Deformation Processing

Question 1

What is deformation processing in metals?

Answer 1

Deformation processing is an alternative to casting to produce shaped components.

Question 2

What material properties make a metal suitable for deformation processing

Answer 2

• Ductile: undergo large amounts of straining without failure
• Strong in compression: most deformation processes are done in compression to avoid necking and tensile failure at large strains.

Question 3

What are the advantages of deformation processing

Answer 3

The advantages of deformation processing are:

• Improved mechanical properties (alloy choice and microstructure control)
• Lower temperatures (lower energy consumption)
• Better surface finish and dimensional accuracy

Question 4

What is cold-working

Answer 4

Cold-working is a deformation processing technique which occurs at room temperature (no external heating). It involves passing a sheet of metal between a pair of rollers (rolling mill).

Question 5

What are the micro-structural changes that occur when cold-working?

Answer 5

• Grains change shape: prior to cold-working the grains are equiaxed, however after passing through the rolling mill they are ‘pancaked’. This is because the plastic deformation in metals occurs at constant volume as it caused by dislocations gliding through the material in shear.
• Work hardening: this increases the yield stress and is related to the dislocation density during plastic straining. The higher the dislocation density, the smaller the spacing between dislocations - these dislocations pin each other increasing the yield stress.

Question 6

What model is used for the stress-strain relationship of a work hardened metal?

Answer 6

stress = a*strain^n (n=0.1-0.3)

Question 7

What are the problems with the use of cold working?

Answer 7

Work hardening during the process brings about the following problems:

• As deformation progresses, the applied forces must increase which makes large plastic strains difficult
• Ductility of the metal reduces and thus the risk of fracture increases

Question 8

What is annealing?

Answer 8

Annealing is a form of heat treatment, where the material is heated in order to reverse the effects of work hardening (reduce yield strength and increase ductility). It is used in conjunction with cold-working to achieve larger plastic strains (e.g. in the production of aluminium foil)

Question 9

What is the driving force for Annealing?

Answer 9

The driving force for micro-structural changes during annealing is elastic stored energy within the material that arises due to the increased dislocation density as a result of cold-working. This is due to dislocations inducing elastic strain in the surrounding lattice. During annealing this stored elastic energy in reduced using diffusional mechanisms.

Question 10

Outline the first Annealing mechanism.

Answer 10

Recovery: one route is for the dislocations to re-arrange into a pattern which reduces the strain in the crystal lattice around them.
- dislocations of opposite orientation annihilate
- dislocations of similar orientations align and form sub-grains.
(see diagrams Pg5. Week 7)
These processes depend on the glide of dislocations and local re-arrangements of the Crystal structure near to the dislocations by diffusion of atoms.

Question 11

Outline the second Annealing mechanism

Answer 11

Recrystallisation: Another route to relieve stored dislocation strain energy is nucleate and grow new crystals within the work hardened grains. The new grains consequently have a much lower dislocation density which causes a drop in the yield stress.
(see diagram Week 7 Pg 6)
This requires more significant atomic re-arrangement so higher temperatures are needed (compared to recovery)

Question 12

How can recrystallisation be used to control grain size?

Answer 12

Re-crystallisation is controlled by plastic strain. No crystallisation occurs below a critical level of plastic strain (epsilon_critical) and the final grain size depends on the plastic strain above the critical level of plastic strain.

(epsilon - epsilon_critical) is the driving force for re-crystallisation.

Question 13

Explain the phenomenon of Grain growth after recrystallisation

Answer 13

If a material is maintained at a high temperature after recrystallisation is completed, grain growth will occur.

• Grain boundaries (discontinuities in the crystal structure) carry an energy penalty
• If atoms are able to re-arrange by diffusion, the grain size will tend to increase thus reducing the surface area to volume ratio if the grains.

Question 14

Define hot-working

Answer 14

In practice, the processes of deformation and annealing are often combined by hot working the material (0.7Tm). This has the following advantages:

• The material yield strength remains low due to a lack of strain hardening and the deformed material remains ductile. This reduces the force on the rolling mill and allows for larger plastic strains over one pass.
• No need for separate annealing (heat treatment) step.