Magnesium alloy offers an outstanding combination of light weight, ease of manufacturing, and good engineering properties. The most common method to manufacture magnesium alloy products is die-casting; however defect rate for magnesium alloy die-casting is still relatively high. Especially in case of thin-sectioned die-casting, mold filling may not be accomplished occasionally due to its fast solidification rate. As a result, fluidity (i.e. the ability of filling a cavity) becomes very essential. In this study, a unique "melt droplet experiment" is proposed and conducted to examine the fluidity performance of magnesium alloys on various mold surfaces treatment, including plain SKD61 molds, vacuum nitration treated SKD61 molds and 4 other types of ceramic coating on SKD61 molds, TiAIN, CrN, AlCrN and CrC. In the beginning a variety of magnesium alloys with known fluidity are examined to validate this method's legitimacy. The effect of mold surfaces coating treatment on fluidity of magnesium alloy AZ91D was studied. Different types of surface coatings are applied on mold to distinguish the influence on fluidity performance. It is realized that surface coatings have certain enhancement towards fluidity length. For example, fluidity on the TiAlN coated molds was about 40 mm, while non treated plain molds exhibited 29 mm of fluidity, although their microscopic structure and surface roughness are similar. The difference of fluidity performance between them is about 35%. From the experimental results and heat transfer point of view, thermal properties of mold surface coating may be related to the fluidity difference.
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