Defects and strength of aluminum alloy die casting parts

Causes for scattering of the tensile strength values were investigated on specimens which were cut from various parts of aluminum castings (ADC10, 12) made by die casting machines (5-20MN). The strengths of non-defect specimens are 200-350MPa. In the case where the distance along the flow of molten metal from the gate is less than 150-200mm, the strength is almost constant. But this value varies with the kinds of castings or their thickness, corresponding to the size of dendrite arm spacing. The strengths at locations beyond the above-mentioned distance are lower, and the apparent specific gravities are also lower. The strengths of specimens containing defects are under 200MPa. These casting defects are classified into lump, thin and scatter structure defect. While the lump defect reduces both apparent specific gravity and tensile strength, the thin and scatter structure defects reduce the tensile strength but hardly affect the apparent specific gravity. The tensile strength varies almost linearly with the fraction of defect area on the fracture surface. The scatter of the strength of aluminum die castings can be reduced, when they are cast under the conditions that produce defects within a certain small quantity.