Describe the method of solution heat treatment of an aluminum alloy.

• A. The metal is heated in a furnace to a specified temperature and immediately quenched in water. It is soft when it is removed from the quench, but regains hardness as it ages.
• B. The metal is heated in a furnace to a specified temperature and quenched in air.
• C. The metal is heated to a temperature below its melting point and aged.
• D. The metal is dissolved in acid and then baked.

Answer: (A)

Solution heat treatment for aluminum alloys involves heating the alloy to a specific high temperature to dissolve alloying elements into a uniform solid solution, then quenching rapidly to lock those elements in as a supersaturated solution. After quenching, the material is soft because there are no precipitates hindering dislocation movement; aging—either naturally at room temperature or artificially at an elevated temperature—then precipitates fine particles that restore and often increase hardness and strength. Quenching in water provides the rapid cooling needed to achieve the supersaturated state; slower cooling, such as in air, can allow premature precipitation and reduce the effectiveness of the treatment. Heating below the melting point and aging without the solution step would not create the supersaturated state required, and dissolving in acid has no relation to this process.