What happens to aluminum after solution heat treatment and quenching?

• A. It remains hard after quenching.
• B. It is soft after quench but regains hardness as it ages.
• C. It becomes permanently brittle.
• D. It melts.

Answer: (B)

In aluminum alloys that respond to precipitation hardening, solution heat treating dissolves the alloying elements, and quenching rapidly cools the material to lock those elements in a supersaturated solid solution. In this state, there are few or no precipitates to hinder dislocations, so the alloy is soft. As aging occurs—either naturally at room temperature or artificially at an elevated temperature—fine precipitates form and impede dislocation motion, increasing hardness and strength. So the material is soft right after quenching and hardens again as it ages.