Effect of Overlap Ratio on Thermal Accumulation Effect in Multi-track Laser Directed Energy Deposition of Martensitic Stainless Steel Coating

This study presents a comprehensive numerical investigation into the laser directed energy deposition (LDED) of homogeneous martensitic stainless steel coatings, with a particular focus on the role of the overlap ratio in governing melt pool dynamics and track morphology. A three-dimensional transient thermal-fluid coupled model was developed using the Fluent platform, incorporating the Volume of Fluid (VOF) method to accurately track the free-surface evolution, while integrally accounting for the coupled mechanisms of heat transfer, phase change, fluid flow, and free-surface deformation. The model was employed to simulate the track geometry, transient temperature fields, and flow field distributions under three representative overlap ratios of 30%, 40%, and 50%. The results demonstrate that the optimal overlap ratio for achieving the best surface planarity of the martensitic stainless steel coating is 40%, which yields the highest flatness coefficient of 0.91, whereas ratios of 30% and 50% result in inferior surface quality characterised by groove formation (flatness 0.87) and localised protrusions, respectively. Increasing the overlap ratio from 30% to 50% markedly intensifies the inter-track thermal accumulation, with the thermal accumulation coefficient rising from 0.43% to 9.5%, accompanied by substantial enlargement of the melt pool dimensions and enhanced recoil pressure at the pool centre. This validated three-dimensional thermal-fluid coupled model reliably captures the intricate interplay between overlap ratio, thermal behaviour, and track morphology, thereby providing a quantitative theoretical basis for optimising LDED process parameters to improve the coating quality and service performance of laser-repaired hydraulic turbine blades.