<p>Tailoring the microstructure, especially controlling the transition between columnar and equiaxed grain structure is of great importance in metal additive manufacturing for achieving favorable mechanical performance. Static magnetic field (SMF) has demonstrated exceptional potential in promoting columnar-to-equiaxed transition in the laser powder bed fusion of multiple alloys, but the underlying mechanism of grain refinement has not yet been well understood due to the lack of multiscale insights bridging molten pool dynamics and dendrite-scale phenomena. To address this knowledge gap, we establish an integrated modeling framework combining a thermoelectric magnetohydrodynamics model and a 3D dendrite growth model incorporating a stochastic nucleation model and a dendrite-fragment-induced nucleation model to investigate how SMFs induce grain refinement. AlSi10Mg is taken as an example alloy, due to availability of experimental results in literature. The simulation results reveal that instead of the change in molten pool features, dendrite fragments induced by thermo-electric-magnetic forces make major contribution to grain refinement. This work provides useful insight into the microstructure evolution and mechanical property optimization in magnetic-field-assisted additive manufacturing.</p>