Chapter 7 Flashcards
What is metal forming?
A manufacturing process in which plastic deformation is used to change the shape of metal workpieces
What does the ‘die’ do?
It is the tool used for metal forming. It applies stresses that exceed the yield strength of the metal.
What is the type of stress that causes metal to plastically deform?
Compressive stress. Examples of compressive stress are rolling, forging, and extrusion.
Desirable properties in Metal Forming :
Low yield strength and high ductility. (Temperature dependent as temp. rises, ductility increases)
Basic Types of Metal Forming
Bulk Deformation and Sheet Metalworking
Types of Bulk Deformation
Wire and bar drawing. Rolling, forging, and extrusion processes.
Types of Sheet Metalworking
Bending operations, shear processes, and deep/cut drawings.
Bulk Deformation Processes is characterized by….
Significant deformations and massive shape changes. Has a relatively low surface area-to-volume ratio. Starting work shapes are simple geometries.
Sheet Metalworking is characterized by….
metal sheets, strips, and coils.
Has a high surface area-to-volume ratio. Often called pressworking.
What is the temp for Cold Working?
performed at room temperature or slightly above.
Advantages of Cold Working
- Better accuracy and surface finish, closer tolerances
-strain hardening increases strength and hardness - grain flow during deformation causes desirable directional properties
- no heating of work required
Disadvantages of Cold Working
- Higher forces and power required for deformation
-Ductility and strain hardening limit the amount of forming.
What was the temp of the Warm Working?
Performed at temperatures above room temperature but below recrystallization temperature
Advantages of Warm Working
- lower forces and power than in cold working
- more intricate work geometries possible
- need for annealing may be reduced or eliminated
Disadvantage for Warm Working
- Workpiece must be heated
- Time and energy to heat workpiece
What is recrystallization?
When deformed grains are replaced by a new set of undeformed grains that nucleate and grow.
- Causes reduction in the strength and hardness and increase in the ductility.
- Nucleation and growth of recrystallized grains depend on the annealing temperature.
What temperatures can hot working be performed in?
In temperatures above the recrystallization temperature
Qualities of hot working
Capability for substantial plastic deformation.
- Ductility is significantly increased
- Strength coefficient (K) is substantially less compared to room temperature
-Strain hardening exponent (n) is zero (theoretically).
- Ductility is significantly increased
Advantages of Hot Working
-Work part shape can be significantly alerted
-Lower forces and power required during forming.
- Metals that usually fracture in cold working can be hot formed.
-Strength properties of product are generally isotropic (the same when measured differently)
Disadvantages of Hot Working
-Lower dimensional accuracy
-Higher total energy required, which is the sum of the thermal energy needed to heat the workpiece and the energy to deform the metal
- Work surface oxidation(scale) - poorer surface finish
- Shorter tool life - e.g. dies and rolls in bulk deformation
What is strain rate?
It is the deformation speed over the instantaneous height of workpiece being deformed.
Flow stress
is based on true strain and true stress. The flow stress is a fucntion of temeprature and, in hot-working, depends on the strain rate.
Strain-rate sensitivity
As strain-rate increases, resistance to deformation increases
Friciton in Metal Forming
- Metal flow is reduced
- Forces and power are increased
- Tools wear faster
