The influence of impurity level and tin addition on the ageing heat treatment of the 356 class alloy

The influence of the addition of 0.5 wt.% Sn to Al–7Si–0.3 Mg alloys (356 and A356) on their ageing behaviour and mechanical properties was evaluated. Adding Sn led to a reduction of the iron rich intermetallics volume fraction, and of hardness. During solution heat treatment, Mg went into the solid solution, and Sn particles grew by competitive growth, concentrating at phase boundaries and interfaces. During aging β″ and Si precipitated. In the alloys with Sn, the β″ precipitation was accelerated and its hardening effect was greater, whereas the Si precipitation did not changed significantly. The mechanical properties of the A356 alloy were compatible with the hardening achieved during the heat treatment and to the amount of defects (pores) present in the microstructure. The yield strength and elongation of the A356 + 0.5% Sn alloy decreased after solution heat treatment and with increasing ageing temperature. These detrimental effects were minimized by treating this alloy in the T5 condition at 150 °C.