Influence of casting technique and hot isostatic pressing on the fatigue of an Al-7Si-Mg alloy

The influence of the casting filling technique and hot isostatic pressing ( hipping) on the fatigue-life distribution of Al-7Si-Mg alloy castings has b een studied. To vary the number density and size of oxide-film defects in t he castings, test bars were cast using bottom-gated filling systems with an d without filtration. Some unfiltered castings were subjected to a hipping treatment of 100 MPa at 500 degreesC for 6 hours. Test pieces were machined from the castings and were fatigue tested in pull-pull sinusoidal loading, at maximum stresses of 150 and 240 MPa under a stress ratio of R = +0.1. T he fatigue lives at any probability of failure and Weibull statistical para meters of the filtered castings were higher than those of the unfiltered an d nonhipped castings, illustrating the importance of the casting technique. However, the unfiltered but hipped castings exhibited higher performance. It is proposed that the significant improvement in fatigue life after hippi ng is due to the deactivation of entrained double oxide-film defects as fat igue-crack initiators.