English: This close-up view of the friction stir weld tack tool used to manufacture of space shuttle external tanks shows the process of tack welding barrel panels together. Barrels were previously fabricated using traditional fusion welding, but friction stir welding is different in that the materials are not melted. A rotating tool pin uses friction and applied pressure to join the 20-foot longitudinal panels together. Friction stir welding is the most recent upgrade to the space shuttle's external tank, the largest element of the shuttle and the only element that is not reusable. The new welding technique utilizes frictional heating combined with forging pressure to produce high-strength bonds virtually free of defects. Friction stir welding transforms the metals from a solid state into a "plastic-like" state, and then mechanically stirs the materials together under pressure to form a welded joint. Invented and patented by The Welding Institute, a British research and technology organization, the process is applicable to aerospace, shipbuilding, aircraft and automotive industries. One of the key benefits of this new technology is that it allows welds to be made on aluminium alloys that cannot be readily fusion arc welded, the traditional method of welding.