| Magnesium alloy is widely used in engineering because of its low density,high specific strength and specific stiffness,good shock absorption and so on.However,with the development of magnesium alloy to thin-walled parts with complex structure,its hot tearing defect has become one of the bottlenecks in the application of magnesium alloy.Therefore,it is of great practical significance to study the hot tearing susceptibility and hot tearing formation mechanism of magnesium alloy for improving the quality of magnesium alloy castings and expanding the application range of magnesium alloy.In this paper,based on Clyne-Davies theoretical model,the stress curve of alloy during solidification with time was collected by using "T" mold testing system.Differential thermal analysis method was used to collect the temperature changes curve of alloy during solidification with time,and collect the characteristic information,such as dendrite coherency temperature,solid fraction during dendrite coherency,?-Mg precipitation temperature,etc The microstructure and fracture morphology of the alloy were observed by optical microscope(OM),x-ray diffractometer(XRD)and scanning electron microscope(SEM),and the phase composition and distribution were determined by EDS and surface scanning.At pouring temperature of 700?and mold temperature of 270?,hot tearing susceptibility of mg-4zn-xSn-0.6zr(x = 0,0.5,1,2,3wt.%)and Mg-4Zn-xSn-1Ca(x=0,0.5,1,2wt.%)alloys were studied.And the influence of electromagnetic fields with different current intensity and frequency on hot tearing susceptibility of Mg-4Zn-1Sn-1Ca were systematically studied.The experimental results show that after adding Sn element to Mg-4Zn-0.6Zr alloy,most Sn is dissolved in ?-Mg matrix,the remaining Sn precipitates and combines with Mg to form the Mg2 Sn phase.The main phase composition in the alloy is ?-Mg,Mg7Zn3,and Mg2 Sn phases.The addition of Sn can refine the grain and reduce the hot tearing susceptibility of the alloy.However,when Sn is added more than 2.0wt.%,a large number of Mg2 Sn phases will aggregate and grow.The coarsed Mg2 Sn particles gather at the grain boundary to affect the feeding of residual liquid phase,resulting in increasing hot tearing susceptibility of the alloy.The hot tearing susceptibility of Mg-4Zn-xSn-0.6Zr series alloys was evaluated by CSC parameters.The numerical simulation results are consistent with the experimental results.The order of the hot tearing susceptibility of the alloys from large to small is:CSC(Mg-4Zn-0.6 Zr)>CSC(Mg-4Zn-0.5Sn-0.6Zr)>CSC(Mg-4Zn-3Sn-0.6Zr)>CSC(Mg-4Zn-2Sn-0.6Zr)>CSC(Mg-4Zn-1Sn-0.6Zr),Mg-4Zn-1Sn-0.6Zr alloy has the least hot tearing susceptibility.With the increase of Sn content,the hot tearing susceptibility of Mg-4Zn-xSn-1Ca alloy first decreased and then increased,and the hot tearing susceptibility of Mg-4Zn-1Sn-1Ca alloy was the smallest.There are mainly ?-Mg,Ca2Mg6Zn3 and Ca Mg Sn phases in the alloy,among which Ca Mg Sn has a great influence on the compensation channel of intergranular residual liquid phase in the final solidification stage of the alloy.Adding Sn element into Mg-4Zn-1Ca alloy can refine grains,improve feeding channels,promote precipitation of Ca element forming a low melting point Ca2Mg6Zn3 phase,and improve feeding effect.However,when the Sn content is more than 1.0wt.%,the acicular Ca Mg Sn phases formed in the microstructure aggregate to form a network structure,which affects the feeding of residual liquid phase and leads to increase in hot tearing susceptibility of the alloy.When the electromagnetic field frequency is 10 Hz,with the increase of current intensity,the hot tearing susceptibility of the alloy decreases.When the magnetic field parameters are 10 Hz 15 A,the alloy has the lowest hot tearing susceptibility.Increasing the magnetic field intensity can decrease dendrite coherency temperature,refine the grains,improve the feeding channel,increase the content of the second phase with low melting point,break the network structure formed by the aggregation of excessive precipitated Ca Mg Sn phase,and enhance the feeding ability of the alloy in the final solidification stage,thus reducing the hot tearing susceptibility of the alloy.When the current intensity is 15 A and the magnetic field frequency is15 Hz,the magnetic field could produce too much joule heat,which leads to grain coarsening,and the hot tearing susceptibility of the alloy increases slightly. |
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