Plastic- Case Study 2: Recrystallisation of Aluminium Alloys Flashcards
Scenario for case study 2
Following case study 1 where we extruded a bar for machining stock, the customer reports that after heat treatment the fracture toughness and corrosion cracking are below specifications. Need to ascertain the reasons for these poor properties and suggest a new processing route for extrusion of the Al alloy so that this problem is avoided.
What did the customer do to the extruded bar?
After machining the component was solution treated for 3 hrs at 480°C and then aged for 8 hrs at 180°C to achieve required strength levels.
What did microstructural analysis of the as extruded material reveal about the grain structure?
Original grains in homogenised billet became drawn out in the extrusion direction. Within the deformed grains is a network of subgrains with an average size of 1.1μm. Subgrains slightly elongated in extrusion direction
What did microstructural analysis reveal about the extruded and solution treated material grain structure?
Light microscopy illustrates how the extruded structure has recrystallised. TEM analysis shows no evidence of subgrain structure.
How does recrystallisation affect fracture toughness and corrosion resistance?
They are both impaired when RX occurs during solution treatment compared to web. The hot worked structure is retained during solution treatment
What are the compressed samples from?
They were used to ascertain the flow stress of the material and have also been analysed in the as-compressed and solution treated conditions
Subgrain size vs strain rate graph for as-compressed samples and what affects the graph
Line starts high at low strain rates and decreases with ever decreasing steepness for increasing strain rate. The line is higher (higher subgrain size) for higher temperature
When does recrystallisation occur for the solution treatment of the compressed samples?
At low temperatures and high strain rates. If the subgrain size is greater than 1.8μm in the deformed material then RX will not occur during solution treatment
Formula for Zener Hollomon parameter (Z)
Aka temperature compensated strain rate.
Z=ε•exp(ΔH/R.T)
Where ε• is strain rate
ΔH is activation energy for hot working
R is universal gas constant 8.314J/mol
T is temperature (K)
What do strain rate and temperature determine in dynamic recovery?
Strain rate determines rate of dislocation generation and temperature determines rate of dislocation removal. Such that if strain rate increases and/or T decreases then steady state dislocation density increases in the material
What is the rate of dislocation annihilation determined by?
Thermally activated processes such as cross slip and climb. Therefore rate of dislocation annihilation related to T through the Zener Hollomon relationship exp(ΔH/R.T). Cross slip and climb rely on atomic rearrangement via diffusion so can expect activation energy to be close to activation energy for lattice self diffusion (140,000J/mol/K for Al)
Formula for change in dislocation density with respect to strain
δρ/δε is proportional to:
a.ε•.ρ^x - b.exp(-ΔH/R.T).ρ^y
Where first part represents dislocation generation.
Second part is dislocation removal.
a and b are constants.
x and y are constants greater than 0.
ρ is the current dislocation density
What is Z a measure of and why is it useful?
Measure of steady state dislocation density in a material deformed at T and ε•. Also the hot working behaviour of a material will be the same if the value of Z is the same even if the strain rate and temperatures are different. Flow stress increases as dislocation density increases and so is also a function of Z
Relationship of flow stress with Z
Graph of flow stress vs Z is straight line with positive gradient as opposed to different concave curves up with strain rate for different temperatures.
Z=A.Sinh(α.σ)^n
Where A, α and n are empirically derived hot working constants and σ is flow stress
What else is a function of Z?
Dynamically recovered subgrain size
