Influence of the microstructure on the fatigue damage behaviour of the aluminium cast alloy AlSi7Mg0.3

The influence of a variation in the heat treatment on the microstructure and the mechanical properties of the aluminium cast alloy AlSi7Mg0.3 (equivalent to A 356), in particular the fatigue mechanism, was studied. Grain refined and sodium-modified specimens were produced by die casting. Temperature and time for the solution heat treatment of cylindrical specimens were chosen in such a way that the strength of the α-solid solution could be varied while keeping the shape and size of the silicon precipitates in the eutectic area constant. The specimens were fatigue tested in a servohydraulic testing system under fully reversed loading (R=−1). Fatigue crack propagation was observed with a long distance microscope and was found to be shear-controlled in stage I. A qualitative correlation between the silicon particles and the crack propagation was found, i.e., eutectic areas cause deflections of the crack path and a decrease of the propagation rate. Specimens that were annealed at the highest solution heat treatment temperature exhibit a longer fatigue life.