How the underwater environment affects the melt pool solidification during underwater laser direct metal deposition of HNS steel?

Abstract

Underwater laser direct metal deposition (UDMD) can be employed to repair and maintain offshore engineering structures. The external factors inherent in the UDMD process could have a significant effect on melt pool solidification, but the corresponding mechanism is poorly understood. In the current research, a series of mathematical models based on the finite element method, lattice Boltzmann method, and cellular automaton were proposed to fill the relative knowledge gap. Results indicated that the enhanced powder carrier gas and elevated ambient pressure in UDMD process can flatten the deposition track, and its aspect ratio increased from the original 3.21 to 5.10. Furthermore, the driving effect of the Marangoni convection on the solute redistribution was calculated, and the nitrogen concentration in the downstream region of the DMD melt pool increased by 16.7 % compared to that in the upstream area. Meanwhile, the dendrite morphology between the upstream and downstream was different. In the UDMD melt pool, the changed melt pool thermodynamics (e.g., reduced peak temperature and elevated thermal gradient) and the external factors reinforced the Marangoni convection. In this case, samples fabricated by UDMD exhibited relatively uniform solute distribution and dendrite morphology, which are expected to possess better comprehensive mechanical properties and corrosion resistance.