Influence of Heat Treatment on High-Cycle Fatigue and Fracture Behaviors of Piston Aluminum Alloy Under Fully-Reversed Cyclic Bending

Abstract

In the present article, the influence of the heat treatment on high-cycle fatigue and fracture behaviors of piston aluminum–silicon alloys was studied. For this objective, standard samples were tested under fully-reversed cyclic bending by a rotary fatigue machine and then, fracture behaviors were investigated by the field emission scanning electron microscopy. Before experiments, two heat treatments were performed on as-cast aluminum specimens, including a cooling process on samples after casting and the T6 treatment (solutioning for 5 h at 500 °C, quenching and ageing for 9 h at 180 °C). Obtained results showed that the heat treatment changed the distribution and the size and of silicon phases in the aluminum matrix and increased the hardness, significantly. In addition, the high-cycle fatigue lifetime increased by the heat treatment, surprisingly as 2146% and 411%, at the highest stress level and the lowest stress level, respectively. This enhancement was more than the improvement by cooling samples after casting. Observing cleavage and quasi-cleavage marks on the specimen fracture surfaces was the reason for the brittle fracture behavior. Besides, cleavage planes were smaller in heat treated samples, with shorter micro-cracks or lower crack densities, which affected the fatigue lifetime of the material.