Longhurst Engineering, PLC and Vanderbilt University propose the innovation of torque control of friction stir welding (FSW) as a replacement to force control of FSW. The value of the torque is significant because it indicates how far the tool is plunged into the work piece. Proper engagement of the tool into the work piece is critical for producing reliable welds. The commercialized innovation will consist of three elements. First, a FSW tool will be developed to produce a linear relationship between the welding torque and the tool's plunge depth into the work piece. Second, the welding torque will be measured from outside the welding environment via the spindle motor current, thus eliminating the need for expensive force sensors associated with force control. Third, a closed-loop architecture will be designed and implemented to control the welding torque. Torque control of FSW can be applied by NASA to increase welding reliability with the Upper Stage of the Ares I launch vehicle. Torque control will also reduce capital investment and operations costs for NASA. The expected TRL is 4 at the beginning of the project and 5 at the end of Phase I.