Titanium alloy powder used for laser-based powder bed fusion (L-PBF) process is costly. One of the solutions is the inclusion of a powder recycling strategy, allowing unused or exposed powder particles to be recuperated post manufacture, replenished and used for future builds. However, during a L-PBF process, powder particles are exposed to high levels of concentrated energy from the laser. Particularly those in close proximity to the melt pool, leading to the formation of spatter and agglomerated particles. These particles can settle onto the powder bed, which can then influence the particle size distribution and layer uniformity. This study analysed extra-low interstitial (ELI) Ti6Al4V (Grade 23) powder when subjected to nine recycle iterations, tracking powder property variation across the successive recycling stages. Characterisation included chemical composition focusing upon O, N, and H content, particle size distribution, morphology and tapped and bulk densities. On review of the compositional analysis, the oxygen content exceeded the 0.13% limit for the ELI grade after 8 recycles, resulting in the degradation from Grade 23 level.

Processes that use moving point heat sources to temporarily create localized melt pools (metal additive manufacture and fusion welding) have a flow phenomenon due to the surface tension gradient. Surface tension of the liquid metal reduces with temperature and this, coupled with the high temperature gradients associated with point heat sources, creates Marangoni convection in the melt. The Marangoni convection tends to reduce the temperature and change the melt pool geometry (increases width but reduces depth). Computational Fluid Dynamics (CFD) models can simulate the phenomenon of Marangoni convection but are computationally intensive. A simpler thermal model involving heat conduction and latent heat, but with the liquid’s thermal conductivity artificially increased by a constant factor, exhibits similar thermal effects to the Marangoni convection. The heat conduction models are computationally less intensive than CFD, but the trial-and-error exercise needed to obtain an appropriate multiplying factor is time consuming. With an aim to improve the process of factor selection, the present study investigates the correlation between the surface tension gradient and correction factors. For a Ti-6Al-4V under typical additive manufacturing parameters, the corresponding correction factor to be applied to liquid thermal conductivity was 1.76.

The properties of Extra Low Interstitials (ELI) Ti6Al4V components fabricated via the laser-based powder bed fusion (L-PBF) process are prone to variation, particularly throughout a powder reuse regime. Interstitial pick-up of interstitial elements within the build chamber during processing can occur, most notably, oxygen, nitrogen, and hydrogen, which can impair the mechanical properties of the built component. This study analyses ELI Ti6Al4V components manufactured by the L-PBF process when subjected to a nine-stage powder reuse sequence. Mechanical properties are reported via hardness measurement and tensile testing. Results showed that from 0.099 wt.% to 0.126 wt.% oxygen content, the mean hardness and tensile strength increased from 367.8 HV to 381.9 HV and from 947.6 Mpa to 1030.7 Mpa, respectively, whereas the ductility (area reduction) reduced from around 10% to 3%. Statistical analysis based on the empirical model from Tabor was performed to determine the strength–hardness relationship. Results revealed a significant direct relationship between tensile strength and Vickers hardness with a proportionality constant of 2.61 (R-square of 0.996 and p-value of 6.57 × 10⁻⁶).