Powder degradation during powder bed fusion processing: impact of processing conditions and alloy composition
Doktorsavhandling, 2022

In the recent decade, powder bed fusion (PBF) metal additive manufacturing (AM) has attracted huge attention both from the industry and the research community. This effort has helped mature PBF technology as a potential alternative to conventional manufacturing processes as casting, machining, forging, etc. However, there remain various challenges hindering the path of large-scale adoption of these techniques in the manufacturing industry. One such challenge affecting the cost, reproducibility of the products and sustainability of the process, is the reusability of unconsumed powder after each build job. The issue during powder reusability is the likelihood of degraded quality of the reused powder compared to virgin powder either by oxidation during exposure to the atmosphere, or accumulation of process byproducts, referred to as spatters, during processing. The quality degradation of the feedstock powder can lead to an increased number of defects in the produced products and affect the robustness and reproducibility of the PBF process. This thesis is focused on determining the dominant powder degradation mechanisms in powder bed fusion laser-beam (PBF-LB) and powder bed fusion electron-beam (PBF-EB) processes. Here, the approach to investigate the degradation of reused powder is based on the dedicated analysis of changes in powder surface chemistry, analysis of oxygen pick-up, and variation in surface morphology.
During the analysis of the powders during the PBF-LB process, three different alloy systems were studied, namely aluminum alloys (AlSi10Mg), nickel-based superalloys (Alloy 718 and Hastelloy X (HX)), and titanium alloys (TiAl6V4). The assessment of powder degradation was initiated with the investigation of AlSi10Mg powder reused for over 30 months. The analysis showed that the powder degradation is mainly triggered by the accumulation of highly oxidized spatter particles in the powder, characterized by the overall greater oxide layer thickness (~75-125 nm) on the surface of powder. These oxidized spatter particles are contributing towards increasing the oxygen content and number of defects in the as-printed components. Analysis of the surface oxide state of spatter particles, generated during the processing of Alloy 718, HX alloy, and TiAl6V4 revealed that the extent of oxidation of spatters from different alloy systems is dependent on the content of oxidation-sensitive elements e.g., Al. Ti, Cr, etc. The impact of the part design in terms of surface to volume ratio of the part on the spatter generation and accumulation was also shown. Results also show an increasing amount of spatter formation with increasing layer thickness per layer deposited. However, the total amount of spatter generated per build job is lower when a higher layer thickness was applied. The results have shown that by employing appropriate processing gas composition containing He the generation of spatter can be reduced. Furthermore, by reducing residual oxygen content in the build chamber, the extent of spatter oxidation can be reduced. Finally, the effect of powder degradation on the quality of fabricated parts was analyzed where the accumulation and redeposition of spatters on the powder bed resulted in a lack of fusion defects, higher porosity, and a decrease in the strength of fabricated parts.
In the PBF-EB process, powder oxidation and sublimation of volatile elements during the processing of Alloy 718 have been investigated. The results have identified powder oxidation during PBF-EB processing, due to the long-term powder exposure to high temperature, as the dominant powder degradation mechanism. Furthermore, the sublimation of the alloying elements such as Al and Cr in the case of PBF-EB processing of Alloy 718 was detected.

residual oxygen

powder bed fusion – electron beam

powder degradation

oxidation

Alloy 718

AlSi10Mg

powder bed fusion - laser beam

TiAl6V4, Hastelloy X.

sublimation

spatter particles

Keywords: additive manufacturing

Virtual Development Laboratory (VDL Room), Hörsalsvägen 7A, Chalmers University of Technology, Gothenburg, Sweden ---- ZOOM Meeting Password: 4545
Opponent: Herbert Danninger, Professor for Chemical Technology of Inorganic Materials at Technische Universität Wien (TU Vienna), Austria

Författare

Ahmad Raza

Chalmers, Industri- och materialvetenskap, Material och tillverkning

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Additive manufacturing (AM) is bringing a paradigm shift in the fundamentals of the manufacturing industry from subtraction- to addition-oriented manufacturing. AM was first patented by a French scientist Alain Le Mehaute in 1984. After AM’s introduction to the industrial world, it was mainly used for prototype fabrication as it reduced the cost and lead time. However, this success was restricted to small batches and mass production has not yet been realized to its full potential. In the last couple of decades, innovations, both in AM technologies and computation, have made it possible to optimize the products and fabrication time. These advancements make it possible to use AM on larger scales rather than just for rapid prototyping. The major advantages that AM can provide cover: creating products with intricated designs to reduce material consumption, providing on-site repair possibilities to reduce the inventory cost in the industry, and cost reduction of the product by producing and finishing at a single fabrication plant.

Among AM techniques, powder bed fusion (PBF) techniques, including powder bed fusion laser beam (PBF-LB) and powder bed fusion electron beam (PBF-EB), are two rapidly growing additive AM processes. 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, the 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 a deviation of the powder quality from an 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 the quality of powder with reuse.

This thesis focuses on investigating the underlying mechanisms of powder degradation during PBF processing in correlation to alloy composition. Moreover, the effect of part geometry, processing parameters (powder layer thickness), processing gas, and its purity on spatter formation and oxidation has been evaluated. Finally, the effect of powder degradation on the properties of the produced parts is examined. This was done by processing AlSi10Mg, Alloy 718, Hastelloy X, and TiAl6V4 in the PBF-LB and Alloy 718 in the PBF-EB process. The acquired knowledge in this thesis can be used for better optimization of part geometry, process parameters, and processing gas to reduce the rate of powder degradation and increase powder reusability. This will eventually help in increasing the robustness and reusability of the AM process.

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

Drivkrafter

Hållbar utveckling

Innovation och entreprenörskap

Ämneskategorier

Materialteknik

Bearbetnings-, yt- och fogningsteknik

Metallurgi och metalliska material

Fundament

Grundläggande vetenskaper

Infrastruktur

Chalmers materialanalyslaboratorium

Lärande och undervisning

Pedagogiskt arbete

Styrkeområden

Materialvetenskap

ISBN

978-91-7905-748-0

Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 5214

Utgivare

Chalmers

Virtual Development Laboratory (VDL Room), Hörsalsvägen 7A, Chalmers University of Technology, Gothenburg, Sweden ---- ZOOM Meeting Password: 4545

Online

Opponent: Herbert Danninger, Professor for Chemical Technology of Inorganic Materials at Technische Universität Wien (TU Vienna), Austria

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Senast uppdaterat

2023-11-13