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ENGINEERING, AE 2025 HSC 27b

Describe the process of artificial age hardening of aluminium alloys.   (3 marks)

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  • The aluminium alloy is first heated to a high temperature to create a homogeneous solid solution, then rapidly quenched to trap alloying elements in the lattice.
  • The alloy is then reheated to a moderate temperature for a controlled period.
  • This allows fine precipitate particles to form within the lattice, significantly increasing the strength and hardness of the alloy.
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  • The aluminium alloy is first heated to a high temperature to create a homogeneous solid solution, then rapidly quenched to trap alloying elements in the lattice.
  • The alloy is then reheated to a moderate temperature for a controlled period.
  • This allows fine precipitate particles to form within the lattice, significantly increasing the strength and hardness of the alloy.

♦♦♦ Mean mark 29%.

ENGINEERING, AE 2017 HSC 26c

In modern aircraft, the external skin is riveted to the frame using solution treated and quenched aluminium 4% copper alloy rivets. These rivets are used immediately to attach the external skin of the aircraft to the frame.

Describe the changes that occur to the structure and properties of these rivets after installation.   (3 marks)

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Changes to structure and properties of the rivet

  • The microstructure of the rivet, after quenching, is a solid unstable solution of copper dissolved in aluminium.
  • On reaching room temperature, the heat energy created initiates the copper’s precipitation out of solid solution as finely distributed precipitates \(\ce{(CuAl2)} \), which strengthens the alloy considerably.
  • Work hardening is produced by cold working the metal when the rivet head is formed.
  • The rivet is significantly strengthened due to the installation, and its hardness will increase until precipitation hardening is finalised.
  • Corresponding decreases in the ductility of the metal will also be evident. 
Show Worked Solution

Changes to structure and properties of the rivet

  • The microstructure of the rivet, after quenching, is a solid unstable solution of copper dissolved in aluminium.
  • On reaching room temperature, the heat energy created initiates the copper’s precipitation out of solid solution as finely distributed precipitates \(\ce{(CuAl2)} \), which strengthens the alloy considerably.
  • Work hardening is produced by cold working the metal when the rivet head is formed.
  • The rivet is significantly strengthened due to the installation, and its hardness will increase until precipitation hardening is finalised.
  • Corresponding decreases in the ductility of the metal will also be evident. 

♦♦ Mean mark 27%.