FAA Aviation Mechanic Airframe (AMA)Airframe StructuresHard
A technician solution heat treats a 2024 aluminum repair skin and, after quenching, allows the part to remain at room temperature for several days before use. During this period, the alloy's strength increases significantly as it reaches its final temper. What is this room-temperature strengthening process called?
- ACase hardening
- BNormalizing
- CNatural aging (precipitation hardening)
- DAnnealing
Show answer & explanationAnswer & explanation
Correct answer: C. Natural aging (precipitation hardening)
After solution heat treatment and quenching, 2024 aluminum alloy undergoes natural aging at room temperature, during which fine precipitates form within the grain structure, increasing hardness and strength until the alloy reaches its stable T3/T4 temper. Annealing and normalizing are softening processes, and case hardening applies to ferrous alloys, not aluminum.
Why the other options are wrong
- A. Case hardening is a surface-hardening process for steel, not applicable to aluminum sheet.
- B. Normalizing is a heat treatment process used for ferrous (steel) alloys, not aluminum.
- D. Annealing softens the metal and is the opposite effect of aging.
Natural Aging (Precipitation Hardening)
The room-temperature strengthening of a solution heat-treated aluminum alloy as fine precipitates form within the grain structure over time.
- Occurs after solution heat treatment and quenching
- 2024 aluminum reaches full T3/T4 strength within about 4 days at room temperature
- Artificial aging uses elevated oven temperatures to accelerate the process (e.g., T6 temper)
Memory trick: Natural aging is the metal quietly getting stronger with age, like fine wine