Technical News｜Discussion on Brazing Technology of Aluminum Heat Sink

Abstract

Radiators have experienced three generations of development, namely copper radiators, aluminum fabricated radiators and aluminum brazed radiators. So far, aluminum brazing radiator has become the trend of the times, and aluminum brazing is a new joining technology in the manufacturing industry of aluminum radiator. This article mainly discusses the basic principles and general process flow of this emerging aluminum brazing technology.

Key words: aluminum brazing radiator; radiator; aluminum brazing process

Author: Qing Rujiao

Unit: Nanning Baling Technology Co., Ltd. Nanning, Guangxi

1. Advantages and disadvantages of aluminum brazing

Brazing is one of the three welding methods of fusion welding, pressure welding, and brazing. Aluminum brazing uses metal solder with a melting point lower than that of the weldment metal. Heat the solder and weldment until it is below the melting temperature of the weldment and above the melting temperature of the solder. It is a method to use liquid solder to wet the metal of the weldment, fill the thin seam of the joint and attract each other with the metal molecules of the base metal to achieve the purpose of connecting the weldment.

advantage :

1) Under normal conditions, the weldment will not be melted during brazing;

2) Multiple parts or multi-layer structure and nested weldments can be brazed at one time;

3) It can braze very thin and thin components, and can also braze parts with large differences in thickness and thickness;

4) The brazed joints of some specific materials can be disassembled and brazed again.

shortcoming :

For example: 1) The specific strength of brazing joints is lower than that of fusion welding, so lap joints are often used to increase the bearing capacity;

2) The requirements for the cleaning degree of the joint surface of the brazing workpiece and the assembly quality of the workpiece are very high.

2. The principle and process of aluminum brazing

The principle of aluminum brazing

Usually, during brazing, there is a dense oxide film on the surface of aluminum and aluminum alloy, which hinders the wetting and flow of molten solder. Therefore, in order to achieve a good brazing joint of the weldment, this layer of oxide film must be destroyed before welding. During the brazing process, when the temperature reaches the required temperature of the flux, the flux begins to melt, and the molten flux spreads on the surface of the aluminum to dissolve the oxide film as the temperature rises further. The Ai-Si alloy begins to melt, and flows to the gap to be welded through capillary movement, wets and expands to form a joint.

Although the brazing principles of aluminum radiators are basically similar, they can be divided into vacuum brazing, air brazing and Nocolok. brazing according to the brazing process. The following are some specific comparisons of these three brazing processes.

Among the three processes, Nocolok. brazing is the core process of aluminum radiator brazing process. The reason why Nocolok. brazing can now become a central part of the aluminum radiator brazing process is mainly due to the good welding quality of this product. And it has the characteristics of low energy consumption, high production efficiency, small environmental impact, and relatively strong corrosion resistance. It is an ideal brazing method.

Nocolok. Brazing Process

Cleaning

There are separate cleaning of parts and cleaning of radiator cores. At this time, controlling the temperature and concentration of the cleaning agent and keeping the temperature and concentration of the cleaning agent at a more appropriate value are the key steps in cleaning. Practical experience shows that the cleaning temperature of 40°C to 55°C and the concentration of cleaning agent of 20% are the best values for cleaning aluminum radiator parts. (Here refers to aluminum environmental protection cleaning agent, pH value: 10; cleaning agents of different models or pH levels need to be verified before use)

If there is enough flux, it is possible to braze the workpiece without cleaning, but a cleaning will result in a more coordinated process, which can reduce the amount of flux used and obtain a good-looking welded product. The cleanliness of the workpiece will also affect the amount of flux coating.

Spray flux

Spraying flux on the surface of aluminum parts is an essential process in Nocolok. The brazing process, the quality of flux spraying will directly affect the quality of brazing. Because there is an oxide film on the surface of aluminum. The oxide film on aluminum will hinder the surface wetting and the flow of molten fiber. The oxide film must be removed or pierced to form a weld.

The role of flux: 1) Destroy the oxide film on the aluminum surface; 2) Promote the wetting and smooth flow of the solder; 3) Prevent the surface from re-oxidizing during the brazing process. After brazing is completed, the flux will form a protective film with strong adhesion on the surface of the aluminum part. This layer of film basically has no adverse effect on the performance of the product, but it can greatly enhance the ability of aluminum parts to resist external corrosion.

The amount of flux attached: During the brazing process, the amount of flux attached: generally 5g of flux per square meter; 3g per square meter is also common nowadays.

Flux addition method:

  1) There are many different methods: low-pressure spraying, brushing, high-pressure spraying, dipping, electrostatic spraying;

2) The most common method of adding flux in controlled atmosphere brazing (c A B ) process is suspension spraying;

3) The physical and chemical properties of flux make wet spraying the first choice;

4) On a global scale, according to statistics: 80% use wet spray, 15% use dry spray, 5% selectively spray or pre-coat;

Wet spraying is still the most common method of fluxing in the industry and gives very good results.

Drying

In order to ensure the quality of brazing parts, the workpiece must be fully dried before brazing to remove moisture from the flux coating. The most critical thing in the drying process is to control the drying temperature and mesh speed; if the temperature is too low or the mesh speed is too fast, the core will not be dried, resulting in a decrease in brazing quality or desoldering. The drying temperature is generally between 180°C and 250°C.

Brazing

The temperature of each zone in the brazing section, the speed of the net and the atmosphere of the brazing furnace control the brazing quality. Brazing temperature and brazing time will have a direct impact on the quality of the product. Regardless of whether the temperature is too high or too low, it will have a negative impact on the product, such as reducing the service life of the product, resulting in poor fluidity of the solder, and weakening the fatigue resistance of the product; therefore, controlling the temperature and brazing time is the key to the production process.

The atmosphere in the brazing furnace is an important factor affecting the welding rate. In order to prevent the flux and the aluminum parts from being oxidized by the air, the speed of the mesh not only determines the length of the brazing time, but also determines the production efficiency. When the volume of the radiator core is large, in order to obtain sufficient heat for each zone (pre-brazing zone, heating zone and brazing zone) during the brazing process. The speed of the network needs to be slower so that the surface temperature can reach the optimum process value. On the contrary, when the volume of the radiator core is small, the speed of the network needs to be relatively fast.

3. conclusion

Radiators have experienced three generations of development, namely copper radiators, aluminum fabricated radiators and aluminum brazed radiators. So far, aluminum brazed radiators have become the trend of the times, with the continuous development and progress of technology, and the development of lightweight automobiles. Aluminum radiators have been widely used due to their strong corrosion resistance, good thermal conductivity and light weight. With the wide application of aluminum radiators, the research on the principle of brazing technology is also developing towards simplification and diversification, and brazing is an emerging welding technology in the manufacturing industry of aluminum radiators. It can be divided into two categories: no flux brazing and flux brazing. Traditional flux brazing uses chloride as flux to destroy the oxide film on the aluminum surface. However, the use of chloride flux will bring potential corrosion problems. To this end, the aluminum company has developed a non-corrosive flux called the Nocolok. method. Nocolok. Brazing is the future development trend, but Nocolok. Brazing also has certain limitations. Since Nocolok. flux is insoluble in water, it is difficult to coat the flux and needs to be dried. At the same time, the fluoride flux can react with magnesium, which limits the application of aluminum materials. Fluoride flux brazing temperature is too high. Therefore, the Nocolok. method still needs to be improved.

【references】

[1] Wu Yuchang, Kang Hui, Qu Ping. Research on Expert System of Aluminum Alloy Brazing Process [J]. Electric Welding Machine, 2009.

[2] Gu Haiyun. New Technology of Aluminum Brazed Radiator [J]. Mechanical Worker, 2010.

[3] Feng Tao, Lou Songnian, Yang Shanglei, Li Yajiang. Research on vacuum brazing performance and microstructure of aluminum radiator [J]. Pressure Vessel, 2011.

[4] Yu Honghua. Brazing process and equipment in air furnace for aluminum radiator. Electronic Technology, 2009.

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