Abstract: An adapter for use in friction stir welding is configured to be mountable in and rotated by a standard milling machine. In one embodiment, the adapter includes an upper housing, a lower housing, and a mounting element for transmitting rotational movement from the milling machine. At least a portion of the mounting element is mounted in the upper housing. The adapter may also include a tool holder connected to the lower housing and rotated by the mounting element; and a displacement mechanism for providing movement of the tool holder relative to the mounting element. The displacement mechanism may be isolated from the rotational movement of the mounting element and the tool holder. The displacement mechanism of the adapter may also include at least one piston cylinder assembly. In one embodiment, the upper housing and the lower housing form a part of the piston cylinder assembly.