Thermodynamics and Optimization of a Slag System for Al and Ti Burning Loss Control of a Φ1100-mm Ni-Based Superalloy Ingot During the Electroslag Remelting Process

A thermodynamic model of slag–metal equilibrium reactions between the GH4706 alloy melt and the CaO-MgO-Al 2 O 3 -TiO 2 -CaF 2 -FeO-SiO 2 slag is established based on the ion and molecule coexistence theory during the electroslag remelting process. First, it is verified that Ni as well as Fe should be regarded as the matrix in Ni-based superalloys, and the reasonable activity coefficients of Al and Ti can be obtained by applying the first-order Wagner equation with suitable values of activity interaction coefficients. The results indicate that the chemical reaction between Al 2 O 3 in the test slag system and Ti reaches a local equilibrium state. The slag composition contains a FeO mass fraction of lower than 0.20%, and a TiO 2 content ranging between 2.00 and 6.00%. In addition, the optimized slag systems were successfully applied in an industrial trial to produce a Φ1100-mm Ni-based superalloy GH4706 ingot. The industrial experiment results verify the calculated results, and the losses for the Al and Ti elements from the bottom to the top within the ingot were determined to be lower than 10%.