should be 2.5 times the sprue entrance diameter. A stainer core restricts the flow
of metal into the sprue and thus helps in quick filling of the pouring basin. It is a ceramic coated
screen with many small holes.

Sprue:-

It is a channel through which molten metal is pours into the parting plane where it enters into the
runner and gates to reach the mould cavity. When molten metal is moving from top to the cope, it
gains velocity and requires a smaller amount of area of cross-section for the same amount of metal to
flow. If the sprue is straight and cylindrical, then a low pressure area will be created at the bottom of
the sprue. Since the sand is permeable, it will aspire atmospheric air into the mould cavity causing
defects in the casting. That is why the sprue is generally made tapered to gradually reduce the cross-
section.

Exact tapering can be obtained by equation of continuity

AtVt= AcVc t denotes top section

 At = Ac c denotes choke section

By Bernoulli’s equation

At = Ac velocity α (potential head)2

The profile of the sprue should be parabolic. Metal at entry of the sprue is moving with a velocity of

V= . Hence At = Ac

H= actual sprue height

ht = h + H

Sprue Base Well :-

This is the reservoir for the metal at the bottom of sprue to reduce the momentum of the molten metal.
Sprue base well area should be 5 times the sprue choke area and well depth should be approximately
equal to that of the runner.
Runner:-

It is located at the parting plane which connects the sprue to its ingates. It traps the slag & dross from
moving into the mould cavity. This is normally made trapezoidal in cross section. For ferrous metals,
the runners should be kept in cope and ingates in drag.

Runner extension:-
This is provided to trap the slag in the molten metal.

GATES/ IN-GATES:-

These are the opening through which molten metal enters into the mould cavity. Depending on the
application, the various types of gates are

Top gate:- The molten metal enters into the mould cavity from the top. These are only used for
ferrous alloys. Suitable for simple casting shape. There may be chance of mould erosion.

Bottom gate:- This type of gating system is used for very deep moulds. It takes higher time for
filling of the mould cavity.

Parting gate:- This is most widely used gate in sand casting. The metal enters into the mould at the
parting plane. This is easiest and most economical.

Step gate:- These types of gates are used for heavy and large casting. The molten metal enters into the
mould cavity through a number of ingates arranged in vertical steps. The size of ingates are increased
from top to bottom ensuring a gradual filling of mould cavity.

RISER:-

Most alloys shrink during solidification. As a result of this volumetric shrinkage, voids are formed
which are known as hot spots. So a reservoir of molten metal is maintained from which the metal can
flow steadily into the casting. These reservoirs are known as risers.
Design considerations:- The metal in riser should solidify at the end and the riser volume should be
sufficient for compensating the shrinkage in the casting. To solve this problem, the riser should have
highervolume.
Types:-

(a) top riser- This type of riser is open to the atmosphere. It is very conventional & convenient to
make. It looses heat to the atmosphere by radiation & convention. To reduce this, insulation is
provided on top such as plaster of paris and asbestos sheets.
(b) blind riser :- This type of riser is surrounded by the moulding sand and looses heat very slowly.
(c) Internal rise:- It is surrounded on all sides by casting such that heat from casting keeps the metal
in the riser hot for a longer time. These are used for cylindrical shapes or hollow cylindrical portions
casting.

Chill:- Metallic chills are used to provide progressive solidification or to avoid the shrinkage cavities.
These are large heat sinks. Use of chill will form a hard spots, which needs further machining.

GATING SYSTEM DESIGN:-

The Liquid metal that runs through various channels, obeys Bernoullis equation according to which
the total energy head remains constant.

h+ + = constant (ignore frictional losses)

h = Potential head, m
P = pressure, Pa
V = liquid velocity, m/s
W= sp. wt. of liquid, N/m3
g = gravitational constant, 9.8 m/s2

According to the Law of continuity, the volume of metal flow at any section is constant.

Q= A1V1 = A2V2

Q= rate of flow, m3/s

A = Area, m2
V = Velocity, m/s

Pouring time:-

It is the time required for complete filing of mould cavity. If it is too long, then it requires a higher
pouring temp. and if is too short, there will be turbulent flow, which will cause defective casting. So
the pouring time depends on casting material, complexity of casting, section thickness and casting
size. Ferrous material requires less pouring time where as non-Ferrous materials require higher
pouring time.

Some Standard methods for pouring time :-

(1) Grey cast iron, mass< 450 kg

t = K (1.41 + ) ,s
K= fluidity of iron, inches/40
T = avg. section thickness, mm
W = Mass of casting, kg
 Pouring time for cast iron

(2) Grey cast iron, mass> 450 kg Casting mass Pouring time, s
20 kg 6 to 10
t = K (1.236 + ) ,s 100 kg 15 – 30
100000 kg 60 – 180
(3) Steel casting
t = (2.4335 – 0.3953 log W) ,s

(4) Shell moulded ductile iron, (Vertical pouring)

t = K1 , s
K1 = 2.080 for thin section
= 2.670 for 10 – 25 mm thick sections
= 2.970 for heavier section
(5) Cu alloy castings
t = K2 ,s
K2 = constant given by
Top gating – 1. 30
Bottom gating – 1.80
Brass – 1.90
Tin bronze – 2.80
T (mm) K3
(6) Intricately shaped thin walled casting – upto 450 kg 1.5 – 2.5 1.62
’
t = K3 ,s 2.5 – 3.5 1.68
’
W =mass of casting with gates and risers, kg 3.5 – 8 1.85
8 – 15 2.2
K3 = constant

(7) Above 450 kg &upto 1000 kg

’ T (mm) K4
t = K4 ,s Upto 10 1
10 – 20 1.35
for mass< 200kg; avg.section thickness – 25mm 20 – 40 1.4
Above 40 1.7
grey cast iron 40s
steel 20s
brass 15 – 45s

Choke area:-

The control area which meters the metal flow into the mould cavity so that the mould is completely
filled up within the calculated pouring time is known as choke area. It is mainly considered at the
bottom of the sprue.
The choke area by using Bernoulli’s equation,
A=
A = choke area
W = casting mass, kg
t = Pouring time, s
d = mass density of molten metal, kg/mm3
g = acceleration due to gravity, mm/s2
H = sprue height, mm
C = efficiency factor
The effective sprue height, H depends on type of gating system.

Top gate, H = h Bottom, H = h - Parting, H = h –

h = height of sprue
P = height of mould cavity in sprue
C = total height of mould cavity

C=

K1, K2 = loss coeff. – at changes of direction

A1, A2 = area down the stream from changes
A = choke area

Gating ratios:-
The gating ratios refers to the proportion of the cross-sectional areas between sprue, runner and
ingate.

There can be Two types of gating system.

(a) Non pressurized gating system:-
This has a choke at bottom of the sprue having total runner area and in gates area >sprue area.
This reduces the turbulence. This is useful for Al and Mg alloys. These have tapered sprue, sprue
base well and pouring basin.

Sprue : runner : ingate :: 1:4:4

Disadvantages :-
-Air inspiration
-casting yield- less

(b) Pressurized gating system:-

In this type, the in gate areas are smallest, thus maintaining a back pressure. Beacause of this, the
metal is more turbulent and flows full with a minimum air aspiration. This has a higher casting yield.
Mostly useful for ferrous castings.

Sprue : runner : ingate :: 1:2:1

SLAG TRAP SYSTEM:-

Runner extension:- This is a blind alley ahead the gates. The clean metal will go into the mould after
filling up the runner extension in which the slags and dross will be remained. This should be twice the
runner width.
Whirl gate:-
It utilizes the principle of centrifugal action to throw the dense metal to the periphery and retain the
lighter slag at the centre. The entry area should be 1.5 times the exit area.

Melting & casting Quality

Melting is a major factor which controls the quality of casting. The different methods for melting
foundry alloys are pit furnace, open hearth furnace, rotary furnace and cupola furnace etc. The choice
of furnace depends amount & type of alloy.

CUPOLA:-
It consists of a cylindrical steel shell with its interior lined with heat resisting fire bricks. There is a
drop door at the bottom after closing which proper sand bed could be prepared. This sand bed
provides proper refractory bottom for molten metal & coke. Above the sand bed, there is a metal
tapping hole which will be initially closed with clay known as ”bot”. Opposite & above the metal
tapping hole, there is a slag hole where slag is trapped. Above the slag hole, there is a wind box which
is connected to air blowers. Air enters to the cupola through the tuyeres. Above the charging platform,
there is a charging hole through which charge is put into the cupola. The charge consists of the pig
iron, scrap iron, coke and fluxes.

Figure 5 Schematic diagram of a cupola

Operation:-

First the drop door at the bottom is closed. Sand bed with slope towards tap hole is rammed. Coke bed
of suitable height is prepared above the sand bed and is ignited through the tap hole. After proper
ignition, alternate layers of charge, flux & coke are fed through the charge door. Then the charge is
allowed to soak in the heat and the air blast is turned on. Within 5 to 10mins, the molten metal is
collected through the tap hole. When enough metal is collected in the well of the cupola, the slag is
drained off through the slag hole. Then the molten metal is collected in the ladles and is transported to
the moulds with a minimum time loss.

Fluxes are added in the charge to remove the oxides & other impurities present in the metal. The flux
commonly used is lime stone (CaCO3) in a proportion of 2 to 4% of the metal charge. Others fluxes
used are dolomite, sodium carbonate, calcium carbide. Flux reacts with oxides to form compounds
having low melting point and lighter so that it will float on the metal pool.

Charge calculations:-

Carbon:- When charge comes through the coke bed, some amount of carbon is picked up by the metal
depending on the temperature and the time when the metal is in contact with the coke. It is of the
order of 0.15% carbon.

Silicon:- It is Oxidised in the cupola and there will be a loss of 10% silicon. It may be as high as 30%.
To increase the silicon content, ferrosilicon is added to the metal.

Manganese:- There is a loss of 15 to 20% manganese during melting process. The content of
manganese can be increased by the addition of ferromanganese.

Sulphur- There will a sulphur pick up in a range of 0.03 to 0.05%.

Other furnaces:

Other furnaces include

 Open hearth furnace

 Rotary furnace
 Crucible furnace
 Immersion heated furnace

Based on the source of heating, they can be classified as

 Electrical heating furnace (arc, resistance or induction)

 Fossil full fired furnace (solid, oil/gaseous fuel)

ELECTRIC ARC FURNACE:

For heavy steel castings, the open hearth type furnace with electric arc/oil fired would be suitable.
These furnaces are suitable for ferrous materials. It consists of a bowl shaped bottom known as hearth
lined with refractory bricks and granular refractory material. Heat is directly transferred to the charge
by electric arc from the electrodes. Tilting mechanism forward is used for metal tapping and backward is for
deslagging.

INDUCTION FURNACE:

This type of furnace is suitable for all types of materials. The heat source is isolated from charge and
slag. The flux gets necessary heat directly from the charge instead of the heat source. The stirring
effect of electric current would cause fluxes to be entrained in the melt.

Figure 6 Induction Furnace

 Module-II
CASTING CLEANING:-

The moulds should be broken at a temperature when no transformation occurs. For example, for
ferrous alloys, breaking should be done below 7000C, for thin and fragile casting, it should be below
4000C and for heavier castings, it should be at 5000C.

The process of cleaning of casting is known as fettling. This includes removal of cores, gates and
risers, cleaning casting surface, chipping of unnecessary projections etc. Dry sand core can be
removed by knocking off with iron bar, by means of core vibrator or by means of hydro blasting. The
selection of the method depends on the size, complexity and core material used.

The gates and risers can be removed by hammering, chipping, hack sawing, abrasive cut off and by
flame or by arc cutting. For brittle materials like grey cast iron, it can be done by hitting with hammer.
For steels and other materials, sawing with hacksaw or band saw is more convenient. For large size
gates and risers, flame or arc cutting is used. Similarly for removal of gates, abrasive cut off can be
used. Fins and other small projections after removal of gates can be chipped off by using hand tools
and pneumatic tools.

CASTING DEFECTS:-

a) Gas defects
b) Shrinkage cavities
c) Moulding material defects
d) Pouring metal defects
e) Metallurgical defects

(a) Gas defects:

A condition existing in a casting caused by the trapping of gas in the molten metal or by mould gases
evolved during the pouring of the casting. The defects in this category can be classified into blowholes
and pinhole porosity. Blowholes are spherical or elongated cavities present in the casting on the
surface or inside the casting. Pinhole porosity occurs due to the dissolution of hydrogen gas, which
gets entrapped during heating of molten metal.

Causes

The lower gas-passing tendency of the mould, which may be due to lower venting, lower permeability
of the mould or improper design of the casting. The lower permeability is caused by finer grain size of
the sand, high percentage of clay in mould mixture, and excessive moisture present in the mould.

 Metal contains gas

 Mould is too hot
 Poor mould burnout
(b) Shrinkage Cavities

These are caused by liquid shrinkage occurring during the solidification of the casting. To compensate
for this, proper feeding of liquid metal is required. For this reason risers are placed at the appropriate
places in the mould. Sprues may be too thin, too long or not attached in the proper location, causing
shrinkage cavities. It is recommended to use thick sprues to avoid shrinkage cavities.

(c) Molding Material Defects

The defects in this category are cuts and washes, metal penetration, fusion, and swell.

Cut and washes

These appear as rough spots and areas of excess metal, and are caused by erosion of moulding sand by
the flowing metal. This is caused by the moulding sand not having enough strength and the molten
metal flowing at high velocity. The former can be taken care of by the proper choice of moulding sand
and the latter can be overcome by the proper design of the gating system.

Metal penetration

When molten metal enters into the gaps between sand grains, the result is a rough casting surface.
This occurs because the sand is coarse or no mould wash was applied on the surface of the mould. The
coarser the sand grains more the metal penetration.

Fusion

This is caused by the fusion of the sand grains with the molten metal, giving a brittle, glassy
appearance on the casting surface. The main reason for this is that the clay or the sand particles are of
lower refractoriness or that the pouring temperature is too high.

Swell

Under the influence of metallostatic forces, the mould wall may move back causing a swell in the
dimension of the casting. A proper ramming of the mould will correct this defect.

Inclusions

Particles of slag, refractory materials, sand or deoxidation products are trapped in the casting during
pouring solidification. The provision of choke in the gating system and the pouring basin at the top of
the mould can prevent this defect.

(d) Pouring Metal Defects

The likely defects in this category are

 Mis-runs and
 Cold shuts.
A mis-run is caused when the metal is unable to fill the mould cavity completely and thus leaves
unfilled cavities. A mis-run results when the metal is too cold to flow to the extremities of the mould
cavity before freezing. Long, thin sections are subject to this defect and should be avoided in casting
design.

A cold shut is caused when two streams while meeting in the mould cavity, do not fuse together
properly thus forming a discontinuity in the casting. When the molten metal is poured into the mould
cavity through more-than-one gate, multiple liquid fronts will have to flow together and become one
solid. If the flowing metal fronts are too cool, they may not flow together, but will leave a seam in the
part. Such a seam is called a cold shut, and can be prevented by assuring sufficient superheat in the
poured metal and thick enough walls in the casting design.

The mis-run and cold shut defects are caused either by a lower fluidity of the mould or when the
section thickness of the casting is very small. Fluidity can be improved by changing the composition
of the metal and by increasing the pouring temperature of the metal.

Mould Shift

The mould shift defect occurs when cope and drag or moulding boxes have not been properly aligned.

Figure 1 Casting defects

CONTINUOUS CASTING:

In this process the liquid steel is poured into a double walled bottomless water cooled mould where a
solid skin is quickly formed having a thickness of 10 to 25 mm and a semi solid skin emerges from
open mould bottom which will be further solidified by water sprays. Molten metal is collected in a
ladle and is kept over a refractory lined intermediate pouring vessel called tundish and then poured
into water cooled vertical copper mould of 450 to 750 mm long. Before starting casting, a dummy
starter bar will be kept at the mould bottom. After starting casting process, as the metal level rises to a
height, the starter bar will be withdrawn at equal rate that of the steel pouring rate. Initially metal
freezes on to the starter bar as well as periphery of the mould. Solidified shell supports the steel liquid
as it moves downwards. The steel shell is mechanically supported by rollers as it moves down through
the secondary cooling zone with water.

Figure 2 Continuous casting plant

Figure 3 use of dummy starter bar at the start of continuous casting process
SQUEEZE CASTING:

It was first developed in Russia. It

is a combination of casting and
forging process. First the punch
and die are separated. The furnace
holds the liquid metal at a
requisite temperature. Then the
metal is put into the die cavity
and the punch is lowered to its
place forming a tight seal. The
metal is under a pressure of 50 to
140 mpa and looses heat rapidly
because of the contact with the
metallic die. Once the casting is
solidified, the punch is retracted.

Adv: very low gas entrapment.

Lower shrinkage cavity.

Lower die costs.

High quality surface.

Application: Mg, Al, Cu alloy

PRECESSION INVESTMENT CASTING:

The investment casting process also called lost wax process begins with the production of wax
replicas or patterns of the desired shape of the castings. A pattern is needed for every casting to be
produced. The patterns are prepared by injecting wax or polystyrene in a metal dies. A number of
patterns are attached to a central wax sprue to form an assembly. The mould is prepared by
surrounding the pattern with refractory slurry that can set at room temperature. The mould is then
heated so that pattern melts and flows out, leaving a clean cavity behind. The mould is further
hardened by heating and the molten metal is poured while it is still hot. When the casting is solidified,
the mould is broken and the casting taken out.

The basic steps of the investment casting process are:

1. Production of heat-disposable wax, plastic, or polystyrene patterns

2. Assembly of these patterns onto a gating system
3. “Investing,” or covering the pattern assembly with refractory slurry
4. Melting the pattern assembly to remove the pattern material
5. Firing the mould to remove the last traces of the pattern material
6. Pouring
 7. Knockout, cut off and finishing.

Adv: complex shapes, very fine details, close tolerance, better surface finish, no machining

Limitation: size and mass – maximum 5 kg

-expensive

Application: jewellery, surgical instruments, vanes and blades of gas turbine, impellers, claws of
movie camera

SHELL MOULD CASTING:

It is a process in which, the sand mixed with a thermosetting resin is allowed to come in contact with a
heated pattern plate (200 oC), this causes a skin (Shell) of about 3.5 mm of sand/plastic mixture to
adhere to the pattern.. Then the shell is removed from the pattern. The cope and drag shells are kept in
a flask with necessary backup material and the molten metal is poured into the mould.

This process can produce complex parts with good surface finish 1.25 µm to 3.75 µm, and
dimensional tolerance of 0.5 %. A good surface finish and good size tolerance reduce the need for
machining. The process overall is quite cost effective due to reduced machining and cleanup costs.
It is a process in which, the sand mixed with a
thermosetting resin is allowed to come in
contact with a heated pattern plate (200 oC),
this causes a skin (Shell) of about 3.5 mm of
sand/plastic mixture to adhere to the pattern..
Then the shell is removed from the pattern. The
cope and drag shells are kept in a flask with
necessary backup material and the molten
metal is poured into the mould.

This process can produce complex parts with

good surface finish 1.25 µm to 3.75 µm, and
dimensional tolerance of 0.5 %. A good surface
finish and good size tolerance reduce the need
for machining. The process overall is quite cost
effective due to reduced machining and cleanup
costs. The materials that can be used with this
process are cast irons, and aluminium and
copper alloys.

Moulding Sand in Shell Moulding Process

The moulding sand is a mixture of fine grained quartz sand and powdered bakelite. There are two
methods of coating the sand grains with bakelite. First method is Cold coating method and another
one is the hot method of coating.

In the method of cold coating, quartz sand is poured into the mixer and then the solution of powdered
bakelite in acetone and ethyl aldehyde are added. The typical mixture is 92% quartz sand, 5%
bakelite, 3% ethyl aldehyde. During mixing of the ingredients, the resin envelops the sand grains and
the solvent evaporates, leaving a thin film that uniformly coats the surface of sand grains, thereby
imparting fluidity to the sand mixtures.

In the method of hot coating, the mixture is heated to 150-180 o C prior to loading the sand. In the
course of sand mixing, the soluble phenol formaldehyde resin is added. The mixer is allowed to cool
up to 80 – 90o C. This method gives better properties to the mixtures than cold method.

Adv: dimensionally accurate

Smoother surface
Lowered draft angle
Thin section
No gas inclusion
Small amount of sand needed
Simple processing
Limitation: patterns are expensive
Size of casting – limited
Complicated shapes
Sophisticated equipments needed.
Application: cylinders
Break beam
Transmission planet carrier
Refrigerator valve plate
Small crank shaft
PERMANENT MOULD CASTING:

For large-scale production, making a mould, for every casting to be produced, may be difficult and
expensive. Therefore, a permanent mould, called the die may be made from which a large number of
castings can be produced. , the moulds are usually made of cast iron or steel, although graphite,
copper and aluminium have been used as mould materials. The process in which we use a die to make
the castings is called permanent mould casting or gravity die casting, since the metal enters the mould
under gravity. Some time in die-casting we inject the molten metal with a high pressure. When we
apply pressure in injecting the metal it is called pressure die casting process.

Adv: fine casting

Good surface finish
Close dimensional tolerance
Small core holes
Limitation: limited size
Not for complicated shapes
High cost
Not for all materials
Application: automobile piston
Gear blanks
Connecting rods
Aircraft fittings

CENTRIFUGAL CASTING:

In this process, the mould is rotated rapidly about its central axis as the metal is poured into it.
Because of the centrifugal force, a continuous pressure will be acting on the metal as it solidifies. The
slag, oxides and other inclusions being lighter, get separated from the metal and segregate towards the
center. This process is normally used for the making of hollow pipes, tubes, hollow bushes, etc.,
which are axisymmetric with a concentric hole. Since the metal is always pushed outward because of
the centrifugal force, no core needs to be used for making the concentric hole. The mould can be
rotated about a vertical, horizontal or an inclined axis or about its horizontal and vertical axes
simultaneously. The length and outside diameter are fixed by the mould cavity dimensions while the
inside diameter is determined by the amount of molten metal poured into the mould.

There are three types of centrifugal casting.

a) True centrifugal casting

b) Semi centrifugal casting
c) Centrifuging

a) True centrifugal casting:

- Hollow pipes, tubes, hollow bushes – axi-symmetric with concentric holes

 - Axis of rotation – horizontal, vertical or any angle.
- Sand moulds/ metal moulds
- Water cooling
Adv: superior mechanical properties
Directional solidification
No cores
No gates and runners
Limitation: - only for axi-symmetric concentric holes
- Expensive
b) Semi-centrifugal casting:
- More complicated- axi-symmetric jobs
- Vertical
c) Centrifuging:
- Not axi-symmetrical jobs
- Small jobs of any shape – joined by radial runners with a central sprue on revolving table.

DIE CASTING:

Die casting involves preparation of components by injecting molten metal at high pressure into a
metallic dies. It is also known as pressure die casting. Narrow sections, complex shapes, fine surface
details can be produced by using this casting process.

The dies have two parts. 1st one is a stationary half (cover die) which is fixed to die casting m/c. The
other one is a moving half (ejector die) which is moved out for extraction of casting. At the starting of
the process, two halves of the die should be placed apart. The lubricant is sprayed on die cavity and
then the dies are closed and clamped. The metal is injected into the die. After solidification, the die
will be opened and the casting will be ejected.

Vacuum die casting:

The major problem of die casting is that the air left in the cavity when the die is closed. Also back
pressure exists on the molten metal in the die cavity. It can be overcome by evacuating the air from
the die after the die is closed and metal is injected.

Adv:- Metal enters much faster – less filling time

No porosity
Parts exposed to air after solidification so no oxidation.
High tolerance
Fine microstructure

These are of two types:

(a) Hot chamber die casting

(b) Cold chamber die casting
(a) Hot chamber die casting:
A Gooseneck is used for pumping of the liquid metal in to the die cavity. It is made up of grey
C.I., ductile iron, cast steel. A plunger made up of alloy C.I., hydraulic operated moves up in the
gooseneck to uncover the entry port for the entry of liquid metal into the gooseneck.

(b) Cold chamber process:

- Used for zinc, lead and tin (low melting temp. alloys)
- Operation same as hot chamber
- Molten metal is poured in to shot chamber of m/c by either manually or by hand ladle / auto
ladle.
Adv: complex casting
Small thickness
High production rate (200 pieces/hr)
Good surface finish (1 micron)
Closer dimension tolerance
Long life of die
Economic

Limitation: maximum size

Not all materials
Air trapped
Application: carburettors
Crank cases
Magnetos
Automobile parts
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Submerged Arc Welding : Working, Equipment

and Its Applications
Submerged Arc Welding was discovered in the year 1935 by Rothermund and Jones, Kennedy. This
welding can be operated in semi-automatic mode otherwise in automatic mode. But generally, the
operation of this SAW can be done in automatic mode. Submerged-arc welding method is fixed and
extremely adaptable. This kind of welding involves in arranging the arc among a constantly fed electrode
as well as the workpiece. A layer of powdered flux generates a protecting gas shield as well as a slag to
protect the weld region. The arc can be submerged below the flux layer & in general, is not noticeable
throughout the welding process. In this, the weld quality is extensively influenced by the submerged arc
welding parameters like welding speed, welding current, arc voltage, electrode stick out which are closely
related to the calculation of the weld bead, This article discusses an overview of submerged Arc welding
method.
What is Submerged Arc Welding?
The definition of submerged arc welding is, it is one type of welding method where this welding arc can
travel under a layer of granular flux. In this type of welding, a tubular electrode otherwise consumable solid
can be fed constantly to the weld region. At the same time, a layer of granular fusible flux can be poured
over the weld zone which immersed the welding arc as well as defends it from atmospheric pollution.

The granulated flux includes compounds like lime, silica, manganese oxide, calcium fluoride, etc.
Whenever the flux is melted, then it turns into conductive as well as offers a current lane among the
workpiece & electrode. The solid layer of flux wraps the melted metal totally and stops the sprinkle and
covers the strong ultraviolet (UV) radiation vapors generated during the procedure.

Equipment of Submerged Arc Welding

The submerged arc welding can be built with main parts or equipment like Welding head, Flux hopper,
Flux, Electrode wire feed unit, Electrode, and Flux recovery unit. Welding head can be used to supply filler
as well as flux metal to the joint for welding.
 Equipment of Submerged Arc Welding

In Flux hopper, the flux can be stored as well as deliver to the welding joint. It controls the rate of
deposition of flux to the welding joint.
The granular flux is used to shield the welding arc, and it includes silica, lime, calcium fluoride, oxides of
calcium, manganese oxide, etc. It fed into the weld zone with the flow of gravity during the welding head
nozzle. Whenever it is melted, then it turns into conductive as well as conducts the current among the
workpiece & electrode.

The granular flux’s solid layer wraps the melted metal totally & stops the sprinkle and flash. It covers up
the UV radiations which is the characteristic of SMAW method. The minor part of the flux obtains melted &
shapes slag on the weld pond. It is detached after the welding method obtains completed. The higher
element of the flux performs like an insulator & encourages the deep transmission of heat toward the
workpiece.

Electrode wire feed unit offers nonstop electrode wire feed toward the welding joint, and it includes a reel
on which the electrode wire can be injured.
A consumable electrode can be used by the submerged arc welding which is a loop of bare round wire
with 1.5 mm to 10 mm diameter. It can be fed routinely throughout the welding gun, and the submerged
arc welding electrode composition depends on the welded material. The electrodes are available to weld
high carbon steel, mild steel, low and special alloy steels, stainless steel, etc. Generally, the electrodes are
covered by the copper to stop rusting & amplify electrical conductivity. They are obtainable within straight
length & coils.

Flux recovery unit is used to gather the not used flux present after welding, and after recovery, it can be
used another time for the joining.

Submerged Arc Welding Working

In this kind of welding, the flux begins for depositing on the joint to be welded. Whenever the flux is cold,
then it acts as an insulator. The arc can be started by moving the tool by the work portion. The arc struck
will constantly remain below a wide coating of flux, and the generated heat by the arc softens the granular
flux.

Once the flux is melted by the heat of the arc, then it will become highly conductive. The flow of current
begins to flow the electrode through the melted flux that can be in contact by the atmosphere. The minor
dissolved flux alters to wastage slag & which is detached after welding method finished.
At a fixed speed, the electrode from the roll is constantly fed toward the joint to be linked. If linking is
partially automatic, then the top of the welding can be moved physically along with the connection. In an
automatic submerged arc welding, a separate drive can be used to move the welding top above the
stationary job otherwise job moves beneath the head of the stationary welding.

With the help of the self-adjusting arc principle, the length of the arc is kept stable. When the arc length
reduces, the arc voltages will increase & this will increase the arc current.

Because of this, the rates of burn-off will increase & the arc length will be increased. The reverse
phenomenon arises when the arc length rises more than the regular length. For straight penetration as
well as for supporting the huge quantity of melted metal a support steel plate otherwise copper may be
used.

Advantages

The advantages of Submerged Arc Welding include the following.

This submerged arc welding process has high (45kg/h) deposit rate.
In automatic applications.
Very small welding smoke can be observed.
No edge training is required.
This method is used in indoor, and outdoor.
No chance of weld sprinkles because it is submerged within flux blanket.
Disadvantages

The disadvantages of Submerged Arc Welding include the following

The process is incomplete to some particular metals.

The application is imperfect to direct seams vessels, and pipes.
The flux usage is hard.
A health problem can be occurred due to the flux.
Slag elimination is desirable after welding.

Submerged Arc Welding Applications

The applications of Submerged Arc Welding include the following

The Submerged Arc Welding can be used to weld pressure vessels like boilers.
A lot of structural outlines, pipes, earth moving tools, shipbuilding, railroad construction, and
locomotives.
This type of welding can be used to repair machine parts.

Thus, this is all about Submerged Arc Welding. From the above information, finally, we can conclude that
this method can be used to produce the welding of metals at high temperature using an arm among the
workpiece as well as a metal. Here is a question for you, what is submerged arc welding defects?
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Gas Tungsten Arc Welding(GTAW)
Tungsten inert gas (TIG) welding is a type of arc welding that uses tungsten
as an electrode and the electrode is non-consumable in nature. This
welding is also known as Gas tungsten arc welding (GTAW).

Today in this paper we will see the Definition, Main parts or Equipment,
Working Principle, Advantages, Disadvantages, and Applications of
Tungsten Inert Gas Welding.

So Let’s start with the Definition first.

>>WHAT IS TUNGSTEN INERT GAS WELDING?
Tungsten inert gas (TIG) welding is one type of arc welding method where
we use a non-consumable tungsten electrode, to weld the two metallic
bodies. The weld spot is protected from contamination by helium, argon
and other inert shielding gases.

With this process, there is no longer a need for filler metal that is typically
used in the process of arc welding. This process makes this kind of weld
highly resistant to the effects of corrosion.

CONSTRUCTION OF TUNGSTEN INERT GAS WELDING MACHINE:

A Tungsten Inert Gas Welding Machine consists of the following
equipment:

● Power Supply
● Inert Gas Supply
● Welding Torch/Holder
● Tungsten Electrode
● Shielding Gas
● Filler Rod
Main Parts of Tungsten Inert Gas Welding Machine, Learn Mechanical

Power Supply:
In TIG welding we need a constant power supply because if there was a
fluctuation of current then it is hard for the welder to weld the joints
properly.
The power supply can be two types:

1. DC Power Supply
2. AC Power Supply
In the DC power supply, we can weld steels, nickel, titanium, etc.
And in AC power supply, we can weld magnesium, aluminum,
etc. materials.

Inert Gas Supply:

In TIG Welding, we need an inert gas supply to provide the
shielding to the weld area from the atmospheric gas (For
example, Oxygen, Nitrogen, and Hydrogen).

In general, Argon is used as an Inert gas supply in TIG Welding. We will

discuss this later on the Shielding gas section.
Welding Torch:
In TIG Welding the welding torch is designed to do either automatic and
manual operations. However, in terms of construction, both are the same,
in the manual torch, they are provided with a handle to hold, and in case of
automatic, they are designed to mount on an automatic machine.

Torches are provided with a cooling system either by water or air.

When the Ampere of the current is less than 200 A generally we use
air-cooling, but if it exceeds 200 A than we use water cooling to decrease
the temperature of the welding torch.

The inside portion of the welding torch is generally made of copper to

increase the conductivity of heat.

And the torches are provided with a holding arrangement (Port) to hold the
Tungsten electrode firmly.

Parts of Welding Torch

Tungsten Electrode:
In TIG Welding we use a non-consumable electrode made of Tungsten or
Tungsten Alloy.

Due to High-temperature resisting capacity (Melting Temp of Tungsten is

3,422 °C ) of tungsten rather than any other metal, that’s why we use the
tungsten electrode.

The diameter of the electrode is generally varies from 0.5 mm to 0.65 mm,
and the length varies between 75 mm to 610 mm.

Bunch of tungsten
Eletrode

Shielding Gas:
Shielding gases are used to protect the welding pool from atmospheric
gases like nitrogen, oxygen otherwise these gases can damage the welding
surface by creating porosity, blowhole, etc.

Choosing of Shielding gases depends on the types of welding as well as

the atmospheric condition along with the type of metal used for the
operation and many more.

However, generally, we use Argon as a shielding gas in TIG Welding.

Sometimes Argon-helium mixtures are also used in this type of welding.

Filler Rod:
As we already know, in TIG Welding, we use a non-consumable electrode
that is Tungsten, so in some cases, we need separate material to fillup the
gap between two joints.

The material of the filler rod can be anything, like carbon steel, aluminium,
etc. It generally depends on the type of joints, the work-piece material,
thickness and also the properties of the workpiece.
A Bunch of Filler Rods

WORKING PRINCIPLE OF TUNGSTEN

INERT GAS WELDING:
When we switch on the machine the high-frequency generator provides an
electric spark.

The electric spark is struck between the Workpiece and the Electrode either
by touching electrode by scrap material or by using a high-frequency unit.
We need to do this operation (Touching with the scrap material) at least 2-3
times to warm up the electrode before the actual operation started. Due to
this, we can save the breaking of the electrode tip.

In actual operation, the heat generated by the electric spark which fuses
the metal from the joint area and it produce a molten weld pool. The size of
the pool depends on the size of the electrode and the amount of the current
supplied by the generator.

The arc area is surrounded by an inert or reducing gas shield to protect the
weld pool and the non-consumable electrode.

The process may be operated autogenously, that means without filler

material or filler material may be added by feeding a consumable wire or
rod into the established weld pool.

Tungsten Inert Gas Welding produces very high-quality welds across a

wide range of materials with thicknesses up to about 8 or 10mm.

APPLICATIONS OF TIG WELDING:

This is specially used in the welding of refractory, sheet, and reactive
materials.

Tungsten Gas welding can be used with such a large variety of metals, the
process can be applied to several industries and aid in the creation and
repair of many items. This form of welding is common in the aerospace,
automotive, repair, and art fields.

● Aerospace: Aircraft and spacecraft are constructed in part by

means of TIG welding.
● Automotive: Safe and secure construction is essential in the
auto industry, as is making vehicles stand the test of time.
● Repair: TIG may be used in a number of repair applications.
From fixing a child’s toy, like a wagon or old-fashioned pedal
 car, to repairing aluminum tools, this welding method comes in
handy.

ADVANTAGES OF TIG WELDING:

The advantages of Tungsten Inert Gas Welding are the following:

● Tungsten welding offers a solution for welding critical joints,

and for situations where small or exceptionally precise welds
are required.
● It can be performed with a wide variety of metals
● And, when done correctly, it produces a high-quality and
high-purity weld compared with other joining processes, which
is crucial in many applications.
● It can be done in both automatic and manual.
● Overall, it is one of the most efficient ways to join two metals.
● No slag is produced.
● TIG Welding can be done in any position.

DISADVANTAGES OF TIG WELDING:

The disadvantages of TIG Welding are mentioned below:

● Tungsten welding can not be used for thicker sheets of metals.

● More complicated-High Skilled and professional workers are
needed.
● The safety issue, welders are exposed to the high intensity of
lights which can cause eye damage.
● The price of TIG welding services is high. However, costs will
vary depending upon the materials being welded, and the scope
of the project.
● It is a slow process welding.

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Gas metal Arc Welding (GMAW)
Plasma Arc Welding (PAW)
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The END unit-2

 UNIT 3: WELDING
Definition:
Welding is a process of joining similar or dissimilar materials by the application of heat and/or
pressure.

Principle of welding:
If two surfaces are brought together in such a way that nothing but the grain boundaries separate
them then the two bodies with adhere with a very large force resulting in what we called
welding.
Types of welding:

1
Fusion Welding Processes
Fusion welding is a joining process that uses fusion of the base metal to make the weld. The
three major types of fusion welding processes are as follows:

1. Gas welding: Oxyacetylene welding (OAW)

2. Arc welding:
Shielded metal arc welding (SMAW)
Gas–tungsten arc welding (GTAW)
Gas–metal arc welding (GMAW)
Submerged arc welding (SAW)

3. High-energy beam welding:

Laser beam welding (LBW)
Electron Beam Welding (EBW)

OXYACETYLENE WELDING

The Process
Gas welding is a welding process that melts and joins metals by heating them with a flame
caused by the reaction between a fuel gas and oxygen. Oxyacetylene welding (OAW), shown
in Figure 1, is the most commonly used gas welding process because of its high flame
temperature. A flux may be used to deoxidize and cleanse the weld metal. The flux melts,
solidifies, and forms a slag skin on the resultant weld metal. Figure 2 shows three different
types of flames in oxyacetylene welding: neutral, reducing, and oxidizing (4), which are
described next.

Three Types of Flames

A. Neutral Flame This refers to the case where oxygen (O2) and acetylene (C2H2) are mixed
in equal amounts and burned at the tip of the welding torch.
A short inner cone and a longer outer envelope characterize a neutral flame (Figure 2a). The
inner cone is the area where the primary combustion takes place through the chemical reaction
between O2 and C2H2, as shown in Figure 3. The heat of this reaction accounts for about two-
thirds of the total heat generated. The products of the primary combustion, CO and H2, react
with O2 from the surrounding air and form CO2 and H2O. This is the secondary combustion,
which accounts for about one-third of the total heat generated. The area where this secondary
combustion takes place is called the outer envelope. It is also called the protection envelope
since CO and H2 here consume the O2 entering from the surrounding air, thereby protecting the
weld metal from oxidation. For most metals, a neutral flame is used.

B. Reducing Flame When excess acetylene is used, the resulting flame is called a reducing
flame. The combustion of acetylene is incomplete. As a result, a greenish acetylene feather
between the inert cone and the outer envelope characterizes a reducing flame (Figure 2b). This
flame is reducing in nature and is desirable for welding aluminum alloys because aluminum
oxidizes easily. It is also good for welding high-carbon steels (also called carburizing flame in
this case) because excess oxygen can oxidize carbon and form CO gas porosity in the weld
metal.
C. Oxidizing Flame When excess oxygen is used, the flame becomes oxidizing because of the
presence of unconsumed oxygen. A short white inner cone characterizes an oxidizing flame

2
(Figure 2c). This flame is preferred when welding brass because copper oxide covers the weld
pool and thus prevents zinc from evaporating from the weld pool.

Figure 3.1 Oxyacetylene welding: (a) overall process; (b) welding area enlarged.

Figure 3.2 Three types of flames in oxyacetylene welding.

3
Figure 3.3 Chemical reactions and temperature distribution in a neutral oxyacetylene flame.

Advantages and Disadvantages of Gas welding

The main advantage of the oxyacetylene welding process is that the equipment
is simple, portable, and inexpensive. Therefore, it is convenient for maintenance
and repair applications. However, due to its limited power density, the welding speed is very
low and the total heat input per unit length of the weld
is rather high, resulting in large heat-affected zones and severe distortion.The
oxyacetylene welding process is not recommended for welding reactive metals
such as titanium and zirconium because of its limited protection power.

SHIELDED METAL ARC WELDING

The Process
Shielded metal arc welding (SMAW) is a process that melts and joins metals by heating them
with an arc established between a sticklike covered electrode and the metals, as shown in Figure
4. It is often called stick welding. The electrode holder is connected through a welding cable to
one terminal of the power source and the workpiece is connected through a second cable to the
other terminal of the power source (Figure 4a). The core of the covered electrode, the core
wire, conducts the electric current to the arc and provides filler metal for the joint. For electrical
contact, the top 1.5 cm of the core wire is bare and held by the electrode holder. The electrode
holder is essentially a metal clamp with an electrically insulated outside shell for the welder to
hold safely.
The heat of the arc causes both the core wire and the flux covering at the electrode tip to melt
off as droplets (Figure 4b). The molten metal collects in the weld pool and solidifies into the
weld metal.The lighter molten flux, on the other hand, floats on the pool surface and solidifies
into a slag layer at the top of the weld metal.

Functions of Electrode Covering

The covering of the electrode contains various chemicals and even metal powder in order to
perform one or more of the functions described below.

A. Protection It provides a gaseous shield to protect the molten metal from air. For a cellulose-
type electrode, the covering contains cellulose, (C6H10O5)x. A large volume of gas mixture of
H2, CO, H2O, and CO2 is produced when cellulose in the electrode covering is heated and

4
decomposes. For a limestone-(CaCO3) type electrode, on the other hand, CO2 gas and CaO slag
form when the limestone decomposes. The limestone-type electrode is a low-hydrogen type
electrode because it produces a gaseous shield low in hydrogen. It is often used for welding
metals that are susceptible to hydrogen cracking, such as high-strength steels.

B. Deoxidation It provides deoxidizers and fluxing agents to deoxidize and cleanse the weld
metal. The solid slag formed also protects the already solidified but still hot weld metal from
oxidation.

Figure 3.4 Shielded metal arc welding: (a) overall process; (b) welding area enlarged.
C. Arc Stabilization It provides arc stabilizers to help maintain a stable arc. The arc is an ionic
gas (a plasma) that conducts the electric current. Arc stabilizers are compounds that decompose
readily into ions in the arc, such as potassium oxalate and lithium carbonate. They increase the
electrical conductivity of the arc and help the arc conduct the electric current more smoothly.

D. Metal Addition It provides alloying elements and/or metal powder to the weld pool. The
former helps control the composition of the weld metal while the latter helps increase the
deposition rate.

Advantages and Disadvantages of SMAW

The welding equipment is relatively simple, portable, and inexpensive as compared to other
arc welding processes. For this reason, SMAW is often used for maintenance, repair, and field
construction. However, the gas shield in SMAW is not clean enough for reactive metals such
as aluminum and titanium. The deposition rate is limited by the fact that the electrode covering
tends to overheat and fall off when excessively high welding currents are used. The limited
length of the electrode (about 35 cm) requires electrode changing, and this further reduces the
overall production rate.

5
 GAS–TUNGSTEN ARC WELDING

The Process
Gas–tungsten arc welding (GTAW) is a process that melts and joins metals by heating them
with an arc established between a nonconsumable tungsten electrode and the metals, as shown
in Figure 5. The torch holding the tungsten electrode is connected to a shielding gas cylinder
as well as one terminal of the power source, as shown in Figure 5a. The tungsten electrode is
usually in contact with a water-cooled copper tube, called the contact tube, as shown in Figure
5b, which is connected to the welding cable (cable 1) from the terminal. This allows both the
welding current from the power source to enter the electrode and the electrode to be cooled to
prevent overheating. The workpiece is connected to the other terminal of the power source
through a different cable (cable 2). The shielding gas goes through the torch body and is
directed by a nozzle toward the weld pool to protect it from the air. Protection from the air is
much better in GTAW than in SMAW because an inert gas such as argon or helium is usually
used as the shielding gas and because the shielding gas is directed toward the weld pool. For
this reason, GTAW is also called tungsten–inert gas (TIG) welding. However, in special
occasions a noninert gas can be added in a small quantity to the shielding gas. Therefore,
GTAW seems a more appropriate name for this welding process. When a filler rod is needed,
for instance, for joining thicker materials, it can be fed either manually or automatically into
the arc.

Figure 3.5. Gas–tungsten arc welding: (a) overall process; (b) welding area enlarged.

6
Polarity
Figure 6 shows three different polarities in GTAW, which are described next.
A. Direct-Current Electrode Negative (DCEN) This, also called the straight polarity, is the
most common polarity in GTAW. The electrode is connected to the negative terminal of the
power supply. As shown in Figure 6a, electrons are emitted from the tungsten electrode and
accelerated while traveling through the arc. A significant amount of energy, called the work
function, is required for an electron to be emitted from the electrode. When the electron enters
the workpiece, an amount of energy equivalent to the work function is released. This is why in
GTAW with DCEN more power (about two-thirds) is located at the work end of the arc and
less (about one-third) at the electrode end. Consequently, a relatively narrow and deep weld is
produced.
B. Direct-Current Electrode Positive (DCEP) This is also called the reverse polarity. The
electrode is connected to the positive terminal of the power source. As shown in Figure 6b, the
heating effect of electrons is now at the tungsten electrode rather than at the workpiece.
Consequently, a shallow weld is produced. Furthermore, a large-diameter, water-cooled
electrodes must be used in order to prevent the electrode tip from melting. The positive ions of
the shielding gas bombard the workpiece, as shown in Figure 7, knocking off oxide films and
producing a clean weld surface. Therefore, DCEP can be used for welding thin sheets of strong
oxide-forming materials such as aluminium and magnesium, where deep penetration is not
required.
C. Alternating Current (AC) Reasonably good penetration and oxide cleaning action can both
be obtained, as illustrated in Figure 6c. This is often used for welding aluminum alloys.

Figure 3.6 Three different polarities in GTAW.

Figure 3.7 Surface cleaning action in GTAW with DC electrode positive

7
Electrodes
Tungsten electrodes with 2% cerium or thorium have better electron emissivity, current-
carrying capacity, and resistance to contamination than pure tungsten electrodes (3). As a
result, arc starting is easier and the arc is more stable. The electron emissivity refers to the
ability of the electrode tip to emit electrons. A lower electron emissivity implies a higher
electrode tip temperature required to emit electrons and hence a greater risk of melting the tip.

Shielding Gases
Both argon and helium can be used. Table 1 lists the properties of some shielding gases (6). As
shown, the ionization potentials for argon and helium are 15.7 and 24.5 eV (electron volts),
respectively. Since it is easier to ionize argon than helium, arc initiation is easier and the voltage
drop across the arc is lower with argon. Also, since argon is heavier than helium, it offers more
effective shielding and greater resistance to cross draft than helium. With DCEP or AC, argon
also has a greater oxide cleaning action than helium. These advantages plus the lower cost of
argon make it more attractive for GTAW than helium.
Because of the greater voltage drop across a helium arc than an argon arc, however, higher
power inputs and greater sensitivity to variations in the arc length can be obtained with helium.
The former allows the welding of thicker sections and the use of higher welding speeds. The
latter, on the other hand, allows a better control of the arc length during automatic GTAW.

Advantages and Disadvantages

Gas–tungsten arc welding is suitable for joining thin sections because of its limited heat inputs.
The feeding rate of the filler metal is somewhat independent of the welding current, thus
allowing a variation in the relative amount of the fusion of the base metal and the fusion of the
filler metal. Therefore, the control of dilution and energy input to the weld can be achieved
without changing the size of the weld. It can also be used to weld butt joints of thin sheets by
fusion alone, that is, without the addition of filler metals or autogenous welding. Since the
GTAW process is a very clean welding process, it can be used to weld reactive metals, such as
titanium and zirconium, aluminum, and magnesium.
However, the deposition rate in GTAW is low. Excessive welding currents can cause melting
of the tungsten electrode and results in brittle tungsten inclusions in the weld metal. However,
by using preheated filler metals, the deposition rate can be improved.

TABLE 3.1 Properties of Shielding Gases Used for Welding

8
 GAS–METAL ARC WELDING

The Process
Gas–metal arc welding (GMAW) is a process that melts and joins metals by heating them with
an arc established between a continuously fed filler wire electrode and the metals, as shown in
Figure 8. Shielding of the arc and the molten weld pool is often obtained by using inert gases
such as argon and helium, and this is why GMAW is also called the metal–inert gas (MIG)
welding process. Since noninert gases, particularly CO2, are also used, GMAW seems a more
appropriate name. This is the most widely used arc welding process for aluminum alloys.

Figure 3.8 Gas–metal arc welding: (a) overall process; (b) welding area enlarged.

Modes of metal transfer: Modes of metal transfer significantly affect the depth of penetration,
stability of weld pool and amount of spatter loss. Various forces cause the transfer of metal
into the weld pool. The mode of transfer depends on the intersection of these forces and governs
the ability of welding in different positions. The major forces which take part in this process
are those due to (i) gravity, (ii) surface tension, (iii) electromagnetic interaction

1. Metal transfer under the influence of gravity: The force due to gravity may be retaining
or detaching force, depending on whether the electrode is pointing upward or
downward.
2. Metal droplet under the action of surface tension: Surface tension always tends to retain
the liquid drop at the tip of the electrode. This force depends on the radius of the
electrode and the density of the liquid metal.
3. Metal transfer under the action of electromagnetic force: The electromagnetic force,
known as Lorenz force, is setup due to the interaction of the electric current with its
own magnetic field. This force acts in the direction of the current when the cross section

9
 of the conductor is increasing in the direction of the current. Similarly, the force acts in
the direction opposite to that of the current if the cross section of the conductor is
reducing in the direction of current. The hydrostatic pressure is created due to the
magnetic force. As a result, the liquid drop is projected along the line of the electrode,
independent of gravity.

All these forces interact in a complicated manner and give rise to two broad classes of metal
transfer.

1. Free flight transfer. (a) Globular, (b) spray transfer.

2. Short circuit transfer.

(A). Globular transfer: Discrete metal drop close to or larger then electrode diameter travel
across the arc gap under the influence of gravity. Globular transfer often is not smooth and
produce spatter at relatively low welding current. G T occurs regardless of the type of shielding
gases.

(b). Spray Transfer: Above a critical current level small discrete metal drops travel across the
arc gap under the action of electromagnetic force at much higher frequency and speed than in
globular mode. Metal transfer is much more stable and spatter free.

(c) Short circuit transfer: In Short circuit transfer the liquid drop at the tip of the electrode gets
in contact with the weld pool before being detached from the electrode. Thus, the arc is
momentarily short circuited. However, due to the surface tension and the electromagnetic force,
the drop is pulled into the weld pool and the contact with the electrode is broken. Short-
circuiting transfer encompasses the lowest range of welding currents and electrode diameters.
It produces a small and fast freezing weld pool that is desirable for welding thin sections and
overhead position welding.

Figure 3.9: Modes of metal transfer (a) Globular (b) Spray

Advantages and Disadvantages

Like GTAW, GMAW can be very clean when using an inert shielding gas. The main advantage
of GMAW over GTAW is the much higher deposition rate, which allows thicker workpieces
to be welded at higher welding speeds. The dual-torch and twin-wire processes further increase
the deposition rate of GMAW (12). The skill to maintain a very short and yet stable arc in
GTAW is not required. However, GMAW guns can be bulky and difficult-to-reach small areas
or corners.

10
 SUBMERGED ARC WELDING

The Process
Submerged arc welding (SAW) is a process that melts and joins metals by heating them with
an arc established between a consumable wire electrode and the metals, with the arc being
shielded by a molten slag and granular flux, as shown in Figure 10. This process differs from
the arc welding processes discussed so far in that the arc is submerged and thus invisible.The
flux is supplied from a hopper (Figure 10a), which travels with the torch. No shielding gas is
needed because the molten metal is separated from the air by the molten slag and granular flux
(Figure 10b). Direct-current electrode positive is most often used. However, at very high
welding currents (e.g., above 900A) AC is preferred in order to minimize arc blow. Arc blow
is caused by the electromagnetic (Lorentz) force as a result of the interaction between the
electric current itself and the magnetic field it induces.

Figure 3.10 Submerged arc welding: (a) overall process; (b) welding area enlarged.

Advantages and Disadvantages

The protecting and refining action of the slag helps produce clean welds in SAW. Since the arc
is submerged, spatter and heat losses to the surrounding air are eliminated even at high welding
currents. Both alloying elements and metal powders can be added to the granular flux to control
the weld metal composition and increase the deposition rate, respectively. Using two or more
electrodes in tandem further increases the deposition rate. Because of its high deposition rate,
workpieces much thicker than that in GTAW and GMAW can be welded by SAW. However,
the relatively large volumes of molten slag and metal pool often limit SAW to flat-position
welding and circumferential welding (of pipes). The relatively high heat input can reduce the
weld quality and increase distortions.

11
 LASER BEAM WELDING

The Process
Laser beam welding (LBW) is a process that melts and joins metals by heating them with a
laser beam. The laser beam can be produced either by a solid- state laser or a gas laser. In either
case, the laser beam can be focused and directed by optical means to achieve high power
densities. In a solid-state laser, a single crystal is doped with small concentrations of transition
elements or rare earth elements. For instance, in a YAG laser the crystal of yttrium– aluminum–
garnet (YAG) is doped with neodymium. The electrons of the dopant element can be selectively
excited to higher energy levels upon exposure to high-intensity flash lamps, as shown in Figure
11a.

Lasing occurs when these excited electrons return to their normal energy state, as shown in
Figure 11b.The power level of solid-state lasers has improved significantly, and continuous
YAG lasers of 3 or even 5 kW have been developed. In a CO2 laser, a gas mixture of CO2,
N2, and He is continuously excited by electrodes connected to the power supply and lases
continuously. Higher power can be achieved by a CO2 laser than a solid-state laser, for
instance, 15kW. Figure 12a shows LBW in the keyholing mode. Figure 12b shows a weld in a
13-mm-thick A633 steel made with a 15-kW CO2 laser at 20mm/s (18). Besides solid-state
and gas lasers, semiconductor-based diode lasers have also been developed. Diode lasers of
2.5kW power and 1mm focus diameter have been demonstrated (19). While keyholing is not
yet possible, conduction mode (surface melting) welding has produced full-penetration welds
with a depth–width ratio of 3 : 1 or better in 3-mm-thick sheets.

Reflectivity
The very high reflectivity of a laser beam by the metal surface is a well-known problem in
LBW. As much as about 95% of the CO2 beam power can be reflected by a polished metal
surface. Reflectivity is slightly lower with a YAG laser beam. Surface modifications such as
roughening, oxidizing, and coating can reduce reflectivity significantly (20). Once keyholing
is established, absorption is high because the beam is trapped inside the hole by internal
reflection.

Advantages and Disadvantages

Like EBW, LBW can produce deep and narrow welds at high welding speeds, with a narrow
heat-affected zone and little distortion of the workpiece. It can be used for welding dissimilar
metals or parts varying greatly in mass and size. Unlike EBW, however, vacuum and x-ray
shielding are not required in LBW. However, the very high reflectivity of a laser beam by the
metal surface is a major drawback, as already mentioned. Like EBW, the equipment cost is
very high, and precise joint fit-up and alignment are required.

12
Figure 3.11 Laser beam welding with solid-state laser: (a) process; (b) energy absorption and
emission during laser action.

Figure 3.12 Laser beam welding with CO2 laser: (a) process; (b) weld in 13-mm-thick A633 steel.

13
 ELECTRIC RESISTANCE WELDING

The electric resistance welding is commonly used. It can be applied to any metals. Electric
current passes through the materials being joined. The resistance offered to the flow of current
results in raising the temperature of the two metal pieces to melting point at their junction.
Mechanical pressure is applied at this moment to complete the weld. Two copper electrodes of
low resistance are used in a circuit.
The mechanical pressure or force required after the surfaces are heated to a plastic temperature
is approximately 0.3 N/m2 at the welded surface.
This method of welding is widely used in modern practice for making welded joints in sheet
metal parts, bars and tubes etc.

Parameter Affecting Resistance Welding

Successful application of Resistance welding process depends upon correct application and
proper control of the following factors.
1. Current: Enough current is needed to bring the metal to its plastic state of welding.

2. Pressure: Mechanical pressure is applied first to hold the metal pieces tightly between the
electrodes, while the current flows through them called weld pressure, and secondly when the
metal has been heated to its plastic state, to forge the metal pieces together to form the weld,
called forge pressure.

3. Time of Application: It is the cyclic time and the sum total of the following time period
allowed during different stages of welding
a. Weld Time Time period during which the welding current flow through the metal pieces to
raise their temp.
b. Forge Time Time period during which the forge pressure is applied to the metal pieces.
c. Hold Time Time period during which the weld to be solidify.
d. Off Time The period of time from the release of the electrodes to the start of the next weld
cycle.

4. Electrode contact area: The weld size depends on the contact area of the face of the
Electrodes

TYPES OF RESISTANCE WELDING

1. Spot welding 2. Seam welding, 3. Projection welding

Spot Welding

Spot welding is used to lap weld joints in thin metallic plates (up to 12.7 mm thick) for
mechanical
strength and not for tightness.
The metallic plates are overlapped and held between two copper electrodes. A high current,
depending upon plate thickness, at a very low volt-age (4-12 volts), is passed between the
electrodes. The contact resistance of the plates causes to heat rapidly to a plastic state.
Mechanical pressure is applied. Supply is cut-off for the metal to regain strength. The pressure
is released. The process is repeated at another portion of the plates.
Thus, spot joints at regular interval depending upon the strength required are obtained. The
electrodes are water cooled to avoid overheating and softening of the tips. Spot welding is

14
mainly used in the manufacture of automobile parts refrigerators, metallic toys, racks, frames,
boxes, radio chassis, etc.

Figure 3.13 (a) Spot Welding

Seam Welding

The metallic plates are held by two copper roller electrodes with one roller driven by motor so that the
plates are moved between the rollers at a suitable speed. The high current is passed between the
electrodes holding metallic plates pressed together with suitable force and pushes together to travel
between the revolving electrodes as showing in Fig. 7.29. The plates between the electrodes get heated
to welding (fusion) heat and welded continuously under constant pressure of rotating electrodes. This
is a quicker operation than spot welding and gives a stronger joint. The process is employed for pressure
tight joints on oil drums, tanks and boiler water pipes, refrigeration parts, motorcar body, utensils,
stoves, etc

Figure 3.13 (b): Seam welding

15
Projection Welding

There are raised projections in the workpiece at all points where a weld is desired as shown in
Fig. 13 (c). As the current is switched on the projection are melted and the workpieces pressed
together to complete the weld. The melted projections form the welds. This method enables
production of several spot welds simultaneously.

Figure 3.13 (c): Projection welding

ULTRASONIC WELDING

3.14

3.14

16
17
 Weld Defects

15
3.15

15
3.15

18
3.16
16

16
3.16

19
 3.17
17

17
3.17

20
 POWDER METALLURGY

• The Characterization of Engineering Powders

• Production of Metallic Powders
• Conventional Pressing and Sintering
• Alternative Pressing and Sintering Techniques
• Materials and Products for PM
• Design Considerations in Powder Metallurgy

©2002 John Wiley & Sons, Inc. M. P. Groover, “

Fundamentals of Modern Manufacturing 2/e”
 Powder Metallurgy (PM)
Metal processing technology in which parts are
produced from metallic powders
• In the usual PM production sequence, the powders
are compressed (pressed) into the desired shape and
then heated (sintered) to bond the particles into a
hard, rigid mass
Pressing is accomplished in a press-type machine
using punch-and-die tooling designed specifically
for the part to be manufactured
Sintering is performed at a temperature below the
melting point of the metal

©2002 John Wiley & Sons, Inc. M. P. Groover, “

Fundamentals of Modern Manufacturing 2/e”
 Why Powder Metallurgy is Important

• PM parts can be mass produced to net shape or near

net shape, eliminating or reducing the need for
subsequent machining
• PM process wastes very little material - about 97% of
the starting powders are converted to product
• PM parts can be made with a specified level of
porosity, to produce porous metal parts
Examples: filters, oil-impregnated bearings and
gears

©2002 John Wiley & Sons, Inc. M. P. Groover, “

Fundamentals of Modern Manufacturing 2/e”
 More Reasons Why PM is Important

• Certain metals that are difficult to fabricate by other

methods can be shaped by powder metallurgy
Example: Tungsten filaments for incandescent
lamp bulbs are made by PM
• Certain alloy combinations and cermets made by PM
cannot be produced in other ways
• PM compares favorably to most casting processes in
dimensional control
• PM production methods can be automated for
economical production

©2002 John Wiley & Sons, Inc. M. P. Groover, “

Fundamentals of Modern Manufacturing 2/e”
 Limitations and Disadvantages
with PM Processing
• High tooling and equipment costs
• Metallic powders are expensive
• Problems in storing and handling metal powders
Examples: degradation over time, fire hazards
with certain metals
• Limitations on part geometry because metal powders
do not readily flow laterally in the die during pressing
• Variations in density throughout part may be a
problem, especially for complex geometries

©2002 John Wiley & Sons, Inc. M. P. Groover, “

Fundamentals of Modern Manufacturing 2/e”
 PM Work Materials

• Largest tonnage of metals are alloys of iron, steel,

and aluminum
• Other PM metals include copper, nickel, and
refractory metals such as molybdenum and tungsten
• Metallic carbides such as tungsten carbide are often
included within the scope of powder metallurgy

©2002 John Wiley & Sons, Inc. M. P. Groover, “

Fundamentals of Modern Manufacturing 2/e”
Figure 16.1 - A collection of powder metallurgy parts (courtesy of
Dorst America, Inc.)

©2002 John Wiley & Sons, Inc. M. P. Groover, “

Fundamentals of Modern Manufacturing 2/e”
 Engineering Powders

A powder can be defined as a finely divided particulate

solid
• Engineering powders include metals and ceramics
• Geometric features of engineering powders:
Particle size and distribution
Particle shape and internal structure
Surface area

©2002 John Wiley & Sons, Inc. M. P. Groover, “

Fundamentals of Modern Manufacturing 2/e”
 Measuring Particle Size

• Most common method uses screens of different

mesh sizes
• Mesh count - refers to the number of openings per
linear inch of screen
A mesh count of 200 means there are 200
openings per linear inch
Since the mesh is square, the count is the same in
both directions, and the total number of openings
per square inch is 2002 = 40,000
Higher mesh count means smaller particle size

©2002 John Wiley & Sons, Inc. M. P. Groover, “

Fundamentals of Modern Manufacturing 2/e”
 Figure 16.2 - Screen mesh for sorting particle sizes

©2002 John Wiley & Sons, Inc. M. P. Groover, “

Fundamentals of Modern Manufacturing 2/e”
 Figure 16.3 - Several of the possible (ideal) particle shapes in
powder metallurgy

©2002 John Wiley & Sons, Inc. M. P. Groover, “

Fundamentals of Modern Manufacturing 2/e”
 Interparticle Friction and
Flow Characteristics
• Friction between particles affects ability of a powder
to flow readily and pack tightly
• A common test of interparticle friction is the angle of
repose, which is the angle formed by a pile of
powders as they are poured from a narrow funnel

©2002 John Wiley & Sons, Inc. M. P. Groover, “

Fundamentals of Modern Manufacturing 2/e”
Figure 16.4 - Interparticle friction as indicated by the angle of repose
of a pile of powders poured from a narrow funnel. Larger angles
indicate greater interparticle friction.

©2002 John Wiley & Sons, Inc. M. P. Groover, “

Fundamentals of Modern Manufacturing 2/e”
 Observations

• Smaller particle sizes generally show greater friction

and steeper angles
• Spherical shapes have the lowest interpartical friction
• As shape deviates from spherical, friction between
particles tends to increase

©2002 John Wiley & Sons, Inc. M. P. Groover, “

Fundamentals of Modern Manufacturing 2/e”
 Particle Density Measures

• True density - density of the true volume of the

material
The density of the material if the powders were
melted into a solid mass
• Bulk density - density of the powders in the loose
state after pouring
Because of pores between particles, bulk density
is less than true density

©2002 John Wiley & Sons, Inc. M. P. Groover, “

Fundamentals of Modern Manufacturing 2/e”
 Packing Factor = Bulk Density
divided by True Density
• Typical values for loose powders range between 0.5
and 0.7
• If powders of various sizes are present, smaller
powders will fit into the interstices of larger ones that
would otherwise be taken up by air, thus higher
packing factor
• Packing can be increased by vibrating the powders,
causing them to settle more tightly
• Pressure applied during compaction greatly
increases packing of powders through rearrangement
and deformation of particles

©2002 John Wiley & Sons, Inc. M. P. Groover, “

Fundamentals of Modern Manufacturing 2/e”
 Porosity

Ratio of the volume of the pores (empty spaces) in the

powder to the bulk volume
• In principle, Porosity + Packing factor = 1.0
• The issue is complicated by the possible existence of
closed pores in some of the particles
• If internal pore volumes are included in above
porosity, then equation is exact

©2002 John Wiley & Sons, Inc. M. P. Groover, “

Fundamentals of Modern Manufacturing 2/e”
 Chemistry and Surface Films

• Metallic powders are classified as either

Elemental - consisting of a pure metal
Pre-alloyed - each particle is an alloy
• Possible surface films include oxides, silica,
adsorbed organic materials, and moisture
As a general rule, these films must be removed
prior to shape processing

©2002 John Wiley & Sons, Inc. M. P. Groover, “

Fundamentals of Modern Manufacturing 2/e”
 Production of Metallic Powders

• In general, producers of metallic powders are not the

same companies as those that make PM parts
• Virtually any metal can be made into powder form
• Three principal methods by which metallic powders
are commercially produced
1. Atomization
2. Chemical
3. Electrolytic
• In addition, mechanical methods are occasionally
used to reduce powder sizes

©2002 John Wiley & Sons, Inc. M. P. Groover, “

Fundamentals of Modern Manufacturing 2/e”
 Gas Atomization Method
High velocity gas stream flows through an expansion
nozzle, siphoning molten metal from below and
spraying it into a container
• Droplets solidify into powder form

Figure 16.5 (a) gas

atomization method

©2002 John Wiley & Sons, Inc. M. P. Groover, “

Fundamentals of Modern Manufacturing 2/e”
 Figure 16.6 - Iron powders produced by decomposition of iron
pentacarbonyl; particle sizes range from about 0.25 - 3.0 microns
(10 to 125 -in) (photo courtesy of GAF Chemicals Corporation,
Advanced Materials Division)

©2002 John Wiley & Sons, Inc. M. P. Groover, “

Fundamentals of Modern Manufacturing 2/e”
 Conventional Press and Sinter
• After the metallic powders have been produced, the
conventional PM sequence consists of three steps:
1. Blending and mixing of the powders
2. Compaction - pressing into desired part shape
3. Sintering - heating to a temperature below the
melting point to cause solid-state bonding of
particles and strengthening of part
• In addition, secondary operations are sometimes
performed to improve dimensional accuracy,
increase density, and for other reasons

©2002 John Wiley & Sons, Inc. M. P. Groover, “

Fundamentals of Modern Manufacturing 2/e”
Figure 16.7 - Conventional powder metallurgy production sequence:
(1) blending, (2) compacting, and (3) sintering; (a) shows the
condition of the particles while (b) shows the operation and/or
workpart during the sequence
©2002 John Wiley & Sons, Inc. M. P. Groover, “
Fundamentals of Modern Manufacturing 2/e”
 Blending and Mixing of Powders
• For successful results in compaction and sintering,
the starting powders must be homogenized
• Blending - powders of the same chemistry but
possibly different particle sizes are intermingled
Different particle sizes are often blended to reduce
porosity
• Mixing - powders of different chemistries are
combined
PM technology allows mixing various metals into
alloys that would be difficult or impossible to
produce by other means

©2002 John Wiley & Sons, Inc. M. P. Groover, “

Fundamentals of Modern Manufacturing 2/e”
 Compaction

Application of high pressure to the powders to form

them into the required shape
• The conventional compaction method is pressing, in
which opposing punches squeeze the powders
contained in a die
• The workpart after pressing is called a green
compact, the word green meaning not yet fully
processed
• The green strength of the part when pressed is
adequate for handling but far less than after sintering

©2002 John Wiley & Sons, Inc. M. P. Groover, “

Fundamentals of Modern Manufacturing 2/e”
Figure 16.9 - Pressing in PM: (1) filling die cavity with powder by
automatic feeder; (2) initial and (3) final positions of upper and
lower punches during pressing, and (4) ejection of part

©2002 John Wiley & Sons, Inc. M. P. Groover, “

Fundamentals of Modern Manufacturing 2/e”
 Figure 16.11 - A 450 kN
(50-ton) hydraulic
press for compaction
of powder metallurgy
components. This
press has the
capability to actuate
multiple levels to
produce complex PM
part geometries (photo
courtesy Dorst
America, Inc.).

©2002 John Wiley & Sons, Inc. M. P. Groover, “

Fundamentals of Modern Manufacturing 2/e”
 Sintering

Heat treatment to bond the metallic particles, thereby

increasing strength and hardness
• Usually carried out at between 70% and 90% of the
metal's melting point (absolute scale)
• Generally agreed among researchers that the
primary driving force for sintering is reduction of
surface energy
• Part shrinkage occurs during sintering due to pore
size reduction

©2002 John Wiley & Sons, Inc. M. P. Groover, “

Fundamentals of Modern Manufacturing 2/e”
Figure 16.12 - Sintering on a microscopic scale: (1) particle
bonding is initiated at contact points; (2) contact points grow into
"necks"; (3) the pores between particles are reduced in size; and
(4) grain boundaries develop between particles in place of the
necked regions

©2002 John Wiley & Sons, Inc. M. P. Groover, “

Fundamentals of Modern Manufacturing 2/e”
 Figure 16.13 - (a) Typical heat treatment cycle in sintering; and
(b) schematic cross-section of a continuous sintering furnace

©2002 John Wiley & Sons, Inc. M. P. Groover, “

Fundamentals of Modern Manufacturing 2/e”
 Densification and Sizing
Secondary operations are performed to increase
density, improve accuracy, or accomplish additional
shaping of the sintered part
• Repressing - pressing the sintered part in a closed
die to increase density and improve properties
• Sizing - pressing a sintered part to improve
dimensional accuracy
• Coining - pressworking operation on a sintered part
to press details into its surface
• Machining - creates geometric features that cannot
be achieved by pressing, such as threads, side holes,
and other details

©2002 John Wiley & Sons, Inc. M. P. Groover, “

Fundamentals of Modern Manufacturing 2/e”
 Impregnation and Infiltration

• Porosity is a unique and inherent characteristic of

PM technology
• It can be exploited to create special products by
filling the available pore space with oils, polymers, or
metals
• Two categories:
1. Impregnation
2. Infiltration

©2002 John Wiley & Sons, Inc. M. P. Groover, “

Fundamentals of Modern Manufacturing 2/e”
 Impregnation

The term used when oil or other fluid is permeated into

the pores of a sintered PM part
• Common products are oil-impregnated bearings,
gears, and similar components
• An alternative application is when parts are
impregnated with polymer resins that seep into the
pore spaces in liquid form and then solidify to create
a pressure tight part

©2002 John Wiley & Sons, Inc. M. P. Groover, “

Fundamentals of Modern Manufacturing 2/e”
 Infiltration

An operation in which the pores of the PM part are filled

with a molten metal
• The melting point of the filler metal must be below
that of the PM part
• Involves heating the filler metal in contact with the
sintered component so capillary action draws the filler
into the pores
• The resulting structure is relatively nonporous, and
the infiltrated part has a more uniform density, as well
as improved toughness and strength

©2002 John Wiley & Sons, Inc. M. P. Groover, “

Fundamentals of Modern Manufacturing 2/e”
 Alternative Pressing and Sintering
Techniques
• The conventional press and sinter sequence is the
most widely used shaping technology in powder
metallurgy
• Additional methods for processing PM parts include:
Isostatic pressing
Hot pressing - combined pressing and sintering

©2002 John Wiley & Sons, Inc. M. P. Groover, “

Fundamentals of Modern Manufacturing 2/e”
 Materials and Products for PM

• Raw materials for PM are more expensive than for

other metalworking because of the additional energy
required to reduce the metal to powder form
• Accordingly, PM is competitive only in a certain range
of applications
• What are the materials and products that seem most
suited to powder metallurgy?

©2002 John Wiley & Sons, Inc. M. P. Groover, “

Fundamentals of Modern Manufacturing 2/e”
 PM Materials –Elemental Powders
A pure metal in particulate form
• Used in applications where high purity is important
• Common elemental powders:
 Iron
 Aluminum
 Copper
• Elemental powders are also mixed with other metal
powders to produce special alloys that are difficult to
formulate by conventional methods
 Example: tool steels

©2002 John Wiley & Sons, Inc. M. P. Groover, “

Fundamentals of Modern Manufacturing 2/e”
 PM Materials –Pre-Alloyed Powders

Each particle is an alloy comprised of the desired

chemical composition
• Used for alloys that cannot be formulated by mixing
elemental powders
• Common pre-alloyed powders:
 Stainless steels
 Certain copper alloys
 High speed steel

©2002 John Wiley & Sons, Inc. M. P. Groover, “

Fundamentals of Modern Manufacturing 2/e”
 PM Products
• Gears, bearings, sprockets, fasteners, electrical
contacts, cutting tools, and various machinery parts
• Advantage of PM: parts can be made to near net
shape or net shape
 They require little or no additional shaping after
PM processing
• When produced in large quantities, gears and
bearings are ideal for PM because:
 The geometry is defined in two dimensions
 There is a need for porosity in the part to serve
as a reservoir for lubricant

©2002 John Wiley & Sons, Inc. M. P. Groover, “

Fundamentals of Modern Manufacturing 2/e”
 PM Parts Classification System

• The Metal Powder Industries Federation (MPIF)

defines four classes of powder metallurgy part
designs, by level of difficulty in conventional pressing
• Useful because it indicates some of the limitations on
shape that can be achieved with conventional PM
processing

©2002 John Wiley & Sons, Inc. M. P. Groover, “

Fundamentals of Modern Manufacturing 2/e”
Figure 16.16 - Four classes of PM parts (side view shown;
cross-section is circular): (a) Class I - simple thin shapes,
pressed from one direction; (b) Class II - simple but thicker
shapes require pressing from two directions; (c) Class III - two
levels of thickness, pressed from two directions; and (d) Class
IV - multiple levels of thickness, pressed from two directions,
with separate controls for each level

©2002 John Wiley & Sons, Inc. M. P. Groover, “

Fundamentals of Modern Manufacturing 2/e”
 Design Guidelines for PM Parts - I

• Economics usually require large quantities to justify

cost of equipment and special tooling
Minimum quantities of 10,000 units are suggested
• PM is unique in its capability to fabricate parts with a
controlled level of porosity
Porosities up to 50% are possible
• PM can be used to make parts out of unusual metals
and alloys - materials that would be difficult if not
impossible to produce by other means

©2002 John Wiley & Sons, Inc. M. P. Groover, “

Fundamentals of Modern Manufacturing 2/e”
 Design Guidelines for PM Parts - II

• The part geometry must permit ejection from die after

pressing
This generally means that part must have vertical
or near-vertical sides, although steps are allowed
Design features such as undercuts and holes on
the part sides must be avoided
Vertical undercuts and holes are permissible
because they do not interfere with ejection
Vertical holes can be of cross-sectional shapes
other than round without significant difficulty

©2002 John Wiley & Sons, Inc. M. P. Groover, “

Fundamentals of Modern Manufacturing 2/e”
Figure 16.17 - Part features to be avoided in PM: side holes and (b)
side undercuts since part ejection is impossible

©2002 John Wiley & Sons, Inc. M. P. Groover, “

Fundamentals of Modern Manufacturing 2/e”
 Design Guidelines for PM Parts - III

• Screw threads cannot be fabricated by PM; if

required, they must be machined into the part
• Chamfers and corner radii are possible by PM
pressing, but problems arise in punch rigidity when
angles are too acute
• Wall thickness should be a minimum of 1.5 mm
(0.060 in) between holes or a hole and outside wall
• Minimum recommended hole diameter is 1.5 mm
(0.060 in)

©2002 John Wiley & Sons, Inc. M. P. Groover, “

Fundamentals of Modern Manufacturing 2/e”
Figure 16.19 - Chamfers and corner radii are accomplished but
certain rules should be observed: (a) avoid acute angles; (b) larger
angles preferred for punch rigidity; (c) inside radius is desirable; (d)
avoid full outside corner radius because punch is fragile at edge;
(e) problem solved by combining radius and chamfer

©2002 John Wiley & Sons, Inc. M. P. Groover, “

Fundamentals of Modern Manufacturing 2/e”
 1

UNIT 8: High Energy Rate Forming (HERF) Processes

Introduction:

The forming processes are affected by the rates of strain used.

Effects of strain rates during forming:

1. The flow stress increases with strain rates

2. The temperature of work is increases due to adiabatic heating.
3. Improved lubrication if lubricating film is maintained.
4. Many difficult to form materials like Titanium and Tungsten alloys, can be deformed
under high strain rates.

Principle / important features of HERF processes:

• The energy of deformation is delivered at a much higher rate than in conventional

practice.
• Larger energy is applied for a very short interval of time.
• High particle velocities are produced in contrast with conventional forming process.
• The velocity of deformation is also very large and hence these are also called High
Velocity Forming (HVF) processes.
• Many metals tend to deform more readily under extra fast application of force.
• Large parts can be easily formed by this technique.
• For many metals, the elongation to fracture increases with strain rate beyond the usual
metal working range, until a critical strain rate is achieved, where the ductility drops
sharply.
• The strain rate dependence of strength increases with increasing temperature.
• The yield stress and flow stress at lower plastic strains are more dependent on strain
rate than the tensile strength.
• High rates of strain cause the yield point to appear in tests on low carbon steel that do
not show a yield point under ordinary rates of strain.
 2

Advantages of HERF Processes

i) Production rates are higher, as parts are made at a rapid rate.

ii) Die costs are relatively lower.
iii) Tolerances can be easily maintained.
iv) Versatility of the process – it is possible to form most metals including difficult to form
metals.
v) No or minimum spring back effect on the material after the process.
vi) Production cost is low as power hammer (or press) is eliminated in the process. Hence it
is economically justifiable.
vii) Complex shapes / profiles can be made much easily, as compared to conventional
forming.
viii) The required final shape/ dimensions are obtained in one stroke (or step), thus
eliminating intermediate forming steps and pre forming dies.
ix) Suitable for a range of production volume such as small numbers, batches or mass
production.

Limitations:

i) Highly skilled personnel are required from design to execution.

ii) Transient stresses of high magnitude are applied on the work.
iii) Not suitable to highly brittle materials
iv) Source of energy (chemical explosive or electrical) must be handled carefully.
v) Governmental regulations/ procedures / safety norms must be followed.
vi) Dies need to be much bigger to withstand high energy rates and shocks and to prevent
cracking.
vii) Controlling the application of energy is critical as it may crack the die or work.
viii) It is very essential to know the behavior or established performance of the work metal
initially.

Applications:

i) In ship building – to form large plates / parts (up to 25 mm thick).

ii) Bending thick tubes/ pipes (up to 25 mm thick).
iii) Crimping of metal strips.
iv) Radar dishes
 3

v) Elliptical domes used in space applications.

vi) Cladding of two large plates of dissimilar metals.

(I) Explosive Forming

Introduction:
A punch in conventional forming is replaced by an explosive charge.
Explosives used can be:
• High energy chemicals like TNT, RDX, and Dynamite.
• Gaseous mixtures
• Propellants.

Factors to be considered while selecting an HERF process:

• Size of work piece

• Geometry of deformation
• Behavior of work material under high strain rates
• Energy requirements/ source
• Cost of tooling / die
• Cycle time
• Overall capital investment
• Safety considerations.

Types of explosive forming:

1) Unconfined type or Stand off technique

2) Confined type or Contact technique

1) Unconfined type (or Stand off technique)

Principle:
The work is firmly supported on the die and the die cavity is evacuated. A definite
quantity of explosive is placed suitably in water medium at a definite stand off distance from the
 4

work. On detonation of the explosive charge, a pressure pulse (or a shock wave) of very high
intensity is produced.

Fig. Unconfined Type Explosive Forming

A gas bubble is also produced which expands spherically and then collapses. When the
pressure pulse impinges against the work (plate or sheet0, the metal is deformed into the die
with a high velocity of around 120 m/s (430km/h).

The vacuum is necessary in the die to prevent adiabatic heating of the work which
may lead to oxidation or melting.

Role of water:

i) Acts as energy transfer medium

ii) Ensures uniform transmission of energy
iii) Muffles the sound of explosion
iv) Cushioning/ smooth application of energy on the work without direct contact.

Process Variables
i) Type and amount of explosive: wide range of explosive sis available.
ii) Stand off distance – SOD- (Distance between work piece and explosive): Optimum SOD
must be maintained.
iii) The medium used to transmit energy: water is most widely used.
iv) Work size:
 5

v) Work material properties

vi) Vacuum in the die

Advantages;
i) Shock wave is efficiently transmitted through water and energy is transmitted effectively
on the work
ii) Less noise
iii) Less probability of damage to work.
iv) Large and thick parts can be easily formed
v) Economical, when compared to a hydraulic press

Limitations:
i) Optimum SOD is essential for proper forming operation.
ii) Vacuum is essential and hence it adds to the cost.
iii) Dies must be larger and thicker to withstand shocks.
iv) Not suitable for small and thin works.
v) Explosives must be carefully handled according to the regulations of the government.

Applications:
• Ship building,
• Radar dish,
• Elliptical domes in space applications

2) Confined System ( or Contact Technique)

Principle:
The pressure pulse or shock wave produced is in direct contact with the work piece
(usually tubular) and hence the energy is directly applied on the work without any water
medium.
The tube collapses into the die cavity and is formed. It is used for bulging and flaring
operations.
 6

Fig. Confined (Contact) type Explosive Forming

Advantages:

i) Entire shock wave front is utilized as there is no loss in water.

ii) More efficient as compared to unconfined type.
iii)
Disadvantages:
i) More hazard of die failure
ii) Vacuum is required in the die
iii) Air present in the work piece (tube) is compressed leading to heating.
iv) Not suitable for large and thick plates.

Applications;
Bulging and flaring of tubes.

(II) Electro hydraulic Forming

Fig. Electro Hydraulic Forming

 7

Principle
A sudden electrical discharge in the form of sparks is produced between electrodes and
this discharge produces a shock wave in the water medium. This shock wave deforms the work
plate and collapses it into the die.
The characteristics of this process are similar to those of explosive forming. The major
difference, however, is that a chemical explosive is replaced by a capacitor bank, which stores
the electrical energy.
The capacitor is charged through a charging circuit. When the switch is closed, a spark
is produced between electrodes and a shock wave or pressure pulse is created. The energy
released is much lesser than that released in explosive forming.

Process Characteristics:
i) Stand off distance: It must be optimum.
ii) Capacitor used: The energy of the pressure pulse depends on the size of capacitor.
iii) Transfer medium: Usually water is used.
iv) Vacuum: the die cavity must be evacuated to prevent adiabatic heating of the work due
to a sudden compression of air.
v) Material properties with regard to the application of high rates of strain.

Advantages:
i) Better control of the pressure pulse as source of energy is electrical- which can be easily
controlled.
ii) Safer in handling than the explosive materials.
iii) More suitable if the work size is small to medium.
iv) Thin plates can be formed with smaller amounts of energy.
v) The process does not depend on the electrical properties of the work material.

Limitations:
i) Suitable only for smaller works
ii) Need for vacuum makes the equipment more complicated.
iii) Proper SOD is necessary for effective process.
 8

Applications:
They include smaller radar dish, cone and other shapes in thinner and small works.

(III) Electromagnetic forming

The electrical energy stored in a capacitor bank is used to produce opposing
magnetic fields around a tubular work piece, surrounded by current carrying
coils. The coil is firmly held and hence the work piece collapses into the die
cavity due to magnetic repelling force, thus assuming die shape.

Fig. Electro Magnetic Forming

Process details/ Steps:

i) The electrical energy is stored in the capacitor bank

ii) The tubular work piece is mounted on a mandrel having the die cavity to produce shape
on the tube.
iii) A primary coil is placed around the tube and mandrel assembly.
iv) When the switch is closed, the energy is discharged through the coil
v) The coil produces a varying magnetic field around it.
vi) In the tube a secondary current is induced, which creates its own magnetic field in the
opposite direction.
 9

vii) The directions of these two magnetic fields oppose one another and hence the rigidly
held coil repels the work into the die cavity.
viii) The work tube collapses into the die, assuming its shape.

Process parameters:
i) Work piece size
ii) Electrical conductivity of the work material.
iii) Size of the capacitor bank
iv) The strength of the current, which decides the strength of the magnetic field and the
force applied.
v) Insulation on the coil.
vi) Rigidity of the coil.

Advantages:
i) Suitable for small tubes
ii) Operations like collapsing, bending and crimping can be easily done.
iii) Electrical energy applied can be precisely controlled and hence the process is accurately
controlled.
iv) The process is safer compared to explosive forming.
v) Wide range of applications.

Limitations:
i) Applicable only for electrically conducting materials.
ii) Not suitable for large work pieces.
iii) Rigid clamping of primary coil is critical.
iv) Shorter life of the coil due to large forces acting on it.

Applications:
i) Crimping of coils, tubes, wires
ii) Bending of tubes into complex shapes
iii) Bulging of thin tubes.

___________________________________ END ________________________________

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VARIOUS FOUNDRY FURNACES
INDUCTION TILTING FURNACE GAS FIRED TILTING FURNACE
CONSTRUCTION OF CUPOLA FURNACE
Steel Shell:
• Cylindrical steel shell : 6-10 mm thick MS
steel plate, riveted/ welded
• Steel shell, Internally lined with refractory
bricks.
• The shell diameter varies from 1 to 2 meters
with a height of about 3 to 5 times the
diameter.

Foundation:
• The whole structure is erected on legs or
steel columns.
• A drop door is hinged to a supporting leg.
When the cupola is full of charge, a prop
support at the bottom door is provided so that
door remains close and do not collapse due to
the heavy weight of the charge.
• If the cupola is not in use, the drop door
allows for maintenance and repair work of the
furnace lining.
CONSTRUCTION OF CUPOLA FURNACE
Charging door:
Towards the top of the furnace there is an opening called charging
door . The charging door is used for feeding the charge containing
metal, coke and flux, into the furnace.
Air Blower:
An air blower is connected to the wind box by means of blast pipe. It
supplies the air to the wind box. A valve is provided in the blast pipe
to control the flow of air.
Tuyeres:
Air, which is needed for the combustion is blown through the tuyeres
located about (0.9 m) above the bottom of the furnace. Total area of
the tuyeres should be 1/5 to 1/6 of the cross-section area of the
cupola inside the lining at tuyere level.
Flow Meter:
The flow meter is installed in a cupola furnace to know the volume of
air passing. The amount of air needed to melt one tone of iron
depends upon the quality and quantity of coke and coke iron ratio.
CONSTRUCTION OF CUPOLA FURNACE
Tap Hole (Molten Metal Hole):
Slightly above the bottom and in the front there is a tap hole to allow
molten metal to be collected.
Slag Hole:
There is also a slag hole located at the side/ rear to take the slag out.
Chimney:
The portion of shell above charging hole is known as chimney. Its height
is generally 4 to 6 m. The chimney is provided with a filter screen and a
spark arrester. This facilitates a free escape of the waste gases and
arrest the sparks.
Modern blast furnaces generally range in size from 15 to
36 m have hearth diameters of 1 to 14 m and can
produce from 1,000 to almost 10,000 tons of pig iron BLAST FURNACE SPECIFICATION
daily.
The largest blast furnace in the
world is in South Korea, with a
volume around 6,000 m3. It can
produce around 5,650,000
tonnes (5.6 MT/ iron per year.)
OPERATION OF CUPOLA FURNACE
Preparation of Cupola:
Any slag around the tuyeres from previous run are cleaned. Any broken bricks are
repaired with a mixture of silica sand and fire clay. A layer of refractory material is
applied over the brunt area over the fire brick lining.

A bed of moulding sand is then rammed on the bottom to a thickness of about 6

inches (15 cm) or more, sloping towards the tap hole to ensure better flow of molten
metal. A newly built cupola should be thoroughly dried before firing.

Firing of Cupola:
A fire of wood is ignited on the sand bottom, when the wood burns well; coke is
dumped on the bed well from top. A bed of coke about 40 inches thick is next placed
on the sand i.e., slightly above the tuyeres.

The air blast is turned on at a lower blowing rate than normal for igniting the coke. A
measuring rod is used which indicates the height of coke bed. Firing is done about 3
hours before the molten metal required.
 OPERATION OF CUPOLA FURNACE
Charging the Cupola
• The charge is fed into the cupola through the charging door. Many factors,
such as the charge composition, quality of coke etc. affect the final
structure of the cast iron obtained.
• The charge is composed of pig iron, gray cast iron scrap, steel scrap,
foundry returns, coke as fuel and limestone as flux.
• These constituents form alternate layers of coke, limestone and metal in the
furnace. Besides limestone, fluorspar and soda ash are also used as flux
material.
• The function of flux is to remove the impurities in the iron and protect the
iron from oxidation.
Soaking of Iron:
• After charging the furnace fully, it is allowed to remain as such for about
1—1.5 hr. During this stage charge slowly gets heated up because the air
blast is kept shut this time and due to this the iron gets soaked.
OPERATION OF CUPOLA FURNACE
Starting the Air Blast:

The air blast is opened at the end of the soaking period. The tap
opening is kept closed till the metal melts and sufficient metal is
collected. As melting proceeds, the contents of the charge move
gradually downwards. The rate of charging must be equal to the
rate of melting so that the furnace is kept full throughout the heat.
Slagging and tapping

Closing the Cupola:

When no more melting is required, the feeding of charge and air

blast is stopped. The prop is removed, so that the bottom door
swings to open. The slag deposited is removed. The melting
period does not exceed 4 hours in most of the foundries. But, it
can be operated continuously for 10 hours or more.
ZONES OF CUPOLA FURNACE:
The following are the six important zones:

(i) Well or Crucible Zone:

It is the zone between top of the sand bed and bottom of the tuyeres. Molten metal
collected in this zone.

(ii) Combustion Zone:

It is situated normally 150mm to 300 mm above the top of the tuyeres and it is also
known as oxidizing zone.

Here, the combustion actually done, consuming all the oxygen from the air blast and
generates huge amount of heat. The temperature range for this zone is about 1500°C
to 1850°C. The heat produced in this zone is sufficient to meet the requirements of
other zones of cupola.
Reducing Zone:

It is the zone between the top of the combustion zone and the top level of
the coke bed. It is also known as protective zone.

The CO2 flowing upward through this zone reacts with hot coke and CO2, is
reduced to CO. Due to this reaction, the temperature gets reduced to about
1200°C. This zone protects the charge against oxidation as it has reducing
atmosphere in it.

(iv) Melting Zone:

It is the zone between the first layer of metal charge and above the
reducing zone. It is between 300 to 900 mm above the bed charge. The solid
metal charge changes to molten state picks up sufficient carbon in this
zone. The temperature attainable in this zone is in the range of 1600°C to
1700°C.
Preheating Zone:

It is the zone from above the melting zone to the bottom level of the charging door.
Charging materials are fed in this zone. The charge is preheated to about 1093°C
before they settle downwards to enter the melting zone. It is also known as
charging zone.

(vi) Stack Zone:

It is the empty portion of this furnace, which extends from above the charging zone
to the top of the furnace. It carries the hot gases generated within the furnace to
the atmosphere.
Capacity of Cupola Furnace:
The capacity of cupola is defined in terms of tones of liquid metal obtained per hour
of heat. It depends upon the dimensions of cupola, the efficiency of combustion,
combustion rate, and constituents of charging, etc.

The output of cupola can be increased by oxygen enrichment of air blast and by
better heat utilization of hot outgoing gases to preheat the furnace to about 180 to
270°C.
 Advantages of Cupola Furnace:
(i) Simple in construction and operation.
(ii) Low cast of construction, operation and maintenance.
(iii) Fast rate of production.
(iv) Does not require very skilled operators.
(v) Requires small floor area as compared to other furnaces.
(vi) Composition of melt can be controlled.
Limitations of Cupola Furnace:
(i) Close temperature control is difficult
(ii) Metallurgical change in chemical composition in various
elements
CUPOLA OPERATION ANIMATION
CUPOLA OPERATION
VDO
 CUPOLA CHARGE

• Pig Iron: 30% (Product of blast furnace)

• Scrap Iron: 30%,Old discarded machinery like automobiles, farming
equipments, machine tools etc.
• Foundry return: 40%, Gating system parts rejected castings
• Coke: 6:1 to 12:1, Iron : Coke ( Coke produced from coal, ash 6-12%
Max. Fixed carbon more than 86%
• Flux: 3-4% of Iron weight, Lime stone, Fluo-spar( Calcium fluoride),
Soda ash
• Ferro-manganese and ferro-silicon
• Air: About 900 m3 per ton of iron
 NUMERICALS
• Estimate the final composition of the cast iron produce with the following
charge compositions. Consider the iron charge weight is 1000 kg.

• [1] A cupola of 75 cm diameter has melting ratio of 10:1. If weight of the

coke (86% fixed carbon) charged is 32 kg, calculate
(a) The volume of air required for complete combustion
(b) The volume of air required to melt 500 kg of iron with same iron to coke
ratio.
• [2] A cupola, 0.75 m in diameter has a melting ratio of 10:1. Assume a
melting rate of 0.562kg/hr/cm, calculate the volume of air requirement
per hr. (Consider fixed carbon in coke as 86%.)
NUMERICAL ON CUPOLA CHARGE

Estimate the final composition of the cast iron produce with the following charge
compositions. Consider the iron charge weight is1000 kg.

C% Si% Mn% P% S%
Pig Iron 1 15% 3.5 2.5 0.7 0.17 0.016
Pig Iron 2 20% 3.5 3.0 0.65 0.11 0.018
Iron scrap 30% 3.4 2.3 0.50 0.22 0.030
Foundry Return 35% 3.3 2.5 0.65 0.16 0.035
Charge % Wt C Si Mn P S
kg
% Wt. % Wt. % Wt. % Wt. % Wt.
kg kg kg kg kg

Pig Iron 1 15 150 3.5 2.5 0.7 0.17 0.016

Pig Iron 2 20 200 3.5 3.0 0.65 0.11 0.018
Iron scrap 30 300 3.4 2.3 0.50 0.22 0.030
Foundry 35 350 3.3 2.5 0.65 0.16 0.035
Return
Total weight 1000
of various
elements
% of total
charge
Melting loss +15% -10 -15 Nil +0.03
or gain % % %
Actual value
Final
composition
A cupola of 75 cm diameter has melting ratio of 10:1. If weight of the coke (86% fixed carbon)
charged is 32 kg, calculate
(1) The volume of air required for complete combustion (take 23% oxygen in air on mass basis
and air density as 1.225 kg/ m3
(2) the volume of air required to melt 500 kg of iron with iron to coke ratio 8:1

• Ans
SOLUTION

• (1)
A cupola, 0.75 m in diameter has a melting ratio of 10:1. Assume a melting rate of
0.562kg/hr/cm2, calculate the volume of air requirement per hr. (Take 23% oxygen
in air on mass basis and air density as 1.225 kg/ m3. and coke has fixed carbon 92%
fixed carbon

• Ans.
LIP LADLES METAL POURING vdo

LIP LADLES
BOTTOM POURED LADLES vdo
SHAKE OUT AND CLEANING OF CASTINGS
• After the solidification of the casting, the mould is knocked out and
solidified casting is taken out of the moulding sand. This operation is
known as shake out.
• The process for removing a casting from a mold begins with determining
the right time and temperature to do the shakeout. After the liquid metal
has been poured, it must freeze before shake out.
• If casting is pulled too soon, the surface of the metal may chemically react
to the cool air with unwanted effects.
• Metal microstructures change based on its rates of heating and cooling.
Pulling a too-hot casting into relatively cold air can cause the structure to
become more brittle.
• There are formulae available for foundries to determine when a casting of
a certain volume and grade of metal should be pulled from the mold, but
many foundry workers use visual clues.
SHAKE OUT METHODS
Manually: Dumping, Striking with metal bar, Hammering
Mechanically: Vibratory platform or conveyor belt, Jolting grids
SHAKING OUT VIBRATORY CONVEYOR BELT
 CLEANING OF CASTINGS

• The shake out cast product is attached with sprues, risers, fins gates
and many times the fused moulding sand also get adhered to the
casting. All those operations which helps to give a casting good
appearance after shake out is known as cleaning of castings or fettling
.
• The cleaning /fettling operations includes
• 1. Removal of sprues, runners, gates, risers etc.
• 2. Removal of core sand
• 3. Removal of adhered/fused sand and oxide scale
• 4. Removal of fins, unwanted projections and Finishing
REMOVAL OF SPRUES GATES, RUNNERS, RISERS, ETC.

• FACTORS:
• Casting material
• Size and shape of the casting
• Size of runners/risers etc.
• METHODS:
• Chipping (chisel and hammers)
• Flogging
• Shearing
• Abrasive wheel slitting
• Flame cutting
• Plasma cutting
 REMOVAL OF FINS, UNWANTED PROJECTIONS AND
FINISHING
Removal of fins, unwanted projections
All unwanted projections like fins/flash/stumps of feeder heads and
ingates are dressed and flush with the surface of casting. Methods
available are
• Grinding : Portable, Swing frame, Pedestal
• Abrasive belt
• Chipping and sawing
Finishing methods:
• Machining
• Grinding
• Rotary filing
• Chemical treatment
• Polishing/buffing
SURFACE CLEANING
Removal of all adhering/ fused sand and oxide layers and to produce
smooth surface finish.
Manually:
• Wire brush, file, crow bar
Mechanically:
• Tumbling
• Sand blasting
• Shot blasting
• Air less blasting wheel abrator
• Hydro blasting
• Chemical cleaning
 Tumbling is the most common
technique employed by most of the
mass finishing process for finishing
TUMBLING parts by making it rub with the abrasive
media.
Vibratory Tumbling Machine
• The industrial vibratory tumbler
machine works on the principle of
vibration.

• It has a hemispherical bowl open at

the top side and the electrical motor
makes the bowl to vibrate.

• This vibration of the bowl, in turn,

vibrates the tumbling media and the
parts inside the bowl.

• Thus, the media will be rubbing the

part continuously for material
removal and does the job.
 Barrel Tumbling Machine

TUMBLING • The industrial barrel tumbler

machine has a barrel that is
designed to rotate along the central
axis.

• This barrel is connected to a motor

for the rotation.

• The machines will either have a

single barrel or multiple barrels.
Here, the media and the parts are
loaded into the barrel tumbler.

• The parts will slide/rub along with

the abrasive media as a result of
this rotation thus, the finishing
process is done.
ROTARY TUMBLER
TUMBLING MEDIA

The wide variety of media includes the plastic

media, ceramic media, stainless steel media,
silicon carbide media, organic media, and the
porcelain media.
SAND BLASTING • Sand-blasting is process of smoothening
and cleaning a hard surface by forcing
Sand particles across that surface at
high speeds using compressed air.
• A sand blasting system includes four
basic components: the air source, the
sand blasting cabinet, the dust collector,
and the blasting media.
• The air source is usually an air
compressor.
• The sand blasting cabinet holds the
object being blasted and the dust
collector removes dust from the cabinet.
• Sand blasting can remove paint, rust, and
residue from oxidation from materials
quickly and efficiently. Sandblasting can
also be used to remove scratches or
casting marks. of a casting surface.
SHOT BLASTING
Shot blasting is the first process carried out after the
casting has been removed from the sand mould. It is
also sometimes carried out a second time after heat
treatment to improve the surface finish.

Firstly, the casting is placed in an enclosed cabinet

on a rotating turntable. As the casting is rotated,
suitable shot media is accelerated towards the
workpiece at high speed and at varying angles
through a pressurised nozzle.

Various shot media can be used in shot blasting,

with different shapes, sizes, densities and materials
(such as ceramic, glass and steel). The choice of
material and size is dependent on the surface
treatment required.
SHOT BLASTING MACHINE vdo