On the role of pre- and post-contour scanning in laser powder bed fusion: Thermal-fluid dynamics and laser reflections

Despite of the promising advantages of laser powder bed fusion (LPBF), the surface quality control of the LPBFproduced parts has been a critical challenge for this technique to have a successful application in the industry. Different from the majority of literatures that pay attention to the effects of process parameters on the surface quality, this work has uniquely explored the influence of the contour scan sequence on the surface roughness magnitude in the LPBF of copper alloy. The experimental results present that the pre-contour scan strategy gives rise to lower roughness magnitude than the common post-contour scan strategy under the majority of contour process parameters. To investigate the underlying mechanisms of this phenomenon, a high-fidelity CFD model with a ray-tracing method is utilized to model the laser-matter interactions and melt pool dynamics in the contour scan. Given this, the multiple laser reflection behaviors are well visualized and discussed and the different laser reflection mechanisms in the different contour scan strategies are correlated with the melt flow behaviors. It is suggested that the presence of powders deposited on the infill region helps enhance the laser absorption during the pre-contour scan, and by extension, improve the melting behavior. On the contrary, the flat surface of the infill region that has solidified prior to the contour scan weakens the absorption of laser energy by metal materials to some extent, which is responsible for the poor wettability and defect formation of the contour track.