Princi-ple and Characteristic of Brazingand Soldering Technique
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- The fundamental difference between the brazing and welding methods is that no melting occurs at the joint of the parent metal during the brazing process. The conventional brazing process uses a brazing material having a melting temperature lower than that of the base material, and is placed at the joint to be brazed, when heated to a solidus line lower than the base material but higher than the melting temperature of the brazing material. At this time, the solder melts, and the liquid solder wets the solid base material, which flows, fills, interacts with the base material (dissolves, diffuses or forms a compound) in the joint of the base material joint, and finally cools and solidifies, thereby Two base metals are joined together. For example, brazed pure aluminum (melting point 660C) uses AI-Si co-product (melting temperature 577C) as the brazing material. When the operating temperature is 590-630C, the molten brazing filler metal and the base metal are dissolved in the brazing joint. The diffusion reaction, after cooling, forms a joint of a eutectic-sub-eutectic structure, and the two base metals are joined together.
- When brazing, the workpiece is often heated by the whole (such as brazing in the furnace) or a large area around the brazing joint. Therefore, the relative deformation of the workpiece and the residual stress of the brazed joint are much smaller than the fusion welding, which is easy to ensure the precision of the workpiece. size.
- The selection of brazing filler metal is wide. In order to prevent the change of the microstructure and properties of the base metal, it is possible to use brazing material with a relatively low liquidus temperature for brazing. There is no room for such selection in fusion welding.
- As long as the brazing process is properly selected, the brazed joint can be made “uneven” without machining, which is difficult to achieve by fusion welding. In addition, proper brazing conditions are also beneficial for simultaneous or continuous brazing of multiple brazing seams or large batches of workpiece.
- Since the brazing reaction is carried out only at the interface of several micrometers to several tens of micrometers on the surface of the base metal, it is unfortunately involved in the deep structure of the mother village, so it is particularly advantageous between the dissimilar metals, even between metal and non-metal, non-metal and non-metal. The connection, which is also not possible with the welding method.
- Brazing also has the advantage that the braze seam can be thermally diffused to enhance the strength of the fiber slit. When the composition of the brazing material and the base material have a certain degree of circumferential solubility, prolonging the holding time can cause some components of the fissure to diffuse into the deep layer of the parent metal, and finally the brazing seam can “disappear” under the microscope.
- The weakness of the brazing method is that the composition and properties of the brazing material and the mother village are unlikely to be very close in most cases, and sometimes they are far apart.
- If the aluminum is brazed with a heavy metal solder, it is easy to produce different degrees of electrochemical corrosion between the joint and the base metal. In addition, there are certain limitations in the choice of solder and the characteristics of the interfacial reaction. When brazing most materials, the brazed joint and the base metal cannot reach the same strength, so the overlap area is increased to solve the problem.
- The word brazing is called Tlaxa in Russian, and the corresponding words are in English, only Bruing (brazed) and Sldering (soft soldering), and Taiwan in China is called “hard scorpion” and “soft stalk”. According to international standards, brazing using a solder liquid having a liquidus temperature of 450 C or more is referred to as brazing, and 450 C or less is referred to as soldering. In some literature reports, it is customary to subdivide into high-temperature, medium-temperature, and low-temperature brazing. For example, aluminum brazing is called high-temperature aluminum brazing in the range of 500-630C, and medium-temperature aluminum brazing is called 300-500C. Welding, while below 300 is called low temperature aluminum brazing. Brazing of steel and other metal alloys sometimes occurs in similar situations, but the temperature range is not the same. This subdivision appears because the types of solder, flux, and brazing methods used in each range are often similar, and the different ranges are quite different. A prominent example is low temperature brazing at temperatures below 300 C, where fluxes of organic compounds are commonly used, and temperatures above this temperature are difficult. As for the classical meaning of “high temperature brazing”, it refers to brazing at a brazing temperature higher than 900 C in a vacuum or a protective gas (without flux)