Microstructural and Mechanical Properties of Thixoformed Al–Si–Cu–Mg Alloy: Effect of Partial Remelting Treatment

Thixoforming is a semi-solid metal processing technique that involves the formation of alloys in the solid–liquid regions. The process exhibits many advantages, such as homogenous microstructure and low shrinkage porosity that results in improved mechanical properties. It relies on thixotropic behaviour of semi-solid slurries having solid spheroidal grains suspended in liquid matrix and can be obtained via a rheocasting process called cooling slope casting. In cooling slope casting, molten alloy is poured onto a tilted metal plate, and the collected solidified alloy is prepared in the form of billets. These billets will later be re-heated into the semi-solid states prior to deformation into shapes. This work investigated thixoforming of an aluminium alloy prepared by the cooling slope process and then subjected to partial remelting heatings at various inductive currents. The effect of these different inductive currents, at 440, 460, 480 and 500 A, to the heating temperature profiles of the billets, the microstructural evolutions and the tensile strengths were investigated. The results demonstrated the importance of employing the correct inductive current in obtaining fine globular solid grain microstructures, hence avoiding excessive grain coarsening that may adversely affect the mechanical strength of a thixoformed part. In general, the work showed that the average solid grain size had significantly increased with increasing inductive currents. Here, an inductive current of 460-A had resulted in acceptable grain size and shape factor of approximately 41μm and 0.82, respectively. The tensile properties yielded the highest values, reaching 194 and 295 MPa for yield stress and the ultimate tensile strength, respectively.