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

According to the global green development situation,magnesium alloy,as a kind of lightweight material with development prospect,is developing towards high yield,low consumption and low pollution sustainability.However,hot cracking in the forming process of magnesium alloy ingot has always been the technical bottleneck that restricts its product quality and yield.Because hot cracking is often accompanied by complex solidification,many underlying causes and microscopic mechanisms remain unclear.Therefore,it is of great significance to study the hot tearing susceptibility of magnesium alloys for obtaining high quality castings.At present,Mg-Y-Zn alloy containing LPSO phase is highly sought after by researchers because of its excellent mechanical properties.It was found that the solid solubility of Gd in Mg was much higher than that of Y in Mg,and the solid solubility decreased significantly with the decrease of temperature,and the aging hardening effect was better.Therefore,it can be inferred that Mg-Gd-Zn alloy may have greater strengthening potential,but there are few studies on its hot-cracking sensitivity.In this paper,Clyne-Davies theoretical model has been used to predict the hot tearing susceptibility of Mg-x Zn-2x Gd(x=0.5,1,1.5,2at.%)and Mg-x Gd-(2-x)Y-Zn(x=0,0.5,1,1.5,2at.%)alloys,respectively.In this paper,the"T"type hot crack mold test system was used to collect the variation curve of solidification shrinkage stress of Mg-Zn-Y(-Gd)alloy with solidification temperature,so as to study the initiation temperature and propagation mechanism of hot crack in the solidification process of magnesium alloy.The characteristic parameters such as dendrite interference temperature and precipitation temperature of precipitated phase of Mg-Zn-Y(-Gd)alloy were collected by double galvanic test system.The fracture morphology and microstructure of Mg-Zn-Y(-Gd)alloy were observed by scanning electron microscopy(SEM)and X-ray diffraction(XRD),and the type and structure of precipitates were confirmed by transmission electron microscopy(TEM).The solidification behavior and hot tearing susceptibility of Mg-x Zn-2x Gd(x=0.5,1,1.5,2at.%)and Mg-x Gd-(2-x)Y-Zn(x=0,0.5,1,1.5,2at.%)alloys were studied by the number and morphology of the second phase,and the microstructure mechanism was revealed.The experimental results show that for Mg-2x Gd-x Zn(x=0,0.5,1,1.5,2at.%)alloy,there are three kinds of microstructure composition,?-Mg,LPSO phase and W phase respectively.With the increase of x,the relative content of residual liquid phase between?-Mg grains after crystallization increases,and the ability of feeding the local shrinkage of solid phase is enhanced.With the increase of x,the?-Mg grains are refined and the number of the second phase increases gradually,which makes the hot tearing susceptibility of the alloy decrease gradually.When the content of Zn and Gd increases in the same proportion,the precipitation temperature of?-Mg phase in Mg-2x Gd-x Zn alloy decreases gradually,and the grain refinement degree and solidification temperature range decrease gradually,which is the fundamental reason for the decrease of hot tearing susceptibility of the alloy.In addition,the LPSO phase precipitated along grain boundaries wedges into both sides of?-Mg matrix in lamellar form,which helps to prevent hot cracks from propagating along grain boundaries,which is another important reason for reducing the alloy's susceptibility to hot cracking.For Mg-x Gd-(2-x)Y-Zn(x=0,0.5,1,1.5,2at.%)alloy,Y element is gradually replaced by Gd element.W phase at grain boundary is distributed from discontinuous to continuous network,and LPSO phase extends from grain boundary to grain interior,which increases interdendritic bonding force and slows down crack initiation and propagation.It can be proved that Gd element is superior to Y element in reducing hot cracking sensitivity.With the increase of x,the number of second phase precipitated in the alloy microstructure increases,which reduces the tendency of hot cracking.At the same time,with the increase of Gd and the decrease of Y in Mg-x Gd-(2-x)Y-Zn(x=0,0.5,1,1.5,2at.%)alloy,the dendrite interference temperature(T_coh)decreases gradually,and the feeding channel between dendrites becomes shorter,which also leads to the decrease of hot cracking tendency of the alloy.