1. Overview of processes
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2. Metal forming Principle of the process Structure and configurtion
Process modeling
Defects
Design For Manufacturing (DFM)
Process variation
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3. Principle of Metal Forming
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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
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5. Metal Forming
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 the solid state in the process
- (temperature can be either cold, warm or hot)
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6. Metal Forming Metal forming is divided into: (1) bulk and (2) sheet
significant deformation
massive shape change
surface area to volume of the work is small
Sheet: Surface area to volume of the work is large
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7. Bulk deformation processes
Forging
Rolling
Traditionally Hot
Extrusion
Drawing
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8. Drawing Bending Shearing Actually Cutting
Sheet deformation processes (Press working/ Stamping)
Drawing
Bending
Shearing
Actually Cutting
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9. General mechanics principle Individual processes: mechanics principles
We discuss:
General mechanics principle
Individual processes:
mechanics principles
design for manufacturing (DFM) rules
equipment
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10. 1. General mechanics principle
The underlying mechanics principle for metal forming is the stress-strain relationship; see Figure 1.
Figure 1
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11. True strain: Instantaneous elongation per unit length of the material
True Stress: Applied load divided by instantaneous value of cross-section area
True strain: Instantaneous elongation per unit length of the material
L0: the initial length of a specimen
L: the length of the specimen at time t
the true strain at time t
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12. Plastic deformation region
More interested in the plastic deformation region
Plastic deformation region
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13. The stress-strain relation in the plastic deformation region
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.
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14. FLOW STRESS
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”)
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15. AVERAGE FLOW STRESS
Average stress: total stress in one complete operation (e.g., extrusion)
Integrating the flow stress along the trajectory of straining, from zero to the final strain value defining the range of interest, divided by the final strain
Strength Coefficient
Max. strain during deformation
Average flow stress
Strain hardening exponent
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16. According to the statement of the problem, we have
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 of
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17. Temperature up  Yield strength down and ductility up
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  Yield strength down and ductility up
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18. h Strain rate (related to elevated temperatures) Rate of the straining
Strain affecting flow stress
Instantaneous height of work-piece being deformed
Speed of deformation (could be equal to velocity of ram)
Strain Rate h h
Flow stress
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19. Strength coefficient but not the same as K
where
C  strength constant
m  strain-rate sensitivity exponent
C and m are determined by the following figure which is generated from the experiment
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20. Lg Lg
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21. C and m are affected by temperatureLg
Temperature Up
C Down
m Up
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22. Even in the cold work, the strain rate could affect the flow stress
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
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23. From these two equations, one can find m=0.1973
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
Solution:
23000=C(10)^m
45000=C(300)^m
From these two equations, one can find m=0.1973
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