UNIT III METAL FORMING PROCESSES

Hot working and cold working of metals – Forging processes – Open, impression and closed
die forging – forging operations. Rolling of metals– Types of Rolling – Flat strip rolling –
shape rolling operations – Defects in rolled parts. Principle of rod and wire drawing – Tube
drawing – Principles of Extrusion – Types – Hot and Cold extrusion.

3. 1 INTRODUCTION

Metal forming process is used to obtain the required shape and sized components by applying
higher stress on the material. Due to that the material deforms plastically and formed the
desired shape. Comparing to other process the production rate is too high.

Casting: There is no grain flow, so that it has poor mechanical properties.

Machining: Here the fibre is affected by interrupted due to machining.

Forging: The fibre of the metal has not been interrupted and continues along the entire length.

Fig shows the Grain flow

3.2 HOT WORKING AND COLD WORKING OF METALS

Hot working: Deforming the material at its recrystallization temperature is hot working. Most
of the metals have high recrystallization temperature. Metals like lead and tin have low
recrystallization temperature. That is equal to room temperature. In hot working mechanical
properties are improved. Some of the hot working process is hammer forging, drop forging
and hot spinning.

Cold working: Deforming the material at a temperature lower than the recrystallization
temperature of the work metal is called cold working. In cold working process greater
pressure is required. In hot working both the strength and hardness increases due to strain
hardening but ductility decreases. Cold working misprocess surface finish and dimensional
accuracy.

S.No Hot working Cold working

Hot working is done above Cold working is done below
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recrystallisation temperature recrystallisation temperature.
2 Refinement of grains takes place Grain structure is distorted.
Impurities and porosity are removed Impurities and porosities exist in metal
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from metals after hot working. after cold working.
4 Rapid oxidation or scaling of surfaces No oxidation and hence good surface

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 occurs which results in poor surface
finish finish is obtained.
5 Residual stresses are eliminated. Residual stresses are not eliminated.
Close dimensional tolerances cannot Close dimensional tolerances can be
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be maintained. obtained.
7 Toolling and handling costs are more. Tooling and handling costs are less
Mecanical properties such as Cold working icreases elongation,reduction
8 Toughness,ductility,elongation are in area , hardness, tensile strength. Fatigue
improved. strength are improved.
FORGING TOOLS -TONGS : Tongs are used mainly for holding work of many section.

FLATTER : Flatter is used to give smoothness & accuracy to articles which have already
been shaped by fullers and swages.

SWAGE : Swage is used to reduce/finish to round, square/hexagonal form. It consists of

two parts-The top part having a handle. The bottom part having a square shank.

ANVIL : Anvil is used for supporting hot job while hammering is done for shaping it into
various shapes. It is made of cast steel.

FULLERS : Fullers are used for necking down/to form depressions.

3.3FORGING PROCESSES

It is one of mechanical working process. Desired shape is obtained by the application of

compressive force. Types of forging process

• Hand forging :-Hand forging is emplayed only to shape a small number of light
forgings chiefly in repair shops.

• Hammer forgings :-Usually used for small item forging.

• Press forging :-Usually used for heavy item forging.

• Machine forging :-For medium sized and large articles requiring very heavy blows.

• Drop forging :-For mass production of identical parts.

3.4.1 OPEN DIE FORGING:

• Open die forging is performed on ingot, billets, bar. The deformation takes place
between flat or shaped dies without completely restricting the metal flow.

• Few “centimeter” workpieces to 30 meter workpiece can be processed.

• Several hundred “kg” workpiece can also be worked in open die forging.

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• The workpiece reduces their height/plastically deform by compressing it.

• The process is used for mostly large objects

• Tooling is simple, inexpensive and allows the production of a large variety of shapes.

Application. Forging ingots, large and bulky forgings, preforms for finished forgings.

3.4.2 CLOSED DIE FORGING

• Complex shapes with great accuracy forgings cannot be obtained in open die forging.

• In thismetal is deformed under high pressure between two dies (called tooling) or in a
closed cavity that contain a profile of the desired part.

• Closed die forging are commonly use where mass production of identical & more
complex shapes of greater accuracy are required.

• The process provide precision forging with close dimensional tolerance.

• Normally used for smaller components.

• Closed dies are expensive

Applications: forging of automobile parts.

3.4.3 IMPRESSION DIE FORGING

• The complex forging can be made in impression die forging.

• Impression die forging both die and punch have impressions, shapes which are
imparted onto the work piece.

• There is more constrained flow in this process. Moreover, the excess metal flows out
of the cavity, forming flash.

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Applications: Aircraft, Railroad and mining equipment

Flash in forging:

It is necessary to achieve complete filling of the forging cavity with out generating
excessive pressure against the die that may cause it to fracture.

Flash acts as a safety value for excess metal. It builds up high pressure to ensure that
the metal fills all recesses of the die cavity

3.5 FORGING OPERATIONS:

1. Upsetting
The thickness of the work reduces and length increases

2. Edging
The ends of the bar are shaped to requirement using edging dies.

3. Fullering

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 The cross sectional area of the work reduces as metal flows outward, away from
centre.
4. Drawing
The cross sectional area of the work is reduced with corresponding increase in length
using convex dies.

5. Swaging:
The cross sectional area of the bar is reduced using concave dies.
6. Piercing:
The metal flows around the die cavity as a moving die pierces the metal.
7. Punching:
It is a cutting operation in which a required hole is produced using a punching die.
Punch

8. Bending:
The metal is bent around a die/anvil.
3.7 DEFECTS IN FORGING:
1.) Unfilled section:

 As the name implies in this type of defect some of the forging section remain
unfilled. This is due to poor design of die or poor forging technic. This is also due to
less raw material or poor heating.

 This defect can be removed by proper die design, proper availability of raw material
and proper heating.

2.) Cold Shut:

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• Cold shut includes small cracks at corners. These defects occur due to improper
design of forging die. It is also due to sharp corner, and excessive chilling in forge
product.

• The fillet radius of the die should be increase to remove these defects.

3.) Scale Pits:

• Scale pits are due to improper cleaning of forged surface. This defect generally
associated with forging in open environment. It is irregular deputations on the surface
of forging.

• It can be removed by proper cleaning of forged surface.

4.) Die Shift:

• Die shift is caused by misalignment of upper die and lower die. When both these dies
are not properly aligned the forged product does not get proper dimensions.

• This defect can be removed by proper alignment. It can be done by provide half notch
on upper die and half on lower die so at the time of alignment, both these notches will
matched.

5.) Flakes:

• These are internal cracks occur due to improper cooling of forge product. When the
forge product cooled quickly, these cracks generally occur which can reduced the
strength of forge product.

• This defect can be removed by proper cooling.

6.) Improper Grain Growth:

• This defect occurs due to improper flow of metal in casting which changes predefine
grain structure of product.

• It can be removed by proper die design

3.8 ROLLING OF METALS:

• Rolling is the process in which the metals and alloys are plastically deformed into
semi finished or finished condition by passing between circular cylinders.

• Due to the frictional forces the metal is drawn into the opening.

• Metal Changes its shape due to high compressive forces.

• Both hot(for drastic shape changing) and cold rolling(for finishing) process are there.

• From the start ingot- blooms- billet- slaps which are further rolled into plat, sheet, rod,
bar, pipe, rails.

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ANGLE OF CONTACT:

• The metal contact each of the roller along the arc(AB), which is known as the “flat
strip” arc of contact.

• This arc corresponds to the central angle “α” called angle of contact or Bite.

FRICTION IN ROLLING:

• It depends on lubrication, work material and also on the temperature.

• In cold rolling the value of coefficient of friction is around 0.1 and in warm working it
is around 0.2.

• In hotrolling it is around 0.4.In hot rolling sticking friction condition is also seen and
then friction coefficient is observed up to 0.7.

• In sticking the hot wok surface adheres to roll and thus the central part of the strip
undergoes with a severe deformation.

3.9 TYPES OF ROLLING MILLS:

Classifications based on number of rolls

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 • Two high rolling mills • Multi roll rolling mills

• Three high rolling mills • Universal rolling mill

• Four high rolling mills • Planetary rolling mill

1)Two high rolling mills(single direction)

• Constant direction rolling

• Upper rolls is moveable one to set the distance.

• For single step reduction it is better one.

• If successive reduction is needed, distance is changed for each operations.

• Least expensive

Two high rolling mill Three high rolling mills

2) Three high rolling mills

• Three rolls with constant direction of rotation are arranged in a single vertical plane.

• Lifting table used to rise or lower the metal after each pass.

• Both top and bottom rolls are driver rolls

3) Four high rolling mills

• The bending of the roller is less if the diameter of the roller is high. At the same time
the power consumption and force P value also very high for big diameter rollers.

• To overcome this problem small Diameter rollers with larger diameter back up
rollers are used. This can be used for both directions.

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 Four high rolling mil Multiple rolling mil

4)Multiple roll mills

• By reducing the work roll diameter will produce bending effect on the back up
rolls.

• So in multiple roll mills a cluster of 6, 12, 20 rolls are used to manufacturing strips.
Form 0.001mm thick and 2000 mm wide.

5)Universal rolling mill

• Metal reduction occurs in both horizontal and vertical rolls.

• Vertical rolls are mounted either one side or both sides

• Horizontal rolls may be either two, three or four high arrangement.

• Used for roll wide strips, sheets, plates and slabs that requires both rolling edges and
also for rolling of Hsections.

Universal rolling mill Planetary roll mill

6)Planetary roll mills

• Consist of a pair of heavy backing rolls surrounded by a large number of

planetary rolls.

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 • Each planetary roll gives an almost constant reduction to the slab as it sweeps out a
circular path between the backing rolls and the slab.

• As each pair of planetary rolls ceases to have contact with the work piece, another
pair of rolls makes contact and repeat that reduction.

• The overall reduction is the summation of a series of small reductions by each pair of
rolls. Therefore, the planetary mill can hot reduces a slab directly to strip in one
pass through the mill.

• The operation requires feed rolls to introduce the slab into the mill, and a pair of
planishing rolls on the exit to improve the surface finish.

7.) Continuous rolling mills (tandem)

• Arrangement is similar to two- high rolling mill.

• Here the metal work piece passes from one roll pass to another and we get continuous
reduction in size.

• Used for mass production

3.10 SHAPE ROLLING OPERATIONS:

• Various shapes can be produced

• Straight and long structural shapes, solid bars, I-beams, channels, railroad rails.

3.10.1 Ring rolling:

• Thick ring is expanded into a large diameter ring with a reduced cross section.

• Ring is placed between two rolls and one of the roll is driven and the ring thickness
is reduced by bringing the rolls closer together as they rotate.

• The reduction in thickness of the ring is compensated by an increase in the rings

diameter.

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Advantages: Short production time, close tolerances and material savings.

Applications:

• Ball and roller bearing races

• Steel tires for railroad wheels

• Rings for pipes

• Flanges and rings

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3.10.2 THREAD ROLLING:

• It is a cold forming process by which straight or tapered threads are formed.

• Threads are formed on the rod or on wire with each stroke of a pair of flat
reciprocating dies. The die is made up of required thread impressions.

• One die is stationary and another is moveable one.

• The thread is formed by rotating and squeezing the blank between hardened steel dies.

• The displaced material forms the roots of the thread and, at the same time it forced
radially outward to form the crests.

• The die penetrated on the work, so this will create the impressions on workpiece.

Advantages:

• Here without loss of material the threads can be prepared.

• Good surface finish and good fatigue life products

• Machining thread cuts the grain flow lines of the material.

• Rolled threads have a grain flow pattern that improves the strength of the thread.

• The threads are rolled on metals in the soft conditions and threaded fastness such as
bolts are made by this process.

• The lubrication is very important in thread rolling process to minimize the defects.

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 • Spur and helical gears can be produced by cold rolling process similar to thread
rolling.

3.11 SEAMLESS TUBE MANUFACTURING PROCESS

It is also named as Mannesmann cross roll piercing OR Rotary tube piercing

STEP 1: the billet piercing

STEP 2: the shell elongation

STEP 3: the tube sizing

STEP 4: the tube finishing

• When a round bar is subjected to radial compressive forces, stresses develop at the
center of the bar.

• When it is subjected continuously to these cyclic compressive stresses, the bar begins
to develop a small cavity at its center, which then begins to grow.

• The axes of the rolls are skewed in order to pull the round bar through the rolls by the
axial component of the rotary motion.

• An internal mandrel assists the operation by expanding the hole and sizing the inside
diameter of the tube.

• The mandrel may be held in place by a long rod, or it may be a floating mandrel
without a support.

• Because of the severe deformation that the bar undergoes, the material must be high
in quality and free from defects (since internal defects may propagate rapidly and
cause premature failure of the part during forming.

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3.12 ROLLING DEFECTS

Two types of rolling defects are there Surface defects and Internal structural
defects

Surface Defects:
• Defects such as scale, rust, scratches, cracks etc.
• Due to impurities and inclusions in the original cast material or various other
conditions related to material preparation and rolling operations.
Wavy edges:
• Wavy edges can be caused because of the elastic deflection of the rolls due to the
rolling force acting on them.
• The edges of the rolled part gets thin and will experience compressive stresses.
• These compressive stresses cause the rolled sheet to be wavy at the edges.

Zipper cracks:
• These are also caused due to the bending of rolls under the rolling pressure similar to
wavy edges.
• Zipped cracks will be seen at the centre due to the bending deformation of the rolls.
Edge cracks:
• Due to the non uniform deformation of metal across the width.
• The lateral spread is more prominent towards the edges which will cause the edge
cracks.
Alligatoring:
• The top and bottom surface of the rolled part expands less compared to the middle
portion of the part. So the middle portion will tear.
• Due to the Defects in the original cast material also reason for alligatoring

3.13 EXTRUSION INTRODUCTION:

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• In extrusion, a cylindrical billet is forced through a die similar to squeezing toothpaste
from a tube in various cross sections.

• Large deformations can take place without fracture because the material is under high
triaxial compression. Products have elongated grain structure.

• Commonly extruded materials are aluminum, copper, steel, magnesium, and lead.

• variables are the temperature of the billet, the speed at which the ram travels, and the
type of lubricant used.

• Hot extrusion - Direct extrusion -Indirect extrusion

• Cold extrusion - Impact extrusion -Solid and hollow objects

• Hydrostatic extrusion

3.13.1 DIRECT EXTRUSION or FORWARD EXTRUSION:

• Alloys that do not have sufficient ductility at room temperature, or in order to

reduce the forces required, extrusion is carried out at elevated temperatures

• The heated billet is placed in a press which is operated by ram and a cylinder.

• The heated metal billet is pushed by ram and with the application of ram pressure the
metal first plastically fills the die.

• The billet is in hot condition so it develops oxide layer. The oxide layer will affect the
flow pattern of the extruded metal.

• It act as anabrasive layer. To prevent this a dummy block is placed ahead of the ram.
It is little smaller in diameter than the container. (it avoids the maximum friction force
resistance)
• As a result, a thin shell (skull) is removed from the chamber.
• Die materials --Hot worked die steels--Zirconia dies
• Lubrications ---Oil or graphite Pressure---- 30 to 700 MPa

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 Advantages:

 cost of equipment and tooling are less

 Used for large reduction
 Long and complex shapes can be produced
Disadvantages:
 lubrication is necessary
 Possibility of oxide scaling is high
 Surface finish and dimensional accuracy is poor
3.13.2 INDIRECT OR REVERSE OR BACKWARD OR INVERTED EXTRUSION

• Indirect extrusion process the die moves towards the unextruded billet.

• The heated metal is placed in the container and the force is applied by the power
operated hollow ram.

• Due to that the extruded metal is passed through the hollow ram.

• Die materials --Hot worked die steels--Zirconia dies

• Lubrications ---Oil or graphite Pressure---- 30 to 500 MPa

Advantages:

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 • Indirect extrusion there is no billet- container friction, since there is no relative
motion.
• Thus, indirect extrusion is used on materials with very high friction, such as high
strength steels.
Applications:
 Long and complex shapes can be produced
 Process is very economical
Disadvantages:
 Surface finish and dimensional accuracy is poor
 Lubrication is necessary
3.13.3 COLD EXTRUSION OR IMPACT EXTRUSION

• In Cold extrusion the working temperature is slightly elevated temperature.

• The working principle of this process is that the work material is placed between the
die and ram.

• The punch is connected with the ram.

• When the sudden impact is given to the ram, the metal flows plastically in the upward
direction, metals such as aluminium, tins are extruded in an impact extrusion.

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 • The metal flows up along the surface of the punch forming a cup shaped component.

• When the punch moves up, the stripper OR compressed air is used to separate the
component from the punch.

Application:

• Cold extrusion is used widely for components in automobiles, motorcycles, bicycles,

and appliances and in transportation and farm equipment.

• The various items of daily use such as tubes for shaving creams, tooth pastes and
paints, condenser cans and thin walled products are impact extruded.

Advantage:
• Cold extrusion process improves the physical properties of a metal and to produce a
good accurate dimensioned objects.
• The production rate is fairly high giving about 60 components per minute.
• The main advantages of this process are, it speeds up the product uniformity and
lowers scrap yield.

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Disadvantage:
 This process suitable for production for light components with soft materials
 Deformation is limited to small reduction.

3.13.4 HYDROSTATIC EXTRUSION:

• In hydrostatic extrusion, the billet is smaller in diameter than the chamber (which is
filled with a fluid), and the pressure is transmitted to the fluid by a ram.

• The fluid pressure results in triaxial compressive stresses acting on the workpiece and
thus improved formability. Brittle materials can also be extruded by this process.

• Pressure involved is more than 1700MPa.

Advantage:

• No friction between container and billet

• Brittle material can be extruded by this method.
• Uniform flow of material
• No billet residue is left on the walls of container
Disadvantage:
• Difficult to contain the fluid under the effects of high pressure (2Gpa)
• Decreased process efficiency in terms of billet to container volume ratio
• Billets have to be prepared by tapering one end so that it matches the die entry angle.

3.14 DRAWING OF WIRE AND ROD

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 • It is a cold working process in which the workpiece is pulled through a tapered hole in
a die so, the diameter is reduced.

• Wire cannot be hot rolled economically smaller than 5mm dia. So cold working is
mainly used in wire production

• The starting material input for drawing is from the extruded or rolled rods of 5 to
9mm.

Wire drawing

• Wire drawing die geometry is typically a bell shaped one. Wire drawing is usually
done in multiple steps using 4 to 12 dies.

• Wire is drawn product having less than 5mm diameter.

• The one end of the wire is gripped with a plier or carriage which pulls the rod through
all zones of the die hole where it under goes deformation or elongation. Then it is
rolled on a power reel.

• Then the power reel rotates at a proper speed and pulls the wire.

• Wire speed is 25m/s.

Rod drawing:

• Here the product must remain straight.

• The maximum length depends upon the carriage movement distance

• Here a moving chain arrangement is used to pull the rod with a help of hook.

• The pull capacity is 10kN to 1500kN.

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 • Drawing speed may be For larger size rod 1500mm/s --For smaller size rod 1.5m/s

Application:

 Used to produce the round, rectangle, square and hexagonal shapes

 Used to make the hydraulic systems of vehicles and industrial machineries.

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