Microstructure Characteristics of the Eutectics of Die Cast AM60B Magnesium Alloy

Abstract

Under the cold-chamber high pressure die casting (HPDC) process, samples were produced with AM60B magnesium alloy to investigate the microstructure characteristics of the eutectics, especially focusing on the constitution, morphology and distribution of the eutectics over cross section of the castings. Attentions were also paid to study the efiect of heat treatment on the eutectics in the die castings. Based on experimental analysis using optical microscopy (OM), X-ray difiraction (XRD), scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS), it was determined that fully divorced eutectics consisting of fi-Mg and fl-Mg17Al12 appeared at the grain boundary of the primary fi-Mg in the as-cast microstructure. Islands and networks of fl-Mg17Al12 phase were observed in the central region of the castings, while the fl-Mg17Al12 phase revealed a more dispersed and granular morphology on the surface layer. The two phases ratio fl/fi in the central region of the castings was approximately 10%, which was higher than that on the surface layer. Besides, the defect bands contained a higher percentage of the eutectics than the adjacent regions. After aging treatment (T6), only fi-Mg phase was detected by XRD in the AM60B magnesium alloy, though a small amount of precipitated fl-Mg17Al12 phase was observed at the grain boundary. In contrast to the microstructure of die cast AZ91D magnesium alloy under the same T6 heat treatment, no discontinuous precipitation of the fl-Mg17Al12 phase was observed in AM60B magnesium alloy die castings.