Study On The Fracture Toughness And Fatigue Of Al-3.7Cu-1.6Mg Alloy

The strength, fracture behavior and fatigue damage of Al-Cu-Mg alloys are associated with the heat-treatment and microstructure revolution. In order to obtain the excellent microstructure for better mechanical properties by optimizing the heat-treatment, the effect of solution heat treatment, pre-stretching and aging on the tensile properties, fracture toughness and fatigue behavior of an Al-3.7Cu-1.6Mg alloy was investigated by XRD, EBSD, TEM and SEM analysis. The microstructure revolution of the alloy was also analyzed. The main conclusions could be summarized as follows:(1) Two step solution-treatment (495℃/1h followed by515℃/1h) of Al-3.7Cu-1.6Mg alloy resulted in an increase of the tensile and fatigue strength, and the reduction of the electrical conductivity. The changes in mechanical properties were related to the dissolution of the residual phase, which provided solute atoms to increase the degree of the solid solution supersaturation.(2) The natural aging response of Al-3.7Cu-1.6Mg alloy increased significantly after pre-stretching due to more introduce of high density dislocations. Meanwhile, the suppression of the formation of GPB zones by pre-stretching, leading to a slow aging response rate and a prolonged time to reach the peak aging condition. Furthermore, an increase of pre-stretching from5%to10%, the yield strength of the alloy was significantly improved by57and121MPa, whereas the variation of the tensile strength was minor. The elongation was found to decrease with the increasing pre-stretching.(3) The fracture toughness decreased as the pre-stretching increased from0%to10%. The unstretched sample possessed the highest fracture toughness of about29.7MPa·m1/2. After pre-stretching, however, the fracture thoughness decrease to27.6and26.9MPa·m1/2with increasing the pre-stretching. The SEM observation showed that the fracture surface possessed a ductile transgranular character as revealed by numerous dimples. Also, both the diameter and depth of the dimples decreased with the increasing pre-stretching, which again demonstrated the less deformation energy consumption and the low fracture toughness of the alloy. The results were in good agreement with the fracture toughness test.(4) The texture of the unstretched Al-3.7Cu-1.6Mg alloy was dominated by cube texture{001}<100>. Three types of texture, which are composed of Copper texture{211}<111>, S texture{123}<634> and Rotating cube texture{001}<110>, appeared with the introduce of5%and10%pre-stretching. Despite little effect was observed on the grain orientation, it was apparent that the pre-stretching facilitated the fatigue crack growth and decreased the intensity of the texture of the natural-aged alloy. The size of GPB zones in170℃/30min artificially aged alloy were larger than that in natural aged alloy. Therefore, the existence of GPB zones eliminated the adverse effect of pre-stretching and improved the fatigue crack propagation resistance.