Thermodynamic analysis of the smelting of Inconel 718 superalloy during electroslag remelting process

Inconel 718, a precipitation strengthening nickel-based superalloy, is widely used in aerospace and other high-temperature process industries because of its adequate strength, ductility, and good workability.The increasing demand for alloys with remarkable comprehensive performance necessitates the development of alloys with highly uniform microstructures and highly homogeneous composition;therefore, electroslag remelting (ESR) has attracted much attention in metallurgical circles.However, strong chemical reactions, such as (Al₂O₃) +Ti= (Ti O₂) +Al, usually occur at the electrode-slag interface (ESI) or the droplet-slag interface (DSI) and the metal pool-slag interface (MSI) ;therefore, the concentrations of Al and Ti cannot be maintained within specifications, or the elements cannot be uniformly distributed along the height of the ingots during the ESR process.Consequently, mechanical property deteriorates and the production cost of nickel-based alloy increases.To overcome these issues, in this study, a thermodynamic model for calculating equilibrium content of Al and Ti in a nickel-based alloy during ESR was developed based on the ion and molecule coexistence theory (IMCT) .The chemical composition of slags plays a key role in the physicochemical properties of slags and controls the loss of alloying element during ESR process.Therefore, the respective relations between the activity and activity ratio of individual slag components with chemical composition of Ca O-Si O₂-MgO-Fe O-Al₂O₃-Ti O₂-Ca F₂slag were studied.Furthermore, effect of the slag composition on the equilibrium content of Al and Ti in Inconel 718 superalloy under various metallurgical temperatures during ESR was investigated.The results indicate that the equilibrium content of Al increases with increasing temperature, however, the equilibrium content of Ti decreases.MgO and Ca F₂have little influence on the control of the loss of active alloy elements.