アルミ鋳造合金A356における型材と鋳造欠陥の延性破壊特性への影響

Abstract

This paper compares mechanical properties of two types of cast aluminum components made in sand mold and cast iron mold, respectively. In addition, this study attempts to establish the correlation between the shear and the tensile ductility and the characteristic size of defects for cast aluminum alloy A356. For each type of the castings, the fracture tests are performed under a wide range of stress states including tensile tests on notched and unnotched round bars and biaxial loading tests on the butterfly specimens. Using a combined experimental-numerical approach, the plasticity and fracture properties are characterized in terms of the true stress-strain curve and the ductile fracture locus. It is found that the sand-molding component is of higher yield resistance and lower ductility than the metal-molding one. Meanwhile, the fractographic study reveals that there exist two competing failure mechanisms: the internal necking of the matrix at high positive stress triaxialities and void sheeting due to shear at negative stress triaxialities. The transition of the failure modes occurs in the intermediate range. In the specimens with porosities, the metallographic observation is performed and the area fraction of defects, the area and the chord length of the largest defect are measured using Matlab's Image Processing Toolbox. The linear function is used to correlate the tensile and shear fracture strain with the characteristic size of defects. It is found that the shear ductility decreases at a faster rate than the tensile ductility with the increasing size of defects.