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Metal Forming Processes
Presentation · May 2020
DOI: 10.13140/RG.2.2.29269.24804
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Metal Forming
Processes
Production Engineering
Dr. Jalil Jabbar Shukur
DEPARTMENT OF PRODUCTION ENGINEERING AND METALLURGY
UNIVERSITY OF TECHNOLOGY BAGHDAD, IRAQ.
METAL FORMING PROCESSES
Metal Forming Processes
1.Introduction
Metal forming processes are used to produce structural parts and components
that have widespread applications in many industries including automobile,
aerospace, appliances. Metal forming processes include a wide range of
operations which deform sheet or tube metal to form the component with the
desired geometry. Deep drawing is one of the most popular metal forming
processes used to produce a cup like cylindrical component by radially
drawing metal blank into the die cavity with the help of a punch. The cup
shaped part thus produced has the depth greater than half of its diameter. The
change in cross-section is achieved by plastic deformation of the initial blank.
Commercial applications of this metal shaping process often involve complex
geometries with straight sides and radii. In such a case, the term stamping is
used in order to distinguish between the deep drawing (radial tensiontangential compression) and stretch-and-bend (along the straight sides)
components.
2.Metal forming classified in two Categories:
❖ Bulk forming: It is a severe deformation process resulting in massive
shape change. The surface area-to-volume of the work is relatively
small. Mostly done in hot working conditions.
❖ Sheet metal forming: Sheet metal forming involves forming and cutting
operations performed on metal sheets, strips, and coils. The surface areato-volume ratio of the starting metal is relatively high. Tools include
punch, die that are used to deform the sheets.
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METAL FORMING PROCESSES
Figure (1). General classification of metal forming processes
3. According to working temperature metal forming classified in
three Categories:
➢ Cold working: Generally done at room temperature or
slightly above recrystallization temperature (RT)
Advantages compared to hot forming:
1. closer tolerances can be achieved.
2. good surface finish.
3. because of strain hardening, higher strength and hardness is seen
in part.
4. grain flow during deformation provides the opportunity for
desirable directional properties.
5. since no heating of the work is involved furnace, fuel, electricity
costs are minimized.
6. Machining requirements are minimum resulting in possibility of
near net shaped forming.
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METAL FORMING PROCESSES
Disadvantages:
1. higher forces and power are required.
2. strain hardening of the work metal limit the amount of forming that can
be done.
3. sometimes cold forming annealing-cold forming cycle should be
followed.
4. the work piece is not ductile enough to be cold worked.
➢ Warm working: In this case, forming is performed at temperatures
just above room temperature but below the recrystallization
temperature. The working temperature is taken to be 0.3 Tm where Tm
is the melting point of the workpiece.
Advantages:
1.
2.
3.
4.
enhanced plastic deformation properties.
lower forces required.
intricate work geometries possible.
annealing stages can be reduced.
➢ Hot working: Involves deformation above recrystallization
temperature, between 0.5T m to 0.75Tm.
Advantages:
1. significant plastic deformation can be given to the sample.
significant change in workpiece shape.
2. lower forces are required.
3. materials with premature failure can be hot formed.
4. absence of strengthening due to work hardening.
Disadvantages:
1.
2.
3.
4.
shorter tool life.
poor surface finish.
lower dimensional accuracy.
sample surface oxidation.
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METAL FORMING PROCESSES
4.Classification of basic sheet forming processes
Sheet metal forming involves forming and cutting operations performed on
metal sheets, strips, and coils. The surface area-to-volume ratio of the
starting metal is relatively high. Tools include punch, die that are used to
deform the sheets.
4.1 Bending process:
Bending of sheets and plates is widely used in forming parts such as
corrugations, flanges, etc. Bending is a forming operation in which a sheet
metal is subjected to bending stress thereby a flat straight sheet is made into a
curved sheet. The sheet gets plastically deformed without change in thickness.
Die and punch are used for bending. If a v shaped die and punch are used, the
bending is called v-bending. If the sheet is bent on the edge using a wiping
die it is called edge bending. In this process, one end of the sheet is held like
a cantilever using a pressure pad and the other end is deformed by a punch
which moves vertically down, bending the sheet. Usually, edge bending is
done in order to obtain an angle of 90ᵒ.
Figure (2). V-bending process
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METAL FORMING PROCESSES
During bending of a strip, the material outward of the neutral axis is subjected
to tensile stress. Material inside is subjected to compressive stress. Bend
radius R is the radius of curvature of the bent sheet inside the bending. The
neutral axis remains at the center of the thickness of the sheet for elastic
bending. For plastic bending, however, the neutral axis shifts towards the
inside of the bend. The rate of elongation of outer fibers is greater than the
rate of contraction of inner fibers. Therefore, there is a thickness reduction at
the bend section.
4.2 Deep drawing process
It is a sheet forming operation, in which the sheet is placed over the die
opening and is pushed by punch into the opening. The sheet is held flat on the
die surface by using a blank holder.
Figure (3). Deep drawing process
The clearance ‘c’ is defined to equal to 10% more than the sheet thickness ‘t’.
If the clearance between the die and the punch is less than the sheet thickness,
then ironing occurs. c =1.1t
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METAL FORMING PROCESSES
4.2.1Stages in deep drawing:
I.
As the punch pushes the sheet, it is subjected to a bending operation.
Bending of sheet occurs over the punch corner and die corner. The
outside perimeter of the blank moves slightly inwards toward the cup
center.
II.
In this stage, the sheet region that was bent over the die corner will be
straightened in the clearance region at this stage, so that it will become
cup wall region. In order to compensate the presence of sheet in cup
wall, more metal will be pulled from the sheet edge, i.e., more metal
moves into the die opening.
III.
Friction between the sheet and the die, blank holder surfaces restricts
the movement of sheet into the die opening. The blank holding force
also influences the movement. Lubricants or drawing compounds are
generally used to reduce friction forces.
IV.
Other than friction, compression occurs at the edge of the sheet. Since
the perimeter is reduced, the sheet is squeezed into the die opening.
Because volume remains constant, with reduction in perimeter,
thickening occurs at the edge. In thin sheets, this is reflected in the form
of wrinkling. This also occurs in case of low blank holding force. If
BHF very small, wrinkling occurs. If it is high, it prevents the
sheet from flowing properly toward the die hole, resulting in stretching
and tearing of sheet.
V.
The final cup part will have some thinning in side wall.
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METAL FORMING PROCESSES
4.4
Shearing process
Cutting processes are those in which a piece of sheet metal is separated by
applying a great enough force to cause the material to fail. The most common
cutting processes are performed by applying a shearing force, and are
therefore sometimes referred to as shearing processes. When a great enough
shearing force is applied, the shear stress in the material will exceed
the ultimate shear strength and the material will fail and separate at the cut
location. This shearing force is applied by two tools, one above and one below
the sheet. Whether these tools are a punch and die or upper and lower blades,
the tool above the sheet delivers a quick downward blow to the sheet metal
that rests over the lower tool. A small clearance is present between the edges
of the upper and lower tools, which facilitates the fracture of the material. The
size of this clearance is typically 2-10% of the material thickness and depends
upon several factors, such as the specific shearing process, material, and sheet
thickness.
Figure (4). Shearing process
The effects of shearing on the material change as the cut progresses and are
visible on the edge of the sheared material. When the punch or blade impacts
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METAL FORMING PROCESSES
the sheet, the clearance between the tools allows the sheet to plastically
deform and "rollover" the edge. As the tool penetrates the sheet further, the
shearing results in a vertical burnished zone of material. Finally, the shear
stress is too great and the material fractures at an angle with a small burr
formed at the edge. The height of each of these portions of the cut depends on
several factors, including the sharpness of the tools and the clearance between
the tools.
5.Classification of basic Bulk forming processes
Bulk forming: It is a severe deformation process resulting in massive shape
change. The surface area-to-volume of the work is relatively small. Mostly
done in hot working conditions.
5.1Forging
It is a deformation process in which the work piece is compressed between
two dies, using either impact load or hydraulic load (or gradual load) to
deform it. It is used to make a variety of high-strength components for
automotive, aerospace, and other applications. The components include
engine crankshafts, connecting rods, gears, aircraft structural components, jet
engine turbine parts etc.
The forging processes divided into three categories based on type of forming:
5.1.1 Open die forging
In open die forging, the work piece is compressed between two flat platens or
dies, thus allowing the metal to flow without any restriction in the sideward
direction relative to the die surfaces.
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METAL FORMING PROCESSES
Figure (5). Open die forging
5.1.2 Impression die forging
In impression die forging, the die surfaces contain a shape that is given to the
work piece during compression, thus restricting the metal flow significantly.
There is some extra deformed material outside the die impression which is
called as flash. This will be trimmed off later.
Figure (6). Impression die forging
5.1.3 Flashless forging
In flashless forging, the work piece is fully restricted within the die and no
flash is produced. The amount of initial work piece used must be controlled
accurately so that it matches the volume of the die cavity.
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METAL FORMING PROCESSES
Figure (6). Flashless forging
5.2
Extrusion
Extrusion is a bulk forming process in which the work metal is forced or
compressed to flow through a die hole to produce a desired cross-sectional
shape. Example: squeezing toothpaste from a toothpaste tube.
5.2.1 Direct extrusion:
A metal billet is first loaded into a container having die holes. A ram
compresses the material, forcing it to flow through the die holes. In direct
extrusion, a significant amount of friction exists between the billet
surface and the container walls, as the billet is forced to slide toward the die
opening. Because of the presence of friction, a substantial increase in the ram
force is required.
Figure (7). direct extrusion.
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METAL FORMING PROCESSES
In hot direct extrusion, the friction problem is increased by the presence of
oxide layer on the surface of the billet. This oxide layer can cause defects in
the extruded product.
In order to address these problems, a dummy block is used between the ram
and the work billet. The diameter of the dummy block is kept slightly smaller
than the billet diameter, so that a thin layer of billet containing the oxide layer
is left in the container, leaving the final product free of oxides.
5.2.2 Indirect extrusion:
In this type, the die is mounted to the ram and not on the
container. As the ram compresses the metal, it flows through the die hole on
the ram side which is in opposite direction to the movement of ram.
Since there is no relative motion between the billet and the container, there is
no friction at the interface, and hence the ram force is lower than in direct
extrusion.
Figure (8). Indirect extrusion: solid billet and hollow billet.
5.2.3 Wire drawing
In this bulk forming process, a wire, rod, bar are pulled through a die hole
reducing their cross-section area. The basic difference between bar drawing
and wire drawing is the stock size that is used for forming. Bar drawing is
meant for large diameter bar and rod, while wire drawing is meant for small
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METAL FORMING PROCESSES
diameter stock. Wire sizes of the order of 0.03 mm are produced in wire
drawing.
Figure (9). Wire drawing process.
Bar drawing is generally done as a single stage operation, in which stock
is pulled through one die opening. The inlet bars are straight and not in the
form of coil, which limits the length of the work that can be drawn. This
necessitates a batch type operation.
In contrast, wire is drawn from coils consisting of several hundred meters
of wire and is drawn through a series of dies. The number of dies varies
between 4 and 12. This is termed as ‘continuous drawing’ because of the
long production runs that are achieved with the wire coils. The segments
can be butt-welded to the next to make the operation truly continuous.
5.2.4 Rolling
Rolling is a metal forming process in which the thickness of the work is
reduced by compressive forces exerted by two rolls rotating in opposite
direction. Flat rolling is shown in figure. Similarly shape rolling is also
possible like a square cross section is formed into a shape such as an I-beam,
L-beam.
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METAL FORMING PROCESSES
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Figure (9). Flat rolling.
Important terminologies:
Bloom: It has a square cross section 150 mm x 150 mm or more. Blooms are
rolled into structural shapes like rails for railroad tracks.
Slab: It is rolled from an ingot or a bloom and has a rectangular cross section
of 250 mm width or more and thickness 40 mm or more. Slabs are rolled into
plates,
sheets,
and
strips.
Hot
rolled
plates
are
generally
used in shipbuilding, bridges, boilers, welded structures for various heavy
machines, and many other products.
Billet: It is rolled from a bloom and is square in cross-section with dimensions
40mm on a side or more. Billets are rolled into bars, rods. They become raw
materials for machining, wire drawing, forging, extrusion etc.
The plates and sheets are further reduced in thickness by cold rolling to
strengthen the metal and permits a tighter tolerance on thickness. Important
advantage is that the surface of the cold-rolled sheet does not contain scales
and generally superior to the corresponding hot rolled product. Later the coldrolled sheets are used for stampings, exterior panels, and other parts used in
automobile, aerospace and house hold appliance industries.
METAL FORMING PROCESSES
Figure (10). Bloom, Slab and Billet.
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