Fundamental Study On The Controlling Loss Of Alloying Elements Of Large IN718 Electroslag Remelting Ingots

Inconel 718,a precipitation-strengthened nickel-based alloy,has been widely applied in aerospace,power energy industry,and national defense military in consideration of its adequate strength,ductility,and fatigue resistance up to 923 K.The increased additions of Al and Ti to Inconel 718 alloy increase the volume of Ni3(Al,Ti,Nb)type ?' phase and Ni3(Nb,Al,Ti)type y" phase,which further improves the mechanical properties of the Ni-based alloy at high temperature.The purpose of electroslag remelting(ESR),widely performed in the secondary refining technique,is to facilitate the production of high clean,fully dense,and homogeneous castings,which is in order to meet the increasing demand for remarkable comprehensive performance of alloys.However,there are remain some typical difficulties in ESR of alloys containing easily oxidized elements,which is due to strong chemical reactions between Al and Ti in Ni-based alloy and weak oxide,such as FeO,SiO2,and TiO2,in slag taking place in the slag-metal interface.Consequently,it is not possible to maintain Al and Ti uniformly from the bottom to top of the relemted ingot.Meanwhile,the presence of sulfur can deteriorate the machine properties of the superalloy due to the formation of sulfocarbide phase M2SC in the matrix and low-melting Ni-Ni3S2 eutectic film along the nickel grain boundaries.ESR acts as a terminal process for producing large Ni-based alloy ingots,the uniform distribution of Al and Ti in the resultant ingots and detrimental gaseous impurities should be strictly controlled in this process.To address the lack of basic understanding of homogenization control of Al and Ti and cleanness during electroslag remelting of Inconel 718 alloy,the research was mainly carried out in the following three aspects:(1)theoretical calculation of activities of Al and Ti in the Ni-based alloy and each component in fluoride-containing slags,which lays the foundation for obtaining the suitable thermodynamic conditions of controlling Al and Ti oxidation during ESR process;(2)Development of proper slag composition for electroslag remelting of Inconel 718 alloy by coupling with thermodynamic and experimental analysis;(3)According to the fundamental equation of heterogeneous reactions kinetics based on the double film theory,mass transfer model for elaborating alloying elements oxidation,deoxidation and desulfurization of Ni-based alloy had been established,which could provide valuable references to the electroslag remelting of Inconel 718 nickel-based alloy.Firstly,the activities of components in the full compositional range of Ni-Al,Ni-Ti,and Al-Ti melts had been calculated by using the relative alloy phase diagrams and the atom and molecule coexistence theory(AMCT),which were consistence with the values reported by the literature.The iso-activity curves of Al and Ti in the nickel-rich range of Ni-Al-Ti melts(xNi>0.9)at 1873 K had also been obtained regardless of the presence of NixAlyTiz complex molecule in the metal melt based on the thermodynamic properties of Ni-Al,Ni-Ti,and Al-Ti binary melts,The relationship between the molar enthalpy of mixing ?mixHm,Ni-Al-Ti of Ni-Al-Ti ternary melts and mole fraction xNi of Ni had also been calculated by means of the GSM(General Solution Model),Toop,Kohler,and Muggianu models using the Redlich-Kister(R-K)parameters for the molar mixing enthalpy ?mixHm of Ni-Al,Ni-Ti,and Al-Ti binary melt at 1873 K.The results showed that ?mixHm,Ni-Al-Ti increased by increasing the mole fraction of Ni,but decreased with an increase in Al/Ti ratio at 1873 K;the results obtained using the Toop,Kohler,and Muggianu models agreed well with those obtained by the GSM when the mole fraction xN;of Ni greater than 0.9,in contrast,the values obtained by the Toop,Kohler,and Muggianu models were lower than the values obtained by the GSM when the concentration of Ni was less than 0.9,the rationale behind these phenomena was that the Ni-Al-Ti ternary melts not belong to the regular solution.The feasibility of the prediction model for calculating the thermophysical properties of Ni-Al-Ti melts,such as viscosity and surface tension,is largely dependent on the accuracy of calculation of enthalpy of mixing ?mixHm and excess Gibbs free energy ?GE.The viscosity and surface tension of Ni-Al,Ni-Ti,and Al-Ti melts had been calculated using the optimized thermodynamic properties of these alloy melts based on the AMCT.The results indicated that the Hirai's model was suitable to calculate the viscosity of the Ni-Al system,while the Kaptay's model was successfully applied to estimate the viscosity of the Ni-Ti and Al-Ti alloy melts;the surface tension of Ni-Al,Ni-Ti,and Al-Ti melts can be reliably calculated by Butler's model.The maximum value of viscosity of Ni-Al and Ni-Ti alloy melts appeared at xAl=0.5 and xTi=0.3,respectively,which resulted from the formation of stable intermetallic phase NiAl and Ni3Ti would lead to short-range ordering of the in Ni-Al and Ni-Ti melts,respectively.The viscosity ?Ni-Al-Ti and surface tension ?Ni-Al-Ti of Ni-Al-Ti showed a similar trend,and ?Ni-Al-Ti as well as ?Ni-Al-Ti increased with Ti content at a given Al/Ni ratio,while decreased with the increase in Al/Ni ratio at a fixed alloy composition.Secondly,the relationship between the calculated equilibrium Al content for a given Ti content(1.13%)and the slag composition of component i in CaF2-CaO-Al2O3-MgO-TiO2 slags in the temperature range of 1773 K to 1973 K had been discussed from the point of thermodynamics,and the theoretical calculations had been verified by the experimental results.TiO2 had a great influence on the controlling loss of Al and Ti during ESR of Inconel 718 alloy compared with other components in CaF2-CaO-Al2O3-MgO-TiO2 slags,viz.,the importance of factors for controlling the equilibrium Al content in Inconel 718 alloy was order as TiO2>Al2O3>CaO>CaF2>MgO.When the TiO2 content in the slags was 2.2%and 10%,the oxidation of Ti and Al in the Ni-based alloy will be occurred respectively at 1773 K,whereas both Al and Ti were not subject to oxidation as the slags containing 4.26%TiO2.The unstable oxides FeO and SiO2 easily reacted with Al and Ti in the Ni-based alloy,and FeO and SiO2 content should be kept low as possible when the smelting temperature dropped.The Al and Ti content in the Ni-based alloy can be designed to be the upper limit in the case where some mechanical properties were met,which was an effective method of controlling Al and Ti during the ESR process.Temperature was also a vital parameter,particularly for the temperature-rise period during the beginning of the ESR process,which influenced the oxidation behavior of Al and Ti in the molten alloy.Ti was prone to be oxidized compared with Al as the temperature rise and a higher amount of TiO2 should be added in CaF2-CaO-Al2O3-MgO-TiO2 slag at elevated temperatures.Low and high slag temperatures can be adopted in the slag composition within low(0 to 4%)and high(13.5 to 15.5%)TiO2 content ranges,respectively.Finally,oxidation behavior of Al and Ti elements and effect of slag composition and alloy content as well as temperature on the deoxidation and desulfurization of Inconel 718 alloy by CaF2-CaO-Al2O3-MgO-TiO2 slag were systematically investigated by the laboratory-scale experiments and the developed mass transfer model based on the fundamental equation of heterogeneous reaction kinetics.The mass transfer rate of Al and Ti in the metal phase was much larger than that of Al2O3 and TiO2 in the slag phase,indicating that the mass transfer of Al2O3 and/or TiO2 in the slag phase likely to be the rate-controlling step(RCS).The developed mass transfer model was suitable for analysis of deoxidation and desulfurization of Inconel 718 alloy by CaF2-CaO-Al2O3-MgO-TiO2 slag with different CaO,MgO,CaF2,and TiO2 contents,and the slag for deoxidation and desulfurization in the Ni-based alloy at 1773 K and slag composition was in order of CaO>CaF2>MgO>TiO2.The slag composition with high CaO content not only decreased oxygen content by facilitating removal of oxide inclusions(MgO·Al2O3)and decreasing oxygen activity at the slag-metal interface,but increased the desulfurization ratio of the Ni-based alloy.TiO2 had the opposite effect,and the oxygen activity at the slag-metal interface increased with the increase of temperature,leading to a decrease in the desulfurization ratio,which can demonstrate that lower temperature promoted desulfurization of the Ni-based alloy.