Light Alloys- Aluminium Casting and Phase Diagrams Flashcards
Describe the Al-Cu phase diagram
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.
Equilibrium solidification sequence of 4wt% Cu
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.
Non-equilibrium solidification sequence of 4wt% Cu
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.
Why doesn’t liquidus line change with faster cooling?
Diffusion in liquid is rapid
What sort of cooling is used in DC casting?
Non-equilibrium because equilibrium would take 3-4 days whereas this could take 1 hour.
How does homogenisation work for Al with 4wt% Cu?
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.
Why is non-equilibrium cooling and homogenising more economical than equilibrium cooling?
Faster cooling (1h vs 3-4 days) and then can homogenise lots of casts at once for up to 24h
Examples of insoluble impurities that remain after homogenisation
Fe-rich (from steel channels before DC casting).
Mg2Si