Why carry out deformation processing?
- Geometry: forming long, thin-walled shapes (i.e. high aspect ratio, difficult to cast).
- Low waste: forming processes mostly “near net shape”.
- Tolerance and surface finish: usually good (and can be corrected by machining)
- Microstructure: cast microstructures usually coarse – need to be refined by deformation (and heat treatment) to enhance properties
. 5. Energy and cost efficient: temperatures below melting
Disadvantage of wrought alloy processing?
- High forces and complex control systems are required: can be high capital cost, with expensive high strength steel tooling.
- Multiple stages (including machining) often needed due to physical limits on achievable shape changes and complexity.
- Metals work-harden with cold deformation and often require intermediate annealing to enable further deformation.
Cold Working
Plastic Yielding T<0.3Tm strain rate around 10^5 s-1
Temperature and strain rate have little influence on yield response
Deformed, elongated grains; anisotropic mechanical properties
Modest forming in tension viable (woek hardening supresses necking)
Good surface finish and tolerance
Hot working
High strain rate creep T>0.5Tm strain rate around 10^3 s-1
Temperature and strain rate have strong influence on yield response
recrystallised, equixaed grains; isotropic mechanical properties
must be worked in compression
poor surface finish
large deformation can be accomlished at a faster rate
What do recovery and recrytallisation address?
- to maintain ductility (enabling large strains without cracking);
- to reduce forming loads (dynamic softening balances work hardening; annealing eliminates prior work hardening);
- to control final grain structure.
What are the two main nucleation mechanisms in recrystallisation?
Grain boundary nucleation -Larger subgrains at grain boundaries act as the nuclei for recrystallised grains.
Particle-stimulated nucleation
Wrought alloys contain fine-scale, hard, second phase particles and dispersoids (e.g. in Al alloys, intermetallic compounds of Al with Mn, Cr, Fe, Zr).
The dislocation density is greater around the hard particles, locally increasing the driving force for recovery and recrystallisation.
What minmum levels does recrystallisation have?
Recrystallisation requires a minimum strain level – typically 5% for cold deformation (but higher for hot deformation). This reflects the need to store sufficient energy in the form of dislocations.
There is also a minimum temperature needed to trigger recrystallisation, and this falls with increasing strain. This reflects the thermal activation needed to nucleate new grains.
Disadvantages of recystallization
Deformation processing is always inhomogeneous (due to geometric complexity, friction and heat transfer). Even in simple geometries such as flat strip rolling it is difficult to maintain uniform deformation across a rolled strip, and from one end of a coil to the other. In forging and extrusion deformation is very inhomogeneous.
Hence different parts of the component will have different grain sizes, or may not recrystallise at all in some places. This can lead to problems with variable properties, anisotropic deformation behaviour, poor surface finish, localised corrosion etc.
Age hardening mechanism
The shape of the ageing curve results from the interaction of a number of effects:
(i) rapid initial fine-scale precipitation from supersaturated solid solution (SSSS).
(ii) particle coarsening (i.e. steady decrease in the number of particles, with an increase in mean size and spacing), through one or more intermediate precipitates, eventually reaching the equilibrium phases.
(iii) decrease in coherency (i.e. crystallographic matching) of the particle-matrix interface, as the particles coarsen and transform.
(iv) transition from dislocations shearing the particles while they are small and coherent (the rising part of the curve), to dislocations bypassing the particles when they are well-spaced and incoherent (the falling part of the curve).
What is artificail and natural ageing?
Artificial ageing: hardness and yield stress rise to a peak in about 5-24 hours (the “T6 temper”) and then fall.
Natural ageing: slow rise to a plateau hardness over 1-28 days (the “T4 temper”).
