Laser-material interaction and its significance in laser directed energy deposition process

Abstract In laser directed energy deposition (L-DED) process, a laser beam passes through the powder cloud generated by a co-axial powder feeding nozzle, where the laser beam is partially absorbed by the powder, while the remaining energy reaches the substrate, creating a molten pool. As the laser moves, the heated powder particles enter this molten pool, melt, and solidify, forming a bead. The bead quality, defined by geometric dimensions like dilution, wetting angle, and bonding, depends on how energy is distributed between the powder and substrate. Therefore, this study examines how carrier gas flow rate, powder mass flow rate, and stand-off distance (SOD) affect powder cloud density and energy apportionment. A novel experimental method using a laser power meter is introduced to measure energy distribution. Increasing carrier gas flow rate from 10 l/min to 40 l/min tightens the powder cloud, reducing energy reaching the substrate and altering bead formation from high dilution to balling. A similar trend is observed with powder mass flow rate (25 g/min to 45 g/min), increasing powder cloud density. The effect of these parameters on powder flow characteristics, like powder footprint diameter and distance from nozzle tip, is analyzed using imaging. SOD variations reveal values lower than the powder focal point lead to high dilution, while higher SOD causes balling. Particle temperatures in-flight, measured via thermal imaging, support these findings. An optimized process window is reported, along with a case study on in-process waviness control based on insights.