Handout 4: Part 1 Flashcards
What are reasons for carrying 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.
- Energy and cost efficient: temperatures below melting.
What are some disadvantages of deformation processes?
- 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.
List the different types of steady state processes (fixed deformation pattern through which materials flows).
Rolling - Hot or Cold
Extrusion - usually hot
Wire drawing - cold
Machining - cold
“Hot” and “cold” refer to whether the material is pre-heated before forming – but all deformation generates heat, so even a cold-worked part may undergo some heating. The average temperature during forming determines the deformation mechanism.
List the different types of non-steady-state processes
Forging - Usually hot
Sheet metal forming - cold
For cold working what is:
- Typical temperatures
- Influence on strain rate
- Types of grains produced
- surface finish
- plastic yielding: T < 0.3 Tm, typical e= 1–105 s-1
- T ,e : little influence on yield response
- deformed, elongated grains; anisotropic mechanical properties
- modest forming in tension viable (work hardening suppresses necking)
- good surface finish and tolerance
For hot working what is:
Typical temperatures
Influence on strain rate
Types of grains produced
surface finish
- high strain–rate creep: T> 0.5 Tm, typical e = 1–103s-1
- T , e: strong influence on yield response
- recrystallised, equiaxed grains; isotropic mechanical properties
- must be worked in compression (dynamic softening: necking in tension)
- poor surface finish (oxidation) and tolerance (differential thermal contraction)
Explain the process carbon steels undergoe when hot working called “dynamic recrystalisation”.
Carbon steels (left) undergo dynamic recrystallisation – new grains forming, growing, work hardening, and recrystallising in a continuous cycle: flow stress peaks then falls.
Explain the process aluminium alloys undergoe when hot working called “dynamic recrystaliisation”.
Al alloys (right) undergo dynamic recovery – dislocation accumulation (work-hardening) balanced by dislocation annihilation (recovery): flow stress reaches a constant steady-state.
Central to metal forming are the mechanisms of recovery and recrystallisation. These may occur both during forming (“dynamic”) and after forming, during annealing (“static”).
These mechanisms fulfill what important purposes?
- 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.
Describe the two main ways (sites) for nucleation of recovery.
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.
The recrystallised grain size after being deformed has a complex dependence on?
deformation strain-rate and temperature
strain during deformation
recrystallisation temperature
Why is deformation processing inhomogeneous and what problems does this arise?
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.
Describe the process of producing a heat treatable aluminium alloy.
Aluminium dissolves up to 10% of Mg, Cu, Zn, Si, Li.
Typical heat-treatable alloys contain at least two major alloy additions.
The steps in age hardening are:
(i) Solution heat treat (SHT), in the single phase region of the phase diagram.
(ii) Quench to achieve a supersaturated solid solution. (iii) Age at a temperature in the 150–250oC range (artificial ageing), or at room temperature (natural ageing).
Draw and describe natural and artificial 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”).
Describe the differents mechanisms and stages of age hardening.
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).
Draw the temperature history of an extruded alloy from coming out as a cast ingot to be being aged hardened.
Quench sensitivity
Tendency for an alloy to suffer from removal of solute from the supersaturated solution (due to dispersoids), and hence lowers peak aged strength.
Quench sensitivity is a particular problem when it is difficult to impose a fast cooling rate – e.g. thick rolled plate, or extrusions of complex shape (which are more likely to distort).
These transmission electron micrographs show coarse n precipitates nucleated on tiny spherical dispersoids during the quench. The fine-scale precipitates were formed during subsequent ageing. Each coarse precipitate has “used up” the surrounding solute, as indicated by the absence of fine-scale precipitation near the n precipitates.
The final microstructure is a micro-composite of: normal peak-aged regions, and very soft precipitate-free regions. The net effect is an intermediate hardness, up to 50% below the peak-aged value.
