Effect of pre-aging on the behavior in the early stage of aging at high temperature for Al–0.81mass% Si–0.64mass% Mg alloy has been studied by means of hardness measurement, electric resistivity measurement, differential scanning calorimetry and transmission electron microscopy.
Pre-aging temperature influenced on the subsequent aging behavior at high temperature. G.P.I zones were formed during the pre-aging below about 343 K, which were stable and remained in the early stage of the subsequent aging at 448 K. Therefore, G.P.I zones coexist with β″ phase in the alloy aged at 448 K for 1.8 ks after pre-aged below about 343 K, and the microstructure is coarse and sparse. This leads to the decrease in hardness.
On the other hand, β″ phases were directly formed during the pre-aging above 343 K. And the microstructure is fine and dense in the alloy aged at 448 K for 1.8 ks after pre-aged above 343 K, which leads to the higher hardness.
In order to get higher paint bake hardenability for the Al–Mg–Si alloy, it is important to suppress the formation of G.P.I zones before the bake hardening treatment.
