Effects of Mg and Mn element on microstructure and mechanical properties of Al-Si alloy

By means of optical micrograph and other instruments,the microstructures and shapes of transcocrystallized Al-Si alloy treated by Mg and Mn complex modifications were studied.The results indicate that the mass coarse and irregular lath-shaped primary silicon and slim strip eutectic silicon phase are found with the Si content increasing.In addition,the tensile strength decreases gradually when the Si content increasing.However,the hardness increases.The primary and eutectic silicon phases are refined when adding 0.9% Mg to Al-20Si alloy,and the mechanical properties are remarkably improved.SEM and OM results show that 0.9Mg-xMn can effectively refine the primary and eutectic silicon in alloy.The morphology of primary silicon after modification is transformed from pentalpha and irregular shape to block.A majority of eutectic silicon is changed from acicular or lamellar morphology to small short rod.By adding 0.9%Mg and 0.5% Mn elements,the tensile strength reaches 210 MPa,which increases by 64% and 37% relative to those of Al-20Si and Al-20Si-0.9Mg alloys,respectively,and the heat treatment tensile strength reaches 345 MPa,increases by 64% relative to that of the as-cast alloy.