Solid-state thermomechanical processing routes such as friction extrusion (FE), friction stir welding (FSW), and friction surfacing (FS) represent advanced techniques for processing lightweight alloys. These processes operate through the combined action of frictional heating and severe plastic deforma- tion, promoting substantial microstructural evolution while maintaining temperatures below the melting point [1]. The extreme stress, strain, and temperature gradients inherent to these processes, however, make in situ experimental characterization of microstructure evolution highly challenging. As a result, robust computational modeling tools are essential for predicting microstructural development and guid- ing process optimization. Depending on the stacking-fault energy of the alloy and the imposed thermomechanical conditions, mul- tiple dynamic recrystallization (DRX) mechanisms may be activated during FE, FS, and FSW. To cap- ture these mechanisms under the exceptionally large strains characteristic of solid-state processing, we develop a fully coupled multiscale modeling framework that integrates a multiphase-field (MPF) formu- lation with a phenomenological crystal plasticity (CP) model [2]. The MPF approach describes grain nucleation, growth, and grain boundary migration, while the CP model accounts for anisotropic plastic deformation, strain hardening, and lattice reorientation at the grain scale. Large deformation kinematics are incorporated through an Arbitrary-Lagrangian–Eulerian formulation specifically adapted for diffuse- interface simulations. Mechanical equilibrium is solved incrementally within a small-strain framework using an efficient FFT based solver, and the resulting velocity field is employed to advect the phase-field variables. Stress updates under finite deformation conditions are achieved by correcting for rigid body rotations via the spin tensor derived from the local velocity gradient. Periodic remeshing enables simu- lations on a fixed computational grid size while maintaining numerical stability and accuracy. The proposed framework enables predictive simulation of microstructural evolution under severe defor- mation conditions during the solid-state processing. It provides detailed insight into the coupling be- tween processing parameters, DRX mechanisms, evolving grain structures, and macroscopic mechanical response in polycrystalline lightweight alloys.