A numerical and experimental investigation of convective heat transfer during laser-powder bed fusion

Parts fabricated using additive manufacturing (AM) methods, such as laser-powder bed fusion (L-PBF), receive highly localized heat fluxes from a laser within a purged, inert environment during manufacture. These heat fluxes are used for melting metal powder feedstock, while remaining energy is transferred to the solidified part and adjoining gas environment. Using computational fluid dynamics (CFD), the local heat transfer between the adjoining shielding gas, laser-induced melt pool and surrounding heat affected zone is estimated. Simulations are performed for the L-PBF of a single layer of Ti-6Al-4 V. Local temperature, temperature gradients, temperature time-rates-of-change (including cooling rates), as well as dimensionless numbers descriptive of important thermophysics, are provided in order to quantify local convective heat transfer for various laser/gas motion directions. Results demonstrate that L-PBF track heat transfer is highly dependent on relative gas/laser direction which can impact the prior β grain sizes in Ti-6Al-4 V material by up to 10%. It is found that when the laser and gas are moving in the same direction, convection heat transfer is the highest and a 'leading thermal boundary layer' exists in front of the laser which is capable of preheating downstream powder for a possible reduction in residual stress formation along the track. Presented results can aid ongoing L-PBF modeling efforts and assist manufacturing design decisions (e.g. scan strategy, laser power, scanning speed, etc.) - especially for cases where homogeneous or controlled material traits are desired.