Abstract: A friction stir welding (FSW) tool includes a pin, a housing, and a shoulder. The pin is configured to extend through a joint line between edges of two work pieces. The pin rotates to perform a FSW process that welds the two work pieces together at the joint line. The housing is coupled to a distal end of the pin to enable rotation of the pin relative to the housing. The pin extends through a support surface of the housing. The support surface contacts respective inner surfaces of the work pieces during the FSW process. The shoulder surrounds the pin and is configured to be rotated during the FSW process. The shoulder contacts respective outer surfaces of the work pieces during the FSW process such that the work pieces are sandwiched between the shoulder and the support surface of the housing.