Plastic- Extrusion of Aluminium Alloys (Def at RT vs Elevated T)

Question 1

Room temperature flow stress vs strain graph

Answer 1

Linear increase then sharp corner to concave curve

Question 2

What does flow stress depend on strain and strain rate for room temperature?

Answer 2

Increases with strain but not strain rate

Question 3

How does work hardening work?

Answer 3

Plastic deformation requires generation and movement of dislocations. As deformation (strain) proceeds the number of dislocations increases. At RT dislocations cannot leave their slip plane. Then get dislocation interaction which increases the stress required for movement. Hence get work hardening and the flow stress increases with strain

Question 4

How does recovery work?

Answer 4

At elevated temperatures cross slip and climb can occur. Dislocations can then rearrange to annihilate or form low angle boundaries or subgrains. This reduces the dislocation density and is called recovery. Recovery can occur during deformation (dynamic) or after deformation (static)

Question 5

Dynamic equilibrium for recovery

Answer 5

Here the rate if dislocation generation equals the rate of dislocation annihilation/rearrangement. Flow stress initially increases as a result of dislocation density increase. Then at equilibrium point no further increase and flow stress stays constant.

Question 6

Elevated temperature flow stress vs strain graph and how it changes

Answer 6

Initially linear increase up from origin then sharpish corner to horizontal. Don’t get any work hardening. The horizontal region gets higher for increasing strain rate, decreasing T, increasing solute content. Horizontal region starts at same strain so linear region gradient changes to accommodate this.

Question 7

Why is there no work hardening at elevated temperatures?

Answer 7

There is dynamic equilibrium so the dislocation generation rate equals the dislocation annihilation/rearrangement rate

Question 8

How does elevated T flow stress vary with strain rate, T and microstructure (explanation)?

Answer 8

Rate of dislocation generation depends on strain rate. Rate of annihilation/rearrangement depends on T. Equilibrium dislocation density and hence flow stress will be a balance of the two. High strain rate and low T gives high flow stress and vice versa.
Any microstructural feature that impedes dislocation motion will increase flow stress (e.g precipitate)

Question 9

In which materials does dynamic recovery usually occur in?

Answer 9

Those with a high stacking fault energy (SFE)

Question 10

How does slow cool or fast cool influence flow stress?

Answer 10

For same temperature and strain rate a fast cool gives higher flow stress

Question 11

Which extrusion mode is the best to use for case study 1?

Answer 11

Indirect extrusion.
Reduced pressure requirements.
As long as billet is scalped the surface quality of the product will be good.