Question

1. Describe the effect of composition and temperature on the equilibrium structure of aluminum alloys containing up to 8% copper?

2. Explain the precipitation hardening of aluminum copper alloys by reference to the heat treatment cycles.

3. Compare the mechanical properties of aluminum 4% copper alloy in the as – cast, solution treated, quenched, quenched and aged and over aged conditions.

Answer 1

1. The maximum solid solubility of Cu in Al is 5.65 wt% Cu. Upto this composition, as we increase the temperature, solubility increases. More specifically at 550°C

If the composition is more than 5.65 wt% Cu, then extra Cu will be rejected. Increase in temperature will increase solid solubility.

2. In precipitation hardening, usually done for the alloys whose composition is less than 5.65 wt% Cu. First of all we homogenise the alloy by raising the temperature to around 550°C. Then we quench the alloy to room temperature. Now we choose a temperature at around 180-230°C and we will age the sample for an optimum ageing period. Then at peak ageing time we take the sample back and thus we achieve peak hardness which is due to the formation of fine CuAl2 precipitates which will lock the dislocations which enhances hardness.

3. Mechanical properties of as cast alloys are superior to solution treated alloys due to normalising treatment. The quenched alloys have better mechanical properties than as cast and solution treated alloys. Quenched and aged alloys have far more superior mechanical properties than all other alloys. Finally overaged alloys have properties inferior to aged alloys but superior to all other alloys.

Finally the order will be

Quenched and aged > over aged > quenched > as cast > solution treated.