Multi-physics multi-scale simulation of microstructure evolution in entire welds of 2219-T6 aluminum alloy: An MPF-LBM study

Abstract

The microstructure of fusion welds significantly affects its mechanical properties. Existing studies on the dynamic evolution of welding molten pools have exposed great limitations in considering the flow field and the solid motion of free dendrites. In this paper, a multi-physics multi-scale coupling simulation of thermal-mass-flow was conducted to study the solidification behavior during the welding of 2219-T6 aluminum alloy. The developed multi-phase field lattice Boltzmann model realized the direct full coupling of phase, temperature, solute, solid motion, and flow fields. It is also combined with a continuous nucleation model to simulate heterogeneous nucleation and columnar-to-equiaxed transition (CET), and a transient growth-condition model to simulate the movement and solidification of the entire molten pool. The model and simulation results were verified by welding experiments, electron back-scattered diffraction, and scanning electron microscopy tests. Good agreements are achieved in terms of grain structure, dendrite morphology, and grain size. The undercooling field, fluid flow, solid motion, dendrite morphology, solute distribution, and quantitative parameters during the whole solidification process were analyzed in detail to reveal the true evolution of thousands of dendrites in fusion welds. The results show that solute convection caused by fluid flow significantly affects the dendrite morphology. The solid motion of equiaxed dendrites squeezes the solute enrichment layer and forms a solute hysteresis transport effect. A band of equiaxed dendrites gathers in front of the columnar dendrites and blocks their growth, forming a mechanical blockade mechanism of the CET process. The equiaxed dendrites dominated the subsequent solidification after CET is complete, and the final microstructure consists of brief-growth columnar dendrites and dense equiaxed dendrites. The top tangential flow causes a band of equiaxed dendrites with large gaps in the upper weld, which was presumed to be the source of surface cracks in welds with high residual stress.