Experimental Investigations of Fatigue Characteristics of AC4CH Cast Aluminum Alloys Fabricated through Rheocast and Squeeze Cast Methods

摘要

This study investigates the fatigue characteristics of AC4CH cast aluminum alloys fabricated through the semi-solid metal (rheocasting) process by employing the inclined cooling plate technique with a wide range of spherical a size (38-160μm) and compares with those of the conventional squeeze casting. Rheocasting process employing an inclined cooling plate is based on the crystal separation theory. The results of measured microstructural parameters indicate that aspect ratio and size of eutectic Si and size of intermetallic compound decrease with the decrease in primary a size. The fatigue strength increases with decreases in primary a size and the material with the minimum primary a size (i.e. 38μm) shows 11.3% higher fatigue strength at 10~7 cycles than that of the squeeze cast material. Although difference in damage accumulation behaviors during tensile loading is quantified by the in-situ studies, however, it does not seem to have discernable effect on the fatigue properties. The Si and intermetallic compound particles remain almost intact below 250 MPa and 200 MPa respectively in all of the materials and damage evolution occurs at the lowest stress level in the squeeze cast material. Moreover, the maximum fraction of the damaged particles depends upon the primary a size and reaches the maximum with the minimum primary a size. The crack growth rates through each phase indicate that the main difference between the rheocast and the squeeze cast material is in the crack propagation across the grain boundaries with the other regions having almost comparable crack propagation rates in the same as well as between different materials. The fact that even decrease in grain size does not lead to the impressive improvement in fatigue strength can not be attributed to the difference in grain structure according to quantitative investigation of misorientation angles between neighboring grains utilizing a scanning electron microscope equipped with electron backscattered diffraction pattern facility.