A method for forming a joint between a tube and a sheet includes forming an anvil at least within the tube, and welding the tube to the sheet in the presence of the anvil. The anvil includes an anchor which is placed within the tube near or at the joint to be formed. At least one washer is placed over the end of the anchor that is near the joint to be formed. A threaded fastener is then placed into the anchor to securely hold the anchor within the tube and to provide a backing substantial enough so that a friction stir weld can be formed. The threaded fastener and the washer can be used as a guide for the friction stir weld. Once the weld is completed, the anvil can be removed. The weld can be further processed to remove burrs and other material.