The effect of low-temperature ageing on the tensile properties of high-pressure die-cast Mg-Al alloys

Data are presented that are relevant to the use of magnesium alloys in the engine bay and interior of automotive vehicles, where temperatures for typical applications can reach 120 degrees C. In this research, high-pressure die castings of the Mg-Al alloys AZ91D and AM60B have been aged at 120 degrees C for times of up to 5000 hours. Results are reported for the effect of this ageing treatment on the tensile properties and on the microstructure. Increases in yield stress of up to 30 MPa were noted for 5 mm thick castings, but the increase for 2 mm thick castings was far less, at about 6 MPa. The ductility of all castings was reduced by more than half. Observations of the microstructure have been carried out by optical and electron microscopy and by nuclear magnetic resonance (NMR). These have shown that precipitation of Mg17Al12 has taken place during ageing, and it is inferred that this has been the cause both of the increase in strength and the decrease in ductility.