Friction spot stir welding

In friction stir welding, no material is melted. A rotating pin is pressed into the material and rubs it until it becomes plastic. Then you move the tool along the joint, and the material is compacted again behind it under high pressure. It's a pure solid-state process, not fusion welding. It's important to guide the pin cleanly and with the right downforce, otherwise you won't get a homogeneous weld. If you travel too fast or the pressure isn't right, voids remain or the weld cracks. The heat is generated only by friction; you don't add any external heat.