Mechanical properties and microstructure evolution of thixoformed aluminum alloys

Abstract

Thixoforming is an attractive process for the manufacture of complex parts withsubstantial savings of time and cost. Different from the conventional metal forming technologies which use either solid metals (forging) or liquid metals (casting) as startingmaterials, thixoforming deals with semi-solid slurries Thixoforming is composed of threemain processes, i.e. thixotropic feedstock production, reheating, and thixoforming. In thisstudy, three steps of the thixoforming process were carried out for A357 and AA6082 alloys. The production of a fine, equiaxed, globular microstructure (often referred to as thixotropic) is a must for the success of the thixoforming process. In the first step, coolingslope (CS) casting process was carried out to obtain such a feedstock. The effect of castingtemperature and pouring length on the formation of thixotropic microstructure were investigated. In the reheating step, as-cast billets were partially remelted at semi-solidtemperature intervals and isothermally held for suitable durations with an inductionfurnace in order to observe the microstructural evolution. In the forming step of theprocess, as-cast billets were inductively reheated to the temperatures practiced in the second step and then thixoformed between dies. Microstructural evolution and mechanicalproperties (hardness, tensile strength, yield strength, elongation to fracture) of thethixoformed samples were investigated. In order to use the advantage of low viscositypresented in the thixotropic feedstocks, thixoforming was performed with lower press loads comparing to the loads used in literature.