Effects of multi-step aging on mechanical property and microstructure of Al-Mg-Si-(Cu) alloy

The relationship between property and microstructure of 6061 alloy treated by multi-step aging were studied by means of transmission electron microscopy(TEM),differential scanning calorimetry(DSC),and hardness testing.The so-called T6I6 temper involves partially aging the alloy at a typical T6 temperature(the pre-aging stage),then holding at 65 ℃ to facilitate further hardening(the secondary aging stage),and finally,aging to peak properties at initial aging temperature(the re-aging stage).120/T6I6 temper is the same with T6I6 except the pre-aging temperature(120 ℃).It was found that an obvious increase of peak hardness as reported in the literature was not achieved by the T6I6 aging treatment.There was a continuous precipitation of GP zones during interrupted aging after pre-aging at 120 ℃,while only a slight change in microstructure was observed during the interrupted aging for the conventional T6I6 temper.The precipitates formed during interrupted aging in 120/T6I6 were mainly GP zones which were not large enough to act as precursors of β″ with enhanced strengthening potential.GP zones and β″ precursors were fully precipitated during pre-aging in T6I6.Neither T6I6 nor 120/T6I6 treatments increased the strength of peak aging.Moreover,120/T6I6 would delay the peak aging time.