论文和技术资料

Although conventional Al 7xxx alloys are widely used in aerospace and automotive applications, their LPBF-processed counterparts exhibit poor weldability and susceptibility to solidification cracking. These challenges are partly due to the limited understanding of the melt pool processes, including melting, thermocapillary-driven fluid flow (FF), conduction, convection, radiation, elemental vaporization, and solidification. Because LPBF involves a highly localized laser–powder interaction volume, short processing time, and rapid heating–cooling cycles, experimental characterization of melt pool behavior remains difficult. However, melt pool behavior directly influences thermal history, defect formation, solidification conditions, microstructure, and final part quality; therefore, accurate simulation is essential for process-window development and reducing costly trial-and-error experiments. For LPBF-processed Al 7xxx alloys, thermal–fluidic transport effects on temperature distribution and melt pool characteristics have received limited attention. Previous LPBF simulations of Al 7075 have largely focused on thermal behavior, melt pool dimensions, process parameters, or thermal–structural responses without accounting for melt pool convection and Marangoni-flow-driven transport. This work addresses that gap by coupling heat transfer (HT) with thermocapillary-driven FF to investigate the effects of positive and negative thermocapillary gradients on seven LPBF-designed Al 7xxx alloys. Herein, COMSOL Multiphysics® Version 5.6 was used to develop three-dimensional finite element continuum models for single-layer, single-track LPBF using coupled transient HT and laminar flow physics (Figure 1). First, a transient HT model simulated laser–powder–substrate interaction, temperature evolution, melting, solidification, and convective/radiative heat losses using a moving Gaussian volumetric heat source based on the Beer–Lambert law. Second, the HT model was extended to a coupled HT–FF model by solving the energy, continuity, and Navier–Stokes momentum equations. The molten pool was modeled as laminar, Newtonian, and incompressible, with gravity, buoyancy, and Darcy damping representing mushy-zone flow resistance. CALPHAD-computed temperature-dependent thermophysical properties and porosity-dependent powder properties were incorporated. Experimentally characterized and CALPHAD-computed surface tension–temperature data were applied as a Marangoni shear-stress boundary condition at the melt pool free surface, enabling the evaluation of thermocapillary effects on flow direction, velocity, temperature distribution, and melt pool dimensions. Model accuracy was verified sequentially through manual mesh refinement followed by time-step sensitivity studies, using fully coupled nonlinear solver settings to balance convergence, stability, and computational cost. The framework was further verified using published experimental melt pool measurements for LPBF-processed Al 6061 alloy, with reasonable agreement observed. The results showed that temperature profiles, melt pool dimensions, maximum velocities, and FF directions were sensitive to thermocapillary gradient variations (Figures 2–4). Negative Marangoni coefficients produced outward radial flow from the hotter melt pool center toward the cooler edges, whereas positive coefficients induced stronger inward radial flow toward the center. These flow reversals altered heat redistribution and changed the predicted melt pool dimensions. By linking alloy composition with experimentally characterized and CALPHAD-computed surface tension–temperature behavior, the developed framework shows how composition- and process-driven thermocapillary variations control Marangoni-driven melt pool dynamics. This study provides a physics-based foundation for improving alloy design, optimizing LPBF process parameters, reducing experimental trial-and-error, and supporting simulation-based qualification of high-performance metal alloys.

Abbreviations (Kindly note that this is not part of the word count for the abstract) CALPHAD Calculation of Phase Diagrams FF Fluid Flow HT Heat Transfer LPBF Laser Powder Bed Fusion MC Marangoni Coefficient