Study on low-cycle fatigue behaviours of the aluminium cast alloys

The effect of aging, Si, Fe and Mn content on the fatigue behavior of two cast aluminium A319 and AS9U3G alloys, in the T5 state, were investigated. The low-cycle fatigue (LCF) was conducted on these alloys and results were analyzed with respect to the evolution of the microstructure and the fatigue fractured surface examination. In the alloy with a high content of Si, Fe and Mn (AS9U3G) the fatigue life is significantly reduces. The high decrease in normalized amplitude and back stresses at the saturation state, just after 200 °C, has been interpreted, in the A319 alloy, as a transition between θ′ in Si, Q and θ precipitates and AS9U3G alloy presents the same transition. Therefore, the increase in Si content and/or residual elements contents does not alter the precipitation mechanism. Aging (100 h at 320 °C) of both alloys has resulted in an increase in fatigue life and secondary cyclic hardening which corresponds to an increase in back stress due to the heterogeneous structure of dislocation formation. The calculation of back stress is in good agreement with the experimental results.