Physical and chemical properties of CaF2-CaO-Al2O3-MgO slag for electroslag remelting of Mg-containing GH3128 superalloy

During electroslag remelting, Mg in the electrode reacts with some components in the slag, resulting in its serve oxidation loss. The CaF2-CaO-Al2O3-MgO slags with high MgO content are generally used for remelting Mg-containing Fe- and Ni-based alloys. However, there is a scarcity of foundational data available on the physical and chemical properties of the slags, posing a challenge in developing a suitable slag for remelting Mg-containing GH3128 superalloy. Hence, this paper conducted a comprehensive study on the melting temperature, viscosity, and reactivity of four known slags. The results show that D2 slag (50%CaF2-20%CaO-20%Al2O3-10%MgO) has the lowest melting temperature of 1588 K due to containing fewer high-melting-point phases and possesses lower viscosity because of simpler aluminate structures. Additionally, the D2 slag has the highest activity ratio, enabling the superalloy to achieve the highest equilibrium Mg content of 0.0174%. The D2 slag exhibits a heightened MgO activity and facilitates the release of more Mg2+ ions, thereby exerting a pronounced inhibitory effect on the diffusion of Mg2+ ions from the slag-metal interface to the slag. The superalloy attains a highest Mg yield of about 30%. Therefore, the D2 slag emerges as the most suitable choice for remelting the Mg-containing GH3128 superalloy.