Effect of Powder Recycling in Electron Beam Melting on the Surface Chemistry of Alloy 718 Powder

Abstract

Process-induced degradation of the powder feedstock in additive manufacturing may have a negative influence on the final properties of built components. Consequently, it may lower the cost-effectiveness of powder bed additive manufacturing, which relies on recycling of the nonconsumed powder. This is especially the case for production of high-performance aero engine components where high material and process reliability is required. This study comprises a detailed investigation on the degradation of Alloy 718 powder during multicycle electron beam melting (EBM). The surface-sensitive analysis methods, X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES), were combined with scanning electron microscopy (SEM) to depict the differences in surface morphology, and surface composition of powder samples exposed to varying numbers of re-use cycles. The results show a significant change in surface characteristics after exposing the powder to the process and the environment in the build chamber. The virgin powder is covered mainly by a relatively thin and homogeneous oxide layer. The re-used powder, however, has undergone transformation to a heterogeneous oxide layer, rich in thermodynamically stable Al-rich oxide particulates, which started already during the first build cycle. Significant growth of the Al-rich oxide occurs via selective oxidation of Al under the conditions in the build chamber, including both pickup of oxygen from the process atmosphere and redistribution of initial surface-bound oxygen from less-stable products like Ni-oxide and/or hydroxide.

Effect of Powder Recycling on Defect Formation in Electron Beam Melted Alloy 718

“…Figure 3 shows the surface of a powder sample collected before build cycle number 30. As was already described in previous research, [23,24] a significant part of the re-used Alloy 718 powder surface is covered by Al-rich oxide particulate features that have formed during exposure of the powder to the process and the environment in the EBM build chamber. In Reference 24, it was also shown that the oxide particulates are confined to the powder surface.…”

Oxygen Balance During Laser Powder Bed Fusion of Alloy 718

“…From EDS analysis, the oxidation features were identified as rich in Al and Cr. The formation of Al-based oxides on Alloy 718 powders has already been reported after powder recycling in electron beampowder bed fusion (EB-PBF) [2].…”

“…Among the materials processable by L-PBF today, many are sensitive to these high temperatures that promote diffusiondriven processes such as oxidation. Rich in Ti, Al, Cr and other elements of high affinity for oxygen [2,3], these materials are likely to form stable oxide species, which are then transferred to the produced parts whose mechanical properties can be compromised. To limit these harmful consequences, an inert gas flow (Ar) covers the build area to remove impurities close to the melt pool and heated material.…”

Effect of atomization on surface oxide composition in 316L stainless steel powders for additive manufacturing

“…If not reduced, these more stable oxsides remain within the consolidated sample 4–6 . Similar correlations have been drawn within the powder bed fusion AM processes, but these have mostly concerned powders that have been recycled 7–9 . There is hence a lack of published papers focusing on the surface oxide of original powder and the effect of atomizing media on surface chemical composition of the powder in the field of additive manufacturing.…”