Friction welding is the process by which two pieces of metal are fused together by rotating one of them at high speed, then pressing the two together.
Another advantage is that it can be used to join dissimilar materials–say, lightweight aluminum and high-strength steel, like you might see in an aerospace application–which have different melting points that would make them impossible to join by conventional welding methods.
A third advantage is you can weld extremely thick-walled parts together. Imagine trying to get the heat from a blowtorch to penetrate down into something several inches thick; it just ain’t happening. But with friction welding, wherever the two pieces are coming into contact will be joined.