Study On Hot-Tearing Of Low-Pressure Die Casting Magnesium Alloy Wheel And The Effects Of Sr

Magnesium alloys have quickly developed in these years for their advancedproperties, such as light weight, high specific strength and favorable environmentalproperties. The light weight effect and high damping capacity of magnesium alloywheel make it widely applied to racing cars, advanced cars and motorcycles indeveloped countries. However, the manufacture techniques of magnesium alloywheel are still under research in China at present. In this paper, the magnesium alloywheel of GN90 motorcycle manufactured in Knight Mg Industry Development Co.by low pressure die casting process was investigated. The characters and causes ofthe casting crack were analyzed. The effect of AM60B alloy's hot tearingsusceptibility to wheel's hot tearing tendency was also investigated.According to the properties of the cracks, it was taken for as hot tearing defect.The hot tearing probability of the magnesium alloy wheel was counted. Theconnections of spoke-spoke, spoke-rim and spoke-hub core were the main hottearing sites. The simulation was applied to analyze the cause of the hot tearing.The simulation result showed that there existed some local isolated melting zonesand stress concentration sites, which were consistent with the main hot tearing sitesof the casting. Feeding resistance and stress concentration led to the hot tearing.Local cooling was suggested to avoid the isolated melting zones. Reduction of thealloy's hot tearing susceptibility was proposed as well.The effects of small quantity of Sr addition (≤0.2wt%) into the AM60B alloy(one of the commonly applied casting magnesium materials to wheel) on hot tearingsusceptibility were investigated. With the suitable amount of Sr addition, the hottearing susceptibility would decrease evidently. The related reasons were as follow:Sr enriched on the grain boundaries of growing crystals, suppressed the grain growth,and delayed the contact-impact behavior which assisted to keep feeding paths openfor longer and facilitate to fill behavior. Moreover, Sr promoted eutectic reaction,decreased the eutectic temperature, which assisted to fill behavior on grain boundaryand high strength of the bridge between the grains resulting in low hot tearingsusceptibility.In the pilot production, the hot tearing in the connection sites of spoke-spokewas almost disappeared by local modification of the die. Although hot tearing stilloccurred, the probabilities of hot tearing at the main hot tearing sites were reducedevidently with addition of 0.1wt%Sr.