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Aluminum alloy vacuum brazing of heat exchangers

Aluminum alloys have been widely used in the field of heat exchangers due to their high specific strength, excellent heat transfer performance, abundant reserves and low cost. The all-aluminization of heat exchangers has become an inevitable trend of development, and its manufacture must be completed by vacuum brazing. In the process of vacuum brazing, the melting point of the solder is much lower than that of the core material due to the large amount of elements that reduce the melting point (usually Si element) in the solder. The brazing temperature is generally higher than the liquidus of the solder and lower than the solidus of the base aluminum alloy. During brazing, the solder melts under the action of capillary force, and the liquid solder flows to the lap joint to form a solid metallurgical joint. During brazing, the melted brazing filler metal will corrode the core material, reducing the mechanical properties of the core material and the fluidity of the liquid brazing filler metal, which will affect the mechanics, corrosion performance and welding effect of the welded product.

1 Conditions and mechanisms of erosion

Erosion was originally only used to describe the dissolution of the core material, and later it was also used to describe grain boundary penetration and liquid film movement. Since these phenomena occur in the interaction of liquid-solid phases, this paper does not make a conceptual distinction and collectively call these phenomena as erosion.

1.1 Conditions for erosion

Taking the brazing of 4×××/3××× composite brazing aluminum alloys at 600°C as an example, Fig. 1 shows the local phase diagram of Al-Si. When the silicon concentration of the solder is greater than the limit solid solubility (point A in Figure 1) at 600 °C, the solder begins to melt. Due to the large concentration gradient of silicon in the brazing filler metal and the core material, Si diffuses into the core material. When the silicon concentration in some places in the core material exceeds the limit solid solubility, melting occurs, which is the generation of ablation. The level of silicon concentration in the solder will have an important impact on the erosion mechanism.

2.Vacuum brazing process system

1) Brazing temperature and holding time

The brazing temperature can directly and effectively affect the erosion process. The general brazing temperature of 4×××/3××× series composite materials is 595℃-605℃. The brazing temperature is an important factor affecting the diffusion rate of Si and the fluidity of the solder. When the brazing temperature is too low, the solder sometimes cannot be completely melted and the fluidity of the solder is affected, resulting in poor brazing effect; the brazing temperature is too high. The accelerated diffusion rate of high Si leads to aggravated the erosion process of the brazing filler metal to the core material, resulting in low post-weld strength of components, reduced service life and even serious phenomena such as erosion and perforation during brazing, resulting in direct scrapping of components. The longer the brazing holding time, the more fully the erosion process is carried out, and the deeper the penetration of the brazing material into the core material, the more serious the erosion is.

2) Heating rate

Some works show that the heating rate during brazing has a significant effect on the recrystallization process of the deformed material, which in turn affects the progress of the erosion process during the brazing process. This is mainly because the heating rate can change the sequence of the recrystallization process and the precipitation precipitation process. If the heating rate is slow, the precipitation precipitation process occurs first, otherwise, the recrystallization process occurs first. Because the size of the precipitated phase is mostly below the micron level, the precipitated phase generally inhibits the recrystallization process, so the slow heating speed will inhibit the recrystallization process, and it is easy to form coarse recrystallized grains, which is beneficial to the control of ablation, and the heating speed is fast. , recrystallization occurs first, generally forming a fine grain structure, resulting in aggravation of erosion.

3 Conclusion

The erosion phenomenon is a key problem in the vacuum brazing of aluminum alloys. With the overall aluminumization and compactness of heat exchangers, the requirements for materials are getting higher and higher. In order to improve the service life and strength of materials, and obtain better brazing effect, the application of aluminum alloy materials for brazing is further expanded. The field must better control the erosion and avoid the influence of erosion on the material to the greatest extent, mainly in the following aspects:

1) Further study the mechanism of erosion, and provide a solid theoretical basis for the control of erosion;

2) Optimize the alloy composition design, make full use of the potential of each element, and develop more excellent anti-corrosion alloys;

3) Explore the influence of each process link of production (such as casting, homogenization, hot rolling, cold rolling and brazing system, etc.) alone or in combination on the influence of the alloy brazing process on the erosion process, and establish factors such as composition, process and structure The relationship with erosion in turn mitigates erosion.

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