Friction Stir Additive Manufacturing (FSAM) is a solid-state additive process particularly suited for aluminum alloys due to their high thermal conductivity, relatively low melting temperature, and strong sensitivity to thermo-mechanical processing. Despite its advantages, numerical modeling of FSAM for aluminum alloys remains challenging because of severe plastic deformation, complex material flow, and strong coupling between temperature evolution and strain-rate–dependent softening. In this study, a three-dimensional numerical model based on the Smoothed Particle Hydrodynamics (SPH) method is developed to simulate the FSAM process of aluminum alloys using both a standard tool with a rotating pin and a pinless tool configuration. The mesh-free SPH framework enables robust modeling of large deformations, free-surface evolution, and interlayer material transport inherent to FSAM. A coupled thermo-viscoplastic formulation is employed, incorporating temperature- and strain-rate–dependent material properties representative of aluminum alloys. Heat generation is modeled through a combination of frictional sliding at the tool–workpiece interface and plastic dissipation within the stirred material. The model explicitly accounts for tool geometry, rotational and traverse speeds, and multilayer deposition to capture the additive nature of the process. Simulation results reveal distinct thermo-mechanical responses for the two tool designs. The standard pin tool produces intensified vertical material flow and enhanced interlayer mixing, promoting metallurgical bonding between successive aluminum layers. In contrast, the pinless tool generates a more uniform surface flow with reduced subsurface strain localization and lower peak temperatures, which may mitigate excessive flash formation and surface defects. Predicted temperature fields, material flow patterns, and deposited layer morphologies show qualitative agreement with experimental trends reported for aluminum FSAM. The developed SPH-based model provides a powerful predictive tool for selecting the proper tool based on the sheet's thickness considered during FSAM for defect-free additive manufacturing.