Numerical study of macrosegregation mechanisms in titanium alloy ingot during vacuum arc remelting using a volume-average approach

Vacuum Arc Remelting (VAR) is a critical process for producing titanium alloys; however, the complex melt flow and its interaction with the mushy zone in the solidifying melt pool often result in macrosegregation. In this study, a volume-average based multiphase solidification model is adapted and extended for this issue. Firstly, a novel method, the Adjusted Material Properties (AMP), is proposed to handle the rising melt pool surface during VAR process. Secondly, the complex multicomponent alloy is reduced to a binary-equivalent system for modeling purposes, with the macrosegregation results subsequently back-calculated to reconstruct element-specific distributions. Thirdly, the developed method for handling the rising melt pool surface requires a modified and more refined coupling approach. Given the dominant columnar structure of most titanium VAR ingots, potential equiaxed crystals from fragmentation are neglected. As an example, the VAR process of an engineering-scale ingot ( ϕ 300 mm × 1000 mm) with a multicomponent composition of titanium alloy is simulated. Various flow mechanisms-driven by thermo-solutal buoyancy, the self-induced Lorentz force, and an externally applied electromagnetic force - are considered. A typical segregation profile, consistent with results from engineering-scale ingot, is obtained. The formation mechanisms of macrosegregation in the ingot are well explained. Macrosegregation is caused by the transport of solute element in the liquid phase. Melt flow in the direction of the liquid concentration gradient leads to negative segregation, whereas melt flow against the concentration gradient results in the positive segregation. Numerical parameters studies, e.g. mesh sensitivity, are also made to ensure the computational accuracy.