AI alloys

Question 1

which is the most significant Al-rich mineral

Answer 1

bauxite

Question 2

extraction of aluminium metal process

Answer 2

✓ The Bayer process to extract alumina (alumium oxide) from bauxite
✓ The Hall-Heroult electrolytic process to produce metallic Al

Question 3

what is secondary alumium and why we use it

Answer 3

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

Question 4

designation of Al wrought alloys, what do the numbers represents

Answer 4

• 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

Question 5

strength of Al alloys range

Answer 5

100MPa-600MPa

Question 6

dispersion strengthening passages

Answer 6

-solution annealing

-aging

Question 7

dispersion strenghtening, describe the steps

Answer 7

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)

Question 8

artificial vs natural aging

Answer 8

artificial aging is much more frequent, we don’t have to wait weeks but just some hours

Question 9

starting point for aluminum work-hardening alloys

Answer 9

The starting point are alloys that already show significant
strengthening by solid solution hardening

Question 10

what can we do if reduction by cold
rolling becomes excessive and ductility is too low in aluminum alloys

Answer 10

Annealing treatment at 250 - 400°C is
adopted to recover original properties

Question 11

why is Al one of the most appropriate metals for casting operations.

Answer 11

low melting temperature, high fluidity in
the molten state, good surface finish of castings

Question 12

possible concerns using Al for casting operations

Answer 12

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

Question 13

effect of silicon in foundry aluminum alloys

Answer 13

• improves alloy fluidity
• lowers the melting point (and solidification range)
• lowers the CTE
• improves the corrosion resistance

Question 14

casting Al alloys: how can we mitigate the loss in ductility

Answer 14

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

Question 15

reaction of Al exposed to air environment

Answer 15

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

Question 16

different types of corrosion of Al alloys

Answer 16

✓ 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

Question 17

what is anodic oxidation

Answer 17

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

Question 18

aluminium applications

Answer 18

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
* …