Prediction of Distortion and Residual Stresses in a Magnesium High Pressure Diecasting with Considerations to the Diecasting Process

Magnesium cold chamber diecastings are steadily making inroads into the automotive industry. Parts that are routinely diecast include instrument panels, valve covers, steering columns, transmission case covers, seat pans, and door frames to name a few. In a bid to take full advantage of weight savings, the majority of the parts are optimized during the design process. This means wall thickness are reduced, and ribs are used to reinforce the part for stiffness. As a part of the design and optimization process, generic load analysis is carried out on the part without taking into account the residual stress developed during the casting process. The iterative process is continued until the design responsible engineer is satisfied with the outcome of the analysis. However, during the diecasting process, variations in the thermal patterns are obtained in the die mainly due to section thickness changes of the part, placement of cooling lines and spray patterns. On ejection from the die, the part will inherit a thermal history, which upon cooling to room temperature in air or in a quenching medium will result in part distortion and development of residual stress. Part warpage, which is a consequence of distortion, results in problems during trimming, machining, and assembly. On the other hand, residual stress beyond certain limits might lead to cracking of the part after production or during service. This paper discusses some of these aspects for a high pressure diecast magnesium automotive part.