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Influence of Brazing Vacuum Degree on Copper and Hafnium Brazed Joints

Copper has high electrical and thermal conductivity. Both silver-based and copper-based solders can wet it well, and the connection with other materials can enhance heat dissipation. Hafnium is a rare metal with high melting point and refractory with good thermal conductivity, electrical conductivity and low electron work function. It is used as a plasma emitter in electrodes for plasma cutting. The element, hafnium and copper matrix are inlaid together, making full use of hafnium’s good anti-oxidation performance, low electron work function and copper’s good thermal conductivity and electrical conductivity, so that the electrode can emit the plasma arc to the maximum extent and at the same time can dissipate heat Conducted as soon as possible, so as not to be burned quickly. The connection methods of hafnium and copper include ordinary inlay, mechanical compression inlay, vacuum brazing and diffusion welding. Among these methods, vacuum brazing has become one of the methods most concerned by researchers due to its simple process, low cost and suitability for mass production. The brazing method of hafnium and copper is to weld hafnium and copper matrix into a whole. Improve its thermal conductivity and electrical conductivity, and at the same time can effectively prevent thermal expansion and cold contraction from generating gaps or oxide films, so that the life of the electrode is greatly extended.

Among the process parameters of vacuum brazing, vacuum degree is an important parameter affecting the quality of vacuum brazing. The brazing vacuum degree affects the quality of the brazed joint from two aspects: the first is the diffusion effect of the elements in the brazing seam; the second is the volatilization of the elements. In the brazing process, the vacuum degree is not as high as possible. If the vacuum is too low, the brazed components are easily oxidized at high temperature; if the vacuum is too high, it will not only waste time and money, but also may cause the volatilization of the brazing material and affect the Quality of brazed joints. In this paper, 72Ag-28Cu solder was used for vacuum brazing of copper and hafnium, and the effect of vacuum degree on the microstructure and properties of copper and hafnium brazed joints was studied.

1 Materials and methods

The purity of copper used is > 99.9%, the hafnium is commercially available industrial hafnium, and the brazing filler metal is commercially available 72Ag-28Cu brazing filler metal. Before brazing, the brazing filler metal was rolled into a foil with a thickness of 0.1 mm, and the surface of the specimen and filler metal was polished with 1200-grit water sandpaper. min. The copper, brazing filler metal and hafnium are stacked in sequence and brazed in a vacuum furnace. During the brazing process, adjust the flow of argon gas and control the degree of vacuum, the range is 5.0X 10-2~8.0 Pa, and the specific parameters are 5.0X10-2, 8.0x 10-2, 2.0X 10-1, 8.0X 10-1 and 8.0Pa. The brazing temperature is 840°C, the brazing holding time is 15 min, and the furnace is cooled to room temperature after welding. In order to study the effect of brazing vacuum degree on the microstructure and properties of brazed joints, the interface morphology of brazed joints was observed by QuantaFEG450 field emission environmental scanning electron microscope (FESEM); the shear strength of joints was observed by ZWICK-Z050 electronic universal material testing machine. carry out testing.

2 Results and Analysis

2.1 Interface morphology

After vacuum brazing, the Hf/Cu joint under the conditions of vacuum brazing temperature of 840 °C and holding time of 15 min and vacuum degree of 2.0 x 10-1 Pa was selected as the analysis object. The interface is clearly identifiable, and the brazed joint is divided into three areas: the left area is hafnium, the right area is copper, and the middle area is the solder layer and diffusion. The boundary between hafnium and solder layer is straight and clear, while the boundary between copper and solder layer is more tortuous.

2.2 Influence of brazing vacuum degree on joint interface structure

When the vacuum degree is 5.0X10-2 Pa, the connection between hafnium and brazing seam is not tight. There are even obvious ravines, indicating that the vacuum is too high, and the solder is seriously volatilized. At 10-1 Pa, the brazing seam is dense and uniform, and the joint interface connection effect is the best. When the vacuum degree is reduced to 8.0X10-1Pa and 8.0Pa, the brazing seam becomes wider and wider. It shows that the vacuum degree of brazing is low, which affects the diffusion effect of brazing elements, and the effect of brazing reaction is getting worse and worse.

2.3 Influence of brazing vacuum degree on joint performance

Table 1 shows the shear strength and fracture position of brazed joints under different brazing vacuum degrees under the conditions of brazing temperature of 840°C and holding time of 15 minutes.

It can be seen from Table 1 and the figure that when the vacuum degree of brazing is 5.0X10-2~8.0 Pa, the shear strength of the joint first increases with the increase of the vacuum degree; when the vacuum degree increases to 2.0X10-1 Pa When the degree of vacuum exceeds 2.0x 10-1 Pa, the shear strength of the joint decreases with the increase of vacuum degree. When the low vacuum degree is 8.0 Pa and the higher vacuum degree is 5.0X10-2 Pa, the shear strength value is quite different from the highest value. This shows that in the vacuum range of 5.0x 10-2~8.0 Pa, the vacuum degree of brazing has a significant effect on the shear strength of the joint.

3 Analysis and discussion

The strength of the brazed joint depends on the interface structure of the joint. In the range of brazing vacuum degree of 5.0x 10-2~8.0 Pa, the shear strength of the joint first increases with the increase of vacuum degree. This is because when the vacuum degree is too low, the diffusion of elements is not ideal, and there is no formation of Good metallurgical bonding, when the vacuum degree is increased to 2.0x 10-1Pa, the shear strength reaches the maximum value of 201 MPa. At this time, the distribution of each phase in the brazing seam is uniform, the element diffusion is good, and the joint interface is uniform and dense. When the vacuum degree exceeds 2.0X 10-1Pa, the shear strength of the joint decreases significantly with the increase of the vacuum degree. This is because the vacuum is too high, and the volatilization loss of elements is intensified, which affects the structure of the brazing joint. When the vacuum degree rises to 5.0x 10-2 Pa, obvious groove marks appear at the joint of hafnium and brazing seam. At this time, the joint between the brazing filler metal and the base metal hafnium is very poor, so the shear strength is low.

4 Conclusion

(1) In the range of brazing vacuum degree of 5.0X10-2~8.0 Pa, the effect of vacuum degree on the shear strength of the joint is significant, and the shear strength of the brazed joint increases with the increase of the vacuum degree. When raised to 2.0×10-1Pa, the shear strength reaches a maximum value of 201 MPa. When the vacuum degree exceeds 2.0×10-1Pa, the shear strength of the joint decreases with the increase of the vacuum degree.

(2) When the brazing vacuum is too low, the diffusion of elements is not ideal, and the copper and hafnium brazing joints do not form a good metallurgical bond. When the vacuum degree exceeds 2.0X 10-1Pa, the vacuum is too high, and the volatilization loss of elements is intensified, which affects the structure of the brazing seam.

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