Thermomechanical fatigue behavior and its life prediction of AlSi9Cu3.5 cast alloy

Elevated temperatures and pressures in high-efficiency engines increases the risk of thermomechanical fatigue (TMF), constraining the utilization of aluminum cast alloys. Comprehensive understanding of TMF behavior is crucial for safety design of engine components. In this study, the out-of-phase thermomechanical fatigue (OP-TMF) performance of a newly developed AlSi9Cu3.5 cast alloy intended for use in critical engine components was investigated with a temperature cycling between 60 and 300 ℃ and strain amplitudes ranging from 0.2 to 0.6%. During the fatigue process, cyclic stress softening occurred in the tested alloy owing to the coarsening of the θʹ particles, which were the dominant precipitates in the T7 state prior to the fatigue test. The applied stress accelerated the coarsening process of the precipitates, resulting in a higher coarsening rate constant per cycle. Under TMF loading, plastic deformation was the predominant mechanism governing the fatigue process. As the strain amplitude increased, a decrease in the lifetime of the TMF was observed. Fracture analysis indicated multiple initiation points of fatigue cracks near the sample’s surface, including pores, fractured plate-like Si particles, and broken intermetallic particles. A hysteresis energy-based model was successfully applied to the prediction of fatigue life, and the predicted fatigue life agreed well with the experimental data. Compared with the commonly used Al–Si alloys, such as 319 and 356, the new AlSi9Cu3.5 alloy exhibited superior OP-TMF performance.