Effects of solution treatment temperature, T ST , and Cu addition on aging behavior of 6063 alloy at 448 K and room temperature were investigated through changes in resistivity r and hardness HV. Calculated complete dissolution temperature of Mg and Si, T CD , was about 760 K for present alloys. Above 773 K, r STQ , the r immediately after the solution treatment and water quenching, increased slightly. Most of the increment could be explained by increase in equilibrium solute Fe concentration.

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... herefore, it is concluded that increase in quenched-in vacancy concentration with the T ST will be small. Solution treatment at 748 K, below the T CD , lowered maximum hardening by the aging at 448 K, also maximum resistivity increment in initial stage of aging at 448 K and the r increment by aging at room temperature until 3Ms. Maximum age-hardening at 448 K slightly increased with T ST above 773 K. However, hardening by 448 K-1.2 ks aging, simulating bake-hardening, showed small dependence on T ST above 773 K. Resistivity increment by aging at room temperature for 3 Ms decreased largely by solution treatment at 748 K. Because elevation of the T ST beyond 773 K gave only small effect on the resistivity increment by aging at room temperature, rate of cluster formation will be suppressed more largely by solute concentration than excess vacancy concentration.