Chemical species mixing during direct energy deposition of bimetallic systems using titanium and dissimilar refractory metals for repair and biomedical applications

Direct energy deposition (DED) is one of the additive manufacturing technologies where the metal powder is laser-melted by the nozzle and then deposited onto the metal substrate layer-by-layer. DED allows dissimilar metals to be fabricated in order to produce high-performance and intricate parts, and to utilise as a repair method of different metals as well. Refractory metals – high-density, high-melting point materials – are used for repair as a thermal shielding of titanium for high-temperature or biomedical applications. Chemical species mixing of the joint interface between Ti and Zr, Nb, Mo, Hf, Ta and W is induced by transport of species, heat and mass during the DED process. In this work, chemical species mixing, solute profile, and thermal-fluid characteristics are studied using the high-fidelity modelling of coupled powder and thermal-solutal-fluid flow dynamics to elucidate the dissimilar mixture of metals and provide insights into the manufacturability of the refractory metals using DED. It is found that low viscosity metals such as Zr and Nb show good repairability indicated by internal species mixing. Other metals also exhibit enhanced mixing when the temperature is properly set to reduce the viscosity. Therefore, consideration of convection and species mixing in the liquid metal phase is significant. Cooling rate, related to cracking, is also discussed and controlling the local cooling and subsequent solidification behaviour should be properly done by setting proper process conditions. This indicates that the computational tool is capable of predicting causal relationship between materials and processing route for DED applications.