ELECTRON BEAM MELTING OF ALLOY 718 - POWDER RECYCLING AND ITS EFFECT ON DEFECT FORMATION
The aim of this study is to investigate the connection between powder recycling, powder chemistry and presence of defects in EBM processed Alloy 718. For this purpose, recycled powder was studied with reference to its virgin counterpart to detect differences in surface morphology, surface chemical composition as well as bulk chemistry as a consequence of powder recycling. The amount of defects and their distribution in samples produced from virgin and recycled powder was studied by means of image analysis and oxygen measurements. Morphological analysis using scanning electron microscopy was performed to understand their origin and formation mechanism.
The results show a significant change in surface characteristics after exposing the powder to the process and the environment in the build chamber. While the virgin powder is covered by a relatively thin and homogeneous oxide layer, the recycled powder has undergone transformation to a heterogeneous oxide layer rich in thermodynamically stable Al-rich oxide particulates. Significant growth of the Al-rich oxide occurs via selective oxidation of Al at the conditions in the build chamber, including both pick-up of oxygen from the process atmosphere and re-distribution of oxygen from less stable oxide products. The increasing amount of oxide is confirmed by an increase in total oxygen level with progressive recycling.
Furthermore, a clear correlation between the powder oxygen level and the amount of oxide inclusions in the EBM fabricated samples was observed. Hot isostatic pressing can be used to reach a near-full densify in samples produced from virgin powder. The samples produced from recycled powder, however, have a higher amount of aluminium-rich oxide inclusions which remain after HIP treatment. A variety of oxide defects was observed, ranging from finely dispersed oxide particulates inside lack of fusion defects to large oxide agglomerates in the bulk metal. Based on their morphology, it is shown that most of them originate from aluminium-rich oxide particulates on the surface of the recycled powder. It is suggested that the quality of EBM processed Alloy 718 is at present dependent on the oxygen level in the powder in general and the surface chemistry of the powder in particular, which needs to be controlled to maintain a low amount of inclusion defects.
electron beam melting
Chalmers, Industri- och materialvetenskap, Material och tillverkning
Effect of Powder Recycling on the Fracture Behavior of Electron Beam Melted Alloy 718
Powder Metallurgy Progress,; Vol. 18(2018)p. 40-48
Artikel i vetenskaplig tidskrift
Gruber, H, Henriksson, M, Hryha, E, Nyborg, L. Effect of Recycling Alloy 718 Powder in Electron Beam Melting
Gruber, H, Luchian, C, Hryha, E, Nyborg, L. Effect of Powder Recycling on Defect Formation in EBM Processed Alloy 718
Bearbetnings-, yt- och fogningsteknik
Metallurgi och metalliska material
Chalmers tekniska högskola
VDL, Chalmers tvärgata 4, 412 58 Göteborg
Opponent: Mattias Thuvander