solution heat treatment
Think of stirring sugar into hot tea: heat lets far more dissolve than the cold drink could hold. Solution heat treatment is heating an alloy hot enough that the alloying element fully dissolves into a single uniform solid solution. It is the essential first step that loads the alloy up before age hardening.
You heat into the single-phase region of the phase diagram and hold until any second-phase particles dissolve and the solute spreads evenly. Then you quench fast enough that the solute has no time to come back out, leaving a supersaturated solid solution at room temperature, holding more dissolved solute than equilibrium permits. That trapped supersaturation is the stored driving force that later aging releases as fine precipitates.
Without a proper solution treat and quench there is nothing dissolved to precipitate later, so aging does nothing. The quench must be fast enough to prevent premature coarse precipitation (especially at grain boundaries, which can hurt corrosion resistance), yet not so drastic that it distorts or cracks the part, which is a real engineering compromise. Overheating risks partial melting; underheating leaves solute undissolved.
An aluminium-copper alloy solution-treated near 500 degrees C, then water-quenched, holds all its copper in a supersaturated solid solution: soft and ready to age-harden.
Dissolve everything, then trap it in place for later precipitation.
Solution treatment by itself leaves the alloy soft; the strength appears only in the following aging step. The quench's job is to trap solute, not to harden.