Study On The Hot Tearing Susceptibility Of Various Heat-Resistant Magnesium Alloys

In order to study the effect of alloy elements and mold temperature on the hot tearingsusceptibility of casting alloys, design two kinds of alloys，Mg-7Al-2Ca-2Si-0.8Zn-0.4Mn-xCa and Mg-7Al-2Ca-2Si-0.8Zn-0.4Mn-xSr. The same length of metal type method wasused to prepare hot tearing sample, the hot tearing susceptibility was evaluated usingcrack-rod critical diameter and hot tearing sensitive coefficient. Scanning electronmicroscopy (SEM), Optical microscopy (OM) and the energy dispersive x-ray (EDX) wereused to examine the microstructure, hot tearing behavior and the composition ofconstituent phase of the alloys, the mechanical properties of alloys were tested by universalelectronic material testing machine. The important results are summarized as following:Ca has a significant effect on the hot tearing susceptibility of Mg-7Al-2Si-0.8Zn-0.5Sr-0.4Mn alloys. In examining the amount of Ca from1.0to3.5wt.%, when the Cacontent is3.0wt.%, both the crack-rod critical diameter and hot tearing sensitive coefficientare at the lowest value. When the Ca content is3.0wt.%, the alloy has the best hot crackingresistance, the lowest probability internal crack existing and the best mechanical properties.The fracture area of α-Mg dendritic crystal or eutectic phase can reflect the hot tearingsusceptibility of alloy, when the content of Ca is3.0wt.%, the fracture area of α-Mgdendritic crystal is almost zero, the fracture area of eutectic phase is almost100%, so thehot tearing tendency corresponds to the minimum solidification interval of alloy.Sr has a significant effect on the hot tearing susceptibility of Mg-7Al-2Si-2Ca-0.8Zn-0.4Mn alloys. In examining the amount of Sr from0to1.5wt.%, when the Sr content is1.0wt.%, both the crack-rod critical diameter and hot tearing sensitive coefficient are at thelowest value. The fluidity of the alloy is improved, the organizations of alloy are refinedand the mechanical properties of alloy are enhanced with the Sr addition. When the Srcontent is1.0wt.%, the alloy has the best hot cracking resistance and the best mechanicalproperties. The fracture area of α-Mg dendritic crystal or eutectic phase can reflect the hottearing susceptibility of alloy, when the content of Sr is1.0wt.%, the fracture area of α-Mg dendritic crystal is almost zero, the fracture area of eutectic phase is almost100%, so thehot tearing tendency corresponds to the minimum solidification interval of alloy.In the crack formation process, eutectic phases have the trend of enrichment to crack.Once the crack formed, the eutectic phases can feed the crack, they are good to improvethe feeding ability of alloy, make alloy get the better feeding. Therefore, they can reducethe crack formation.When the pouring temperature stays in the constant condition, the fluidity of the alloyare improved through raising the mold temperature, so that the hot tearing susceptibility ofalloys can be reduced.