A two-dimensional axisymmetric thermo-mechanical model of the Refill Friction Stir Spot Welding (RFSSW) process was developed and validated with experimental data. Welding temperatures, tool forces, and material flow including defect formation, were accurately predicted by the model. Qualified repair techniques are critical for successful implementation of a welding process for use on large weldments with a significant number of spot joints, and this work demonstrates a repair technique for RFSSW that is validated both experimentally and numerically. Repaired properties are shown to exceed 90% of the original mechanical properties of the RFSSW process. RFSSW has different process parameters for every combination of material alloy, material thickness, weld duration, and machine force limits. Numerical modeling develops the process parameters for any RFSSW iteration in a fraction of the time with the same amount of accuracy. The model can effectively simulate how to determine the optimal weld duration given any experimental parameters.