Melt pool evolution on inclined NV E690 steel plates during laser direct metal deposition

The melt pool evolution during the laser direct metal deposition (DMD) is intricate. However, most of the research focused on a horizontal substrate, and only a little literature has reported the studies on an inclined substrate with a non-vertically irradiated laser beam. This investigation aims to investigate the melt pool evolution on a substrate with different inclination angles. A paraxial high-speed camera and a coaxial industrial camera were applied to obtain the instantaneous images of the melt pool. Besides, the image processing technique was implemented to extract the coaxial melt pool area. The results showed that several small melt droplets were generated in front of the melt pool on a horizontal substrate; however, a similar phenomenon was not observed on an inclined substrate. Besides, the melt droplets dynamics were driven by the surface tension, resisted by the inertia, and the spreading process was much faster than the solidification process. When the substrate was inclined, the laser energy distribution on the substrate was demonstrated. The "critical angle" was defined as the maximum inclination angle when the single track without any humps. In our experiments, the critical angle existed between 36 degrees and 38 degrees, and a periodic appearance of the humping formation was observed at the inclination angle of 40 degrees. Our obtained results revealed the melt droplets dynamics on a horizontal substrate, the range of the critical angle, the laser energy distribution on an inclined substrate, and the humping formation mechanism.