Defects in metal forming operations

1. Defects in Metal
Forming Processes
Prepared by:-
Amitkumar R. Shelar
1

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
2

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.
3

4. Bulk Metal Forming
Rolling Forging
Extrusion Drawing
4

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.
5

6. Sheet Metalworking
6

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
7

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
8

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.
9

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
10

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.
11

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
12

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
13

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
14

15. 15

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
16

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.
17

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
18

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
19

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.
20

21. Figure shows the Wrinkle Defects
21

22. Thank You
22