Investigations On Hot Tearing Susceptibility Of Two Magnesium Alloys

As a light environmental-friendly metallic material, magnesium alloys have been attracted great attentions for application in aerospace, automotive, electronic production and other industry fields because of their low density, high specific strength, excellent machinability etc. However, magnesium alloys have the tendency to form hot tearing during the casting process and then deteriorate the performance of magnesium alloy casting, results in the application limitations of magnesium alloy casting products. So the investigations on magnesium alloy hot tearing behaviors have important significance and practical value.The rare earth elements are widely used in magnesium alloys because of their grain refinement effect, as well as the performance improvement effect, not only the room temperature mechanical properties but also the high temperature mechanical properties, such as excellent creep properties. As a new developed magnesium alloy, EW75 has good high temperature mechanical properties. ZK60 magnesium alloy has higher tensile strength than other ZK series magnesium alloys. However, EW75 and ZK60 magnesium alloys are susceptible to hot tearing in the casting process. The hot tearing susceptibility of EW75 and ZK60 magnesium alloys are predicted using thermodynamic calculations based on Clyne-Davies’ model. The microstructures of hot tearing regions are observed by optical microscopy and scanning electron microscopy. Effects of mold temperature and pouring temperature on hot tearing of alloys are investigated. The hot tearing susceptibility of ZK60 and EW75 alloys are compared with that of the conventional AZ91 magnesium alloy.The results show that, the hot tearing susceptibility of the investigated alloys were found as follows: AZ91<EW75<ZK60. The hot tearing susceptibility of ZK60 magnesium alloy was the highest, the AZ91 magnesium alloy was the lowest. The theory predicted result was consistent with the experimental result. Through experimental measurement the hot tearing volume confirm that the hot tearing susceptibility was decreased with increasing the mold and pouring temperature.The hot tearing mechanism of EW75 magnesium alloy is liquid film, solidification shrinkage repairing and dendritic bridging. When the liquid film strength is not enough to resist solidification shrinkage stress, the hot tearing is formed because of the solidification shrinkage stress. The amount of liquid fraction is much more than ZK60 magnesium alloy at the end of the solidification, the hot tearing can be refilling by the liquid. Meanwhile, the regions of solid cohesion are established between the adjacent dendrites. This can occur to a sufficient extent, then the strength of the casting is greater than that required for preventing the inter-dendritic separation. So, the hot tearing tendency of the alloy will reduce.Hot tearing susceptibility of ZK60 magnesium alloy was the highest due to its large vulnerable temperature range, coarsen dendritic structure, small liquid fraction at end of the solidification and the bad feeding performance.Hot tearing susceptibility of AZ91 magnesium alloy was the smallest due to its narrow solidification temperature range and temperature range of vulnerable regions. Meanwhile, the amount of remainder liquid in the final stage of solidification can be refilling cracks.