Abstract
Defects in parts processed by laser powder bed fusion (LPBF) are often triggered by laser/plasma plume interference and spattering. The implementation of a LPBF process in vacuum has been suggested to possibly reduce these effects. Within this study, the effects of process pressure variations between 1 mbar and atmospheric pressure on the generation of single tracks and on the surrounding layer of loose powder particles were studied for CP titanium grade 2 and the Maraging steel 1.2709. Below 10 mbar no single tracks could be generated and the powder layer adjacent to the track was effectively denuded. It was found that the essential mechanism for incorporating powder into the melt pool begins to work at process pressures above 10 mbar and its effectiveness increases with increasing pressure. The amount of powder incorporated into the melt pool depends on the material and the scanning conditions. With identical scanning conditions, this amount of powder is significantly larger for titanium than for steel. For process pressures above 200 mbar, no significant change in the amount of spattering could be found. In this pressure range improved process stability could be possible due to a reduced laser/plasma interaction and an increased laser penetration depth.
Highlights
Defects in parts processed by laser powder bed fusion (LPBF) are often triggered by laser/plasma plume interference and spattering
There is no study investigating LPBF tracks and the surrounding affected powder layer for process pressures above 20 mbar, Bidare et al [12] already concluded that the pressure regime between 50 mbar and 1 bar could provide an interesting window, but that further investigation is required, as no evaluations of the spattering and LPBF tracks are available for this process pressure range
Between 1 mbar and 10 mbar, the width of the denuded zone decreases with increasing pressure, while the shape of the powder layer surface remains similar: A zone entirely free of loose powder adjacent to the single track accompanied by a pile up
Summary
Laser powder bed fusion (LPBF) is a production method in the field of metal additive manufacturing, which allows to fabricate parts in unique shapes with outstanding mechanical properties [1, 2]. Lowering the process pressure is widely accepted to be beneficial for laser welding [4], a process closely In one of these studies, Matthews et al [8] found for Ti64 that the key mechanism for the consolidation of powder particles to a melt track is governed by the process pressure. There is no study investigating LPBF tracks and the surrounding affected powder layer for process pressures above 20 mbar, Bidare et al [12] already concluded that the pressure regime between 50 mbar and 1 bar could provide an interesting window, but that further investigation is required, as no evaluations of the spattering and LPBF tracks are available for this process pressure range. The findings are discussed in the light of the previously presented literature to assess whether a process with reduced process pressure can provide benefits over a process at atmospheric pressure
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