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Influence of Vacuum Brazing Process on Microstructure of Tungsten and Steel

Tungsten has the characteristics of high melting point, high hardness, high wear resistance and low vapor pressure. It is not only a rare metal, but also a strategic metal. It has a wide range of applications, such as hard alloys, high-strength wear-resistant alloys for aerospace rockets, alloys for contact materials, and high-density alloys. In production applications, it is often necessary to connect tungsten and other dissimilar metal materials together to form a weldment of tungsten and metal to achieve complementary performance. Steel is the most widely used ferrous metal in the industry. It has good weldability, thermal stability, and mechanical properties. The application prospects of tungsten/steel composite structural parts are very broad. Due to the large difference in physical and chemical properties such as the melting point and thermal expansion coefficient of tungsten and steel, it cannot be welded by fusion welding, and defects such as pores and cracks are easy to occur in the joint. Usually, vacuum brazing can be used to realize the connection of tungsten and dissimilar materials. Vacuum brazing has obvious advantages in product precision, production efficiency and cost.

The research process of the effect of vacuum brazing process on the structure of tungsten and steel

The materials used in the test are 20# steel and pure tungsten sheet, and the solder is made of pure copper thin solder sheet. Use a grinding wheel cutting machine to cut the tungsten rod into tungsten flakes with a size of 5 mm×1.0 mm, and then use 600#, 1000#, 1200# sandpaper to grind and polish the vacuum brazing surface in order to remove the processing lines and oxidation on the surface layer. The pure copper brazing filler metal is in the form of flakes with a thickness of 0.1 mm. All raw materials need to be cleaned with acetone on the surface, degreased and decontaminated, and finally cleaned with alcohol, dried and vacuum packed for later use.

The test uses a SIMUWU vacuum brazing furnace for brazing to obtain the required test samples. The stacking sequence of the parts during the brazing process is shown in the figure. In order to reduce the stress of welded joints, copper sheets are used as the connecting layer. Considering the melting point of the solder itself and the characteristics of the test materials, the test vacuum brazing temperature is 1120-1200 ° C, and the vacuum brazing time is 10-30 min. Under the above process conditions, the vacuum brazing test was carried out on the pretreated tungsten sheet and 20# steel to obtain the required tungsten/steel weldment.

According to the comprehensive test and analysis, when the vacuum brazing temperature is 1150°C, with the prolongation of the holding time, the melting degree of the copper solder is getting higher and higher, gradually wetting the surface of the tungsten sheet, and entering into the tungsten sheet through capillary action. in micropores. When the holding time is 20 min, the w-cu interface can form a dense layer with “serrated” mechanical occlusal layer, and the fracture of the weld shows a large number of dimples, and its shear strength is the highest. When the vacuum brazing temperature is further increased or the holding time is prolonged, due to the overburning of the brazing filler metal layer, the brazing filler metal will easily overflow the surface of the workpiece, resulting in serious loss of the brazing filler metal in the weld seam, which is not conducive to the adhesion of the brazing filler metal to the surface of the tungsten sheet. The wetting effect causes the fracture of the weld to appear as a plane tear or a river-like brittle fracture structure, and finally the shear strength of the entire weldment is relatively low.

Vacuum brazing process conclusion

(1) The tungsten/steel weldment was successfully prepared by the vacuum brazing process. The surface of the weldment is smooth and has a good interface with the low carbon steel matrix. , the interface exhibits a mechanical occlusal structure.

(2) The change of vacuum brazing temperature and holding time has a significant effect on the microstructure of the tungsten/steel interface. When the vacuum brazing temperature is 1150℃, as the holding time prolongs, the melting degree of the solder becomes higher and higher, and more and more cu solder diffuses to the interface. When the holding time is 20 min, a dense mechanical occlusal layer with a “serrated” structure can be formed at the tungsten/copper interface. When the vacuum brazing temperature is further increased or the holding time is prolonged, the solder is prone to overburning, causing the solder to overflow the surface of the workpiece, resulting in serious loss of the solder in the weld.

(3) The difference in mechanical properties of different tungsten/steel weldments determines that the fractures present different microstructures. When the vacuum brazing temperature was 1150 ℃ for 20 min, a large number of shear-type elongated dimples appeared in the fracture, and the shear strength reached a maximum of 268 MPa. And when the fracture of the weldment is a plane tearing brittle fracture structure or a river-like cleavage structure, its corresponding mechanical properties are also poor.