Characterisation of Residual Stresses of Rheocast Al Alloy

Abstract

Rheocasting is a semisolid casting process allowing to obtain near-net shape parts. Through the Rheocasting process, it is possible to achieve aluminium castings having a low grade of porosity if compared to traditional die-casting methods, encouraging the production of automotive frame parts. However, casting processes, as commonly known, may cause tensile residual stresses inside the parts. On the other hand, compressive stresses inside castings can significantly increase the life of components: residual compressive stresses increase the material's resistance by counteracting crack initiation and propagation. The cracks propagate when the material is under tensile stress, while the Rheocasting technique seems to promote compressive stresses inside the castings. This work aims to analyse an aluminium rheocasted frame component for race cars in both the as-cast and heat-treated conditions. First, the mechanical properties of the components were evaluated in terms of tensile tests and microhardness. Then, residual stresses were measured at specific points of the casting. Finally, the evolution of the residual stresses inside the component before and after heat treatment led to assessing the effect of the Rheocasting process condition and the heat treatment, proving the marked advantage of using such a technology.