Light Alloys- Aluminium Casting and Phase Diagrams

Question 1

Describe the Al-Cu phase diagram

Answer 1

Left most is the α phase. Eutectic temperature around 550C. Above this and right of α is liquid + α. Below eutectic T is α+Al2Cu. Eutectic point around 34wt% Cu. Further right and above solidus is liquid+Al2Cu. Common composition is 4wt% Cu so only really concerned with left of eutectic point. 4wt% Cu just crosses into α region for about 450-650C.

Question 2

Equilibrium solidification sequence of 4wt% Cu

Answer 2

Start in liquid region. When hit boundary to liquid+α (liquidus line), some solid α will form with a low concentration of Cu (liquid still 4wt% Cu). As progress down this region, negative gradients of boundaries mean concentration increase in the solid and liquid (although liquid turns into solid). Then once hit boundary to pure α region all is solid α with 4wt% Cu.

Question 3

Non-equilibrium solidification sequence of 4wt% Cu

Answer 3

Faster cooling than for equilibrium. Start in liquid region. When hit boundary of liquid+α region some solid α starts to form with low concentration of Cu (liquid still 4wt% Cu). As cool through liquid+α region concentration in liquid increases. α nuclei grow and outer regions become more concentrated in Cu than inner regions. Faster cooling means the boundary to pure α region becomes more steep negative gradient so never cross into α region before eutectic temperature. When this is reached all remaining liquid does eutectic reaction resulting in eutectic at GBs with high Cu content.

Question 4

Why doesn’t liquidus line change with faster cooling?

Answer 4

Diffusion in liquid is rapid

Question 5

What sort of cooling is used in DC casting?

Answer 5

Non-equilibrium because equilibrium would take 3-4 days whereas this could take 1 hour.

Question 6

How does homogenisation work for Al with 4wt% Cu?

Answer 6

After non-equilibrium solidification, heat treat the alloy to a T below the eutectic T (like 500C) and hold for long time (like 24 hours) to remove the eutectic and homogenise the solute content.

Question 7

Why is non-equilibrium cooling and homogenising more economical than equilibrium cooling?

Answer 7

Faster cooling (1h vs 3-4 days) and then can homogenise lots of casts at once for up to 24h

Question 8

Examples of insoluble impurities that remain after homogenisation

Answer 8

Fe-rich (from steel channels before DC casting).
Mg2Si