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.
In the metal additive manufacturing (AM) process of laser powder bed fusion (LPBF), there are a limited number of materials suitable for producing parts with high density and desired mechanical properties. To establish novel materials, it is essential to determine optimized process parameters in order to overcome process-related challenges and mitigate defects such as lack of fusion, keyholing, and balling. Scaling laws based on thermophysical properties and process parameters can be used to transfer knowledge from other materials or LPBF systems. In this work, a scaling law is used to adjust process parameters for single-track experiments over a wide range, which are laser power PL (100–1000 W), scan speed vs (300–2500 mm/s), and laser spot size ds (0.08–0.25 mm). Compared to existing studies, the parameter range is thus extended towards large laser spot sizes and high laser powers. The scaling law used is based on the calculation of the normalized enthalpy $$\frac{\Delta H}{{h}_{s}}$$
Δ
H
h
s
. The ratio of the deposited energy density $$\Delta$$
Δ
H and the melting enthalpy hs correlates with the dimensions of the melt pool. According to the aspect ratio $${\delta }_{\mathrm{c}}$$
δ
c
of the melt pool of each single track, the respective melting mode—conduction, transition, and keyhole mode—was identified. The process parameters of the single tracks in transition mode were used to optimize the density of the LPBF specimens with varying hatch distance hd (0.06–0.12 mm), resulting in specimens with a relative density of > 99.8%. The proposed methodology can accelerate the process parameter finding for new alloys and avoid process-related defects.
Graphical Abstract
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