Hot Tearing Susceptibility Of Mg-Al-Ca Alloy With Different Sn Additions

Magnesium alloys have been recognized as a light environmental-friendly material because of their low density,high specific strength and good casting performance.However,magnesium alloys have the tendency to form hot tearing during the casting process,and then reduce the performance of magnesium alloy casting,which results in the application limitation of magnesium alloy casting product.Therefore,investigation on the hot tearing susceptibility of magnesium alloys is very important.In order to clarify the effect of Sn element on the hot tearing susceptibility of Mg-Al-Ca alloy,the hot tearing susceptibility of MgAl₇Ca₁Snx?x=0,0.5,1,2?alloys is evaluated by thermodynamic calculations based on Clyne Davies model.The solidification temperature and shrinkage stress during the solidification of MgAl₇Ca₁ Snx alloys in the “T” type hot tearing permanent-mold are acquired with the attached computer.The microstructure and morphology of hot tearing regions of the alloys are observed by X-ray diffraction?XRD?and scanning electron microscopy?SEM?.The affecting factors of hot tearing susceptibility of MgAl₇Ca₁ Snx alloys,such as the variation of solid fraction in vulnerable region,the residual liquid fraction in the final stage,the type of the second phase of the alloys are discussed based on the above calculation and observation.The hot tearing susceptibility of MgAl₇Ca₁ is compared with AZ91 alloy.The results demonstrate that the hot tearing susceptibility in the investigated alloys is found as follows:CSC?MgAl₇Ca₁Sn1?> CSC?MgAl₇Ca₁Sn2?> CSC?MgAl₇Ca₁Sn0.5?> CSC?MgAl₇Ca₁?> CSC?AZ91?.The highest hot tearing susceptibility of MgAl₇Ca₁Sn1 alloy resulted from the following factors: the largest freezing range,the biggest changing of the variation of solidification temperature in vulnerable region,the least liquid film in the final stage of solidification.In addition,the formation of the second phase worsened the liquid flow and interdendritic feeding after dendrite coherency point.For MgAl₇Ca₁ Snx alloys?x=0,0.5,1,2?,when the x=1 wt.%,the hot crack volume and the hot tearing susceptibility both are largest.When the x=0 wt.%,the hot crack volume and the hot tearing susceptibility are smallest.When Sn content is low,the main mechanism of hot tearing is dendritic bridging theory.The quantity of the bridge is too less to resist the solidification shrinkage stress,and then the intergranular cracks is formed by fracture of the bridge under the solidification shrinkage stress which causes the hot tearing propagation.When Sn content is high,the main mechanism of hot tearing are liquid film and solidification shrinkage repairing theory.The liquid film around the grain boundaries is thick and the interval of separation dendrite is fed sufficiently resulting the casting without hot tearing.