Plunge depth is one of the most important process parameters that affect the joint strength in refill friction stir spot welding (refill FSSW). In this study, a three-dimensional numerical model is developed using the Lagrangian incremental formulation in DEFORM-3D to simulate the refill FSSW process of thin AA7075-T6 sheets. The numerical model is verified by comparing the obtained temperatures at specific locations with the temperatures from the previous experimental studies. Material flow and temperature behaviors at three different plunge depths are analyzed using the numerical model. The temperatures and effective strains in the weld zone increased with an increase in plunge depth. The movement of the material in the stir zone is enhanced and a larger extent of material from the bottom sheet is involved in the stirring with the increase in plunge depth. The width and thickness of the stir zone are identified from the numerical model and are consistent with the experimental study from the literature. The increase and decrease in joint strength with the increase in plunge depth reported in the literature are correlated to the material flow behavior in the numerical models. © 2022 by the authors. Licensee MDPI, Basel, Switzerland.