In Friction Stir Welding (FSW) processes the axial force provides a forging action that affects the part's microstructure and, hence, its mechanical properties. Controlling this force provides good, consistent welding quality. The aim of this paper is to provide a systematic method to design and implement axial force controllers for FSW processes. The axial force is modeled as a nonlinear function of the measured FSW process parameters (i.e., plunge depth, traverse rate, and rotation speed) and the equipment is modeled as a pure delay from the commanded to the measured process parameters. Based on these dynamic models, a nonlinear feedback controller for the axial force is designed using Polynomial Pole Placement. This controller is implemented in a Smith Predictor-Corrector structure to compensate for the inherent equipment delay and the controller parameters are tuned to achieve the best closed-loop response given the equipment limitations. Experimental implementations verify the controller is able to maintain a constant axial force, even when gaps are encountered during the welding process.