1. Fundamentals of
Metal Forming

2. Bulk deformation vs Sheetmetal working
Fundamentals of Metal Forming
Bulk deformation vs Sheetmetal working
Bulk deformation * Low surface area to volume ratios * Operations increase surface area to volume ratios
Operations are: rolling extrusion forging drawing
This group of machines - which are over a half mile long - roll the slab into sheet steel using tremendous pressure.
Aluminum extrudate – a simple angled shape. The die shown is more complex.

3. Fundamentals of Metal Forming Bulk deformation
Rolling characteristics
* compressive deformation * 2 or more rolls * strong frictional relationship
Extrusion characteristics
* compressive deformation * force metal through die opening * strong frictional relationship * hot and cold working process

4. Fundamentals of Metal Forming Sheet metalworking
Characterized by: * large surface area to volume ratios * little ratio change after process * machines called presses * parts called stampings * tools are punch and die
Operations:
bending drawing shearing

5. Material Behavior – Flow Stress
= K en
Concept of flow stress, Yf – the instantaneous stress required to deform the material
Yf = K en
Process force considerations:
* during compression, determine instantaneous force from Yf * max force is often all that is required, typically at end of stroke * force analysis may be based on average stresses and strains
Average flow stress = Yf = K en /(1 + n)

6. Fundamentals of Metal Forming Temperature and Metal Forming
K and n depend on working temperature, characterized by 3 ranges:
Cold working
Warm working
Hot working

7. Fundamentals of Metal Forming
Fundamentals of Metal Forming Temperature and Metal Forming – cold working
Advantages:
* good accuracy and tolerances
* better surface finish
* strain hardening increases strength and hardness
* directional properties
* energy economy
Disadvantages:
* higher forces/power
* strain hardening limits deformation
* may need to anneal

8. Fundamentals of Metal Forming Temperature and Metal Forming – warm working
Advantages:
* above room temp., but below re-crystallization temp. (0.3 Tm)
* lower forces and power
* reduced strain hardening
* more difficult geometry
* no need for annealing
Disadvantages:
* more energy
* limited geometry

9. Fundamentals of Metal Forming Temperature and Metal Forming – hot working
Advantages:
* above re-crystallization temp. (0.5 Tm < T < 0.75 Tm)
* lower forces and power
* no strain hardening
* difficult geometry
* isotropic properties
Disadvantages:
* more energy
* poorer surface finish
* shorter tool life

10. Fundamentals of Metal Forming - Strain Rate
Note: Strain rate a strong function of working temperature
(high values -> 1000 s-1)
where
v = speed of testing head
h = instantaneous height of part being worked
Strain rate equation:
C = strength constant
m = strain-rate coefficient
Strain rate
Flow stress
Room temp.
400º C
800º C
1200º C
We will mostly assume that strain rate is negligible at room temperature!
Yf = C em

11. Friction and lubrication
* undesirable
* retards metal flow (residual stresses & defects)
* increases force and power requirements
* wears tooling
* high temperature stiction
Lubricants:
* reduce frictional effects
* remove heat and material
* lengthen tool life
* may react chemically with tool or work
* concerns about toxicity, flammability….(mineral oils, emulsions, oils, graphite, glass..)
Lubricants are usually oil-based, and when used in extreme pressure situations, sulfur, chlorine and phosphorus in the lubricant may chemically react with the metal surfaces (tool, chip). The reactions form a surface boundary layer that is more effective than the lubricant itself in reducing friction. Lubricants are most effective at cutting speeds < 400 ft/min.

12. Example 20.6
For a metal with K = 50,000 lb/in2 and n = 0.27, determine the average flow stress that the metal experiences if it is subjected to a stress that is equal to its strength coefficient K.
Solution:
Yf = K = 50,000 = 50,000 en
Thus e = 1
Yf = 50,000 (1) 0.27/1.27 = 39,370 lb/in2

13. What have you learned?