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Vacuum brazing in automotive heat exchangers

1. Basic features and characteristics of vacuum brazing technology
Vacuum brazing technology can be simply understood as a welding technology in which the base material (the metal and non-metal to be connected) does not melt and the brazing material (the connecting metal, also known as solder) melts, which is different from the general melting of the base material and solder. Both melt and pressure welding base materials melt, so vacuum brazing technology can be considered a precision connection technology. Some data show that the dimensional deviation of vacuum brazing connection technology is less than 0.02mm, and complex parts can be connected. Vacuum soldering technology has high production efficiency. Multiple welds can be completed at one time. In the electronics industry, vacuum soldering technology (wave soldering, reflow soldering) can be used to complete thousands of solder joints at one time. At the same time, vacuum brazing technology also has the disadvantage of generally low strength. Through a preliminary understanding of the characteristics of vacuum brazing technology, vacuum brazing method should be given priority for connection workpieces that require precision, small size, and complex structure.
In the vacuum brazing process of automobile heat exchangers, hundreds of welds between the heat dissipation belts and heat dissipation pipes are also connected in one go through the vacuum brazing furnace, which has high production efficiency.

2. Classification of vacuum brazing methods
According to different characteristics and standards, vacuum brazing methods can be divided into various types. In summary, there are mainly the following classification methods:
1) Classification according to melting point of solder
Brazing can be divided into two categories. When the melting point (or liquidus line) is lower than 450°C, it is called soldering, and when its temperature is higher than 450°C, it is called brazing.
2) Classification according to the type of substrate to be connected
Aluminum brazing, stainless steel brazing, ceramic brazing, composite material brazing, copper brazing, etc.
3) Classification according to differences in environmental media
Gas shielded brazing, vacuum brazing, etc.
4) Classification according to the method of removing the oxide film on the surface of the substrate
Flux brazing, flux-free brazing, self-flux brazing.
The commonly used vacuum brazing method for automotive heat exchangers is aluminum brazing in a nitrogen-protected furnace, which is hard vacuum brazing.

3. Reflection of the key points of vacuum brazing in the production of automotive heat exchangers

a. Vacuum brazing process
During vacuum brazing, the flux (flux: a solvent used during vacuum brazing) is used to remove the oxide film on the surface of the brazing material and the base material, and to protect the vacuum brazed parts from re-oxidation during the vacuum brazing process. The melting point of the liquid solder (which enhances the wettability and spreadability of the liquid solder to the base material) is lower than that of the solder. The flux flows into the gap between the base material after heating and melting, and at the same time, the melted flux interacts physically and chemically with the surface oxide of the base material. , thereby cleaning the surface of the base metal (the process of removing the oxide film) and creating conditions for the solder to fill the gaps. As the heating temperature continues to increase, the solder begins to melt, wet, and spread. While the solder removes flux residue and fills the gap between the base material, physical and chemical interactions occur between the melted solder and the solid substrate. When the solder fills the gap and is kept warm for a certain period of time, it begins to cool and solidify, forming a vacuum brazing joint, thereby completing the entire vacuum brazing process. To sum up, the entire vacuum brazing process should be summarized into several processes: flux removal, solder melting, solder wetting and spreading, solder filling, and physical reaction between the solder and the base material to form a vacuum brazing joint.

b. Key points of vacuum brazing
The fundamental problem in vacuum brazing is how to obtain a high-quality joint. Such a joint can only be obtained if the liquid solder can fully flow into and densely fill all the soldering gaps and interact well with the base material. Not any molten flux or solder material can smoothly fill the gap between any welding parts. Therefore, to ensure an excellent joint, the following points must be strictly considered during the actual vacuum brazing operation. Key points:
(1) Wetting and spreading of the base metal by the brazing filler metal
What is the wetting and spreading of the base metal by the solder? There are many examples of this in nature. For example, if a drop of water is dropped on a clean glass plate, the water drop can completely spread on the glass plate. At this time, it can be said that the water completely wets the glass plate; if the drop is If you drop a drop of oil, the oil drop will form a ball and spread to a limited extent. At this time, it can be said that the oil drop can wet the glass plate; if you drop a drop of mercury, the mercury will form a sphere rolling on the glass plate. At this time, This shows that mercury does not wet glass. The same principle applies to the wetting and spreading of the base material by the solder. When the solder melts on the base material without adding flux, the solder rolls in a spherical shape. At this time, the solder does not wet the base material; when adding flux, , the solder will spread on the base material, which means that the solder (solder) is wetted and spread on the base material.
(2) Capillary filling process of liquid solder in the gaps between parts
Capillary action is the inherent characteristic of liquids flowing in narrow gaps. There are many such examples in real life. For example, if two parallel glass plates or a clean tube with a very small diameter are inserted into a certain liquid, two phenomena will occur when the liquid is between the plates or in the thin tube: one is the liquid The liquid rises along the gap or small inner diameter to a certain height h above the liquid level; the other is that the liquid drops along the gap or small inner diameter to a certain height h below the liquid level. This phenomenon is called “capillary action”.
(3) Influence of joint gap size
The size of the joint gap has a considerable impact on the capillary filling effect of the solder, the density of the solder joint, and even the strength of the joint. When other process conditions are determined, the smallest feasible gap should be selected to obtain the joint with the maximum strength. At this time, the capillary effect is significant and the liquid metal easily fills the entire joint gap, thereby obtaining a joint with fewer void defects.
(4) Heating and cooling of weldments
The vacuum brazing operation process starts from heating, reaches a certain temperature and stays there, and finally cools to form a joint. In this process, the most important process parameters involved are the vacuum brazing temperature and holding time, which directly affect the filler joints of the solder and the interaction between the solder and the base material, thus determining the quality of the joint. In addition, heating speed and cooling speed are also important process parameters, and they also have a non-negligible impact on the quality of the joint.
(5) Interaction between solder and base material
During the vacuum brazing process, the liquid solder interacts with the base material while capillaryly filling the seams. This interaction includes two aspects: one is the dissolution of the base material into the liquid solder; the other is the components of the solder. Diffusion into the substrate.
During the vacuum brazing process, these five points affect the quality of vacuum brazed parts. Vacuum brazing workers should have a full understanding of these five points and be able to apply them flexibly. Only in this way can we obtain high-quality vacuum brazing joints and improve the vacuum brazing quality of the product.
When it comes to vacuum brazing production of automotive heat exchangers, several aspects that should be paid special attention to include: vacuum brazing temperature, flux dosage, nitrogen flow, surface cleaning, core drying, tooling, heat sink wave height, heat pipe and heat dissipation Belt, core assembly, heating speed, cooling speed, holding time, etc. In order to obtain a qualified vacuum brazing joint, special attention should be paid to the control of these aspects.