A Holistic Approach to Understand Laser Additive Manufacturing from Melt Pool to Microstructure

The melt pool is the core of Additive Manufacturing (AM) technologies. While the size and its lateral velocity implicitly influence the productivity of the process, the resulting microstruc-ture depends on the local solidification conditions. The scope of this work is to present melt pool models for the Metal Laser Direct Energy Deposition (L-DED) and Laser Powder Bed Fusion (L-PBF) process, with which the energy absorption and energy-matter interactions can be modelled representatively. For L-DED the focus lies in the particle-laser radiation interaction during the particle's time of flight from the nozzle towards the melt pool. The free surface of the melt pool is calculated to assess the resulting track geometries. For L-PBF the keyhole formation, by the recoil pressure of evaporating material, due to high laser radiation intensities, a gas capillary is formed. In this capillary, multiple reflection of the incident laser radiation lead to an increased energy absorption during the process. In-situ measurements are performed to quantify the absorption change in a keyhole-transition process mode. Due to the increased absorption the depth of the keyhole increases with on-setting of the keyhole mode. The developed model can predict melt pool sizes (especially the depth) over a large range of process parameters (from heat conductive- to keyhole-dominated processes). The melt pool size (depth, width and height) in dependence of the main process parameters (scanning velocity and laser power), for both AM processes, is investigated. The results are compared to experimental data, the relative error for the melt pool size is found to be roughly 10 %, thus the description is assumed to be accurate enough. The relative error of the so-lidification conditions is assumed to be in the same order of magnitude.he hard to measure solidification conditions are extracted from the simulations and compared for both processes. As expected, (due to larger melt pool sizes and smaller scanning velocity), the cooling rates, thermal gradients, and solidification velocity are smaller for the L-DED than the L-PBF process. The solidification conditions vary strongly along the solidification front and with the used process parameter, those influences are quantified and a representative volume for the microstructure simulations is picked carefully. The phase field simulation of the microstructure yields similar results (with respect to the primary and secondary dendrite arm spacing) as the performed microstructure analysis. The material investigated is Inconel 718 a γ’-precipitation strengthened nickel-base alloy often used in turbo machinery and AM.