The negative effect of solution treatment on the age hardening of A356 alloy

The effect of solution treatment on the age hardening of an A356 (Al-7.0Si-0.3Mg) alloy was investigated by hardness measurements and electron microscopy. This study presents experimental evidences that a prolonged solution treatment has a detrimental influence on age hardening due to the porosity in the cast A356 alloy. The enrichment of Mg on the internal surface of the porosity was observed. Nanoscale particles of magnesium oxides exist in the samples that have been solution treated with an extended duration. Our results indicate that Mg atoms would diffuse toward the porosity pre-existed in the A356 alloy during solution treatment. This phenomenon is consistent with the degraded aging kinetics and hardening potential of the A356 alloys solution treated for a long time, since the reduction of Mg solutes in the Al matrix shall lead to a decrease of the volume fraction of the Mg-Si-containing strengthening precipitates, which are mainly the monoclinic β″ phase. Our findings emphasize the significance of controlling porosities for the A356 alloys and may provide useful information for designing thermal treatment parameters to reach the optimum balance between strength and ductility.