the combination of fast joining times (on the order of a few seconds) and direct heat input at the weld interface yields relatively small heat-affected zones. friction welding techniques are generally melt-free, which mitigates grain growth in engineered materials such as high-strength heat-treated steels. another advantage is that the motion tends to clean the surface between the materials being welded, which means they can be joined with less preparation. during the welding process, depending on the method being used, small pieces of the plastic or metal will be forced out of the working mass (flash). it is believed that the flash carries away debris and dirt.