UNit 2 MEC 305
UNit 2 MEC 305
UNit 2 MEC 305
2.1. INTRODUCTION
Metal forming can be defined as a process in which the desired size and shape are obtained
through the deformation of metals plastically under the action of externally applied forces.
Metal forming processes like rolling, forging, drawing etc. are gaining ground lately. It is due
to the fact that metal forming is the wasteless process which is highly economical. They give
high dimensional accuracy, easy formability for complex shapes and good surface finish with
desired metallurgical properties.
The metal forming is based upon the plastic deformation of metals. For finding out the
complete information of the stresses and strains that developed in the metal due to application
of loads, comprehensive study and calculations are required.
To start with, there are three conditions to be satisfied, while going for stress estimation:
• There should be equilibrium at all points.
• The volume should remain same before and after the forming.
• Stress-strain relationship of material should be maintained.
The main objective is to find out the yield stress developed in the material body and its
distribution in the material. This helps in estimating the load required for the initiation of the
process and its maximum value that a body can bear. If the body is under single load e.g., only
tensile load or only compressive load is applied to a body, then the yield stress can be measured
easily by stress-strain diagram, but in reality different loads are there on body which make the
process complex and thus also make it difficult to find out the yield stress distribution in the
body.
Elastic deformation of a material is its power of coming back to its original position after
deformation when the stress or load is removed i.e., deformation completely disappears after
removal of load.
The plastic deformation means that the material undergoes some degree of permanent
deformation without failure on application of load. Plastic deformation will take place only
after the elastic range has been exceeded. Plastic deformation is important in case of forming,
shaping, extruding and many other hot and cold working processes.
Due to this various metal can be transformed into different products of required shape and size.
This conversion into desired shape and size is affected either by the application of pressure,
heat or both.
The plastic deformation of metals may occur in the following ways
(1) By slip
(2) By formation of twins
(3) Deviations from regular positions of atoms
(4) Breakdown of structure.
2. Since the working is done in cold state, hence no oxide formation on the surface and
consequently,
good surface finish is obtained.
3. Greater dimensional accuracy is achieved.
4. Easier to handle cold parts and also economical for small sizes.
5. Better mechanical properties are achieved.
I. OPEN-DIE FORGING
Open-die forging is a hot forging process in which metal is shaped by hammering or pressing
between flat or simple contoured dies (see Fig. 2.1). In open die forging the dies do not
completely cover the workpiece. Instead, there are open spaces that allow various aspects of
the workpiece to move from direct hot die contact, and to cooler open areas. In this type of
forging, metals are worked above their recrystallization temperatures. Because the process
requires repeated changes in workpiece positioning. The workpiece cools during open die
forging below its hot-working or recrystallization temperature. It must be reheated before
forging can continue.
(b) Applications
Open-die processes can produce:
1. Step shafts, solid shafts (spindles or rotors) whose diameter increases or decreases at multiple
locations along the longitudinal axis.
2. Hollow cylindrical shapes, usually with length much greater than the diameter of the part
Length, wall thickness, internal and outer diameter can be varied as needed.
3. Contour-formed metal shells like pressure vessels, which may incorporate extruded nozzles
and other design features.
Most engineering metals and alloys can be forged via conventional impression-die processes,
among them: carbon and alloy steels, tool steels, and stainless, aluminum and copper alloys,
and certain titanium alloys.
Applications
1. Part geometry’s range from some of the easiest to forge simple spherical shapes, block-like
rectangular solids, and disc-like configurations to the most intricate components with thin and
long sections that incorporate thin webs and relatively high vertical projections like ribs and
bosses.
2. Although many parts are generally symmetrical, others incorporate all sorts of design
elements (flanges, protrusions, holes, cavities, pockets, etc.) that combine to make the forging
very non-symmetrical.
3. In addition, parts can be bent or curved in one or several planes, whether they are basically
longitudinal, equidimensional or flat.
Impression die forging is further classified as drop, press and machine forging:
(i) Drop forging: It gets its name from the fact that the upper half of the die is dropped onto
the lower half. Drop forgings are made by squeezing the metal at forging heat into shaped
impressions cut in heavy steel blocks called dies. The job is divided equally in upper and lower
die block. When the upper die block falls on the lower die, block metal is squeezed in the die
cavity due to impact force. The die block falls from a height of 3 to 5 m. The bottom die block
is held by set screws on to the base and top is raised by certain mechanism after its free fall. A
workpiece may be forged by a series of punch and die operations (or by several cavities in the
same die) to gradually change its shape.
The process involves several steps:
1. The first two steps are called fullering and edging. Here, the cross-sectional area of the metal
is reduced in some areas and gathered in other areas. This also starts the fibrous grain flow.
2. The third step is referred to as blocking. The shape of the part is not pronounced hence, it
may take several drops in the blocking cavity of the die. In step three, flash begins to appear.
3. The fourth step is called finishing. Here, the final shape of the part is completed.
4. The last step is called trimming. Holes are cleared and the flash is removed from the forging.
Drop forging requires machining to obtain dimensional tolerances and good surface finish.
(ii) Press forging: Press forging is a process similar to kneading, where a slow-continuous
pressure is applied to the area to be forged. The pressure will extend deep into the material and
can be completed either cold or hot. A cold press forging is used on a thin, annealed material,
and a hot press forging is done on large work such as armor plating, locomotives and heavy
machinery. In this type, only one blow is given as compared with number of blows in drop
forging. In press forging number of stages are used and only in last stage die cavity is used to
get finished forging. Dies may have less draft, and the forging comes nearer to the desired
sizes. Press forging are shaped at each impression with a single smooth stroke and they stick
to the die impression more rigidly. Unless some provision is made, the escape of air and excess
die lubricant may be difficult. Thus, press-forging dies require a mechanical means for ejecting
the forging.
Press forging are generally more accurate dimensionally than drop forging. The cost of the
process is three to four times than that in drop forging but with press forging, unskilled labour
can be used and production rate is higher. The working conditions with the press are better as
there is no noise and vibrations.
(iii) Machine forging: The chief difference between hand forging and machine forging is that
in the latter technique various types of machine powered hammers or presses are used instead
of hand sledges. The power hammer can be mechanical or pneumatic type. The stroke of the
hammer varies from 350 mm to 1000 mm and corresponding speeds range from 200 to 800
blows per minute. These machines enable the operator to strike heavy blows with great rapidity
and thus to produce forgings of large size and high quality as swiftly as required by modern
production-line methods. Another advantage of machine forging is that the heavier the blows
struck during forging, the greater the improvement in the quality of metallic structure. Fine
grain size in the forging, which is particularly desirable for maximum impact resistance, is
obtained by working the entire piece. With large, hand-forged metal, only the surface is
deformed, whereas the machine hammer or press will deform the metal throughout the entire
piece.
Machine forging operations are frequently accomplished by use of a series of dies mounted on
the same press or hammer. The dies are arranged in sequence so as to form the finished forging
in a series of steps. After the piece has been partially formed by one stroke, it is moved to the
next die for further shaping on the next stroke.
Mechanical forging presses of the crank type have found wide application in forging practice.
The operative units of the press are powered from motor mounted on the press frame. They are
used for the production of rivets, screws, and nuts where a high operating speed is desired. In
capacity, they range from 50,000 to 8,000,000 kg and speeds from 35 to 90 strokes per minute.
Most engineering metals and alloys can be forged with closed die forging processes; among
them are carbon and alloy steels, aluminum alloys and copper alloys.
Applications
Precision forgings, hollow forgings, fittings, elbows, tees, etc.
2.5. ROLLING
It is the process of reducing the thickness or changing the cross-section of a long workpiece by
compressive forces applied through a set of rolls. One effect of the hot working rolling
operation is the grain refinement brought about by recrystallization, which is shown in Fig. 2.4.
Coarse grain structure is broken up and elongated by the rolling action. Because of the high
temperature, recrystallization starts immediately and small grains begin to form. These grains
grow rapidly until recrystallization is complete. Growth continues at high temperatures, if
further work is not carried on, until the low temperature of the recrystalline range is reached.
(a)
(b) (c)
(1) Two-high rolling mill: It is basically of two types i.e., non-reversing and reversing rolling
mill. The two high non-reversing rolling stand arrangements is the most common arrangement.
In this the rolls always move in only one direction, while in a two-high reversing rolling stand
the direction of roll rotation can be reversed. This type of stand is particularly useful in reducing
the handling of the hot metal in between the rolling passes. About 30 passes are required to
reduce a large ingot into a bloom. This type is used in blooming and slabbing mills.
(2) Three-high rolling mill: It is used for rolling of two continuous passes in a rolling sequence
without reversing the drives. After all the metal has passed through the bottom roll set, the end
of the metal is entered into the other set of the rolls for the next pass. For this purpose, a table-
tilting arrangement is required to bring the metal to the level with the rolls. Such type of
arrangement is used for making plates or sections.
(3) Four-high rolling mill: It is generally a two-high rolling mill, but with small sized rolls.
The other two rolls are the backup rolls for providing the necessary rigidity to the small rolls.
It is used for both hot and cold rolling of wide plates and sheets.
(4) Cluster rolling mill: It uses backup rolls to support the smaller work rolls. In this type of
mill, the roll in contact with the work can be as small as 1/4 in. in diameter. Foil is always
rolled on cluster mills since the small thickness requires small-diameter rolls.
2.6 EXTRUSION
Extrusion is the process that forces metal to flow through a shape-forming die. The metal is
plastically deformed under compression in the die cavity. Extrusion produces only compressive
and shear forces in the stock without any tensile force, which makes high deformation possible
without tearing the metal. It is a hot-working process which, like forging, rolling, etc., uses the
good deformability of heated metallic materials for shaping them. A metal billet heated to the
appropriate temperature is fed into the cylindrical container of the extrusion press and is forced
by the action of a ram through a steel die whose orifice has the desired shape to produce the
solid or hollow section (Fig. 2.7).
The process is generally used to produce profiled sections, thin-walled tubular parts with heavy
flanges, straight tubular shapes, and hollow bar products. Typical products produced are bolts,
screws or stepped shafts.
2. Indirect (reverse, inverted or backward) extrusion: Indirect extrusion is a process that
forces the metal confined in the cavity to flow in a direction opposite to that of the ram travel
(see Fig. 2.9).
Fig. 2.9: Indirect Extrusion
Here, the die moves toward the billet; thus, except at the die, there is no relative motion at the
billet-container interface. As a consequence, the frictional forces are lower and the power
required for extrusion is less than for direct extrusion. In practice, a hollow ram carries a die,
while the other end of the container is closed with a plate. Frequently, for indirect extrusion,
the ram containing the die is kept stationary, and the container with the billet is made to move.
Backward extrusion is useful in forming a variety of cylindrical shapes such as nuts, sleeves
and tubular rivets.
3. Combined extrusion: Combined extrusion uses a combination of forward extrusion and
backward extrusion. The metal is confined inside a matrix between the lower and upper
punches. This forces the metal to flow both up and down. The extruded part is lifted from
the die on the upward stroke of the slide by a lift out on the bed of the press. Some aspects
of combined extrusion are:
1. It is fast
2. It can complete parts in few steps
3. It can produce large quantities with low unit costs
4. It wastes little material
5. It can make parts with small radii
6. It requires mirror tooling
4. Hydrostatic extrusion: In this process, the chamber is filled with a fluid that transmits the
pressure to the billet, which is then extruded through the die. (Fig. 2.10) There is no friction
along the walls of the container. Because the billet is subjected to uniform hydrostatic pressure,
it does not upset to fill the bore of the container as it would in conventional extrusion. This
means that the billet may have a large length to diameter ratio (even coils of wires can be
extruded) or it may have an irregular cross section. Because of the pressurized fluid, lubrication
is very effective, and the extruded product has good surface finish and dimensional accuracy.
Since friction is nearly absent, it is possible to use dies with very low semicone angle which
greatly minimizes the redundant deformation. The only limitation with this process is the
practical limit of fluid pressure that may be used because of the constraint involving the
strength of the container and the requirement that the fluid does not solidify at high pressure.
2.7 SHEARING
Before a sheet-metal part is made, a blank of suitable dimensions first is removed from a large
sheet (usually from a coil) by shearing. This sheet is cut by subjecting it to shear stresses,
generally using a punch and a die (Fig. 2.12a). The typical features of the sheared edges of the
sheet and of the slug are shown in Fig. 2.12b and c, respectively. Note that the edges are not
smooth nor are they perpendicular to the plane of the sheet.
Shearing generally starts with the formation of cracks on both the top and bottom edges of the
workpiece (at points A and B, and C and D, in Fig. 2.12a). These cracks eventually meet each
other and complete separation occurs. The rough fracture surfaces are due to the cracks; the
smooth and shiny burnished surfaces on the hole and the slug are from the contact and rubbing
of the sheared edge against the walls of the punch and die, respectively.
The clearance is a major factor in determining the shape and the quality of the sheared edge.
As the clearance increases, the zone of deformation (Fig. 2.13a) becomes larger and the sheared
edge becomes rougher. The sheet tends to be pulled into the clearance region, and the perimeter
or edges of the sheared zone become rougher. Unless such edges are acceptable as produced,
secondary operations may be required to make them smoother (which will increase the
production cost). Edge quality can be improved with increasing punch speed; speeds may be
as high as 10 to 12 m/s (30 to 40 ft/s.
Fig. 2.12. (a) Schematic illustration of shearing with a punch and die, indicating some of the
process variables. Characteristic features of (b) a punched hole and (c) the slug. (Note that the
scales of (b) and (c) are different.)
As shown in Fig. 2.13b, sheared edges can undergo severe cold working due to the high shear
strains involved. Work hardening of the edges then will reduce the ductility of the edges and
thus adversely affect the formability of the sheet during subsequent operations, such as bending
and stretching. The ratio of the burnished area to the rough areas along the sheared edge (a)
increases with increasing ductility of the sheet metal and (b) decreases with increasing sheet
thickness and clearance. The extent of the deformation zone in Fig. 2.13 depends on the punch
speed. With increasing speed, the heat generated by plastic deformation is confined to a smaller
and smaller zone. Consequently, the sheared zone is narrower, and the sheared surface is
smoother and exhibits less burr formation.
A burr is a thin edge or ridge, as shown in Fig. 2.12 b and c. Burr height increases with
increasing clearance and ductility of the sheet metal. Dull tool edges contribute greatly to large
burr formation. The height, shape, and size of the burr can significantly affect subsequent
forming operations.
Fig. 2.13. (a) Effect of the clearance, c, between punch and die on the deformation zone in
shearing. As the clearance increases, the material tends to be pulled into the die rather than be
sheared. In practice, clearances usually range between 2 and 10% of the thickness of the sheet.
(b) Microhardness (HV) contours for a 6.4-mm (0.25-in.) thick AISI 1020 hot-rolled steel in
the sheared region.
2.7.1. Punch Force. The force required to punch out a blank is basically the product of the
shear strength of the sheet metal and the total area being sheared along the periphery. The
maximum punch force, F, can be estimated from the equation
F = 0.7TL(UTS),
where T is the sheet thickness, L is the total length sheared (such as the perimeter of a hole),
and UTS is the ultimate tensile strength of the material. As the clearance increases, the punch
force decreases, and the wear on dies and punches also is reduced. Friction between the punch
and the workpiece can, however, increase punch force significantly. Furthermore, in addition
to the punch force, a force is required t strip the punch from the sheet during its return stroke.
This second force, which is in opposite direction of the punch force, is difficult to estimate
because of the many factors involved in the operation.
Example: Estimate the force required for punching a 1-inch (25-mm) diameter hole through a
1
𝑖𝑛𝑐ℎ (3.2-mm) thick annealed titanium-alloy Ti-6Al-4V sheet at room temperature.
8
Solution: The force is estimated from Eq. (16.1), where the UTS for this alloy is found from
Table 6.10 to be 1000 MPa or 140,000 psi.
1
F = 0.7 (8) (π)(1)(140,000) = 38,500 lb = 19.24 tons = 0.17 MN.
Die Cutting. This is a shearing operation that consists of the following basic processes (Fig.
2.14b):
• Perforating: punching a number of holes in a sheet
• Parting: shearing the sheet into two or more pieces
• Notching: removing pieces (or various shapes) from the edges
• Lancing: leaving a tab without removing any material.
Parts produced by these processes have various uses, particularly in assembly with other
components. Perforated sheet metals with hole diameters ranging from around 1 mm (0.040
in.) to 75 mm (3 in.) have uses as filters, as screens, in ventilation, as guards for machinery, in
noise abatement, and in weight reduction of fabricated parts and structures.
2.7.3. Characteristics and Type of Shearing Dies
Clearance. Because the formability of the sheared part can be influenced by the quality of its
sheared edges, clearance control is important. The appropriate clearance depends on
• The type of material and its temper
• The thickness and size of the blank
• Its proximity to the edges of other sheared edges or the edges of the original blank.
Clearances generally range between 2 and 8% of the sheet thickness, but they may be as small
as 1% (as in fine blanking) or as large as 30%. The smaller the clearance, the better is the
quality of the edge. If the sheared edge is rough and not acceptable, it can be subjected to a
process called shaving, whereby the extra material from the edge is trimmed by cutting. As a
general guideline, (a) clearances for soft materials are less than those for harder grades; (b) the
thicker the sheet, the larger the clearance must be; and (c) as the ratio of hole diameter to sheet
thickness decreases, clearances should be larger. In using larger clearances, attention must be
paid to the rigidity and the alignment of the presses, the dies, and their setups.
Punch and Die Shape. Note in Fig. 2.13a that the surfaces of the punch and of the die are both
flat. Because the entire thickness is sheared at the same time, the punch force increases rapidly
during shearing. The location of the regions being sheared at any particular instant can be
controlled by beveling the punch and die surfaces. This shape is similar to that of some paper
punches, which you can observe by looking closely at the tip of the punch.
Compound Dies. Several operations on the same sheet may be performed in one stroke at one
station with a compound die (Fig. 2.14). Such combined operations usually are limited to
relatively simple shapes, because (a) the process is somewhat slow and (b) the dies rapidly
become much more expensive to produce than those for individual shearing operations,
especially for complex dies.
Progressive Dies. Parts requiring multiple operations to produce can be made at high
production rates in progressive dies. The sheet metal is fed through as a coil strip, and a
different operation (such as punching, blanking, and notching) is performed at the same station
of the machine with each stroke of a series of punches (Fig. 2.14c). An example of a part made
in progressive dies is shown in Fig. 2.14d; the part is the small round piece that supports the
plastic tip in spray cans.
FIGURE 2.14 Schematic illustrations (a) before and (b) after blanking a common washer in a
compound die. Note the separate movements of the die (for blanking) and the punch (for
punching the hole in the washer). (c) Schematic illustration of making a washer in a progressive
die. (d) Forming of the top piece of an aerosol spray can in a progressive die. Note that the part
is attached to the strip until the last operation is completed.
Transfer Dies. In a transfer-die setup, the sheet metal undergoes different operations at
different stations of the machine that are arranged along a straight line or a circular path. After
each step in a station, the part is transferred to the next station for further operations.
Tool and Die Materials. Tool and die materials for shearing generally are tool steels and (for
high production rates) carbides. Lubrication is important for reducing tool and die wear, thus
improving edge quality.
2.7.4. Miscellaneous Methods of Cutting Sheet Metal
There are several other methods of cutting sheets and, particularly, plates:
• Laser-beam cutting is an important process typically used with
computer-controlled equipment to cut a variety of shapes consistently, in various thicknesses,
and without the use of any dies. Laser-beam cutting also can be combined with punching and
shearing. These processes cover different and complementary ranges. Parts with certain
features can be produced best by
one process; some with other features can be produced best by the other process. Combination
machines incorporating both capabilities have been designed and built.
• Water-jet cutting is effective on many metallic as well as nonmetallic materials.
• Cutting with a band saw; this method is a chip-removal process.
• Friction sawing involves a disk or blade that rubs against the sheet or plate at high surface
speeds.
• Flame cutting is another common method, particularly for thick plates; it is used widely in
shipbuilding and on heavy structural component.