Thermal behaviour and fluid dynamics during pulsed-wave laser powder bed fusion of 18Ni-300 maraging steel

Abstract

Laser powder bed fusion (LPBF) enables the fabrication of metallic components with complex geometries directly from raw powders. The process typically employs continuous- or pulsed-wave lasers, which significantly impact the thermal-fluid dynamics and subsequently affect the microstructure. However, the behaviour during pulsed-wave LPBF remains inadequately understood. This study developed a high-fidelity multi-physics modelling framework to simulate the evolution of point-by-point laser exposure during pulsed-wave LPBF. The effects of laser power and exposure time on thermal-fluid behaviour in single-/multi-track and multi-layer pulsed-wave LPBF were investigated and validated against experiments. The results reveal that variations in either laser power or exposure time can result in similar molten pool morphology during a single exposure, though their dynamic behaviours exhibited marked differences. Increased laser power augmented the drilling rate of the molten pool, while exposure time exhibited a minimal effect on the depth growth rate, thereby enhancing the predictability of its behaviour. Additionally, the critical molten pool depth at which the drilling rate changes remained nearly constant, irrespective of laser power or exposure time. During point-by-point scanning of a single melt track, gaps formed between exposures due to mismatches in laser power, exposure time and point distance, resulting in track discontinuities. In subsequent scanning, deep gaps arose from poor bonding within intra-tracks and insufficient melting between inter-tracks and inter-layers. Keyhole pores primarily formed during the laser-off period of the pulse cycle at high laser powers or exposure times, as surface tension and gravity drove molten material forward, but solidification pinned the keyhole tip, leading to defects. These findings significantly advance the understanding of melt pool dynamics and defect formation in pulsed-wave LPBF.