Process Microstructure Coupling in Reduced Gravity Laser Welding via Open-Source Multiphysics Simulation Framework

TL;DR

This work addresses the feasibility of in-space laser welding by introducing an open-source thermo-fluid-microstructure framework that couples CFD and cellular automata to capture how gravity and vacuum conditions affect melt pool dynamics and solidification. The authors implement a high-fidelity workflow using LaserbeamFoam for transient melt pool and keyhole physics and ExaCA for grain growth, validating against terrestrial experiments and IN625 benchmarks. A twelve-case parametric study across conduction, transition, and keyhole modes in Earth, Mars, Moon, and ISS environments reveals that micro-scale melt pools show limited sensitivity to gravity, with capillary and velocity contributions governing keyhole stability and microstructure more than hydrostatic effects. The framework provides a reproducible platform for predicting process outcomes in in-space manufacturing and supports future exploration of alloys, process windows, and environment-specific weld guidelines, enabling power-efficient, robust space fabrication strategies.

Abstract

Supplying spare parts from Earth for in space repair is economically prohibitive and logistically slow, posing a major barrier to sustainable space operations. As lunar and Martian missions accelerate in the coming decades, the feasibility of in-situ repair methods, particularly laser based welding, must be rigorously evaluated. The micro scale physics governing weld quality are fundamentally altered by variations in gravity and ambient pressure, yet their coupled influence across different welding regimes remains poorly understood. This work introduces a fully open-source thermo-fluid-microstructure modeling framework with computational fluid dynamics (CFD) and cellular automata (CA) to quantify how gravitational conditions reshape weld pool behavior across multiple welding regimes and spatial scales. The framework further enables prediction of the resulting microstructure from imposed process parameters, providing an accessible pathway for future research in in space manufacturing (ISM). As a demonstration, the article analyzes laser welding of Al6061 across three process regimes (conduction, transition, and keyhole) and four gravitational environments (Earth, Moon, Mars, and International Space Station (ISS)). Analysis reveals that reduced gravity suppresses buoyancy-driven convection, thereby altering melt pool geometries. Vacuum conditions increase laser energy deposition, while microgravity promotes equiaxed grain formation via reduced melt convection. The framework captures keyhole dynamics, thermal histories, and grain morphologies with high fidelity, validated against experimental data. These findings establish process microstructure relationships critical for reliable metallic fabrication in space and provide a reproducible platform for future in-space manufacturing research.

Figures (12)

• Figure 1: Schematic overview of the open-source one-way coupled CFD-CA framework for simulating laser welding processes in terrestrial and extraterrestrial environments. The computational framework integrates fluid dynamics and solidification modeling to predict meltpool behavior and microstructure evolution under varying gravitational and welding conditions.
• Figure 2: The workflow that connects different modules of the open source software implementation.
• Figure 3: Validation of thermo-fluid model (a--d) and microstructure model (e--g) against experimental results.a, Comparison of predicted keyhole depth during laser welding of $AL6061$ with experimental data reported by Zhang et al. zhangAccurateEfficientPredictions2025; process parameters: laser power $P$ = 500 W, scanning velocity $v$ = 0.7 m/s, beam radius $r_0$ = 50 $\mu$m; results shown at $t$ = 1 ms after establishment of quasi-steady keyhole morphology. b, Qualitative comparison of keyhole morphology with experimental X-ray image under identical process parameters. c, Quantitative comparison of melt pool depth and width under atmospheric and vacuum (1 kPa) conditions. d, Melt pool cross-sections comparing the melt pool depth and width with experimental measurements from Lee et al. leeVacuumLaserBeam2021 for laser welding of $AL6061$; dashed lines indicate experimental melt pool boundaries; process parameters: laser power $P$ = 2100 W, scanning velocity $v$ = 2 m/min, spot diameter $d_0$ = 356 $\mu$m. e, Comparison of simulated grain structure for laser-welded IN625 with experimental EBSD data from NIST AM Bench 2018 Challenge 02; process parameters: laser power $P$ = 137.9 W, scanning velocity $v$ = 0.4 m/s, beam radius $r_0$ = 50 $\mu$m. f, Dendrite tip velocity as a function of undercooling temperature for IN718 predicted using the LGK model, compared with results provided by Lian et al. lianCellularAutomatonFinite2019. g, Misorientation angle distribution along the build direction for the NIST AM Bench 2018 case compared with experimental measurements from Yuan et al. yuanUncoveringGrainSubgrain2024.
• Figure 4: Meltpool characteristics as a function of volumetric energy density ($E_v$ = 57.7, 106.8, and 166.8 J/mm$^3$) across four gravitational environments. Bars represent meltpool width ($\mu$m, left axis), circles indicate depth ($\mu$m, left axis), and squares show the depth-to-width ratio (right axis) for Earth, Mars, ISS, and Moon conditions. Meltpool morphology for each case is shown above the corresponding bar group, where the red region depicts the meltpool shape under Earth conditions and the white dashed line indicates the meltpool shape under lunar conditions.
• Figure 5: Comparison of keyhole depth between Earth atmospheric conditions and vacuum conditions at reduced scanning speed ($P = 250~\mathrm{W}$, $V = 0.15~\mathrm{m\,s^{-1}}$, $r = 50~\mu\mathrm{m}$), demonstrating the emergence of pressure-driven penetration enhancement when velocity contributions are minimized.