On the influence of process variables on the thermal conditions and properties of high pressure die-cast magnesium alloys

Abstract

The influence of pressure and velocity in high-pressure magnesium die casting on the thermal conditions and on the casting properties is studied. Specimens with the shape of a tensile test plate with a thickness of 12 mm and a length of 295 mm were cast using the alloys AM20HP, AM50HP, AS41, AE42, AZ91HP. Two gate velocities of the liquid metal 40 and 80 m s −1, were used for die filling and two pressures, 30 and 70 MPa, were applied to the metal during the solidification phase. Other processing conditions were kept constant. Temperature measurements at different positions in the die and at the metal/die interface were made during the operating cycle. Temperature distributions obtained from a simple two-dimensional numerical heat-flow model were found to agree generally with the measured values. The temperature distribution did not change significantly when varying the pressure and/or velocity. Calculated cooling curves for the alloy AZ91 indicate that the specimens solidified completely within 4.5 s. The measured bulk density of the casting was found to increase with velocity and/or pressure: Consequently, the tensile strength also increased. The density distribution along the specimen was examined, the density being found to decrease as the distance from the gate increases. The surface hardness for each alloy was generally similar at all positions in the test specimen and did not vary much with velocity or pressure. However, the surface hardness was always higher than the hardness inside the specimen, being due to the fine structure at the surface and the coarse structure inside the specimen resulting from different solidification times.