Additive manufacturing of Ti64 alloy in-situ modified with additions of boron or oxygen

Abstract

Selective laser melting (SLM) is a metal additive manufacturing process that enables the production of complex, near net shape, parts with fully functional mechanical properties. The use of titanium alloys in SLM is a good fit due to a high material utilisation and net shape production with this expensive material. While titanium alloys have attractive properties of high specific strength and excellent corrosion resistance, these and other attributes may be modified through various methods for further improvements of the material. Strengthening mechanisms of interstitial hardening and particulate reinforcement are viable approaches for enhancing titanium alloys, however these often increase processing difficulty. Near net shape processes, like SLM, have the potential to produce functional parts with these modified properties. This study explored the use of SLM for the fabrication of titanium parts, modified in-situ during processing with small amounts of boron or oxygen to develop enhanced properties. The in-situ incorporation of these different elements was performed during SLM using blended powders. Simple mixing of feedstock Ti64 alloy powder with small amounts of either amorphous boron or TiO₂, and subsequent SLM consolidation, has the potential to develop interstitial (oxygen) and particulate (boron) strengthened materials. This work analytically compares the properties of samples created using different process parameters, related through energy density. By examining variation in porosity and hardness with respect to these process parameters, it was possible to establish optimum processing conditions for manufacturing the in-situ modified material. Novel samples were developed for SLM that simplify the multi-level, multi-factor investigation of these process parameters into a single specimen for easy preparation and testing. Further investigation of how processing may influence the distribution and homogeneity of the modified material was studied through the influence of stress relief and sub-beta transus annealing heat treatments. Characterisation of these materials and treatment conditions were performed to determine the effect the additives have on the mechanical and wear properties. Most significantly this work demonstrated that small amounts (approx. 0.2 wt%) of boron or oxygen could be effectively combined with Ti64 via SLM processing. Optimised parameters (resulting in energy density of 54.4 J/mm³) were able to produce high density parts with uniform incorporation. Typical microstructures were developed for SLM produced Ti64, consisting of columnar colonies of fine lamellar martensite that extend in the build direction. Additional oxygen changed this very little, however boron had a significant effect, refining the grain structure and preventing the formation of columnar colonies. Enhancements in compressive strength and hardness resulted from boron or oxygen addition with compressive yield strength increased by up to 10% and 16% respectively. Improvements in these properties were retained in the different heat treatment conditions when compared to unmodified Ti64. While strengthening of Ti64 was achieved by inclusion of the additives, little improvement in wear performance was found. This work identifies a viable method of modifying properties of titanium through in-situ processes during SLM. The findings of this study will have implications for the production of Ti64 alloys during the SLM processes and may guide future work on developing materials specific for additive manufacturing.