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Research status and development of high temperature brazing technology

1.Application of high temperature vacuum brazing technology

High-temperature alloys have excellent high-temperature properties and are widely used in various engineering fields such as nuclear power, petrochemicals, transportation, energy, and aerospace. Nickel-based superalloys are mainly used in combustion chambers, high and low pressure turbine blades, turbine disks and other components in aeroengines, and are called the “cornerstone of advanced engines”. Under the action of high temperature, high pressure and erosion corrosion, hot-end components are prone to thermal fatigue cracks and thermal ablation. In order to extend the service life, the damaged parts need to be repaired. Especially for crack damage in high and low pressure turbine blades, only high-temperature brazing can be used. Repairs are carried out by welding. High-temperature brazing has now developed into an important connection technology for the manufacturing and repair of high-temperature alloy parts. With the development of aerospace technology, in order to improve the efficiency of engines, the working temperature of hot-end components is getting higher and higher. The requirements for connection technology have become higher, and welding technology has also received greater attention.

The connection temperature of high-temperature vacuum brazing is generally higher than 900°C, and the liquidus temperature difference between the solder used and the liquidus of the base metal is small. There is interdiffusion of elements between the base metal and the solder, which will affect the properties of the base metal. performance. Therefore, the type of brazing material and the vacuum brazing process have an important impact on the quality of high-temperature vacuum brazing joints.

There are many parameters that affect high-temperature vacuum brazing, including solder composition, brazing temperature, holding time and protective atmosphere, etc., which will all have an impact on the microstructure and properties of the welded joints; at the same time, with the development of computational materials science, based on first principles, etc. Research methods that combine calculation principles with experimental results to verify the rationality of high-temperature solder compositions and optimize welding process parameters have gradually received attention. This article will summarize the development of high-temperature brazing in recent years from the aspects of solder compositions, welding processes, etc. Comb.

2.1 Solder composition and preparation

During brazing, the weldments are connected in solid phase by heating the filler metal to the melting temperature and then filling the weld with liquid filler metal. During the vacuum brazing process, the properties of the filler metal and its interaction with the base metal are important factors. An important factor in the structural properties of welded joints.

2.1.1 Solder composition

High-temperature vacuum brazing weldments usually work in a high-temperature vacuum state, requiring the welded joints to have good thermal strength and thermal stability. Therefore, metal elements such as Mn, Co, and W are often added to the solder to improve Performance of brazed joints. The BNi7 nickel-based solder is improved by adding alloy element Cu. Compared with the traditional BNi7 solder, with the increase of Cu element, the improved solder not only has an obvious precursor film on the wetting front of the solder, but also improves the solder quality. It also improves the wettability of the solder and reduces the content of the hard and brittle phase formed by P, Ni, Cr and other metal elements in the solder, which is beneficial to improving the mechanical properties of the brazed joint.

In addition, some elements are often added to the solder for high-temperature vacuum brazing to lower its melting point, so that the liquidus temperature of the solder is lower than the liquidus temperature of the base metal. Currently, the common melting-lowering elements are mostly non-metallic elements Si and B wait. The addition of melt-reducing elements can significantly reduce the melting point of the solder, but it will also bring about corresponding problems, such as the large size of Si atoms, slow diffusion rate, and insufficient diffusion. It is easy to form a brittle phase. When Si is added to the nickel-based solder, When the content in is high, the toughness and plasticity of the welded joint will be reduced, resulting in a reduction in the fatigue resistance of the welded joint.

2.1.2 Preparation of solder material

In order to adapt to various vacuum brazing conditions and production and processing needs, brazing materials are often processed into strips, wires, cast rods, amorphous or ordinary foils, powders, rings, pastes, etc. In the actual vacuum brazing process, rational selection of the geometry of the brazing material can simplify the brazing process and improve the brazing quality.

2.2 Welding process parameters

Welding process parameters include vacuum brazing temperature, holding time and other aspects, which are important factors affecting the microstructure and properties of brazed joints. From the perspective of diffusion alone, the brazing process is analyzed according to the Arrhenius equation. There is an exponential relationship between the diffusion of elements during the brazing process and the brazing temperature. The higher the brazing temperature, the greater the diffusion coefficient, and the easier diffusion is. The holding time plays an important role in the isothermal solidification process of alloy materials and the homogenization of welded joints during the brazing process.

2.2.1 Vacuum brazing temperature

The diffusion of elements in the brazed joint changes differently with the change of brazing temperature, and the proportions of various compounds also change accordingly. When selecting the brazing temperature, the main considerations are the melting performance of the brazing material and the interaction with the base metal. Therefore, the welding temperature should be appropriately higher than the melting point of the brazing material, which is conducive to the melting, spreading, wetting and gap filling of the brazing material. , but too high a brazing temperature will cause changes in the properties of the base metal, such as causing the evaporation of low boiling point components, recrystallization and coarse grains of the base metal, and the rapid diffusion of certain components in the solder, resulting in the base metal at the joint Corrosion occurs, etc.

2.2.2 Keeping time

The melting-reducing elements in the solder are based on diffusion and form compounds or solid solutions to remove the melting-reducing effect. In addition to the brazing temperature, appropriate holding time needs to be combined to achieve sufficient diffusion of the melting-reducing elements to reduce the low-melting point eutectic in the brazed joint. The formation of phases can also uniformly distribute joint elements, thereby improving the overall performance of the joint.

3 Conclusions on research on high temperature brazing technology

1) The preparation of new solder materials and the improvement of solder compositions have an important impact on the development of high-temperature brazing. In addition to adding alloying elements to the composition to improve the performance of brazed joints, the most important improvement in brazing filler metal is to improve the performance of the hard and brittle phase formed by melting-reducing elements: First, the development of metal-based melting-reducing elements, such as Hf, Zr, etc. The formed product has better toughness and can reduce the stress concentration of the brazed joint, but the disadvantage is poor diffusion ability; second, melting is reduced through physical effects, such as nanoparticle solder, which is reduced through the Gibbs-Thompson effect The melting point of the solder can avoid the introduction of melting-lowering elements into the resulting low-melting-point eutectic phase. Compared with traditional solder, the new nano-solder does not need to add melt-reducing elements, has good wettability and strong diffusion ability, and can also improve high-temperature brazing efficiency and welding joint performance;

2) Vacuum brazing process parameters are still an important means to control the performance of welded joints. However, the higher the brazing temperature and holding time, the better. Especially considering the impact of interdiffusion of active elements in the base metal on isothermal solidification due to activation of active elements in the base metal, reasonable brazing process parameters need to be matched to each other to obtain the best brazed joint performance. , At this stage, the research on brazing process is no longer limited to cumulative experiments, but is gradually combined with simulation calculation material science. Especially in the related research on brazing temperature, after combining the solder phase diagram calculated by Thermo-Calc software, it can be significantly reduced. Experimental quantities, and combined with diffusion-related theories, are of guiding significance for understanding the component distribution of brazed joints; in addition, improving brazing methods is still of great significance for improving the high-temperature strength and corrosion resistance of brazed joints.