Effects Of Solidification Behavior Of Several Mg-Zn-RE(Y,Gd) Series Alloys On Hot Tearing Mechanism

Compared with aluminum alloys and steels,magnesium alloys have attracted more and more attention in industrial applications due to their high specific strength and excellent functionality.However,due to high shrinkage force and wide solidification range,most commercial magnesium alloys usually exhibit high hot tearing susceptibility during traditional casting processes.Therefore,hot tearing is to be one of the bottleneck problems of improving the production yield,reducing the cost and enlarging its application.The hot tearing susceptibility of magnesium alloys must be an important casting characteristic to be studied.In recent years,Mg-Zn-RE series alloys have become one of the most interesting alloys due to their excellent mechanical properties and multiple types of second phase.Relevant studies have shown that the excellent properties of the alloy system are derived from the existence of different ternary intermetallic phases.However,the research on Mg-Zn-RE alloys is mainly focused on mechanical properties,and there are few reports on hot tearing.In this paper,the Cylne-Davis model was used to predict the hot tearing susceptibility of several Mg-Zn-RE(Y,Gd)alloys.The prediction results were further verified by the'T'-shaped hot tearing mold with load,temperature and time signal acquisition system.At the same time,the curve of solidification shrinkage force with time was obtained by testing,and the hot tearing behavior of the tested alloys was analyzed.The solidification characteristic parameters of the alloys,such as dendrite coherency temperature of?-Mg(T^?_coh),corresponding solid fraction of dendrite coherency(f^?_coh)and phase precipitation temperature,were obtained by thermal analysis system,and the solidification behavior of the alloys and its effect on hot tearing susceptibility were analyzed.XRD,OM,SEM and EDS were used to determine the type of second phase precipitated in the solidification process of the alloys.The microstructure and fracture morphology of the alloys were also observed.The effect of the type of the second phase on the hot tearing susceptibility and the hot tearing behavior of the alloys were further discussed.The grain morphology and second phase characteristics of the alloys were analyzed in-depth by EBSD and TEM,and the effect of solidification behavior on hot tearing susceptibility was further studied.The results show that among Mg-6.5Zn-x Y-0.5Zr(x=1,2,4,6wt.%)alloys with high Zn/Y ratio,Mg-6.5Zn-2Y-0.5Zr alloy has the highest hot tearing susceptibility,and Mg-6.5Zn-6Y-0.5Zr alloy has the lowest hot tearing susceptibility.Among Mg-6.5Zn-x Y-0.5Zr(x=9,12,18wt.%)alloys with low Zn/Y ratio and Mg-5Zn-13.5Y-0.5Zr alloy,Mg-6.5Zn-18Y-0.5Zr alloy has the highest hot tearing susceptibility,and Mg-5Zn-13.5Y-0.5Zr alloy has the lowest hot tearing susceptibility.Among Mg-x Zn-7Y-0.5Zr(x=1.3,2.5,3.7wt.%)alloys,Mg-1.3Zn-7Y-0.5Zr alloy has the highest hot tearing susceptibility,and Mg-3.7Zn-7Y-0.5Zr alloy has the lowest hot tearing susceptibility.In addition,with the increase of initial mold temperature,hot tearing susceptibility of the alloys decreases.Among Mg-x Zn-7Gd-5Y-0.5Zr(x=0,3,5,7wt.%)alloys,Mg-7Gd-5Y-0.5Zr alloy has the highest hot tearing susceptibility,and Mg-7Zn-7Gd-5Y-0.5Zr alloy has the lowest hot tearing susceptibility.Among Mg-x Zn-2x Y(x=1,1.67,2.67wt.%)alloys,Mg-1Zn-2Y alloy has the highest hot tearing susceptibility,and Mg-2.67Zn-5.34Y alloy has the lowest hot tearing susceptibility.The crystallization behavior of?-Mg,grain morphology,precipitation of second phase and solidification conditions all effect the hot tearing susceptibility.When T^?_coh is low(f^?_coh is high),the solidification enters the stage of intergranular feeding later.It indicates that the solidification shrinkage will be fully fed and hot tearing susceptibility of the alloys will be reduced.When f^?_coh is higher,the stronger the intergranular bonding force is,the higher the ability to prevent hot tearing initiation and propagation is.When the average grain size and aspect ratio of?-Mg are small,the risk region of hot tearing will be small.However,the increase of aspect ratio caused by abnormally coarse grains will make the solidification enter hot tearing dangerous region earlier.Moreover,the decrease of residual liquid feeding ability at the end of solidification and long feeding channel will make feeding difficult,and then improve hot tearing susceptibility of the alloys.When the precipitated phase of the alloys is only I-phase or I-phase and W-phase,hot tearing susceptibility increases with the increase of I-phase content,and decreases with the increase of W-phase content.When the precipitated phases of the alloys are W-phase and LPSO phase,hot tearing susceptibility decreases with the increase of LPSO phase content.However,when the second phase content is too high(replacing?-Mg as matrix),even LPSO phase will increase hot tearing susceptibility of the alloys.When the initial mold temperature increases,the fluidity of the residual liquid phase at the end of solidification increases and the viscosity of the residual liquid phase decreases,thus reducing hot tearing susceptibility of the alloys.Combined with the experimental results,it is helpful to analyze hot tearing mechanism of the alloys,and a full understanding of hot tearing mechanism can better understand hot tearing behavior of the alloys.When the hot tearing mechanism of the alloys follows liquid film theory,the intergranular bonding force is mainly provided by the liquid film before hot tearing initiation.When the strength of the liquid film is lower than the solidification shrinkage stress,the liquid film will be torn to form hot tearing.When the hot tearing mechanism of the alloys follows intergranular bridging theory,the intergranular bonding force is mainly provided by the intergranular bridging.When the strength of intergranular bridging is lower than the solidification shrinkage stress,the bridging will break to form hot tearing.Generally,hot tearing susceptibility of the alloys following intergranular bridging theory is lower than that of the alloys following liquid film theory.Especially for the alloys bridged by precipitation of LPSO phase during solidification,due to the special orientation and interface relationship between LPSO phase and?-Mg matrix,it can play a strong pinning role on both sides of the grain,and then can significantly reduce hot tearing susceptibility of the alloys.