The present invention provides a tool for forming a friction stir weld joint in a workpiece. According to one embodiment, the tool includes a rotatable pin having first and second ends and defining a stirring portion therebetween structured to frictionally engage the workpiece so as to at least partially form the friction stir weld joint. The tool includes a rotatable first shoulder defining an aperture therethrough structured to slidably receive the first end of the pin. The tool also includes a second shoulder defining an aperture structured to receive the second end of the pin such that the pin extends between the first and second shoulders and such that the second shoulder is in rotatable communication with the pin. The first shoulder is structured to rotate independently of the pin and the second shoulder.
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