Hot Tearing Behavior And Mechanism Of Mg-7Zn-xCu-0.6Zr Alloy

Owing to the low density,high specific strength and stiffness,good damping and noise reduction,good thermal and electrical conductivity,magnesium alloys have been widely used in aerospace,transportation,electronic communication,medical and defense industries.Many new high-strength magnesium alloys have been developed at home and abroad to counter the low mechanical properties of existing cast magnesium alloys.The mechanical properties of magnesium alloy itself have been significantly improved,however,due to the high hot tearing susceptibility(HTS),it was hard for the casting magnesium alloys components to take the advantage of the improved mechanical properties of magnesium alloys.Therefore,it is a challenge to improve the hot tearing resistance and castability of magnesium alloys.In the present work,a kind of low HTS and high fluidity magnesium alloy is designed.The solidification behaviors of Mg-7Zn-x Cu-0.6Zr(x=0,1,2,3)alloys were investigated by the method of combining thermodynamic calculation and experimental investigation.The results showed that Mg Zn Cu phase precipitated with the addition of Cu element.The precipitation amount of the Mg Zn Cu phase increased,and the coherent temperature(T_coh)was decreased and the solidification range was shortened with the increasing of Cu content.The HTS of Mg-7Zn-x Cu-0.6Zr alloys were studied by modified Clyne-Davies model and homemade T-shaped hot tearing test mold.The numerical simulation was also applied.The results showed that the HTS of ternary alloys can be well predicted by the modified Clyne Davies model.The HTS of the Mg-7Zn-x Cu-0.6Zr alloys decreased with the increasing of Cu content and mold temperature.Mg-7Zn-3Cu-0.6Zr alloy presented the lowest HTS at the mold temperature of 250°C,which was agreed well with the simulation results.The fluidity of Mg-7Zn-x Cu-0.6Zr(x=0,1,2,3)alloys was studied by homemade spiral fluidity test mold,combining with numerical simulation.The results revealed that the fluidity of the alloys increased with the increasing of Cu content and mold temperature.The content of Mg Zn Cu intermetallic phase increased in high Cu-containing alloy.When the mold temperature is 250°C,Mg-7Zn-3Cu-0.6Zr alloy presented the best fluidity,which was agreed with the simulation results.The grain size of the alloys decreases with Cu addition,and the microstructure exhibits discontinuous to continuous and/or semi-continuous meshes owing to the precipitations increase along the grain boundaries.The addition of Cu promotes the Mg Zn₂ phase precipitating homogenization and increases the mechanical properties.However,excessive Cu addition leads to high precipitation content of the Mg Zn Cu phase at the grain boundary,which is a favorable site for crack initiation and propagation deteriorating the mechanical properties.A quality index(QI)is introduced to characterize the comprehensive mechanical properties more accurately.The result shows that Mg-7Zn-1Cu-0.6Zr alloy exhibits the highest QI of the studied alloys with the UTS of 258.9 MPa,YS of 126.2 MPa,EL of 14.2%,and micro-hardness of 57.9VHN.Under the influence of alternative magnetic field(AMF),Mg Zn Cu intermetallic phase precipitated with the form of near-sphere in Mg-7Zn-x Cu-0.6Zr alloy,and the grain was refined in the alloy,which is the main reason of HTS decreasing of the alloy.The numerical simulation results revealed that the applied magnetic field rendered two forced convection in the molten metallic pool,which increased the crystalline core in the liquid phase and inhibited the dendritic coherence,resulting in decreased hot tearing susceptibility of AMF-treated Mg-7Zn-1Cu-0.6Zr alloy.