Influence of copper and heat treatments on the pit formation of cast AlSi10Mg alloys

The demand for recycled cast alloys in electric motors has grown rapidly in recent years. However, cast alloys from combustion engines are not designed for the lower temperatures found in electric motors. Since stagnant water does not evaporate, the high copper and iron contents present in cast recycled aluminium products contribute to pitting corrosion. To counteract this, a combination of heat treatments and copper content must be employed to regulate pit depth by altering the morphology and occurrence of intermetallic phases. Therefore, we studied the corrosion behaviour of two AlSi10MgCuX (X = 0.1 and 2.0 wt%) alloys in the T5 and T6 conditions, correlating the local Volta potential with the nature and distribution of the intermetallic phases. We found that when the copper content is 2.0 wt.% for the T5 condition, pits initially form at θ-Al2Cu and advance to the α-AlFeMnSi and Si-eutectic. Additionally, copper increases the Volta potential of iron and silicon-bearing phases by altering their chemical composition and producing precipitates around them. This increment of the Volta potential leads to pits growing continuously along the eutectic network in the T6 condition. At Cu0.1, pits develop at the α-AlFeMnSi first, followed by the Si-eutectic. Although solution heat treatment followed by ageing can increase corrosion resistance and delocalise pitting, the alloying elements involved have significant effects on the pit formation sequence.