Laser Powder Bed Fusion of Ti-6Al-4V: A Round Robin Analysis of Powder Reuse and Process By-Products

Abstract

Metal Additive Manufacturing (AM) provides immense value as a manufacturing method to produce high complexity parts for stress-critical applications. Metal AM though Powder Bed Fusion (PBF) technologies can produce fully dense metal parts with mechanical properties on par with wrought counterparts, and with geometries unachievable with conventional manufacturing methods. Widespread adoption of this technology requires thorough understanding of the contributors to part variability, and robust machine and part qualification.This dissertation evaluates the effects of powder reuse in L-PBF over the course of ten builds, split into two phases. The study presents a novel approach to powder reuse through a “round robin” style study involving six independent participants following the same operational procedure and, importantly, starting with the same lot of plasma-atomized grade 5 Ti-6Al-4V metal powder. Important metrics of powder quality are monitored throughout the study and compared to mechanical properties of the metal produced over time and between participants. Several categories of variability are considered, including intra-build (referring to variability that arises within individual builds), inter-build (variability that arises from one build to another on the same machine), and inter-machine (variability that arises between different machines run under the same nominal conditions). Byproducts of the L-PBF process, including spatter and metal vapor condensate, are investigated and characterized as important components of variability in powder quality. Phase I of the study involved six builds performed by six participants. Samples of powder were taken prior to each build and analyzed to characterize the particle size distribution, morphology, bulk chemistry, and flowability. The mean particle size of the powder as well as its flowability increased slightly with reuse at all sites, whereas the powder chemistry did not change appreciably over the six builds conducted. Quasi-static mechanical properties of the metal produced from the reused powder were found to correlate with carbon and aluminum content, but not with reuse number. In Phase II of the study, three of the original six participants continued with an additional four builds. Powder was again collected, analyzed, and compared between partners and against Phase I. Extended reuse was found to not intensify the trends observed in Phase I, and instead many of the measured properties stayed at, or returned to, nominal values. Powder chemistry did not change over Phase I or Phase II, although variability was observed between partners. The mean particle size of the powder samples, which had increased in Phase I, decreased back to nominal values.