Question 4 (20 marks) For a binary alloy system, state the two conditions that m

Question

Question 4 (20 marks) For a binary alloy system, state the two conditions that must be met for precipitate strengthening (a) (2 marks) (b) Explain the heat treatment stages that are used to precipitate harden an aluminium alloy. Draw a schematic of the microstructures that result at the end of each stage assuming that the alloy is a binary system. (9 marks) (c) The wheels of a light-weight car are to be made either of the following aluminium alloys: 5052 O, 5052 H32, 6061 - O or 6061 T6. (i) Explain briefly what the temper grades O, H and T mean. (3 marks) (ii) The wheels are to be shrink-fitted using a proposed procedure that involves heating the component to 500°C for 60 mins before fitting the wheels to the car axles and followed by slow cooling. For each of the four aluminium alloys listed above, discuss briefly how the proposed procedure would affect the mechanical properties of the wheel if the wheel is made of the alloy. State any assumptions that you make, if any. You may also assume that the melting temperature of the alloys are in the range 590 - 650°C. (6 marks)

Explanation / Answer

a)The conditions that must be met for precipitate strengthening are :
- Alloy with a composition X is heated to a temperature T between solvus and solidus temperatures and soaked there.
- The sample is cooled rapidly to a temperature by quenching.

b) The heat treatment stages are:
- Homogenization- Alloy is heated to a certain temperature between solvus and solidus temperatures and soaked until all solutes dissolves. The solutes dissolve into alpha-phase. this gives a uniform solid-solution structure.
- Quenching- The sample is rapidly cooled to room temperature by water. The alloy becomes supersaturated and the solute cannot easily diffuse out of phase. This preserves uniform solid solution structure.
- Aging- Here incoherent precipitates form supersaturated solution of solid. At room temperature it is natural aging and above room temp it is artificial.

c) 1) The H temper means strain hardened aluminium alloy. If the product is strengthened through cold working. Also it can denote thermal treatment.
2)T- The T stands for Thermally treated aluminium alloys. They are stable tempers and are heat treated.
3) The O temper denotes Annealed product which has been heat treated to get lowest strength.

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