Aluminium Flashcards
What is the youngs modulus of aluminium alloys?
Greater than that of concrete but smaller than that of steel.
What is the specific stiffness and specific strength of aluminium alloys?
Similar to steel
Why aluminium alloys and not aluminium?
1) Aluminium is not strong, very ductile
2) Aluminium alloy increases strength and loses some ductility.
What type of aluminium alloy has both high strength and high ductility?
Wrought aluminium
What can alloys be?
1) Cast
2) Wrought
3) Heat-treatable or non-heat-treatable
How do aluminium alloys gain strength?
By precipitation hardening (age hardening). A good example is Duralumin (4% copper).
How can age-hardening be sped up?
By heating up to 150 degrees C .
What happens when heated up to temperatures approx 250 degrees C?
Overaging occurs- Fewer particles precipitated. Ductility increases and strength decreases.
Name 4 advantages of Al alloys?
1) Light-weight
2) Strength, stiffness and ductility comparable to steel
3) Very recyclable
4) Corrosion resistant
How is Al alloys corrosion resistant?
Al reacts fast with oxygen in the air (dry corrosion) but this produces a compact protective layer of alumina.
Name 4 disadvantages of Al alloys?
1) More expensive than other construction materials
2) Higher environmental footprint
True or false “Aluminium is among the most abundant substances on Earth’s surface?”
True
Why was pure metal aluminium produced only recently? (3)
1) Aluminium is not found natively in pure form, only mixed as an oxide.
2) The melting temperature of alumina us 2,072 degrees C.
3) Smelting aluminium from alumina is difficult because carbon atoms are unable to steal oxygen from the alumina.
What does the Hall-Heroult process do?
Obtains pure aluminium from alumina.
What does the Bayer process do?
Separates alumina from the ore.
Describe the Bayer process. (6)
1) The ore from alumina is bauxite. We crush and mill this.
2) Heating this to 170/180 degrees C while adding NaOH in aqueous solution, only the aluminium and Si in bauxite will dissolve to Na based ions.
3) Calcium oxide and hydroxide can be added to react with Si ions in solution to precipitate solid calcium-silicate-hydrate.
4) Now left with the NaAL(OH)4 ions and the solid is left behind can be filtered away called “red mud”.
5) Add small crystals of solid AL(OH)3 instead of CO2 (was done) which accelerates precipitation of the solid gibbsite.
6) The solid is finally heated at 1000 degrees C to decompose.
What type of problem is red mud?
Environmental problem.
Describe the Hall-Heroult process?
1) Alumina does not dissolve in water and dissolves and forms ions in liquid cryolite.
2) Leaving a solution of Na+ and Al-based anions.
3) The pumping of electrons and subsequent non-spontaneous reactions occur in an electrolytic cell.
What does aluminium recycling involve?
Melting scrap aluminium and requires up to 95% less energy than primary aluminium production.
