The role of defects in the fracture of an Al–Si–Mg cast alloy

A study of the tensile behaviour of test coupons that were extracted from cast plates of D357 aluminium is described. The effects of specimen thickness, heat treatment, structural integrity and hot isostatic pressing (HIP) were investigated. Chevron marks were found on the fracture surfaces of the specimens that failed under plane strain conditions. In most cases, structural defects, such as shrinkage pores and oxide films, were found at the initiation sites for fracture. It is shown that the structural defects that were found at the origin of the chevron patterns are large enough to explain the fractures, and that the tensile failure of cast Al–7Si–Mg alloys can be explained in terms of the principles of fracture mechanics. The influence of structural defects on the elongation to fracture is discussed, together with some implications for quality control programs involving cast Al–7Si–Mg components.