Thermal stir welding (TSW) is a solid state welding process which has shown promise in joining high strength, high temperature metals needed for space launch systems. Although TSW offers an approach which allows more precise control of the temperature, better measurement of temperature in the weld zone is needed. The Industrial Measurement System Inc. (IMS Inc.) and University of Alabama in Huntsville (UAH) team propose to demonstrate the feasibility of ultrasonic thermometry technique to measure temperature, in real-time, in the fusion zone, during the TSW process. Using sensors attached to the containment plates, precise time-of-flight (ToF) measurements of ultrasound propagating through the fusion zone will be used to estimate temperature. This temperature measurement is non-intrusive and does not influence the thermal transport in the weld zone. Temperature data can be obtained at data rates as high as 1 kHz with the precision of a few degrees Centigrade. Thus, these measurements can be used as feedback controls in in-situ process control strategy for the TSW process. Precise temperature control will enable superior mechanical properties in the weld joint and thus maximize the capability of the TSW weld process.