Investigation On Hot Tearing Susceptibility Of Mg-4.5Zn-xY Alloys

The Cracking Susceptibility Coefficient?CSC?,Hot Tearing Coefficient?HTC?improved by CSC,the Hot Tearing Indicator?HTI?in Procast software and cracking volume measured by wax penetration method demonstrate that HTS of investigated alloys were found as follow: HTS(MgZn_4.5Y₁Zr_0.5)> HTS(MgZn_4.5Y_0.5Zr_0.5)> HTS(MgZn_4.5Y₂Zr_0.5)> HTS(MgZn_4.5Y₄Zr_0.5)> HTS(MgZn_4.5Y₆Zr_0.5),namely HTS of MgZn_4.5Y₁Zr_0.5 alloy is the highest and HTS of MgZn_4.5Y₆Zr_0.5 alloy is the lowest among the MgZn_4.5Y_xZr_0.5 alloys.The parameters such as temperature and shrinkage contraction force of MgZn_4.5Y_xZr_0.5 alloys during the solidification were tested by ‘CRC'.The tested data is stored in the computer at intervals of 0.1 s.‘CRC' hot-tearing curves can be drawn after processing data.Some information about hot-tearing initiation and propagation can be gained from ‘CRC' hot-tearing curves.The central and edge solidification curves of MgZn_4.5Y_xZr_0.5 alloys in the same crucible were collected by double-thermocouple test.The solidification characteristic parameters of the studied alloys were obtained by differential thermal analysis.The results of differential thermal analysis show that when Y content is 1 wt.%,the first characteristic temperature of primary crystal nucleation T_n1 is the highest and the second characteristic temperature of primary crystal nucleation T_n2 is the lowest.With increasing Y content,T_n1 decreases and T_n2 increases.The solid fraction of dendritic coherent?fs-DCP?of MgZn_4.5Y_xZr_0.5 alloys are 0.46-0.64 and the dendritic coherent temperature?TDCP?are 602.8-636.7 °C.MgZn_4.5Y₁Zr_0.5 alloy has the lowest fs-DCP and the highest Ts-DCP.In addition,MgZn_4.5Y₁Zr_0.5 alloy has the highest HTS due to the largest temperature difference in vulnerable region,the less residual liquid in the last stage during the solidification,the thinner film and poorer feeding capability,the smallest anti-contraction force and the highest propagating speed of crack.The microstructure and morphology of hot tearing regions of the studied alloys were observed by X-ray diffraction,ZEISS optical microscope and scanning electron microscopy.It can be seen from the microscopic observation that the hot tearing behavior of MgZn_4.5Y_xZr_0.5 alloys has a great correlation with the amount of low melting point phase.When the Y content is low,HTS of tested alloys is high because of less formed low melting point phase,leading to thin liquid film and not smooth filling channel.The role of low melting point phase is mainly a damaging effect on intergranular binding force and the main mechanism is the fracture of intergranular bridges.When the Y content is high,HTS of tested alloys is low due to more formed low melting point phase,leading to thick liquid film and smoother filling channel.The role of low melting point phase is mainly an intergranular feeding effect on formed separated dendrites and the main mechanism is the collective effect of intergranular bridges,intergranular feeding and film.