Principles and characteristics of brazing

    Brazing is to use a metal material with a lower melting point than the base material as the brazing filler metal. The weldment and the brazing filler metal are heated to a temperature higher than the melting point of the brazing filler metal and lower than the melting point of the base material at the same time, so that the brazing filler metal is melted into a liquid. The brazing material liquid wets the base material, and under the action of capillary flow, the brazing liquid fills the gap between the joints of the base material and diffuses with the base material. The dissolution and diffusion between the base metal and the brazing filler metal change the composition of the brazing seam and the interface base metal. The composition, structure and performance of the brazing joint are greatly different from those of the brazing filler metal and the base metal. Crystals, thus forming a strong joint.

    The biggest difference between brazing and fusion welding methods is that during brazing, the work is often heated as a whole (such as brazing in a furnace) or a large area around the brazing seam is evenly heated. Therefore, the relative deformation of the workpiece and the residual stress of the brazed joint are compared The fusion welding is much smaller, and it is easy to ensure the precise dimensions of the workpiece. In addition, the selection range of brazing filler metal is wide. In order to prevent the change of the structure and characteristics of the base material, brazing filler metal with relatively low liquidus temperature can be selected for brazing. During the brazing process, as long as the brazing process is selected properly, the brazing joint can be made without processing. In addition, as long as the brazing conditions are appropriately changed, it is also conducive to simultaneous or continuous brazing of multiple brazing seams or large quantities of workpieces.

    Since the brazing reaction only proceeds at the interface of the base material from a few microns to several tens of microns or less, it has nothing to do with the deep structure of the base material. Therefore, it is particularly advantageous for the connection between dissimilar metals, even between metal and non-metal, and between non-metal and non-metal. This is also impossible with fusion welding.

    The brazing seam can be used for thermal diffusion treatment to strengthen the strength of the brazing seam. When the components of the brazing filler metal have a certain degree of solid solubility with the base material, extending the heat preservation time can make certain components of the brazing seam diffuse to the base material and improve the bonding strength between the brazing seam base materials.

    The main disadvantage of brazing is that the composition and properties of the brazing filler metal and the base metal cannot be very close, and sometimes the difference is large. For example, brazing aluminum with heavy metal brazing filler metal will inevitably cause different degrees of electrochemical corrosion between the joint and the base metal. In addition, when most materials are brazed, the brazed joint and the base metal cannot achieve the same strength, and can only be improved by increasing the overlap area.

(Huiputech)