In the field of large-scale energy storage in China, liquid cooling technology has been promoted since 2021. A few domestic leading liquid cooling factories have made breakthroughs in the manufacturing process of large liquid cooling plates.

After 1-2 years of exploration by various companies, three types of energy storage cooling plates have been developed: inflated liquid cooling plates, brazed liquid cooling plates, and aluminum profiled liquid cooling plates.

Welding is a crucial process in the fabrication of water-cooled plates. Currently, welding processes for water-cooled plates mainly include electric diffusion bonding, vacuum brazing, and friction stir welding. Due to its flexible design structure and high welding efficiency, vacuum brazing liquid cooling plates are widely used in the processing of aluminum alloy products.

Today, we will discuss vacuum brazing, its advantages and disadvantages, and finally provide a detailed introduction to the principle and brazing materials used for aluminum alloy vacuum brazing.

Principle

1. Refers to heating and welding of workpieces in a vacuum chamber, mainly used for welding products requiring high quality and materials prone to oxidation.

2. Brazing belongs to solid-phase bonding, where the base metal does not melt. A lower melting point brazing material is used, heating at a temperature below the solidus line of the base metal but above the liquidus line of the brazing material.

3. The parts to be joined and the brazing material are heated until the brazing material melts. Liquid brazing material wets and spreads on the surface of the base metal, dissolves and diffuses into the base metal gaps, wetting and capillary flowing, filling and bonding with the base metal.

4. Vacuum brazing removes oxide films using different mechanisms in a vacuum environment, does not require flux, significantly enhancing product corrosion resistance.

5. Brazing material has good wettability and fluidity, allowing welding in complex and narrow passages with high yield rates and safety conditions.

Advantages

1. Multiple adjacent weld seams can be brazed simultaneously, resembling a continuous surface. Multiple components can be brazed simultaneously in a furnace, improving welding efficiency.

2. Welded products do not deform under high pressure.

3. General-purpose tooling and fixtures reduce initial investment costs without the need for special product designs.

4. Uniform heating of workpieces with low thermal stress minimizes deformation, achieving minimal residual stress and ease of machining.

5. Workpieces are under vacuum conditions, preventing oxidation, carbonization, decarburization, contamination, ensuring aesthetically pleasing brazing seams with high corrosion resistance.

6. Complex flow channels can be designed according to parameters, optimizing product heat dissipation and stability.

Disadvantages

1. Material hardness decreases after high-temperature welding, requiring reheat treatment to increase hardness, increasing costs.

2. High requirements for process during welding operation, high technical difficulty, energy consumption, and time-consuming.

3. High costs for pre-weld cleaning and environmental protection.

Principle and Brazing Materials for Aluminum Alloy Vacuum Brazing

Advantages of Aluminum Alloy

Aluminum alloy is widely used in aerospace, construction, electrical, automotive, and marine sectors due to its light weight, good corrosion resistance, excellent thermal and electrical conductivity. Its usage continues to increase and has long been considered a structural material for fabricating heat exchangers, waveguides, and many other complex components. The processing technology of aluminum alloys plays a crucial role in their manufacturing, with vacuum brazing technology without any flux being one of the important methods. Because of its many advantages—eliminating the need for complex pre- and post-weld cleaning, simplifying operations, avoiding slag inclusion in structures, not leaving brazing agents in the structure and ensuring high corrosion resistance and high productivity—the application of aluminum alloy vacuum brazing will inevitably become more widespread, considering current and future environmental protection concepts.

Principle of Aluminum Alloy Vacuum Brazing

Aluminum alloy surfaces have a dense and stable oxide film Al2O3, a major barrier to wetting the base metal with molten brazing material. It is difficult to remove oxide film solely under vacuum conditions and must be assisted by certain metal activators such as magnesium Mg, bismuth Bi, etc. Early on, it was believed that Mg could achieve film removal. This is because Mg reacts with residual O2 and H2O in vacuum, eliminating their harmful effects on aluminum; moreover, Mg also reduces Al2O3 on the base metal surface through reduction reactions. However, extensive subsequent research has shown that the base metal oxide film is not completely removed. Therefore, a new view on film removal has been proposed. In addition to eliminating O2 and H2O in the environment, Mg vapor infiltrates the subfilm layer with Si, forming a low-melting Al-Si-Mg alloy on the surface layer, disrupting the bond between the surface oxide film and the base metal, enabling wetting of the base metal with molten brazing material, floating the surface film and removing it. Although a large amount of research and experimentation has made aluminum alloy vacuum brazing widely used in industry, it is still impossible to provide a reasonable explanation for many scrapped brazed joints in actual production. Aluminum alloy vacuum brazing is sensitive to subtle variations in parameters, and although the composition of brazing alloys prefabricated by different manufacturers is within specified ranges, significant differences in brazed joint appearance often occur. To date, due to the lack of a truly accurate understanding of the mechanism of aluminum alloy vacuum brazing, the development of ideal vacuum brazing processes is greatly limited and largely based on accumulated practical experience.

Brazing Materials for Aluminum Alloy Vacuum Brazing

Most brazing alloys are based on the Al-Si system, with w(si) generally ranging from 7% to 12%. This series of brazing alloys excel in brazing properties, strength, consistency of base metal color, coating properties, and corrosion resistance. They are excellent brazing alloys, particularly since they can undergo tempering treatment, significantly enhancing the toughness and flexural strength of brazed joints. The Al-Si system with w(si) of 11.7% is a eutectic system with a eutectic temperature of 577°C, a standard brazing alloy widely used in production, suitable for brazing various high-melting-point aluminum alloys such as 3A21. New brazing alloys can be formulated by adding Mg and other elements to Al-Si brazing alloys; however, low-boiling-point elements such as Zn cannot be added to aluminum brazing alloys, as they would heavily contaminate the vacuum brazing furnace.

Brazing Materials for Vacuum Brazing

In addition, composite brazing plates are widely used, where a layer of brazing alloy is single-sided or double-sided coated on the structural base metal.

Vacuum Brazing Composite Plates are much more convenient to use than wires, strips, or powders, making them ideal for fabricating complex components. When the coating metal on the brazing plate melts, it can directly wet and bond with the base metal, immediately filling gaps. Joints require only slight diffusion to form. To lower the melting point of aluminum brazing alloys, Al-Si-Ge system brazing alloys have been developed; these alloys can lower their melting point to 423°C and offer excellent fluidity and spreading properties. However, the addition of Ge significantly increases the brittleness of brazing alloys, deteriorates corrosion resistance, and darkens the color, making them impractical as vacuum brazing alloys. Studies have shown that the performance of these alloys can be significantly improved through tempering with elements such as Na, Sr, and La, making them meaningful alloy brazing alloys with excellent brazing properties. Further research could lead to brazing alloys with better comprehensive performance. Additionally, literature has reported Al-Si-Ge brazing alloys; the phase diagram of this system shows a eutectic point with a crystalline eutectic point, w(Si)=13%, w(Be)=0.5%. The selected composition includes a crystalline eutectic point, with a temperature of 571°C, 6°C lower than the Al-Si eutectic. After joint tempering with Sr and La, the strength of the brazing alloy significantly improves. However, its melting point remains relatively high and is unsuitable for brazing high-strength aluminum alloys such as 6061. If the melting point can be further reduced, the brazing alloys in this system would have promising applications. Currently, although some low-melting-point vacuum brazing alloys suitable for laboratory use have been developed, brazing alloys with certain corrosion resistance and good mechanical properties have yet to meet industrial production requirements.

Lori is a professional manufacturer of vacuum brazing water-cooled plates with extensive thermal management experience. We can provide customized water-cooled plate services for you. Contact me if you are interested!