Transition mechanism of melt depth during Laser Powder Bed Fusion using in-situ X-ray and thermal imaging

<jats:title>Abstract</jats:title> <jats:p>This research clarified the transition mechanism of melt depth in Ti powder bed during Laser Powder Bed Fusion process using <jats:italic>in</jats:italic>-<jats:italic>situ</jats:italic> X-ray and thermal imaging. A fiber laser beam of 150 W was irradiated on a powder bed at a scan speed of 15 mm/s for 3.5 s in a vacuum chamber. The obtained X-ray images showed a keyhole depth <jats:italic>L</jats:italic><jats:sub>d</jats:sub> increased immediately after laser irradiation, gradually decreased, and became constant. It also showed a keyhole width <jats:italic>L</jats:italic><jats:sub>w</jats:sub> increased immediately after laser irradiation and decreased afterward, after that, <jats:italic>L</jats:italic><jats:sub>w</jats:sub> increased again, and became constant. Furthermore, thermal images that measured the temperature on the powder bed showed the high temperature width <jats:italic>L</jats:italic><jats:sub>h</jats:sub> gradually increased and become constant. The model of the driving force which pushed the molten droplet was examined by analyzing the volume and scattering speed of the molten droplet. The model indicated the recoil pressure caused by the vaporization of powder metal was a driving force for the molten droplet scattering. The transition mechanism of keyhole depth was considered as follows. The increase of <jats:italic>L</jats:italic><jats:sub>d</jats:sub> at the beginning is due to the increase of the recoil pressure <jats:italic>P</jats:italic><jats:sub>T</jats:sub>. This is because the decrease of <jats:italic>L</jats:italic><jats:sub>w</jats:sub> and large quantity of vaporization. Next, the decrease of <jats:italic>L</jats:italic><jats:sub>d</jats:sub> is due to the decrease in <jats:italic>P</jats:italic><jats:sub>T</jats:sub>. This is because the increase of <jats:italic>L</jats:italic><jats:sub>w</jats:sub> and decrease of quantity of vaporization. At last, the transition to the constant <jats:italic>L</jats:italic><jats:sub>d</jats:sub> is caused by stabilization of <jats:italic>L</jats:italic><jats:sub>w</jats:sub> and<jats:italic> L</jats:italic><jats:sub>h</jats:sub> followed by stabilization of <jats:italic>P</jats:italic><jats:sub>T</jats:sub>.</jats:p>