Fatigue properties of a semi-solid cast Al-7Si-0.3Mg-T6 alloy

Abstract

The room temperature fatigue properties of a semisolid cast A356 alloy have been defined by determining a Wšhler curve for lives between 104 and 107 cycles. The fatigue tests were carried out in axial tension/compression cycling at zero mean stress (stress ratio, R, of -1). The initiation sites of the fatigue cracks were studied in detail and it was observed that cracks generally initiated from shrinkage pores or oxide films at the surfaces of the machined fatigue specimens. Measurements of the sizes of these defects are reported, and evidence is presented to suggest that oxide defects have less effect on the fatigue life than shrinkage pores of the same size, presumably reflecting differences in the ease of fatigue crack initiation at the two types of defects. The fatigue properties of our particular semisolid castings are compared with those of casting made by other processes. It is shown that the semisolid castings performed slightly better than squeeze castings made in our laboratory using the same machine and the same die. The properties of both the semisolid and squeeze castings are significantly better than those of gravity diecastings.