Structural and age hardening characteristics of near eutectic Al-Si alloys

In the present work structural features of near eutectic Al-Si and Al-Si-Cu alloys prepared by a normal melting and casting route are studied. It is observed that morphology of Si plays an important role in enhancing the mechanical properties of Al-Si alloys. In order to modify the Si network in the matrix of Al some immiscible elements such as Pb, Sn, Cd and Bi have been added in small quantities to the alloy. Since the structure developed after casting governs the mechanical properties, it is essential to understand the structural features before testing the mechanical properties. Taking into consideration the age hardening characteristics of the alloys, microstructural assessment and a thermal study of these alloys have been done. Thermal analysis shows that immiscible elements are homogeneously distributed in the alloys. It is observed that the addition of Pb leads to a rosette type structure, whereas in the case of Cd and Sn blunt and twisted Si needles can be seen. The structural features of the eutectic change to hypoeutectic completely on addition of Bi. The shift in the eutectic depression on addition of a ternary element causes a change in the morphology of Si. Addition of Cu to the Al-Si alloys gives precipitation hardening characteristics.