Effect of Solidification Rate on Microstructure and Mechanical Properties of Die Casting AZ91-1.5Si-0.4Ca Magnesium Alloy

Specimens of AZ91-1.5Si-0.4Ca and AZ91 with different section thickness were prepared by HPDC (high pressure die-casting). The solidification rate of the HPDC specimens was numerically simulated. The microstructure, tensile properties and fracture morphology at the ambient temperature and 180 degrees C of the specimens were investigated. The results show that Si and Mg form Mg2Si phase and Ca mainly dissolves in the matrix. Besides, Si and Ca have some effect on grain refinement of the Mg matrix. With the increase in solidification rate, the Mg2Si phase in AZ91-1.5Si-0.4Ca turns from bulky polygons and Chinese script type to fine polygons and short rod. The mechanical properties of AZ91-1.5Si-0.4Ca and AZ91 at ambient temperature and 180 degrees C increase with the increase of solidification rate. The influence of section thickness on tensile strength of AZ91-1.5Si-0.4Ca is greater than that of AZ91, but the influence on elongation is less than that of AZ91. Furthermore, the tensile strength of AZ91-1.5Si-0.4Ca at 180 degrees C is higher than that of AZ91 when the section thickness is less than 2.5 mm, and the tensile strength of AZ91-1.5Si-0.4Ca at ambient temperature is higher than that of AZ91 when the section thickness is less than 3 mm.