Powder degradation during powder bed fusion processing

Licentiate thesis, 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.