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

Magnesium alloys play an indispensable role in the automotive,aerospace and consumer electronics industries.However,with the development of lightweight technology and the complication of structure of magnesium alloy,the hot crack in the solidification process of magnesium alloy has become one of the factors affecting its product quality.therefore,an indepth study of the thermal cracking behavior and its mechanism of magnesium alloy is of great significance to improve the application range of magnesium alloy in practical production and life.In recent years,in the study of the mechanical properties of Mg-TM-RE alloys,researchers at home and abroad have found that controlling the appropriate ratio of TM to RE in magnesium alloys can obviously improve the mechanical properties of magnesium alloys.However,there are few studies on the hot tearing susceptibility of Mg-Cu-RE alloys.In order to further study the mechanism of hot tearing susceptibility of Mg-Cu-RE series magnesium alloys,this paper discusses the effect of increasing rare earth content on hot tearing susceptibility of Mg-Cu-Gd series alloys under the condition of the same Cu/RE ratio.Compared with traditional Mg-Y-Zn series alloys,Cu element is gradually replaced by Zn element,and the difference of hot tearing susceptibility and microstructure of different alloys are discussed.In this paper,based on the Clyne-Davies theoretical model,the hot crack sensitivity of Mgx Cu-2x Gd-0.6Zr(x = 0.5,1.0,1.5,2.0,2.5 at.%)alloy and Mg-x Cu-(2-x)Zn-4Y-0.6Zr alloy is predicted,and this trend is verified by the hot crack volume of "T" castings.The solidification path,solidification fraction,dendrite interference temperature,primary crystal nucleation temperature,low melting point phase precipitation temperature and solidification end temperature of the above experimental alloys were tested by central couple thermal analysis and double couple differential thermal analysis,and the phase composition of different composition alloys was analyzed by XRD,and the microstructure and fracture morphology near hot cracks were analyzed by SEM.The experimental results show that with the increase of x to 2.0 at.%,?-Mg,the grain refinement,the dendrite interference point temperature decreases and the solid content increases,resulting in the decrease of hot tearing susceptibility(HTS)of the alloy.When x increases to 2.5 at.%,the over-thick liquid film on the grain boundary after ?-Mg solidification leads to the increase of hot tearing susceptibility.It is found that LPSO phase will precipitate with the substitution of Y for Gd,and the hot tearing susceptibility of the alloy decreases with the increase of x,and the hot tearing susceptibility of the alloy decreases with the increase of The CSC* reaches the minimum value at%.It can be seen that Cu element is better than Zn element in reducing hot tearing susceptibility.At the same time,with the gradual increase of Cu element,the number of Cu as the second phase in the alloy is more and more,and the precipitation temperature of the second phase is higher and higher,the solid fraction of dendritic overlap decreases,the residual liquid phase increases,the hot crack initiation becomes more difficult,and the possibility of hot crack decreases.In addition,the LPSO phase precipitates along the grain boundary,which may help to prevent the hot crack propagation along the grain boundary and effectively reduce the hot tearing susceptibility of the alloy.