SOME OBSERVATIONS OF THE EFFECTS OF OVERHEATING IN A HYPOEUTECTIC AL-SI ALLOY

Microstructure and related property changes taking place as result of heating a hypoeutectic Al-Si (LM 16) alloy at higher temperatures of 540-degrees-C and 560-degrees-C have been investigated. Partial melting occurred in the alloy at both temperatures and the extent of partial melting was a function of soaking time and temperature. The resolidified regions possessed a considerably refined structure and higher hardness compared to the unaffected matrix. However, inspite of this, the alloy lost its overall hardness compared to that of the corresponding as-cast alloy. Further, with increasing soaking time and temperature, also density and hardness of the alloy were reduced and mechanical properties such as strength and ductility became poorer as the soaking time increased. This deterioration in the physical and mechanical properties is attributed to a secondary porosity produced. The latter outweights the positive effects of the refined structure and higher hardness of the resolidified regions. Increasing electrical resistivity of the alloy with soaking time and temperatures also supports the major role the secondary porosity played.