Effect of Heat Treatment and Loading Path on Non-proportional Fatigue Behavior and Fracture Characteristics of an Al-Si Casting Alloy

This paper investigated the microstructure and mechanical properties of a typical Al-6.2Si-3.4Cu-0.4 Mg cast alloy under four heat treatment processes and compared the non-proportional fatigue behavior of completely reversed stress cycles under circular and elliptical loading paths. The solution treatment significantly reduced the size and aspect ratio of the eutectic Si phase, achieved excellent and balanced mechanical properties, and yielded the highest non-proportional fatigue life, while the aging treatment mainly acted on the precipitation phases, especially the size of the needle-like Mg 2 Si phase. The cyclic deformation behavior indicated that the four heat-treated alloys exhibited two different stress responses and plastic damage forms. Surrounding alloys with these two damage forms, the fracture characteristics were analyzed and summarized. The differences between the two loading paths for the same heat-treated alloy were also discussed in terms of cyclic deformation behavior, crack propagation paths and fracture characteristics. The dislocation evolution revealed the development of plastic damage as the distance decreased from the fatigue section.