Powder degradation during powder bed fusion processing
Licentiatavhandling, 2021

Powder bed fusion (PBF) techniques, including laser-based powder bed fusion (LB-PBF) and electron beam powder bed fusion (EB-PBF), are two rapidly growing additive manufacturing (AM) processes due to their ability to produce complex geometries in near-net shapes. To attain reproducibility and repeatability in PBF processes, a consistent set of powder properties is vital. This is achievable by using virgin powder in every new build cycle. However, considering the amount of unconsumed powder after a build cycle in PBF techniques, reusability of unconsumed powder is imperative to reduce the cost and increase the sustainability of the process. Still, upon reuse, the quality of the processed powder gets degraded by surface oxidation or accumulation of by-products often referred to as spatters. The increase in impurities in the powder feedstock can lead to deviation of the powder quality from the initial state and cause stochastic flaws in the produced components such as inclusions and porosity. Therefore, it is important to study the powder degradation mechanisms and extent of degradation upon processing to track the changes in quality of powder with reuse. This thesis focuses on the analysis of powder degradation mechanisms and their effect on processed components in the case of both, LB-PBF and EB-PBF processes.

In the LB-PBF process, powder degradation for AlSi10Mg and Alloy 718 powders has been investigated. The examined AlSi10Mg powder was used for over 30 months, and the fabricated parts exhibited an increase in porosity and decrease in tensile strength with increased reuse of powder. The analysis of reused powder samples showed that spatter accumulation is a dominant mechanism in powder degradation. Spatters are an inevitable by-product of the process, and the number of generated spatters depends upon the material, process parameters, atmosphere, and geometry of the part. The role of part geometry in spatter generation and powder degradation was further revealed by fabricating specially designed capsules from Alloy 718. Obtained results showed that surface-to-volume ratio and overhang structures tend to increase the number of generated spatters. The analysis of produced Alloy 718 spatters put in evidence the severe surface oxidation with thick Al- and Cr-based oxide patches and particulates formation. By employing an external atmosphere purity system connected to the LB-PBF machine, it was revealed that even if spatters are oxidized particles that can´t be fully avoided, their oxidation can be significantly limited by reducing the oxygen partial pressure in the process chamber. The obtained results showed that spatters generated at <20 ppm residual oxygen content showed only a 30 % increase compared to the spatters generated at 1000 ppm, which showed a 300 % increase in oxygen content. This is a very promising approach to slow down the rate of powder degradation and increase powder reusability for the LB-PBF process.

In the EB-PBF process, the effect of powder bed oxidation and sublimation of volatile elements from Alloy 718 due to the long-term powder exposure to high temperature and high vacuum level on powder degradation was investigated. It was found that in the case of Alloy 718 powder, Cr was dominantly sublimated during the process, which can be detrimental to the superior oxidation and corrosion resistance properties of Alloy 718 components typically preferred for their high-temperature performances. Hence, it is important to monitor Cr and Al content both in powder and built parts while processing of Alloy 718 in the EB-PBF process.

Laser-based powder bed fusion

Sublimation.

Additive manufacturing

AlSi10Mg

Oxidation

Residual oxygen

Electron beam powder bed fusion

Powder degradation

Spatter particles

Alloy 718

Virtual Development Laboratory (VDL-room), Hörsalsvägen 7A, Chalmers University of Technology in Gothenburg, Sweden (Zoom meating ID: 63542102683 Password: 035060)
Opponent: Dr. Dimitris Chasoglou Höganäs AB, Sweden

Författare

Ahmad Raza

Chalmers, Industri- och materialvetenskap, Material och tillverkning

Degradation of AlSi10Mg powder during laser based powder bed fusion processing

Materials and Design,; Vol. 198(2021)

Artikel i vetenskaplig tidskrift

Tobias Fiegl, Martin Franke, Ahmad Raza, Eduard Hryha, and Carolin Körner, "Effect of AlSi10Mg0.4 long-term reused powder in PBFLB/M on the mechanical properties"

Zhuoer Chen, Ahmad Raza, and Eduard Hryha, "Spatter formation during Laser Powder Bed Fusion of Inconel 718 alloy Monitored by Optical Tomography"

Oxygen balance during laser powder bed fusion of Alloy 718

Materials and Design,; Vol. 201(2021)

Artikel i vetenskaplig tidskrift

Ahmad Raza, C. Pauzon, E. Hryha, Andreas Markström, and P. Forêt, "Spatter oxidation during laser powder bed fusion of Alloy 718: dependence on oxygen content in the process atmosphere"

Ahmad Raza, and Eduard Hryha, "Characterization of spatter and sublimation in Alloy 718 during electron beam melting "

Våga printa tunt

VINNOVA (2019-02631), 2019-09-01 -- 2022-08-31.

Drivkrafter

Hållbar utveckling

Ämneskategorier

Bearbetnings-, yt- och fogningsteknik

Kemiska processer

Metallurgi och metalliska material

Fundament

Grundläggande vetenskaper

Infrastruktur

Chalmers materialanalyslaboratorium

Styrkeområden

Materialvetenskap

Utgivare

Chalmers tekniska högskola

Virtual Development Laboratory (VDL-room), Hörsalsvägen 7A, Chalmers University of Technology in Gothenburg, Sweden (Zoom meating ID: 63542102683 Password: 035060)

Online

Opponent: Dr. Dimitris Chasoglou Höganäs AB, Sweden

Mer information

Senast uppdaterat

2021-09-27