On the chemical composition, microstructure and mechanical properties of a Nitrogen-contaminated Ti-6Al-4V component built by Wire-Arc Additive Manufacturing
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Additive manufacturing (AM) using recycled Ti-6Al-4V (Ti64) feedstock material from wrought waste streams is a novel process that can reduce the overall energy cost and carbon (CO2) footprint when compared to primary-production routes. The potential contamination of recycled feedstock material (e.g. C, O, N and Fe) can affect the microstructure and mechanical properties of the component. In this work, a Ti64 test wall built using wire arc AM (WAAM) was studied, where the top half only was contaminated by N through the shielding gas during deposition. This allowed a direct comparison of Ti64 WAAM material with high and low N content, deposited under otherwise identical conditions, to replicate the worst-case scenario of N contamination from using recycled swarf. The hardness of the N-contaminated section was found to be 25% higher than the uncontaminated section of the wall, demonstrating the N solid solution strengthening in Ti64. The room temperature transformed microstructure was found to have a 25% coarser α-lath thickness, which was proposed to be an effect of the AM cyclical heating and increasing of the β-transus temperature due to a higher level of N. Additionally, the outer layer of the N-contaminated sample section was found to have a refined parent β grain structure.
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The authors are appreciative of the EPSRC program grant Sustainable Additive Manufacturing (EP/W01906X/1) for supporting this research. The authors acknowledge the use of equipment within the Advanced Metals Processing and Characterisation themes of the Henry Royce Institute for Advanced Materials, funded through EPSRC grants EP/R00661X/1, EP/S019367/1, EP/P025021/1 and EP/P025498/1.