Study On Preparation Of High Purity FGH4096 Master Alloy By Electron Beam Smelting And Its Purification Behavior

The hot-end components of aero-engine have been working for a long time under condition of high temperature,high speed and high stress,so it has high requirements for the performance of the material.Among them,the turbine disc is the core component of the aero-engine,which is made of wrought superalloy and powder superalloy.Compared with wrought superalloy,powder superalloy represented by FGH4096 for eliminating macrosegregation show characteristics of uniform structure,fine grain size,stable workpiece performance,good hot working deformation performance,especially yield strength and fatigue resistance,so it is used to prepare advanced aero-engine turbine disc.However,oxygen,nitrogen and inclusions in superalloys have important effects on its mechanical.The improvement of superalloy purity and mechanical properties is a directon of future development.The oxygen,nitrogen and inclusions formed by them in superalloys come from raw materials and the pollution of refractories during melting.At present,almost all master alloys of powder superalloys are melted by vacuum induction melting for primary melting.In order to improve the purity of master alloy,some measures such as slag-blocking filtration and high temperature treatment for the melt are adopted in the process of vacuum induction melting,but there are still some problems in the purity of the melt.And duplex or triple melting technology such as electroslag remelting or vacuum consumable remelting was taken on the basis of vacuum induction melting.However,there is a certain gap between China and advanced countries,such as United States,in the controlling of impurity contents.According to the characteristics of oxygen and nitrogen impurities,the condition of high temperature and high vacuum is conducive to the removal of impurities and inclusions during melting process.The problem that oxygen and nitrogen impurities and inclusions is difficult to remove with traditional melting technology in nickel-based powder superalloys is that they are limited by temperature and vacuum degree during melting.In order to solve these problems,the idea of preparing high-purity FGH4096 nickel-base powder superalloy master alloy by electron beam smelting is put forward in this paper.In the process of electron beam smelting of FGH4096 master alloy,water-cooled copper crucible is used,and non-metallic inclusions such as ceramics are not introduced,which avoids the falling off of crucible materials and the reaction of raw materials with crucibles in the smelting process.Thus,the purity and metallurgical quality of FGH4096 master alloy can be improved from the source.The purpose of deep removal of the impurities through enhancing the dissolution,transport and removal of inclusions was achieved by means of high temperature and high vacuum environment of electron beam smelting and electron beam bombardment of inclusions.However,accompanied with the removal of impurities,the evaporation of alloy elements affects the chemical composition of the ingot after electron beam smelting,which makeselements difficult to control.And the removal mechanism of inclusions during electron beam smelting process is not clear.Therefore,the specific research contents and conclusions of electron beam smelting of FGH4096 master alloy are as follows:Aim at the problem that alloy elements were difficult to control,the volatilization rate was calculated by using the Langmuir equation,activity coefficient of element and saturated vapor pressure were also calculated through thermodynamic analysis.The relationship between the element volatilization rate and temperature was determined.The effect of smelting time and smelting temperature on the chemical composition of the alloy was studied theoretically and experimentally.The results show that the evaporation loss of Cr was the most serious among the various elements.The composition of the FGH4096 alloy could be controlled by controlling the evaporation of Cr,and the boundary value of smelting temperature and smelting time for controlling the composition of element Cr was calculated.The elements Cr in the form of elementary substances were proportioned with the raw materials,and the corresponding smelting process was matched to obtain the FGH4096 master alloy ingots that meet the requirements of the composition.According to the distribution of elements in FGH4096 master alloy after electron beam smelting,the segregation behavior of alloy elements in the ingot was studied.Compared with other nickel-based alloys which were melted by traditional melting,the macro effective segregation coefficient of the alloying elements in ingot was relatively small.Aim at the problem that the removal mechanism of inclusions during electron beam smelting was not clear,the surface of melt reach very high temperature for energy concentration during electron beam smelting is beneficial to the floating and decomposition of oxide inclusions and the solidification control of metals.The content of oxygen and nitrogen are decreased to 4 ppmw and the inclusions above 8 ?m can be effectively removed with the smelting power of 9 kW and smelting time of 30 min during electron beam smelting.The removal of oxygen in electron beam smelting can be divided into three stages:the floating of oxide inclusions in melting process,the decomposition of oxide inclusions in refining process and the enrichment of oxide inclusions in induced solidification process.The oxide inclusions migrate to the melt surface due to buoyancy;the oxide inclusions floating on the melt surface are decomposed into vapor phase under the irradiation of electron beam spot;the inclusions on the surface of melt are concentrated in the final solidification region under surface tension during the induction solidification process.Based on the above research,FGH4096 master alloy by electron beam smelting was applied to industrial electron beam equipment.Firstly,the experimental parameters are designed.The time of smelting for FGH4096 master alloy of 200 kg with the power of 500 kW by two electron guns is calculated to be 15 min on the basis of the energy per unit area of the melt surface in the pilot plant smelting process is the same as that of the laboratory electron beam smelting process.Then,FGH4096 master alloy of 200 kg was smelted on the pilot plant electron beam equipment according to the designed technological process and parameters.About 200 kg of FGH4096 master alloy plate was obtained after electron beam smelting.The composition distribution,microstructure,the removal effect of impurity oxygen and inclusion of the ingot were analyzed.The results show that the inclusions are concentrated in the last solidification region,and the oxygen and nitrogen contents of some regions in the matrix of master alloy are reduced to 2 ppmw and 4 ppmw,respectively.It provides theoretical basis and data support for the preparation of high purity FGH4096 master alloy by industrial electron beam smelting technology.