Study On Solidification Behavior And Hot Tearing Susceptibility Of Mg-Zn-Y Alloy

Magnesium alloys have attracted great attention because of their high specific strength,stiffness and good casting properties,which have become an important choice in engineering applications.However,the hot tearing behavior of magnesium alloys has become a bottleneck hindering the development of thin-walled,large and complex castings.Therefore,in order to expand the application of magnesium alloys,it is of great theoretical and practical significance to study the hot cracking formation mechanism and hot cracking susceptibility of magnesium alloys.Based on the Clyne-Davies model,the hot tearing susceptibility?HTS?of Mg-1Zn-xY?Zn/Y=1,0.75,0.5at.%?alloys with different Zn/Y atomic ratios were predicted.The MgZn₁Y₂ ternary alloy with fixed Zn/Y atom ratio was used as the experimental alloy.The grain refinement was achieved by adding trace alloying elements Zr,Ti and different kinds of carbonate refiners.By comparing the HTS of the alloy before and after refinement,the characteristics of the solidification path,the microstructure,the change of dendrite coherency temperature and the effect on the hot tearing susceptibility of Mg-Zn-Y alloy were studied.In this paper,the characteristic parameters of Mg-xZn-yY alloy and MgZn₁Y₂ alloy with different refiners during solidification were measured by double thermocouple method.Curves of solidification shrinkage stress with temperature?or time?of Mg-xZn-yY alloy and MgZn₁Y₂alloy treated with different refiners were collected by T type hot cracking testing system to study the mechanical behavior of alloy solidification process and the mechanism of hot crack initiation and propagation.X-ray diffraction?XRD?and scanning electron microscopy?SEM?were used to observe the microstructure and fracture surface morphology of alloys.The relationship between the grain boundary precipitation types,solidification paths,thermophysical properties,microstructures,dendrite coherency temperature and the hot tearing susceptibility of Mg-Zn-Y alloys with different Zn/Y ratios before and after grain refinement was studied.The microcosmic mechanisms of dendritic skeleton solidification shrinkage,residual liquid phase feeding on the grain boundary,second phase precipitation,hot crack initiation and propagation in MgZn₁Y₂ alloy at the end of solidification were revealed.For the MgZn₁Y₂ alloy,with the addition of the alloying elements Zr,Ti and the carbonate refiners Na₂CO₃ and C₂Cl₆,the grain refining effect was remarkable.It was found that some trace C₂Cl₆ and Na₂CO₃ addition significantly reduced the grain size of the MgZn₁Y₂ alloy.In particular,the addition of Zr was the most obvious,which can reduce the average grain size from 107.4um to 20.64um and the average aspect ratio decreases from 1.859 to 1.49.Further research results show that the hot tearing susceptibility of Mg-1Zn-2Y alloy was decreased with decreasing the grain size.According on the alloy solidification dendrite morphology nucleation and growth process,the solidification shrinkage of liquid phase and analysing feeding behavior,it was found that refinement and spheroidization of grain reduced the dendrite coherency temperature,prolonged the overall liquid feeding time,and increased the number of grain per unit volume,which leaded to a contraction stress dispersion,thereby reducing the hot tearing susceptibility of alloy.The research results show that the atomic ratio of Zn/Y in Mg-Zn-Y alloy was closely related to the formation of alloy phase.When Zn/Y ratio changes from 1 to 0.75 and then becomes 0.5,the secondary phase changed from W-phase,LPSO phases to W+LPSO phase.The results show that the hot tearing susceptibility of Mg-1Zn-xY?Zn/Y=1,0.75,0.5at.%?alloys decreases first and then increases slightly.The hot tearing susceptibility was the lowest when the Zn/Y ratio was 0.75,because the precipitation temperature of LPSO phase was not only higher than that of W-phase and also coherent with?-Mg matrix and had better pinning effect on grain boundaries.