2. Overview
Process Classification
◦ Bulk Deformation Process
◦ Sheet Metalworking
Material Behaviour in Metal Forming
◦ Flow Stress
◦ Average Flow Stress
Temperature in Metal Forming
Effect of Strain Rate
Friction & Lubrication
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3. Bulk Metal Forming
Rolling - compression process to reduce
the thickness of a slab by a pair of rolls.
Forging - compression process performing
between a set of opposing dies.
Extrusion - compression process sqeezing
metal flow a die opening.
Drawing - pulling a wire or bar through a
die opening.
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5. Sheet Metalworking
Forming on metal sheets, strips, and coils.
The process is normally a cold working
process using a set of punch and die.
Bending - straining of a metal sheet to
form an angle bend.
Drawing - forming a sheet into a hollow or
concave shape.
Shearing - not a forming process but a
cutting process.
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7. Material Behavior in Metal Forming
Yf K n
K n
Yf
1 n
Yf Flow Stress
Maximum strain
for forming process
K Strength coefficient
Average flow stress
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8. Temperature in Metalworking
Cold working
◦ Pros
better accuracy
better surface finish
strain hardening increases strength and
hardness
grain flow during deformation provides
directional properties
no heating is needed
◦ Cons
higher forces and power are required
surface should be cleansed
ductility and strain-hardening limits the extent of
forming
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9. Temperature in Metalworking
Warm working - temperature between
room temperature and recrystallization
temperature, roughly about 0.3 Tm
◦ Pros against cold working
Lower forces and power
more intricate work geometries possible
need for annealing may be reduced/eliminated.
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10. Temperature in Metalworking
Hot working - Deformation at temperature
above recrystallization temperature
typically between 0.5Tm to 0.75Tm
◦ Pros
larger deformation possible
lower forces and power
forming of room temperature low ductility material
is possible
isotropic properties resulted from process
no work hardening
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11. Temperature in Metalworking
Isothermal Forming - preheating the tools
to the same temperature as the work
metal. This eliminates the surface cooling
and the resulting thermal gradient in the
work part.
Normally applies to highly alloyed steels,
titanium alloys and high-temperature
nickel alloys.
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12. Effect of Strain Rate
Y f C m
strain rate
The strain rate is strongly
affected by the temperature.
Y f A n m
A = a strength coefficient
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13. Friction and Lubrication
Friction is undesirable:
◦ retard metal flow causing residual stress
◦ increase forces and power
◦ rapid wear of tooling
Lubrication is used to reduce friction at
the workpiece-tool interface
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14. Defects in Metal Forming
Springback Effect-
In Bending ,after plastic deformation there is an elastic
recovery this recovery is called spring back.
Spring back can be calculated approximately in terms if
radii Ri and Rf
Ri/Rf = 4 ( Ri Y / ET )3 – 3 (Ri Y /ET) + 1
Spring back Increases as (R/T ratio & yield stress of
material ) increases as elastic modulus E decreases
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16. Compensation for spring back
Over bending of part
Bottoming the punch – coin the bend area by subjecting it to
high localized compressive between the technique tip of the
punch and the die surface.
Stretch bending – Part is subjected to tension while being
bent.In order to reduce spring back bending may also be
carried to reduce spring back bending may also be carried out
at elevated temperatures
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17. Springback cannot be avoided but can be
minimized by several methods such as
Applying tension,
Overbending,
Warm and hot forming . Finite element method
(FEM) is widely used in industry to predict metal
flow and springback.
Based on the springback predictions obtained from
FEM, the tool geometries are virtually modified to
compensate for the springback before the tool is
manufactured. Thus, tool manufacturing time and
cost are significantly reduced.
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18. Maximum bending force, P = KYLT2
W
K – constant ranges from 0.3(wiping die) – 0.7(u-die)-
1.3(V-die)
Y – yield stress
L- length of the bend
T- thickness of sheet
For a V-die
Max bending force, P = (UTS)LT 2
W
UTS – Ultimate tensile strength
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19. Wrinkles in Deep Drawing operation
Wrinkles can be prevented by using a blank
holder, the function of which is to facilitate
controlled material flow into the die radius.
The main geometric parameters of the die which
influence the wrinkling are:
diameter of the punch and punch edge radiuses.
In the case of friction between the piece and the
tool, the increase of the coefficient of friction
determines the wrinkling to reduce, but high
values of the coefficient can cause cracks and
material breakage
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20. Variables in Deep drawing
operations:
Many variables affect the deep drawing
process, these include
Material properties, die design, friction
conditions.
Drawing ratio , punch corner radius,
punch speed etc.
A properly chosen BHF can prevent
wrinkling.
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