Study On Hot Tearing Behavior,Microstructure And Properties Of Mg-Al-Ca Alloys

Mg-Al-Ca alloy is one of the most interesting heat resistant magnesium alloy and has a promising prospect due to low cost,rich phase constitution,better elevated-temperature creep property,and well corrosion resistance.However,larger hot tearing susceptibility deeply restricts its development.Therefore,based on the Clyne-Davies model,the solidification process and hot tearing susceptibility of Mg-Al-Ca alloy were simulated using ProCAST software.Meanwhile,the effects of Ca additions,mold temperature and alloying on hot tearing behavior,microstructure and mechanical properties of Mg-Al-Ca alloy were also investigated using hot tearing test,solidification test,wax penetration method,OM,SEM and XRD.The influences of dendrite formation during solidification,precipitates and grain size on hot tearing behavior of alloy were discussed in this study.The solidification and hot tearing simulation results of Mg-5Al-xCa alloys（x=0.5,1,2,3,4 and 5.0 wt.%）show that hot tearing susceptibility,effective stress at hot spot and solidification temperature range decrease with the increase of Ca addition at a pouring temperature of 700℃and a mold temperature of 200℃,which indicating that Ca addition can help to increase the hot tearing resistance of alloy.Experimental results show that Ca addition has obvious effects on hot tearing susceptibility of Mg-5Al-x Ca alloys.With the increase of Ca additions,the hot tearing susceptibility of Mg-5Al-xCa alloys first decrease and then slightly increase.Among them,Mg-5Al-0.5Ca alloy has the highest hot tearing susceptibility,while Mg-5Al-4Ca has lowest hot tearing susceptibility.The appropriate Ca addition can reduce the precipitation temperature ofα-Mg in Mg-5Al-xCa alloys,inhibit precipitation of Mg₁₇Al₁₂2 phase,narrow solidification temperature range of the alloy and increase eutectic content,which are helpful for the alloy having a stronger ability of compensation at the solidification end to decrease hot tearing susceptibility.But excessive Ca addition will increase the numbers of brittle phases containing Ca and coarsen the microstructure,resulting in the increase of hot tearing susceptibility.The simulation results are agreed well with that of hot tearing test when Ca addition is less than 4 wt.%.However,there is a deviation when the addition of Ca is 5wt.%,which is caused by ignoring influence of microstructure change during the simulation process.During the hot tearing test,increasing mold temperature can decrease the hot tearing susceptibility of Mg-5Al-xCa alloys.The Mg-5Al-1Ca alloy has best ultimate tensile strength（UTS）,yield strength（YS）and elongation（EL）with the values of 152.4MPa,75.2MPa,and 5.3%,respectively.However,when Ca addition is excessive,the reticular eutectic phases can be formed in the microstructure,which results in splitting matrix and the decrease in mechanical properties of Mg-5Al-xCa alloys.The study results of AXJ530（Mg-5Al-3Ca-0.17Sr）alloy alloying with Sn and Y elements show that Sn and Y can prior combine with Ca or Al to form CaMgSn and Al₂Y phases.The formation of Al₂Y and CaMgSn phases can consume parts of Al and Ca elements and reduce the contents ofα-Mg+C36（（Mg,Al）₂Ca）andα-Mg+C14（Mg₂Ca）eutectic phases,further increase the difficulty of feeding and hot tearing susceptibility of alloy.Compared with Ca addition,the effect of Al addition on hot tearing susceptibility of Mg-Al-Ca-0.5Sn alloy is not obvious.The mechanical properties of AXJ530 can be significantly improved by the addition of Sn and Y.The UTS,YS,and EL of AXJ530-1Sn alloy reach the best values of 153.2MPa,75.2MPa,and 4.9%,which are approximately23.3%,6.2%and 17.1%higher than those of AXJ530,respectively.The UTS,YS,and EL of AXJ530-2Y alloy reach the best values of 167.4MPa,92.5MPa,and 5.6%,which are approximately 34.8%,30.6%,and 36.6%higher than those of AXJ530,respectively.The improvement in mechanical properties of AXJ530-（Sn,Y）alloy results from the microstructure refinement,precipitation strengthening,the decrease in content and optimization of morphology of eutectic phase.