Friction welding is a solid phase welding with mechanical energy as energy source. It uses the heat generated by mechanical friction between the two surfaces to realize the connection of metal. The heat of friction welding is concentrated at the joint surface, so the heat affected zone is narrow. Two pressure must be applied to the surface, most of which is to increase the pressure at the end of heating, so that the hot metal will be bonded by forging and the general joint surface will not melt. Friction welding has higher productivity. Almost all metals that can conduct hot forging can be welded by friction. Friction welding can also be used for welding of different kinds of metals. It should be applied to the workpiece with a maximum diameter of 100mm with a circular cross-section. By using the heat produced by the friction between the contact surfaces of the welding parts, the end face reaches the thermoplastic state, and then the top forging force is applied quickly to achieve a solid phase welding method for welding. Friction welding has the following advantages: (1) the welding quality is stable, the dimension accuracy of the weldment is high, and the reject rate of the joint is lower than that of the resistance butt welding and flash butt welding. (2) the welding productivity is high, which is 5~6 times higher than that of flash butt welding. (3) it is suitable for welding dissimilar metals such as carbon steel, low alloy steel and stainless steel, high speed steel, and the connection between copper and stainless steel, copper – aluminum, aluminum – steel, steel – zirconium and so on. (4) the processing cost is low, the electricity is saved, and the welding parts do not need special cleaning. (5) it is easy to realize mechanization and automation, simple operation, no spark, arc and harmful gas in welding workplaces. Disadvantages: it is difficult to weld non circular section by rotating workpiece. Disc parts and thin-walled pipe fittings are difficult to weld because they are not easy to clamp. Limited by the power of the spindle motor of the welding machine, the maximum section of friction welding can be welded at 20000mm2. The friction welding machine has a large investment cost and is suitable for mass production. Applications: heterologous metal and dissimilar steel products, such as copper – aluminum transition joints in electric power industry, high speed steel cutting tools for metal cutting, structural steel cutters, etc., structural steel products such as snake tube, valve and tractor axle bush of power plant boiler.