Powder Reuse in Laser-Based Powder Bed Fusion of Ti6Al4V—Changes in Mechanical Properties during a Powder Top-Up Regime

Abstract

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).

Development and Validation of Empirical Models to Predict Metal Additively Manufactured Part Density and Surface Roughness from Powder Characteristics

“…These changes in the powder material were found to translate into changes in the resulting part properties, such as an increase in the surface roughness and increased porosity. These conclusions agreed with other studies across a range of different metal powders commonly used in the L-PBF process [4][5][6][7][8]. The resulting part quality from the L-PBF process is highly dependent on several factors, including the laser parameters, CAD geometries, and processing conditions, for example [9].…”

“…As a result of the changes in the powder particle size and shape during powder The resulting part quality from the L-PBF process is highly dependent on several factors, including the laser parameters, CAD geometries, and processing conditions, for example [9]. It is well understood that the characteristics of the raw input material, in this case a metal powder, affect the resulting qualities of the parts produced [1,3], research in previous years has reported the interdependent nature of the powder properties and resulting output parts manufactured [6,7]. Figure 2 The resulting part quality from the L-PBF process is highly dependent on several factors, including the laser parameters, CAD geometries, and processing conditions, for example [9].…”

“…The combination of powder particle size and shape [17] influences the flowability of the powder material, which is an important characteristic in L-PBF due to the requirement for the powder to flow freely for the deposition of powder layers. As a result of the changes in the powder particle size and shape during powder Previous work by the authors [3] and other researchers [6][7][8][9][10] have provided a greater knowledge of the effects of powder recycling and rejuvenation processes, allowing L-PBF operators to improve utilisation of the powder material. Researchers have found that the overall powder recycling process leads to a degradation in the powder characteristics such as an increase in the particle size [10][11][12][13][14][15], a decrease in the circularity of the particles [9][10][11][12]15] as well as an increase in the oxygen concentration on the surface of the powder particles [9,14,16].…”

“…The combination of powder particle size and shape [17] influences the flowability of the powder material, which is an important characteristic in L-PBF due to the requirement for the powder to flow freely for the deposition of powder layers. As a result of the changes in the powder particle size and shape during powder recycling, the powder flowability improves [6,18,19]. The resulting manufactured part properties from the L-PBF process are dependent on the powder characteristics [3].…”

“…The resulting manufactured part properties from the L-PBF process are dependent on the powder characteristics [3]. These changes in the powder characteristics have an effect on the resulting manufactured part properties such as increased surface roughness [3,6], increased porosity [17][18][19][20] and changes in the mechanical properties [6,17,18]. With this increased understanding of the effect of the powder recycling process, greater utilisation of the powder material used can be achieved.…”