brazing technology summary

Brazing is to use the metal material with lower melting point than the weldment as filler metal. The weldment and filler metal are heated to a temperature higher than the melting point of the filler metal and lower than the melting point of the weldment. The liquid solder is used to wet the base metal, fill the joint gap and diffuse with the base metal to realize the connection of the weldment. According to the melting point of solder, it can be divided into: soft soldering, that is, the melting point of solder is lower than 450 ℃; and brazing, that is, the melting point of solder is higher than 450 ℃. According to the brazing temperature, there are: high temperature brazing; medium temperature brazing; low temperature brazing. For different materials, the classification temperature is different. According to the heating method, it can be divided into flame brazing, furnace brazing, induction brazing, resistance brazing, soldering iron brazing, etc.

Brazing has wide adaptability and can weld most metals and some non-metallic materials; the accessibility of brazing is good, and it can be completed by brazing for the welding seam which is inaccessible in space; the accuracy of brazing is high, for high-precision and complex parts, multiple welds can be welded at one time, with high efficiency; the heating temperature of brazing is much lower than that of fusion welding, which has little influence on the microstructure and properties of base metal; The deformation of weldments is small, especially by using uniform heating brazing, the welding deformation can be reduced to the minimum, and the dimensional accuracy of weldments can be easily ensured. Brazing also has its disadvantages, mainly including: low strength; poor heat resistance; due to most of the overlapped (LAP) joints, increasing the consumption of base metal, the joint is bulky.

Filler metal is the filler metal used to form brazing seam during brazing. According to the melting point, it can be divided into soft solder (lower than 450 ℃), hard solder (higher than 450 ℃), high temperature solder (higher than 950 ℃); according to the chemical composition (metal elements), it is called "×" based solder, such as nickel based solder, tin based solder, silver based solder, etc. Flux, also known as flux, is used to remove the oxide on the surface of base metal and liquid solder, protect the base metal and solder from further oxidation during heating, and improve the wettability of solder on the surface of base metal. In the same way, the matching flux can also be divided into soft flux and brazing flux; according to the use, it can be divided into: aluminum flux; powder flux; liquid flux; gas flux; paste flux; no cleaning flux, etc.

Most of brazing is carried out with filler metal and flux, but there are also direct brazing without flux. The strength of the front seam is lower than that of the base metal. In order to increase the strength, the lap joint is often used. The length of the lap joint is 3-4 times of the plate thickness, but not more than 15mm. Most pipes are brazed by sleeve joint. Before brazing, the oxide film and dirt on the surface of the weldment shall be cleaned by mechanical or chemical methods. In order to prevent the free flow of liquid solder, flow blocking agent is often applied on the surface of the weldment. If the assembly time gap is too large or too small, it will affect the function of capillary tube, which will reduce the strength of brazing seam and waste solder if the brazing seam is too large.

The main technological parameters of brazing are brazing temperature and holding time. The brazing temperature is generally 25-60 ℃ higher than the melting point of brazing filler metal. If the temperature is too high or too low, it is not conducive to ensure the quality of brazing joint. The holding time of brazing should make enough action between the filler metal and the filler metal, and the action between the filler metal and the base metal should be shorter; the larger the gap and the larger the size of the weldment, the longer the effect. After welding, most of the flux residue will corrode the brazed joint and hinder the observation of the brazed joint, so it must be cleaned up.