Research On Purification Of K417 By Electron Beam Technology

With the continuous improvement of aero-engine power and thermal efficiency,the service temperature of turbine blade and other key components is required to be higher and higher.The severe service environment makes high temperature fatigue and creep become the main factors of component failure,which leads to disastrous consequences.As the key part of aeroengine,the research on high purity of cast nickel base superalloy becomes very significant.Electron beam refining technology has great potential for the purification of superalloys.However,on the one hand,the principle and parameter optimization of electron beam refining are still unclear;on the other hand,the reliability of the alloy prepared by electron beam refining technology needs to be further verified.In this paper,the activity coefficients of the main alloying elements are calculated by means of the weighted mixing enthalpy through thermodynamics analysis.The theoretical evaporation of the alloying elements at different temperatures and smelting times is estimated;the volatilization mechanism of different alloy elements,especially for the abnormal volatilization of element Al,is analyzed.The mechanism and removal of gas impurities N and O are studied.The influence of the quality of raw material on the evaporation and segregation of main alloying elements and impurity elements in the process of electron beam smelting K417is studied.In addition,the mechanical properties of the alloy after the remelting of Ni based superalloy by electron beam technology are studied,and the advantages and disadvantages of electron beam smelting of superalloys are revealed.The results show that it is feasible to calculate the activity coefficients of Ni,Cr,Ti,Co,Mo and Al in multi-component alloy by using the weighted mixing enthalpy.The theoretical volatilization results of Ni,Cr,Ti,CO and Mo calculated by the weighted mixing enthalpy are in good agreement with the actual experiments.In addition,Al,Cr and Ni are the main volatile elements,and the volatilization of other alloy elements such as Mo,Ti,Zr and CO is far less than that of the three main volatile elements.The volatilization rate of Al is controlled by the mass transfer process“from the melt to the liquid boundary layer”and“in the liquid boundary layer”.The flow caused by Marangoni effect will hinder the diffusion of Al.According to the relationship between the actual volatilization rate and the theoretical one,a compensation factorω_Al is introduced as 0.1.After compensation,the theoretical volatilization rate of each element is basically consistent with the actual value,and this compensation factor can be used in other Ni based alloys with similar composition.The inhomogeneous temperature field will cause the segregation of elements from low temperature region to high temperature region,which will promote the removal of impurity elements and cause the segregation of alloy elements.Gas element O decreased from 0.0008wt.%to minimum 0.0004 wt.%;N content decreased from 0.0006 wt.%to minimum 0.0001wt.%.The turbulence of melt caused by EB will hinder the evaporation of impurities,especially for the surface active element O.With the increase of refining power,the residual content of O increases.Al,Co,Ni and Mo elements have no obvious segregation after refining,while Ti and Cr elements have obvious deviation after refining.Cr element is the most serious segregation element.The mass of raw materials will affect the impurity removal and segregation of alloy elements.The mass of raw materials affects the specific surface area of melt during refining.The smaller the specific surface area is,the higher the melt effective temperature is.After refining for 10 min with 15 k W EB,the content of O in 300,1500 and 2000 g alloy is 0.0006,0.0005 and 0.0000.0005 wt.%,and the content of N is 0.0003,0.0001 and 0.0001 wt.%,respectively.However,with the increase of raw material mass,Cr segregation becomes more serious.The cooling rates of K417 ingots obtained by EB refining and VIM smelting are 218℃and 62℃/min,respectively,which results in the change of the dimension ofγ′.With the change of phase size,the Vickers hardness of K417 alloy increases from 376 HV_0.5 to 395（top）,412（middle）and 420 HV_0.5（bottom）.After refining,the Vickers hardness of top and middle region decreases rapidly due to the change of solid solution strengthening caused by the segregation of Cr and other elements,while the hardness of bottom increases slightly.After refining for 30 min,the Vickers hardness of top and middle decreases rapidly,and the hardness of the middle and bottom region is 393,388 and 397 HV_0.5 respectively.The low cycle fatigue（LCF）life（strain controlled,0.4%）of EBRed-K417 alloy at 750℃is 2324 cycles,6840 cycles and 6925 cycles respectively,which is much higher than that of traditional K417 alloy（1050 cycles）.The LCF behaviors of EBRed-K417 alloy are different at750℃and room temperature:at room temperature,the dislocation movement mechanism is consist of some SCD dislocation shearing and a small amount of Orowan bypassing;At 750℃,the mechanism of dislocation movement is a small amount of WCD dislocation cutting and a large amount of Orowan bypassing and cross slip.Because the increase of NMIs content in the alloy,the fatigue source of the alloy changes from the inclusion in the traditional alloy to the interdendritic or surface in EBRed-K417.Therefore,the generation time of the fatigue source is increased,and the fatigue life of the alloy is improved.