Filler metal is the filler metal forming brazing joint, and the quality of brazed joint depends largely on filler metal. The solder should have suitable melting point, good wettability and filling ability, which can be diffused with the parent material, and also have certain mechanical and physical and chemical properties to meet the application performance requirements of the joint. Depending on the melting point of solder, brazing can be divided into two main categories: soldering and brazing. (1) soft soldering. The brazing of the solder with a melting point of less than 450 degrees is called soft soldering. The solder is a tin lead solder. It has good wettability and conductivity. It is widely used in electronic products, electrical machinery and electrical appliances and auto parts. The joint strength of brazing is generally 60 ~ 140MPa. (2) hard brazing. Brazing of solder at melting point above 450 C is called brazing, and brazing filler metal is usually used as brazing filler metal and silver base filler metal. The joint with silver base filler metal has high strength, electrical conductivity and corrosion resistance. The melting point of the filler metal is low and the technology is good, but the price of the solder is high, and it is used for higher welding parts. Brazing is mostly used for brazing steel and copper alloy workpiece with high force and tools. The joint strength of the hard brazing is 200 ~ 490MPa. Note: the contact surfaces of base metal should be very clean. Therefore, fluxes should be used. The effect of the brazing agent is to remove the oxide and oil impurities on the surface of the base material and the filler metal, and to protect the contact surface of the brazing material and the base material not to be oxidized, and to increase the wettability and capillary fluidity of the solder. The melting point of the flux should be lower than that of the filler metal, and the corrosion of the flux residue to the base metal and the joint should be smaller. Rosin or zinc chloride solution is commonly used as brazing flux for soft soldering, while borax, boric acid and basic fluoride are commonly used as brazing flux for hard soldering. According to different heat sources or heating methods, brazing can be divided into flame brazing, induction brazing, furnace brazing, dipping brazing, resistance brazing and so on. Because the heating temperature is relatively low during brazing, it has little effect on the material properties of the workpiece, and the stress and deformation of the weldments are also small. However, the strength of brazed joints is generally low and the heat resistance is poor. Brazing heating method: almost all heating sources can be used as brazing heat source, and brazing is classified according to this. Flame brazing: It is used to braze carbon steel, stainless steel, cemented carbide, cast iron, copper and copper alloys, aluminum and aluminium alloys. Induction brazing: a thermo – heated welding piece with an alternating magnetic field that produces an induced current in a part, used for soldering with a symmetrical shape, especially a tube shaft. Soaking brazing: immerse the welded part or the whole in molten salt mixture or solder melt, and rely on the heat of the liquid medium to realize the brazing process, which is characterized by rapid heating, uniform temperature and deformation of the welding parts. Furnace brazing: welding parts are heated by resistance furnace, which can be protected by vacuum or by reducing gas or inert gas. In addition, there are soldering iron brazing, resistance brazing, diffusion brazing, infrared brazing, reactive brazing, electron beam brazing, laser brazing and so on.