Al alloy

Question 1

Production of Aluminium:

Answer 1

• It is contained in bauxite the most Al rich mineral in nature
• There are two steps to extract it the Bayer process to extract alumina and the Hall Heroult to produce metalic Al.

Question 2

Primary and secondary Al production

Answer 2

• Primary is the extraction of new Al from bauxite.
• Secondary is the recycling of Al from already existing product that is very much cheaper and eco friendly.

Question 3

Mechanical properties of Al

Answer 3

• The strengh can vary from 100 to 600 MPa

Question 4

Dispersion strengthening

Answer 4

It’s a process consisting of three phases that exploit the increase of solubility with the increasing of temperature:
- Solution annealing at high temperature
- Water quencing down to room temp.
- Controlled re-precipitation of sub-micrometric precipitates with natural or artificial aging.

Question 5

Work-hardening alloys

Answer 5

• Are used alloys (Mn,Mg) that show significant strengthening by solution hardening (a process in which is add a component to the base metal is heated to liquid phase, at this point the adding elements is dissolved and the solution is cooled down to freeze the dissolved atoms.
• If the metal is too brittle an annealing is needed

Question 6

Casting Al alloys
Pros n cons

Answer 6

• Al is one of the most suitable casting elemnets due to low melting point, high fluidity and good surface finish
• Issues: Large solidification shrinkage, high thermal expansion, tendecy of absorbing hydrogen.

Question 7

Si in Al alloy

Answer 7

• Si: improves fluidity,lowers melting point, lowers CTE, improves corrosion resistance.
• Less ductility–> to mitigate it we add Na or Sr.

Question 8

Corrosion resistance

Answer 8

Al surface forms a passivate layer (5nm) when exposed to air.
- Despite high reactivity of Al this layer bears corrosion from 4 to 9 ph.
- Is suitable for rural,industrial,marine,chemical enviroments.
- exception for some strong acid and bases

Question 9

Kind of corrosion

Answer 9

• Uniform corrosion: due to solubility of oxide layer
• Galvanometric corrosion: coupling to another more noble metal in aerated acqueous solution
• Pitting corrosion damage of passivating film
• Intergranular corrosion: selective corrosion at grain boundaries due to electrochemical phenomena induce by precipitates
• Exfoliant corrosion: along longitudonal planes of cold rolled sheets
• Stressed corrosion cracking: mechanical stess combinet with corrosion that promote intergranular attack

Question 10

Surface treatments

Answer 10

• Anodic oxidation: exploiting electrochemical processes the oxide layer can be more thick than natural
• The part to be anodized is immersed in an electrolitic cell
• Initially the oxide layer is porous to allow the current flow
• than is sealed