FAA Aviation Mechanic General (AMG)Materials, Hardware and ProcessesMedium
A repair station heat-treats 2024 aluminum sheet by heating it to solution temperature and quenching in water. A technician notices the material continues to gain hardness over the following days at room temperature without further processing. What process accounts for this?
- AStress relieving
- BCase hardening
- CAnnealing
- DNatural aging (precipitation hardening)
Show answer & explanationAnswer & explanation
Correct answer: D. Natural aging (precipitation hardening)
After solution heat treatment and quenching, many aluminum alloys such as 2024 undergo natural aging at room temperature, in which alloying elements precipitate within the grain structure, gradually increasing hardness and strength over several days—this is why some rivets are refrigerated to delay the process.
Why the other options are wrong
- A. Stress relieving reduces internal stress but does not increase hardness this way.
- B. Case hardening adds a hard surface layer to steel, not aluminum aging.
- C. Annealing softens metal and requires slow cooling, the opposite effect.
Aluminum Heat Treatment (Aging)
After solution heat treatment and quenching, aluminum alloys harden over time (naturally at room temperature or artificially at elevated temperature) as alloying elements precipitate within the grain structure.
- Solution treatment dissolves alloying elements at high temp
- Quenching locks elements in solid solution
- Aging (natural or artificial) precipitates hardening compounds
Memory trick: 'Heat, quench, then quietly harden'