Influence of non-planar orientations on solidification microstructure during robot-assisted laser-wire directed energy deposition

Abstract

This work investigates the effect of temperature gradient on the solidification morphology during the non-planar laser-wire directed energy deposition (LWDED) process, abbreviated as DED-LB/w according to ISO/ASTM 52900:2021 standard. The LWDED abbreviation is used further in this work. The novelty of this study lies in the independent variation of substrate tilt angle (STA) and wire feed angle (WFA), which presents a comprehensive understanding of non-planar depositions. The temperature distribution and solidification parameters were computed using a customized 3D transient heat transfer model. This numerical model was introduced considering the pulsed laser beam, laser spot shape and size change due to different non-planar orientations. Solidification time, microstructural changes, and heat-affected zone (HAZ) morphology were discussed by correlating the stainless steel 316 L temperature distributions. A numerical and experimental analysis was presented for single-layer deposits. The STA and WFA significantly influence the cooling rates during solidification, affecting the microstructure of the beads. Lower STA (0°-15°) and WFA (10°-20°) result in higher cooling rates. The change in the laser beam spot size affects the solidification rate in the tilt direction due to the lower heating concentration. Smaller WFA (10°-20°) enables the wire to be positioned closer to the molten pool. It results in better energy absorption and efficient melting by increasing temperature. It increased the initial temperature difference and cooling rate. The fraction of equiaxed solidification morphology from the centre to the tilt direction increased with a reduced thermal gradient. The main outcome of this work is the validated solidification map for non-planar LWDED for optimizing deposition strategies in supportless additive manufacturing. The present approach will help suggest the deposition orientations to achieve consistent quality and reliability in deposited parts at non-planar orientations. This work is required to decide deposition strategies for supportless additive manufacturing.