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Effect of filler metal on vacuum brazing of cemented carbide and steel

Vacuum brazing

Brazing is the most traditional connection method for welding carbide and steel, and its joint performance mainly depends on the choice of filler metal. Therefore, current research mainly focuses on the selection and research and development of solder, among which the most commonly used solders are copper-based solder, nickel-based solder and silver-based solder.

The role of solder

Cu-based brazing material has good plasticity and toughness, can wet WC-Co cemented carbide well, and matches well with the thermal expansion coefficient of steel. The residual stress between Cu alloy and steel during vacuum brazing can be almost Negligible, it has attracted close attention from academia and industry. Compared with pure Cu, Cu-based solder containing Sn, Zn, Al and other alloying elements has better wettability to the matrix, and the formed Fe-Co-based solid solution improves the interfacial bonding strength of the brazed joint, thereby obtaining mechanical properties. Excellent performance welded joints. Among them, CuZn-based solder has small surface tension and viscosity on the surface of the weldment material, and the shear strength of the resulting joint can reach 412 MPa. However, when a large amount of Fe and Cr elements in the steel diffuse into the Cu-based solder, a large amount of y-austenite solid solution phase will be formed. The elastic modulus of this phase is much higher than that of Cu(s,s), which will cause The stress is concentrated in the joint area. Therefore, the use of functionally graded WC-Co/Ni composition (FGWC) cemented carbide and steel for vacuum brazing is beneficial to inhibit the diffusion of Fe and Cr into the bonding area, thereby reducing the residual stress of the joint. In addition, adding elements such as Co and Ni to Cu-based solder can also improve the performance of the joint. On the one hand, the Co in the solder can supplement the loss of the binder phase Co in the cemented carbide during the brazing process. On the other hand, Co, The addition of Ni is beneficial to the formation of Fe-Co-Ni single-phase solid solution and reduces the residual stress of the joint. It can be seen that Ni-containing filler metal can better improve the performance of brazing joints and promote the widespread use of Ni-based filler metal in the brazing process of cemented carbide and steel.

Ni-based filler metal has high brazing temperature and good plasticity. It is completely miscible with steel and can better buffer the thermal stress generated during the welding process. It is one of the preferred filler metals in the vacuum brazing process of cemented carbide and steel. Compared with using Cu foil as the brazing intermediate layer material, the Ni foil intermediate layer can not only significantly reduce the residual stress of the joint, but also greatly increase the joint strength, showing excellent comprehensive performance.

A certain thickness of Ni intermediate layer is added to the Cu-based solder to form a sandwich structure layer, and the bending strength of the resulting brazed joint is increased by about 27%. Using a Cu and Ni stack to fill the solder, in which the Cu alloy is in contact with the steel and the Ni alloy is in contact with the cemented carbide, can minimize the formation of brittle intermetallic compounds on the interface. In addition, electroplating Ni as an intermediate layer on CuZn alloy solder can promote interdiffusion between elements, form a solid solution layer, and improve the bonding strength of the joint. Usually, Ni-based solder is only suitable for situations where the brazing temperature is high. When the brazing temperature is low, Ag-based solder can be used.

Ag-based solder has a relatively low melting point and good plasticity, and has good affinity with cemented carbide. It can effectively reduce the residual stress of welded joints and is one of the commonly used solder materials. Adding Ni and Co elements to Ag-based solder metal, on the one hand, the N element can improve the wettability of the silver-based solder metal to cemented carbide, on the other hand, the addition of Co can inhibit the dissolution of Co in the cemented carbide base material, thereby Reduce the width of the Co diffusion zone on the WC-Co interface and inhibit the growth of n-phase in the weld, thereby improving the bonding strength of the joint. In addition, adding Cu, Zn, Mn and other elements to Ag-based solder can also improve the mechanical properties of the joint.

When vacuum brazing was used to connect WC-10Co cemented carbide and AISI 1020 steel, it was found that the weld structure was dominated by copper-rich phase, silver-based solid solution and Ag-CuZn eutectic structure. The results show that with the increase of heat input, the Cu(s,s) content decreases, the Ag-CuZn eutectic structure coarsens, and the copper precipitates have higher hardness and elastic modulus, showing obvious dispersion strengthening effect.

Effect of vacuum brazing filler metal

In summary, it can be seen that for vacuum brazing of cemented carbide and steel, selecting the appropriate brazing material can improve the overall performance of the welded joint to meet the requirements in practical applications. However, due to the large performance differences between cemented carbide and steel, the interface reaction of the brazing joint is difficult to control, especially the formation and growth of the brittle n-phase, which brings great difficulties to the selection of brazing filler metal. Traditional copper-based and silver-based solders have low melting points, which limits the use temperature of soldered joints. Although nickel-based solder has a high melting point and can greatly improve the mechanical properties of brazed joints at high temperatures, excessively high brazing temperatures will affect the properties of the base metal and thus the performance of the joint. Among various brazing methods, improving the wettability of filler metal and reducing the stress of the brazed joint between cemented carbide and steel still need to be studied. Therefore, the brazing of cemented carbide and steel is generally used for specimens with relatively simple shapes and low joint performance requirements. When the joint performance requirements are high, other process measures need to be used to achieve the connection between them.