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Module 8 Principle of Metal Forming 3
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Module 8
Overview of processes
1
Module 8 2
Metal forming
Principle of the process
Structure and configurtion
Process modeling
Defects
Design For Manufacturing (DFM)
Process variation
Module 8
Principle of Metal Forming
3
Module 8 4
Metal Forming
• Metal forming includes a large group of manufacturing processes in which plastic deformation is used to change the shape of metal work pieces
• Plastic deformation: a permanent change of shape, i.e., the stress in materials is larger than its yield strength
• Usually a die is needed to force deformed metal into the shape of the die
Module 8 5
• Metal with low yield strength and high ductility is in favor of metal forming
• One difference between plastic forming and metal forming is
Plastic: solids are heated up to be polymer melt
Metal: solid state remains solid status in the process
- (temperature can be either cold, warm or hot)
Metal Forming
Module 8 6
Metal forming is divided into: (1) bulk and (2) sheet
Metal Forming
Bulk: significant deformation
massive shape change
surface area to volume of the work is small
Sheet: Surface area to volume of the work is large
Module 8 7
Bulk deformation processes
RollingForging
Extrusion Drawing
Traditionally Hot
Module 8 8
Sheet deformation processes (Press working/ Stamping)
BendingDrawing
Shearing
Actually Cutting
Module 8 9
We discuss:
1. General mechanics principle
2. Individual processes:
mechanics principles
design for manufacturing (DFM) rules
equipment
Module 8 10
1. General mechanics principle The underlying mechanics principle for metal forming is the
stress-strain relationship; see Figure 1.
Figure 1
Module 8 11
True strain: Instantaneous elongation per unit length of the material
0ln
0 LL
LdLL
L
L0: the initial length of a specimen
L: the length of the specimen at time t
the true strain at time t
True Stress: Applied load divided by instantaneous value of cross-section area
AF /
Module 8 12
More interested in the plastic deformation region
Plastic deformation region
Module 8 13
The stress-strain relation in the plastic deformation region
nK
where
K= the strength coefficient, (MPa),
= the true strain, σ=the true stress,
n= the strain hardening exponent.
Remark: Flow stress (Yf) is used for the above stress (which is the stress beyond yield). The equation is called flow curve.
Module 8 14
As deformation occurs, increasing STRESS is required to continue deformation
Flow Stress: Instantaneous value of stress required to continue deforming the material (to keep metal “flowing”)
FLOW STRESS
nKfY
Module 8 15
Average stress: total stress in one complete operation (e.g., exclusion)
Integrating the flow stress along the trajectory of straining, from zero to the final strain value defining the range of interest
nkY
n
f
1
AVERAGE FLOW STRESS
Average flow stress Max. strain during deformation
Strength Coefficient
Strain hardening exponent
Module 8 16
Example 1:Determine the value of the strain-hardening exponent for a metal that will cause the average flow stress to be three-quarters of the final flow stress after deformation.
According to the statement of the problem, we have
4/3fY of fY
333.075.0)1/(1
75.0)1/(
75.0
nn
KnK
YYnn
ff
Module 8 17
The above analysis applicable to the cold working, where the temperature factor is not considered
The metal forming process has three kinds in terms of temperature: (1) cold, (2) warm, (3) hot
In the case of warm and hot forming, the temperature factor needs to be considered, in particular
Temperature up The (yield) strength down and ductility up
Module 8 18
Strain rate (related to elevated temperatures)
- Rate of the straining
- Strain affecting flow stress
hv /h
Speed of deformation (could be equal to velocity of ram)
Instantaneous height of work-piece being deformedh
mf CY
Flow stress
Strain Rate
Module 8 19
mf CY
whereC strength constantm strain-rate sensitivity exponent
C and m are determined by the following figure which is generated from the experiment
nKfY
Strength coefficient but not the same as K
Module 8 20
Module 8 21
C and m are affected by temperature
Temperature Up
C Down
m Up
Module 8 22
mnf AY
Even in the cold work, the strain rate could affect the flow stress. A more general expression of the flow stress with consideration of the strain rate and strain is presented as follows:
A is a strength coefficient, a combined effect of K, C
All these coefficients, A, n, m, are functions of temperature
Module 8 23
Example 2:
A tensile test is carried out to determine the strength constant C and
strain-rate sensitivity exponent m for a certain metal at 1000oF. At a strain
rate = 10/sec, the stress is measured at 23,000 lb/in2; and at a strain rate
= 300/sec, the stress=45,000 lb/in2. Determine C and m
23000=C(10)^m45000=C(300)^m
From these two equations, one can find m=0.1973
Solution: