Sustainable Manufacturing of Clean Ti-Ni Alloy by Remelting Titanium Scrap with Calcium and Halide Fluxes

Abstract

The possibility of the direct removal of oxygen from the liquid Ti-30%Ni alloy through the thermodynamic equilibrium reactions at 1623-1723 K was investigated. The oxygen removal efficiency of Ti-30%Ni alloy by adding different content of metallic calcium (Ca) was evaluated in conjunction with the binary halide fluxes of CaCl2-LiF and CaF2-MgCl2 systems, respectively. For the CaCl2-LiF flux, the oxygen content in the alloy decreased from about 530 to 400 ppm when 0.9% Ca was added to the Ti-30%Ni alloy. Alternatively, the effect of flux compositions, i.e., CaCl2/LiF ratio in the CaCl2-LiF flux and CaF2/MgCl2 ratio in the CaF2-MgCl2 flux on the oxygen content was also investigated. The flux system with 75%CaCl2- 25%LiF system exhibited a good deoxidation capability, i.e., the total oxygen content in the alloy was reduced from 900 to 670 ppm (oxygen removal efficiency 25.6%). The 80%CaF2-20%MgCl2 flux exhibited the best deoxidation capability, i.e., total oxygen content in the alloy decreased from 1450 to 550 ppm (oxygen removal efficiency 62.1%) due to the lower activity of CaF2 indicating that it can promote the absorption capability of the deoxidation product, i.e., CaO. The microstructure of the alloys manufactured by the present method revealed a coarse alpha-Ti phase intermixed with Ti2Ni matrix phase, and the TiC phase also distributed non-uniformly within matrix, which was confirmed by EBSD and SEM-EDS as well as nanoindentation mechanical testing.