AI alloys Flashcards
which is the most significant Al-rich mineral
bauxite
extraction of aluminium metal process
✓ The Bayer process to extract alumina (alumium oxide) from bauxite
✓ The Hall-Heroult electrolytic process to produce metallic Al
what is secondary alumium and why we use it
re-melting and refining Al scraps (secondary Al) is cheaper than production of Al from minerals.
secondary Al requires only 5% of the energy for smelting primary Al
designation of Al wrought alloys, what do the numbers represents
- first number = defines main family
- second number = depend on different evolutions of the same alloy, originally it’s 0, if we have an improvement becomes 1,….)
- third and fourth = means nothing apart in group 1 where they repesents the degree of purity
strength of Al alloys range
100MPa-600MPa
dispersion strengthening passages
-solution annealing
-aging
dispersion strenghtening, describe the steps
first phase of dispersion strenthening, it consists in:
✓ solution annealing at high temperature (homogeneous solid solution)
✓ water quenching down to room temperature (supersaturated solid solution)
✓ controlled re-precipitation from the solid solution of dispersed sub-micrometric precipitates by natural aging (at room temperature) or artificial aging (at moderately high temperature)
artificial vs natural aging
artificial aging is much more frequent, we don’t have to wait weeks but just some hours
starting point for aluminum work-hardening alloys
The starting point are alloys that already show significant
strengthening by solid solution hardening
what can we do if reduction by cold
rolling becomes excessive and ductility is too low in aluminum alloys
Annealing treatment at 250 - 400°C is
adopted to recover original properties
why is Al one of the most appropriate metals for casting operations.
low melting temperature, high fluidity in
the molten state, good surface finish of castings
possible concerns using Al for casting operations
large solidification shrinkage (3,5-8,5%), the high coefficient of thermal expansion
(residual stresses build up) and from marked tendency to absorb Hydrogen in the liquid state
effect of silicon in foundry aluminum alloys
- improves alloy fluidity
- lowers the melting point (and solidification range)
- lowers the CTE
- improves the corrosion resistance
casting Al alloys: how can we mitigate the loss in ductility
To mitigate the loss in ductility promoted by the coarse Si Crystals, the eutectic constituent is usually modified in its morphology to achieve smaller and globular Si particles formed during solidification, by addition of Na or Sr in the melt, before casting
reaction of Al exposed to air environment
exposed to air environment, the Al surface spontaneously forms a continuous and compact oxide layer
about 5 nm thick under compression, that prevents further interaction and chemical attack (passivation).
passive layer is responsible for the excellent corrosion resistance
different types of corrosion of Al alloys
✓ Uniform corrosion: general attack due to solubility of the oxide layer
✓ Galvanic corrosion: coupling to another more noble metal in aerated acqueous solution, ruled by the corrosion
potentials of the two metals
✓ Localized pitting corrosion; generally insoluble oxide is attacked at weak points owing to electrochemical phenomena
at cathodic micro-constituents (eg. CuAl2, FeAl3, Si)
✓ Intergranular corrosion: selective corrosion attack at grain boundaries due to electrochemical phenomena
induced by the presence of precipitates (increasing importance in alloys 6xxx, 2xxx, 7xxx)
✓ Exfoliation corrosion: developed along longitudinal planes of highly reduced (by cold rolling) sheets, along flattened grain boundaries. On sheet surfaces thin detached layers and blisters are generated due to expansion of reaction products
✓ Stress corrosion cracking: combined effect of a mechanical stress and of the corrosion action that promote an intergranular attack, particularly in 2xxx, 5xxx (with high Mg content) and 7xxx alloys
what is anodic oxidation
By electrochemical processes, it is possible to extend the growth of the oxide layer on Al surfaces much more than the natural thickness, that can better resists mechanical loads, wear and corrosion
aluminium applications
usually due to its ductility aluminium is subjected to
* extrusion
* rolling
* founderies
applications:
* applications that features a very low density
* used in transpostation because we need light veichles (plane, boats, trains, cars, bikes)
* used for small motors (cars motors)
advatages:
* good for casting
* good for machining
* good heat dissipation
* …
