Read about it in my current issue of C&D. Essentially using friction, the material is melted. there is supposedly less distortion from extreme heat and you can weld dissimilar materials (steel to aluminum). It's currently being used in some auto manufacturing processes. I've never heard of it but found it very interesting. I don't see it replacing traditional welding but still cool regardless.
Per Wikipedia :
http://en.wikipedia.org/wiki/Friction_stir_welding
