Oxygen balance during laser powder bed fusion of Alloy 718

Camille Pauzon,Ahmad Raza,Eduard Hryha,Pierre Forêt

doi:10.1016/j.matdes.2021.109511

Camille Pauzon, Ahmad Raza + Show 2 more

Open Access

https://doi.org/10.1016/j.matdes.2021.109511

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Abstract

The generation of spatters during laser powder bed fusion (L-PBF) of Alloy 718 is known to be detrimental for the re-use of the feedstock powder and the quality of the final product. In this study, dedicated powder sampling from different positions within the build chamber and powder bed was performed. The results clearly indicate the importance of the surface-to-volume ratio of the built components on powder degradation, as demonstrated by the analysis of the powder using capsules filled with dense lattice structures. Extensive formation of Al- and Cr-rich oxides on the entrained powder deposited on the gas inlet and outlet was detected. Significant oxygen pick-up by spatter particles compared to the virgin powder was measured (>300 ppm O2); while the as-built material experienced a slight loss (~30 ppm O2). The change in the microstructure of spatter particles in comparison to the virgin powder, namely the primary dendrite arm spacing, indicates significantly higher cooling rate during spatter solidification, estimated to be of about 108 K/s, compared to around 106 K/s for the virgin powder and 107 K/s for the L-PBF component. These findings allows to evaluate the extent of powder degradation during L-PBF and establish the oxygen balance of the process.

Highlights

Spatters may arise from two distinct origins
Since the build job was designed to be symmetrical, and the laser scanning is rotated between each layers, it may be connected to local variations in the gas flow over the baseplate, indicating needs for further studies of the gas flow properties on the spatter formation and removal during laser powder bed fusion (L-PBF) processing
Powder landing on the gas inlet consists mostly of large particles belonging to the high end of the virgin powder particle size distribution (PSD), entrained during the process and travelling against the gas flow

Summary

Introduction

Spatters may arise from two distinct origins. They can be produced as direct ejections from the unstable melt pool. Spatters are likely to be rich in oxygen and oxides [10,11], and be precursors for structural defects in the produced part as they re-deposit on the powder bed They are undesirable for part's quality, powder recyclability and the lifetime of the recirculation filter system of the L-PBF machine. The current work is connecting these aspects by establishing an impurity balance (oxygen and nitrogen) during the L-PBF processing of Alloy 718 which is sensitive to both, oxygen and nitrogen This is the reason why Alloy 718 is usually cast under vacuum [18]. The detailed analysis of the morphology and microstructure of spatter particles explained their origin and criticality for defect formation in L-PBF and powder re-use

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Results

Conclusion