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How to improve the brazing of the header of the tube and strip heat exchanger?

The main brazing defects in vacuum brazing of tube and strip heat exchanger are that the brazing joint between header and coil is not dense, faulty welding and incomplete fusion, which seriously affects the first pass rate and service life of the brazing of heat exchanger. The resulting product scrap rate is high in the actual production process. Therefore, taking effective technological measures to improve the brazing conditions of brazed joints is an important way to improve the first pass rate of header brazing and product reliability.

1.Brazing materials

When the header is brazed with the coil, it mainly depends on the composite filler metal or auxiliary filler metal on the surface of the header to fill the brazing seam after melting. Therefore, the selection of header and auxiliary filler metal has a great impact on the brazing quality.

As the core material of the header (double-sided composite pipe or single-sided composite pipe) does not melt during vacuum brazing, the liquid filler metal after melting of the skin material (filler metal) is mainly used to fill the brazing seam. Therefore, the uniformity of the composite layer on the header surface and the composite mode are important factors affecting the brazing quality. According to production experience, the brazing performance of double-sided composite pipe is better than that of single-sided composite pipe. The use of auxiliary brazing filler metal can enhance the filling ability of brazing filler metal. The commonly used brazing filler metal ring is mainly used. The process modification method is to use the brazing ring (double-sided composite plate) as the auxiliary brazing filler metal. Since the core material of the brazing ring is not melted during brazing, and the molten liquid filler metal of the surface composite layer forms a brazing surface with the header and coil, the area of the brazing seam is increased, which can significantly improve the one-time qualification rate and reliability of brazing.

2.Brazing clearance

The brazing clearance between the header and the coil is a key factor affecting the quality of the brazed joint. It is necessary to strictly control the punching size of the header and the necking size of the coil brazed part to ensure the brazing clearance of the brazed joint, so as to obtain excellent strength of the brazed joint.

It is important to determine the optimal clearance value for brazing process. If the clearance is too small, it is easy to cause defects such as incomplete fusion, incomplete penetration, and too small size of the solder joint due to the difficulty of solder flow; When the gap is too large, due to the weakening of the capillary effect of the brazing seam or the insufficient filler metal of the composite pipe, it is easy to cause defects such as unfilled brazing seam, densified brazing seam, and intergranular segregation. Therefore, too small or too much clearance is an important reason for reducing the strength and reliability of brazed joints. According to the production experience, the quality of the brazed joint is good when the unilateral brazing gap between the header and the coil is controlled at 0.08~0.10mm.

There are many human factors during brazing. In addition to strictly controlling the brazing gap, the excess flux around the brazing seam should also be removed, otherwise it is easy to cause loss of solder and fin burning.

3.Joint type

When the header is brazed with the coil, selecting the appropriate joint type is conducive to improving the strength of the brazed joint and the reliability of the product.

The joint form can be changed by using solder rings. At the same time, the melting of filler metal ring during brazing can make up for the lack of filler metal in the header, enhance the ability of filler metal to fill the joint, achieve the ideal size of the solder joint, and improve the joint strength. The production practice has proved that the use of double-sided composite brazing ring can significantly improve the first pass rate of brazing, avoid secondary damage during transportation and use, and is an ideal form of header brazing joint.

4.Workpiece placement mode

The proper placement of the workpiece is conducive to improving the filler metal inflow conditions and ensuring the forming quality of the brazing seam between the header and the coil.

Brazing equipment selection:

In order to achieve high quality and high yield brazing, in addition to mastering the above points, it is also necessary to select appropriate brazing equipment, that is, vacuum brazing furnace. At present, the most applicable product series of SIMUWU is RVB-L low temperature vacuum brazing furnace. It has the characteristics of high temperature accuracy, high vacuum degree, and good temperature homogeneity in the heating zone, and is the ideal equipment for vacuum brazing of aluminum alloy chassis.