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Vacuum brazing process of Ag-Cu eutectic solder

Copper and copper alloys usually have excellent electrical conductivity, thermal conductivity and excellent corrosion resistance in some media, and some copper alloys also have high strength, so they are used in electrical, chemical, oxygen, brewing, food, Industrial sectors such as power and transportation have been widely used. In its application, it is inevitable to connect it. At present, the connection methods of red copper mainly include gas welding, arc welding, TIG welding, submerged arc welding, diffusion welding, electron beam welding, plasma welding and other welding methods.

But there are following problems in these welding methods:

1) The high thermal conductivity of copper makes the base metal extremely difficult to melt in fusion welding, so that it is difficult to fuse;

2) The shrinkage rate and expansion coefficient of red copper are large, and the welding stress is large, which makes the joints prone to thermal cracks;

3) Hydrogen, water vapor, and oxygen will cause pores in the joint;

4) The mechanical properties and electrical conductivity of the joint and the base metal are different, so it is advisable to use the method of brazing to connect it. Since the melting temperature of the silver-based solder is not very high, it has good wettability to copper. And it has good strength, ductility, thermal conductivity, electrical conductivity and corrosion resistance, and is widely used in the welding of various metals. In this paper, Ag-28Cu solder is used for vacuum brazing of red copper, and the microcosmic properties of joints under different brazing temperatures are studied. Microstructure, shear strength and fracture characteristics.

1 test device

The vacuum brazing test was carried out in a high-temperature vacuum brazing furnace produced by SIMUWU. Vacuum brazing has the following advantages: During the heating process, the parts are in a vacuum atmosphere, and there will be no oxidation, decarburization, pollution, etc.; the parts are heated as a whole, and the thermal stress Small, the amount of deformation can be controlled to a minimum; the low pressure around the base metal and the solder can discharge the volatile impurities and gases released by the metal at the brazing temperature, and the performance of the base metal itself is also improved; vacuum brazing does not use Flux, there will be no porosity, slag inclusion defects, etc.

2 Experimental results and analysis

2.1 Microstructure

The figure below shows the microstructure of Cu/AgCu/Cu joints obtained when the holding time is 5min and the vacuum brazing temperatures are 820, 850, 900 and 950°C, respectively. It can be seen that under the given vacuum brazing process conditions, the structure obtained by the joint is a columnar phase near the copper side, while the center of the brazing seam is a pattern-like eutectic structure composed of white and black structures.

In order to determine the types of reaction products, energy spectrum analysis was performed on the joints with a brazing temperature of 900 °C using an energy dispersive X-ray spectrometer (EDS). The results are shown in the table below.

It can be seen from the table that position A is mainly composed of Cu-based solid solution, which contains a very small amount of Ag atoms. According to the Ag-Cu binary phase diagram, when the brazing temperature exceeds the temperature of the Ag-28Cu solder, the base metal Cu will dissolve into the liquid solder, and in the subsequent cooling process, the Cu-based solid solution will be precipitated first, so the position A is a Cu-based solid solution. Position B is located in the center of the brazing seam. It is a structure with eutectic patterns on the white base, mainly containing Ag and Cu elements. Since this test uses Ag-28Cu solder, some solder remains in the center of the brazing seam, so The white tissue in position B is a silver-based solid solution, and the black is a Cu-based solid solution. It can be seen that the interface structure of the joint is Cu/Cu-based solid solution/silver-copper eutectic structure/Cu-based solid solution/Cu when Ag-28Cu solder is used for vacuum brazing of copper.

It can also be seen from the figure that as the vacuum brazing temperature increases, the thickness of the columnar Cu-based solid solution layer increases, and the thickness of the silver-copper eutectic structure layer in the center of the brazing seam decreases; while the entire brazing seam width decreases. This is because as the vacuum brazing temperature increases, the amount of Cu base material dissolved into the solder increases, resulting in an increase in the thickness of the columnar Cu-based solid solution; in addition, as the vacuum brazing temperature increases, the fluidity of the solder increases, and the solder There is more loss, so when the brazing temperature is higher, the silver-copper eutectic structure layer in the center of the brazing seam is thinner, and the width of the entire brazing seam is reduced.

2.2 Influence of vacuum brazing temperature on mechanical properties of joints

The figure below shows the shear strength curves of the joints under the conditions of a fixed holding time of 5 min and vacuum brazing temperatures of 820, 850, 900 and 950 °C, respectively. It can be seen from the figure that as the vacuum brazing temperature increases, the shear strength of the joint increases, and reaches the maximum at 850 °C, which is 238 MPa; if the brazing temperature continues to increase, the shear strength of the joint increases. When the temperature exceeds 900 °C, the shear strength of the joint does not change much.

3 Conclusion

1) Using Ag-28Cu solder to braze copper, columnar Cu-based solid solution and silver-copper eutectic structure are mainly formed in the joint. With the increase of vacuum brazing temperature or the prolongation of holding time, the thickness of the columnar Cu-based solid solution in the joint increases, while the thickness of the silver-copper eutectic structure in the center of the brazing seam decreases, and the width of the entire brazing seam decreases.

2) When the vacuum brazing temperature is 850 ℃ and the holding time is 5 minutes, the shear strength of the joint is the largest, which is 238 MPa.

3) The main fracture of the joint is at the junction of the columnar Cu-based solid solution and the silver-copper eutectic structure.