Numerical Study on Dendritic Growth During Solidification in Vibration-Assisted GMAW of Q450NQR1 Steel Using a Coupled Temperature–Cellular Automaton Model

We developed a multiscale numerical model that couples a macroscopic temperature field with a cellular automaton (CA) model to investigate the evolution of the solidification microstructure in the molten pool of Q450NQR1 weathering steel during low-frequency vibration-assisted gas metal arc welding (GMAW). First, finite element analysis was used to obtain the transient temperature field of the molten pool, and temperature data from a representative cross-section were transferred to the CA computational domain. We then incorporated a vibration model and a dendrite fragmentation mechanism into the CA framework to simulate dendrite nucleation, growth, and microstructural evolution under different vibration frequencies and amplitudes. Welding experiments were also conducted to assess the model’s ability to reproduce the response trends associated with changes in vibration parameters. Without vibration, pronounced solute segregation occurred in the molten pool, and the weld microstructure was dominated by columnar grains and coarse equiaxed grains. After vibration was applied, low-frequency mechanical vibration enhanced dendrite fragmentation and promoted the conversion of fragmented dendrites into stable nuclei, thereby facilitating equiaxed grain formation and grain refinement. At a vibration amplitude of 0.10 mm, increasing the vibration frequency from 0 to 50 Hz reduced the average grain size from 68.5 μm to 35.2 μm, corresponding to a grain refinement rate of 48.6%. At 50 Hz, increasing the vibration amplitude further reduced the grain size and improved solute redistribution. These results indicate that, within the parameter range investigated in this study, low-frequency mechanical vibration refines the weld microstructure by promoting dendrite fragmentation and equiaxed grain formation. It also enhances effective solute diffusion during solidification. The simulated microstructural evolution trends agreed well with the experimental observations. These findings provide a basis for optimizing vibration parameters in vibration-assisted GMAW and for controlling the solidification microstructure of welded joints.