THIS PHASE I PROGRAM ADDRESSES THE NASA NEEDS FOR TECHNICAL AREA TA12 WHICH PROVIDES TECHNOLOGIES SUPPORTING ADVANCED MANUFACTURING, STRUCTURES, AND MATERIALS. IN THIS PROGRAM, ULTRASONIC THERMOMETRY METHODS WILL BE ADAPTED TO ENABLE REAL TIME, TEMPERATURE MEASUREMENTS DURING A FRICTION STIR WELD, FSW. WHILE FSW IT HAS BEEN SUCCESSFULLY USED AT NASA-MSFC IN THE ROBUST JOINING OF THINNER PANELS OF ALUMINUM ALLOYS, THE PRODUCTION OF HIGH QUALITY WELDED JOINTS IN THICKER PANELS OR HIGHER MELTING TEMPERATURE MATERIALS REMAINS CHALLENGING. KNOWING AND CONTROLLING THE TEMPERATURE IS ESSENTIAL TO OPTIMIZE THE PROCESS TO ENSURE THE PRODUCTION OF REPEATABLE, ROBUST COMPONENTS. USING SENSORS ATTACHED TO THE STIR WELD TOOL, PRECISE TIME-OF-FLIGHT (TOF) MEASUREMENTS OF ULTRASOUND PROPAGATING THROUGH THE WELD ZONE WILL BE USED TO ESTIMATE TEMPERATURE. THIS TEMPERATURE MEASUREMENT MADE IN THE WELD ZONE IS NON-INTRUSIVE AND DOES NOT INFLUENCE THE THERMAL TRANSPORT. TEMPERATURE DATA CAN BE OBTAINED AT DATA RATES AS HIGH AS 1 KHZ WITH THE PRECISION OF A FEW DEGREES CENTIGRADE. ONCE THIS SMART STIR WELD TOOL TEMPERATURE SENSOR IS DEMONSTRATED, THIS TECHNIQUE CAN BE INCORPORATED INTO A CONTROL STRATEGY AND READILY MIGRATED TO ANY OF THE FSW VARIATIONS UNDER DEVELOPMENT AT THE NASA-MSFC.