Microstructure Evolution,high Temperature Mechanical Properties And Hot Tearing Behavior Of Mg-Al-Ca-Sm Alloys

As the lightest and strongest metal structural material,Mg-Al series alloys has been rapidly applied in automobile industry.However,due to the poor high temperature mechanical properties of Mg-Al alloy,its application scope is severely limited.In addition,most Mg-Al alloy parts used in automobile industry are complex in shape.In the casting process,the cooling rate of the same casting varies at different positions.Moreover,the temperature span of the solid-liquid phase interval is large during the solidification of Mg-Al alloy.As a result,hot tearing occurs easily at the end of solidification.Therefore,it is of great significance to develop Mg-Al alloy with excellent high temperature mechanical properties and hot tearing resistance.Both rare earth and alkaline earth elements can improve the mechanical properties and hot tearing resistance of Mg-Al alloy.The new alloys developed by the combination of rare earth and alkaline earth elements should have great potential.Hence,in this paper,the alkaline earth element Ca and rare element Sm were used to desine Mg-Al-Ca-Sm alloy.The microstructure and high temperature mechanical properties of Mg-Al-Ca-Sm alloy at different cooling rates were studied in detail by optical microscope(OM),scanning electron microscope(SEM),transmission electron microscope(TEM),X-ray diffraction(XRD),room-temperature and hith-temperature tensile test.The hot tearing behavior of Mg-Al-Ca-Sm alloy under different cooling rates was systematically studied in combination with the solidification-stress curve which was obtained from the hot tearing test.The effect of addition of Ca and Sm on the microstructure and mechanical properties of Mg-6Al-x Ca-y Sm alloys were investigated systematically.The experimental results show that the grain size of the alloy decreased first and then increased by increasing Ca and decreasing Sm content.The minimum grain size was 41.91±0.72?m when the Ca and Sm content both was 2%.By increasing Ca and decreasing Sm content,the content of?-Mg₁₇Al₁₂ and Al₂Sm phases in the alloy decreased gradually,while the content of Al₂Ca and(Mg,Al)₂Ca phases increased continuously.The tensile strength at room temperature and high temperature increased first and then decreased.With the increase of Ca and decrease of Sm,the size and number of cleavage surfaces in the fracture morphology of the alloy continue to increase.The larger the Ca content and the smaller the Sm content,the lower the toughness of the alloy.The effects of different cooling rates and Al content on the microstructure and properties of Mg-x Al-2Ca-2Sm alloy were studied in detail.The results show that with the increase of cooling rate and Al content,the nucleation temperature and size of grains decrease,and the volume fraction of the second phases increases gradually.The Al₁₁Sm₃ phase,Mg₂Ca phase and Mg₁₇Al₁₂ phase can only be formed in a specific cooling rate range or under a certain Al content.By increasing the cooling rate,the tensile strength and elongation of the alloy increase at room temperature and high temperature.However,with the increase of Al content,the tensile strength of the alloy at room temperature and hig h temperature tended to increase first and then decrease.While the tensile strength teached the peak value when the Al content was 9%at room temperature and was5%at high temperature.With the increase of cooling rate,the plasticity of the alloy increases at both room temperature and high temperature.However,with the increase of Al content,the plasticity of the alloy at room temperature and high temperature increases first and then decreases.In addition,the plasticity of the alloy increases wtih the increase of tensile temperature.The change of microstructure of Mg-x Al-2Ca-2Sm alloys after isothermal treatment was systematically studied.The experimental results show that when the isothermal treatment temperature was 175??275?,the nanoscale phase was precipitated in the alloy structure,which improves the mechanical properties of the alloy at room temperature and high temperature,and the plasticity of the alloy is improved.When the isothermal treatment temperature was 300??375?,the Al₁₁Sm₃ phase formed under rapid cooling rate was decomposed and transformed into granular Al₂Sm phase.As a result,the mechanical properties of Mg-3Al-2Ca-2Sm alloy decrease with the increase of isothermal treatment temperature.In addition,the Mg₁₇Al₁₂ phase also decomposed under this isothermal treatment temperature,so that the tensile strength and elongation of Mg-9Al-2Ca-2Sm alloy at room temperature decreased,while the tensile strength at high temperature increased,the elongation at high temperature gradually decreased.When the isothermal treatment temperature was 400??505?,the Mg-9Al-2Ca-2Sm alloy was overheated at 445?.When the isothermal treatment temperature is higher than 415?,the Mg₁₇Al₁₂ phase will be completely decomposed,the tensile strength and elongation at room temperature will decrease,while the tensile strength at high temperature will increase.Both Mg-3Al-2Ca-2Sm and Mg-5Al-2Ca-2Sm alloys overburned at the 505?isothermal treatment temperature.The(Mg,Al)₂Ca phase decomposed into granular Al₂Ca phase,and fish-bone Al₂Ca phase is transformed into granular phase.However,granular Al₂Sm phase does not change.On the basis of Mg-x Al-2Ca-2Sm alloy,starting from the effect of cooling rate and Al content on the hot tearing performance of the alloy,through the study of solidification-stress curve,phase composition and distribution,the formation mechanism of the hot tearing was revealed,and the change rule of hot tearing sensitivity was clarified.The results show that the hot tearing behavior of alloys is closely related to the grain size and grain morphology,and in different cases,one of the two factors is dominant.With the increase of grain size,the hot tearing tendency of the alloy increases,and with the dendrite morphology,the hot tearing tendency of the alloy also increases.By decreasing the cooling rate,Mg ₂Ca phase will be formed in Mg-3Al-2Ca-2Sm alloy,which increases the solidificationm temperature range of the alloy and thus increases the hot tearing tendency.Wit h the increase of Al content,?-Mg₁₇Al₁₂ phase will be formed in the alloy,which also increases the solidification temperature range,leading to the increasing of hot tearing tendency.