Abstract: The present invention provides a self-backing friction stir welding tool for backing up weld joints formed by friction stir welding large workpieces and workpieces having curvilinear geometries. The backing device includes a rotatable shoulder adapted to at least partially engage the weld joint and a probe having first and second ends and an intermediate portion therebetween. At least a portion of the intermediate portion of the probe frictionally engages the workpiece to form the weld joint. The first end of the probe defines a backing member, at least a portion of which engages the weld joint opposite the rotatable shoulder. The second end of the probe is adapted to rotatably and axial translatably communicate with the rotatable shoulder to thereby rotate the probe and urge the backing member toward the weld joint, thus constraining the plasticized material within the weld joint.