some kinds of welding do not use heat to make the weld. these kinds of welding can get hot, but they do not make the metal melt. forge welding is an example of this. friction stir welding is a special kind of welding that does not use heat. it uses a very powerful motor and a special spinning bit to mix the metals together at the edge. this seems odd because metals are a solid. this is why it takes a lot of force to do and is very hard. the energy for this kind of welding is mechanical energy from the spinning bit.