A comparative study of defects and mechanical properties in high pressure die cast and gravity die cast aluminium alloys

Defects such as pores, hot tears, entrained oxides or macrosegregation can occur in aluminium die castings and will impair the mechanical properties. The nature, extent and distribution of such defects will, however, differ between die casting processes. To investigate these differences, a comparative study between gravity castings of an A356 alloy and high pressure die castings of an A380 alloy has been carried out. The defect distributions of the castings were investigated by metallography, radiography and fractography, and the tensile properties were measured. The gravity die castings were produced in a step mould with and without filter and at different controlled hydrogen concentrations in the melt. The U-shaped pressure die castings were produced with systematic variations of process parameters such as plunger speed, commutation point between first and second phase and pouring temperature. It has been found that both castings contain defects, primarily pores and oxides, and that the presence and distribution of these defects are highly sensitive to the process conditions. Significant variations of the defect distribution have, however, also been found in castings produced under the same conditions, particularly in the pressure die castings indicating the stochastic nature of defects in die castings. The dominating defect type in the gravity die casting is hydrogen porosity mainly at high hydrogen melt concentrations, whereas in the high pressure die castings, oxides and entrapped air porosity dominate. The tensile properties in both types of castings are affected by the amount and distribution of defects. This effect is particularly prominent for the pressure die castings where the defect area fraction has been found to determine the tensile strength. In the gravity castings, hydrogen porosity decreases the tensile strength, but this effect becomes significant only at quite high hydrogen melt concentrations. The tensile properties as well as the porosity also depended on the cross section of the castings.