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

Magnesium alloys have been used widely in automotive and aerospace and other project applications because of their low density,high specific strength,high specific stiffness.However,magnesium alloys casting products involves thin wall and complicated structure,hot tearing becomes one of the bottleneck for commercial applications.Therefore,it is of great significance to study the hot tearing susceptibility and its formation mechanism for improving the range of magnesium alloys in practical applications.Based on the Clyne-Davies model,the hot tearing susceptibility of Mg-x Zn-0.5Y-0.5Zr?x=1.5%,2.5%,3.5%,4.5%??Mg-4.5Zn-xY-yNd?x+y=6,x=0,1,3,6 wt.%?and Mg-4.5Zn-1Y-xNd-0.5Zr?x=0,1,2,3wt.%?alloys was predicted.By using the double thermocouple test method,some important characteristic parameters were collected during solidification process of three different compositions alloys,such as the dendrite coherency temperature,characteristic temperature of primary nucleation and solid fractions at different solidification time,etc.The curves of solidification shrinkage stress with temperature?or time?of alloys based on the“T”type hot tearing permanent-mold were collected.The microstructure and morphology of hot tearing regions of the alloys were observed by X-ray diffraction and scanning electron microscopy.The effect of grain refinement on the hot tearing susceptibility of Mg-4.5Zn-1y-xNd-0.5Zr alloy was studied by EBSD.The hot tearing susceptibility of Mg-4.5Zn-1y-xNd-0.5Zr alloy was studied by analyzing the changes of microstructure,average grain size,average grain axial ratio and grain growth orientation after grain refinement.The experimental results show that the main phases of Mg-xZn-0.5Y-0.5Zr alloy are?-Mg,W-phase?Mg₃Zn₃Y₂?and I-phase?Mg₃Zn₆Y?.With the increase of Zn content from 1.5%to4.5%,the precipitated phase content at low melting increases obviously,which improves the feeding ability of residual liquid between dendrites,effectively prevents crack initiation and propagation.When the Zn content is low,liquid membrane theory and solidification shrinkage compensation theory are the main theoretical basis for the interpretation of crack initiation.With the increase of Zn content and sufficient feeding of residual liquid,bridging theory is the main mechanisms of hot tearing formation.The order of hot tearing susceptibility of alloys is Mg-1.5Zn-0.5Y-0.5Zr,Mg-2.5Zn-0.5Y-0.5Zr,Mg-3.5Zn-0.5Y-0.5Zr,Mg-4.5Zn-0.5Y-0.5Zr.When the total content of?Y+Nd?in the alloy is constant?6wt.%?,the predicted and measured HTS of Mg-4.5Zn-x Y-yNd alloys changes with the relative content of Y and Nd.The order from high to low is Mg-4.5Zn-3Y-3Nd,Mg-4.5Zn-1Y-5Nd,Mg-4.5Zn-6Nd and Mg-4.5Zn-6Y.Moreover,The precipitations have an important effect on the hot tearing tendency of Mg-4.5Zn-x Y-yNd alloys.The latent heat peak and precipitation temperature of the second phase of Mg-4.5Zn-3Y-3Nd alloy,Mg-4.5Zn-1Y-5Nd alloy,Mg-4.5Zn-6Nd alloy and Mg-4.5Zn-6Y alloy increase sequentially,which not only improves the feeding capacity of the residual liquid phase between dendrites,but also increases the local temperature between dendrites to alleviate the solidification shrinkage stress and reduce the HTS of the alloys.According to the fracture temperature and solid fraction of the alloys,hot tearing occurs in the middle and late solidification stages,and the interdendritic liquid phase and its crystallization behavior have a great influence on the hot tearing.The residual liquid phase with eutectic composition feeding and the coherent extension of the second phase precipitated at high temperature to the matrix can effectively pin grain boundaries and inhibit the nucleation and growth of hot cracks.For Mg-4.5Zn-1Y-xNd-0.5Zr alloys,The hot tearing susceptibility of Mg-4.5-Zn-1Y-xNd-0.5Zr?x=0,1,2,3wt.%?alloy decreases first then increases with Nd content increase,reaches the minimum value at the Nd content of 2wt.%.Moreover,the crystallization behavior of?-Mg has a great influence on the formation of intergranular feeding channels,which can be characterized by thermal analysis parameters such as dendritic coherence temperature in macro-scale and also grain size and its axis ratio in micro-scale.The precipitation behavior of eutectic phase with low melting point at grain boundary has an important influence on residual liquid phase feeding and hot tearing resistance in solidification fragile zone.The more abundant residual liquid phase is,the more difficult the crack will be to initiate;the higher the precipitation temperature is,the greater the bridging effect on grain boundaries will be,and the less likely the thermal cracking will occur.For Mg-4.5Zn-1Y-2Nd-0.5Zr alloy,fine and uniform?-Mg grains make it have smooth feeding channels.The residual liquid phase at grain boundary can fully feed the solid phase shrink.Moreover,the precipitation temperature of W phase is high and the precipitation phase has a certain crystallographic relationship with the matrix leads to the lowest hot tearing susceptibility of the alloy.