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Vacuum brazing process of electrical contact brazing rate

With the development of the national economy, the number of electrical equipment has increased rapidly. In the low-voltage power supply system, the demand for high-current switch products is increasing.

Vacuum brazing process is one of the most important processes to ensure the quality of electrical contacts. The fundamental problem of vacuum brazing process of electrical contacts is how to form a high-quality joint between the contact and the conductive plate. The quality of brazing is closely related to the composition of the contact and the conductive plate, the composition of the flux and solder, the pre-welding treatment of the brazing surface, the welding time, the welding pressure, the post-welding treatment and the brazing temperature. The firm connection with the conductive plate puts forward very high requirements, and the following vacuum brazing process is explored for the brazing process of this large-scale electrical contact.

1 Material and preparation before welding

(1) Materials. The contact material used is AgNi30C3, the specification is 78mm x 15mm x 2.3 mm, and the conductive plate is T2 copper; the solder silver pads are BAg30CuZn, BAg50CuZn, the thickness is 0.2mm, and its brazing temperature is respectively 760~850C.750~890C; The brazing flux is 102.

(2) Preparation before welding. The base metal and brazing filler metal are first cleaned and degreasing with acetone, and the copper base metal is soaked in acid solution to remove the surface oxide film.

2 Vacuum brazing process method

The welding equipment used is a projection welding machine with a DC 200kV·A, the rated pressure of the compressed air is 0.5 MPa, the T-shaped joint is used for the joint of the test piece, and the silver solder tab is cut to 73 mm x 13 mm. The contact surface with the base metal is coated with a certain amount of flux, and by adjusting the test process parameters, the welding process is completed through the preloading stage, the electric welding stage and the maintenance pressure stage. Use the JTUIS ultrasonic imaging non-destructive testing system to scan the test piece to obtain the electrical contact brazing rate of different test process parameters, as shown in the following table. The scanning images of the JTUIS ultrasonic imaging non-destructive testing system are shown in Figures 1 to 4.

3 Detection method of brazing rate

Detected by JTUIS ultrasonic imaging nondestructive testing system. The JTUIS ultrasonic imaging non-destructive testing system is a non-destructive testing of the bonding quality of electrical switch contacts. It uses the principle of ultrasonic non-destructive testing and image processing technology, and uses a water immersion focusing probe to scan the bonding surface of the switch contact point by point. The ultrasonic reflection echo detection signal is sampled to form an ultrasonic scanning image of the joint surface, and an appropriate threshold is selected. Image processing is used to separate the joint area and the unbonded area in the ultrasonic image, and the interface joint rate is calculated. The detection system consists of an ultrasonic signal generator, a probe, a three-dimensional precision scanning platform, and an image acquisition card. It consists of a stepping motor drive card, a stepping motor drive power supply, an industrial control computer and system software.

4 Results Analysis

(1) Analysis of temperature parameters. ①When the welding temperature is low and the welding heat is small, the thermoplastic deformation of the joint surface is small, and the oxide film is difficult to rupture. In addition, the temperature of the joint area is low, and the activity of the flux is not enough. Both of these two aspects affect the removal of the oxide film, thereby hindering the The wetting of the base metal by the brazing filler metal is improved, and the brazing rate of the joint is low. ② When the welding temperature is too high and the welding heat is large, although the oxide film on the surface of the base metal can be removed well, the brazing filler metal melts rapidly due to the high temperature, and the liquid brazing filler metal is squeezed out of the joint surface under the action of the welding pressure. And the loss, the brazing rate of the joint is also low.

(2) Analysis of welding pressure parameters. ① Since the welding area of the electrical contact is relatively large, a large welding pressure is required, but the welding pressure is too large, on the one hand, serious macroscopic deformation of the weldment will occur, and the size of the weldment cannot be guaranteed; on the other hand, the molten The solder is rapidly extruded from the joint surface, resulting in direct contact between the solder and the low-temperature base metal at the joint surface. Due to the low temperature of the base metal, it is difficult to achieve sufficient diffusion of atoms between the liquid solder and the solid metal base material, which is not conducive to the crystallization process. Therefore, a joint with a higher brazing rate cannot be obtained, as shown in Figure 1. ②During the welding process, we adjusted a moderate welding pressure to avoid the rapid increase of transient current due to the micro-local contact of the joint surface and the rapid melting of the brazing material. A certain amount of thermoplastic deformation is generated, and the oxide film at the joint surface is ruptured, so that the flux can penetrate between the base metal and the oxide film through the rupture of the oxide film, and achieve the purpose of removing the film by peeling and dissolving the oxide film. See Table 3 No. 4 compressed air pressure value: Figure 4 is the brazing rate obtained under moderate welding pressure.

(3) Analysis of surface roughness. Since the silver contact for the test is made of powder metallurgy mixed powder after sintering and molding, the surface roughness value is small, and the surface area of the electrical contact is large, which will make the liquid solder fill the gap unevenly and the flow path is disordered. It is easy to flow out from the welding seam, which will directly affect the quality of the brazed joint, resulting in the formation of a non-dense brazing seam, resulting in defects such as air inclusions. During the welding process, we treated the welding surface of the silver contacts, and processed small grooves to increase the surface roughness value of the base metal, which fully improved the wetting and spreading of the brazing material on the base metal. The grooves are shown in Figure 5 and Figure 6, and the test results of the soldering rate from Table 3, No. 3.4, also prove this point.

(4) The choice of two kinds of brazing materials. The brazing test was carried out on the large electrical contact with BAg30CuZn and BAg50CuZn respectively. Through the metallographic observation of the two brazed joints, it can be seen that the welding seam of the brazed joint with BAg50CuZn solder should be continuous and dense. It can be seen from the test process that the brazing process performance of BAg50CuZn solder is better, and the BAg50CuZn solder contains more silver, and the electrical conductivity of the solder joint is better. Considering the high requirements of the product, we choose BAg50CuZn.

5 Conclusion

According to the above test and analysis, the brazing of large-scale electrical contacts should be based on the size of the surface area of the electrical contacts, the surface of the contacts should be molded or an appropriate amount of grooves should be processed, and the surface roughness value of the silver contacts should be increased to improve the solder in the solder. Wetting and spreading on the base metal, and by adjusting the appropriate temperature and welding pressure and other process parameters, a joint with a high brazing rate of more than 90% can be obtained.

Vacuum brazing furnace equipment selection:The RHVB series vacuum brazing furnace produced by SIMUWU is a high-quality product for the vacuum heat treatment of tooling and molds. Good temperature control accuracy and temperature control uniformity ensure the effective progress of the vacuum brazing process. SIMUWU specializes in the manufacture of vacuum furnaces, has more than ten years of relevant experience, and has a good reputation in the field of vacuum furnace manufacturing.