Research On Influencing Factors Of Hot Cracking Sensitivity Of Mg-4Y-3Nd-xAl Magnesium Alloy

Mg-Y-Nd series magnesium alloys have been widely used in aerospace and transportation because of their excellent room temperature properties and high temperature creep resistance.However,when the alloy is prepared by sand mold casting,it is easy to have thermal cracking defects,which leads to casting scrap.In the past two years,research on Al refining Mg-Y-Nd alloy has attracted much attention,and Al refining effect is remarkable.Mg-Y-Nd-Al alloy has excellent comprehensive performance and broad application prospect,but its thermal cracking performance is rarely reported.Therefore,taking Mg-4Y-3nd alloy as the research object,this paper adopts optical microscope(OM),scanning electron microscope(SEM),X-ray diffraction analysis(XRD)and other tissue analysis,thermal analysis curve,solidification stress curve and other testing methods to study the influencing factors of thermal cracking during the solidification process of Mg-4Y-3Nd-xAl(x=0 wt.%,0.5 wt.%,1.0 wt.%,1.5 wt.%,2.0 wt.%)alloy.Firstly,the effect of Al content on the thermal cracking sensitivity of alloys is discussed: when Al content is 0～0.5 %,with the increase of Al content,the average grain size of alloys increases slightly,the vulnerable temperature range and the temperature of hot crack initiation increase,and the solid phase volume fraction of hot crack initiation decreases,resulting in the increase of the thermal cracking sensitivity of alloys.With the further increase of Al content,the average grain size decreases significantly,the alloy structure gradually changed from dendritic crystal to equiaxial crystal,the second phase tended to be reticulated,the alloy vulnerable temperature range gradually decreases,and the alloy thermal cracking sensitivity gradually decreased.The thermal cracking sensitivity of this series of alloys is in the order of Mg-4Y-3Nd-0.5Al,Mg-4Y-3Nd,Mg-4Y-3Nd-1Al,Mg-4Y-3Nd-1.5Al and Mg-4Y-3Nd-2Al from large to small.Based on the analysis of the residual liquid supplementation marks on the side of Mg-4Y-3NdxAl alloy fracture surface and the trace of liquid film being stretched at the fracture surface,the main theories of the formation of the alloy thermal cracking are liquid film theory and solidification shrinkage compensation theory.Secondly,by changing the casting temperature and the sand molding material to adjust the cooling rate,the influence of the cooling rate on the thermal cracking sensitivity of Mg-4Y-3Nd-1.5Al alloy was studied.It was found by changing the casting temperature of Mg-4Y-3Nd-1.5Al alloy that: With the increase of pouring temperature,cooling rate of alloy decreases,the thermal cracking sensitivity of the alloy decreases first and then increases,when pouring temperature is too low,the ability of the residual liquid phase to fill the microcracks decreased,when pouring temperature is too high,the cooling rate of alloy is reduced,but larger solidification shrinkage,intercrystalline binding force is difficult to resist stress to solidification shrinkage,causing cracking sensitivity of the alloy increase,when pouring temperature is 725 ℃ ～750 ℃ cracking sensitivity of the alloy minimum;the overall cooling rate of the casting is changed by replacing the molding material,it can be found that: molding materials for quartz sand,chromite sand,graphite volume than 3:1 and chromite sand,the cooling rate of alloy were 0.97 ℃ /s,1.34 ℃ /s and 1.78 ℃ /s respectively,with the increase of cooling rate,shorten the feeding time on the crack of the residual liquid,the alloy hot cracking sensitivity increase.When the casting temperature is between 725 ℃ ～750 ℃,and the cooling rate is lower than 0.97 ℃/s,there is no hot cracking in Mg-4Y-3Nd-1.5Al alloy.Finally,the influence of the variable section size of the constraint bar on the thermal cracking sensitivity of Mg-4Y-3Nd-1.5Al alloy was studied.It was found that the casting was completely fractured at the variable section when the constraint bar was cut from 10-35 mm.When the size of the constraint rod is cut from 15-35 mm,cracks appear at the variable section.When the size of constraint rod is changed from 20-35 mm,there is no hot crack.With the increase of the ratio of the variable section size of the constraint bar,the solidification speed of the alloy decreases,the increase of the solidification shrinkage stress slows down,the stressed area at the variable section of the constraint bar increases,and the alloy thermal cracking sensitivity decreases.