Thermal Modeling of the Laser-Powder Bed Fusion Process

Abstract

Parts fabricated using laser-based additive manufacturing (AM) methods, such as laser-powder bed fusion (L-PBF) receive very high, localized heat fluxes from a laser within a purged, shielding gas (i.e. argon) environment during manufacture. To aid the development/design of AM processes, it is important to have some form of predictive simulation capability in order to reduce trial-and-error experimentation and build iterations. In this work, a continuum-scale modeling approach is generated and employed with three-dimensional finite element analysis (FEA), for simulating the temperature response of parts during L-PBF. Additionally, using computational fluid dynamics (CFD), the local heat transfer between the adjoining shielding gas, laser-induced melt pool and surrounding heat affected zone (HAZ) is estimated. The model has been validated using experimental measurements from the literature and in-house experiments. Validated simulations have been performed for the L-PBF of stainless steel (SS) 17-4 precipitation hardened (PH) and Ti-6Al-4V parts. Results demonstrate that the temperature response of and around the part, for constant laser power and scan speed, is dependent on part volume, substrate size and scan pattern. Since the heat transfer during L-PBF is directly coupled with encumbered part microstructural properties, the presented results demonstrate the importance of process-parameter and geometry/size-coupling. Properties of fabricated parts are not only a function of process parameters, but also of scan pattern, part volume, substrate size and intra-layer time interval. The discretized temperature, temperature time rate of change (i.e. cooling rate) and local temperature gradient have been investigated for various scan strategies and number of lasers, i.e. one, two or four. The number of sub-regional areas of the powder bed dedicated to individual lasers, or ‘islands,’ was varied. The average maximum cooling rate, the average maximum temperature gradient per layer, and the spatial standard deviation, or uniformity, of such metrics are presented and their implications on microstructure characteristics and mechanical traits of Ti-6Al-4V are discussed. Results demonstrate that increasing the number of lasers will reduce production times, local cooling rates and residual stress magnitudes; however, the anisotropy of the residual stress field and microstructure may increase based on the scan strategy employed. In general, scan strategies that employ reduced track lengths oriented parallel to the part’s shortest edge, with islands ‘stacked’ in a unit-row, prove to be most beneficial for L-PBF. The generated model was also used to study the effects of convection on thermal response during fabrication of Ti-6Al-4V. Results indicate that by increasing the speed of argon, effects of convection become more prominent and that the convective heat flux is highest when the laser and gas are moving in the same direction. The direction of the laser scan path relative to the gas flow direction clearly impacts local convection heat transfer. Results suggest such variation can impact the prior β grain size in Ti-6Al-4V material by up to 10%. When the laser and gas are moving in the same direction, there also exists a ‘leading boundary layer’ that can preheat upcoming powder to reduce residual stress. 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. In order to learn how to modify AM designs and processes to ensure lab-scale specimens and final components have similar properties, it is important that process-property relationships be established through thermal simulations. In this study, a unique numerical method for efficiently predicting the thermal history of additively manufactured parts via simulation is presented and validated. The numerical method makes use of an idealized, constant/uniform heat flux which is applied at each new layer and ‘bulk-layers’ which consist of several layers and allow the use of coarser meshes and longer time steps. To demonstrate and test the numerical methods, they are used to simulate the L-PBF of SS 17-4 PH parts with different volumes. Simulations indicate how to modify L-PBF process parameters, specifically time intervals, to better ensure a similar thermal history, temperature, temperature gradient and cooling rate, of different sized/shaped parts. Finally, the effects of thermal response on mechanical properties of fabricated parts were studied. Neutron diffraction was employed to measure internal residual stresses at various locations along SS 17-4 PH specimens additively manufactured via L-PBF. Of these specimens, two were rods (diameter = 8 mm, length = 80 mm) built vertically upward and one a parallelepiped (8 × 80 x 9 mm3) built with its longest edge parallel to ground. One rod and the parallelepiped were left in their as-built condition, while the other rod was heat treated. Data presented provide insight into the microstructural characteristics of typical L-PBF SS 17-4 PH specimens and their dependence on build orientation and post-processing procedures such as heat treatment. Results indicate that residual stress in parts with horizontal orientation are higher compared to parts with vertical orientation.