The Study Of Fatigue Properties For A357-T5 Casting Aluminum Alloy

The tensile and fatigue properties of A357 casting aluminum alloy by T5/T6 heat treatment are studied in this paper. The fatigue load is sinusoidal wave form with frequency of 40Hz, the stress ratios are -1, 0.05 and 0.4 respectively, the experimental environments are room temperature and 150℃high temperature. The morphology of fracture surfaces and fracture mechanisms are observed and analyzed by scanning electric microscope (SEM).The Quality Index of A357-T5 casting aluminum alloy is significantly higher than that of A357-T6 casting aluminum alloy. At the environments of room temperature and high temperature, the material properties of A357-T5 casting aluminum alloy is higher than A357-T6 casting aluminum alloy.The fatigue limits of A357-T5 casting aluminum alloy are 75MPa at r=-1, 110MPa at r=0.05 and 145MPa at r=0.4 respectively at room temperature, while 60MPa at r=-1 and 100MPa at r=0.05 respectively at 150℃high-temperature. At both room temperature and high temperature conditions, the fatigue strengths of A357-T5 casting aluminum alloys increase with the increasing of stress ratios, and the trend of the S-N curves at different stress radios are all the same. In the temperature range of this research, the temperature effects on fatigue strengths of A357-T5 casting aluminum alloys can be seen. The fatigue strengths of A357-T5 casting aluminum alloys at room temperature are larger than that at high temperature,and the deviation of the fatigue strengths at different temperature increases with increasing the stress ratio. By comparison of the S-N curves of A357 cast aluminum alloys with T5/T6 heat treatment, we can found that the fatigue strengths with T5 heat treatment increase obviously than that with T6 heat treatment with the increasing stress ratios. on the contrary,at 150℃high-temperature, the fatigue strengths of A357 casting aluminum alloy with T6 heat treatment are larger than that with T5 heat treatment.A357 casting aluminum alloys have good plastics because of lots of dimples on tensile fracture surfaces. Temperature and heat treatment have no significant effects on the fatigue fracture surfaces of A357 casting aluminum alloys. The fatigue cracks initiate from casting inclusions and casting defects. The crack propagation areas of fatigue fracture surfaces have obviously secondary cracks, tear ridge, silicon particles, holes because of debonding Si particles and fatigue straits. It means that the fatigue cracks propagate along the dendrite boundaries and features of brittle cleavage fracture.