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To produce Ti sponge for aerospace applications, the inflow process of Fe, Ni, and Cr impurities has been investigated by obtaining the distribution of the concentrations of these impurities, analyzing the microstructure and elemental composition of specimens, and calculating the formation enthalpies of the Mg–Ti–Fe, Mg–Ti–Ni and Mg–Ti–Cr ternary systems via the Miedema and Troop models. Fe, Ni, and Cr impurities are heterogeneous enriched, and the sides of the sponge mass have relatively higher impurity contents. This is caused by contamination from the stainless-steel reaction retort. The inflow process of impurities consists of two steps: the dissolution of impurities in liquid Mg and the formation of alloys with the Ti sponge. The retort material, the temperature of the reaction zone, and the uniformity and thickness of the Ti film are the key factors that directly influence the impurity contents.