Vacuum Brazing Process of HNSAE18202 Battery Cooler

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1.The Mechanism and Process Characteristics of Vacuum Brazing

When aluminum alloy is brazed, the oxide film on its surface affects the wetting and flow of the liquid solder. During the brazing process, if the oxide film on the surface cannot be effectively removed, it is difficult to form a high-quality brazed joint. In order to remove the oxide film and prevent the aluminum from forming an oxide film during the brazing process, a small amount of magnesium is added as an activator in the solder, which can effectively promote the fragmentation and dispersion of the oxide film on the surface of the part, and the wetting and spreading of the liquid solder.

The vacuum brazing of aluminum alloy is a complicated process, which includes both physical changes and chemical changes. Because the linear expansion coefficient of aluminum oxide film is only about one-third of that of aluminum. According to calculations, when the temperature is higher than 400℃, the thermal stress is enough to cause the alumina film to crack locally, exposing the pure aluminum surface. The magnesium in the brazing filler metal begins to volatilize in large quantities above 550°C.

A magnesium-containing atmosphere is formed in the heating chamber space to protect the exposed aluminum surface from oxidation. On the other hand, the surface of the aluminum alloy is penetrated through the gaps in the aluminum oxide film to form an alloy, which is partially melted, and further digs into the oxide film. The liquid solder produces wetting and flow on the fresh surface without oxide film, and fills the weld to form a weld corner under the action of capillary force.

From the above analysis, it can be seen that the brazing of aluminum alloy must be carried out in a high vacuum state, generally in the order of 10-3 Pa. If the vacuum degree is low, the magnesium will immediately combine with the oxygen and water in the environment after evaporation, and a protective magnesium atmosphere cannot be formed, so that the exposed fresh aluminum surface is immediately oxidized, and the oxide film cannot be removed, causing the liquid solder to be unable to wet and flow.

For heat exchanger products, the surface area of the workpiece is large, and the amount of adsorbed gas is also a lot. A large amount of gas will be released during the heating process, which will reduce the vacuum degree. In order to maintain a positive vacuum, a vacuum unit with a higher pumping speed must be selected.

The melting point of Al-Si-Mg brazing filler metal is close to that of aluminum, so its brazing temperature range is very narrow, generally between 595 and 615°C. For heat exchanger structures-the general size is large and the spatial distribution is very unbalanced, and most of them are thin-walled materials, so the temperature uniformity of the hot zone is very strict, generally not exceeding 5C, Therefore, the temperature needs to be controlled in zones.

2. Vacuum Brazing Process Technology

In order to ensure that the workpiece is heated evenly, a staged heating is adopted. After the vacuum brazing furnace is evacuated, there are three stages of heating and heat preservation. The process curve is shown in Figure 1. Namely: the first stage (a) preheating, constant temperature, and heat preservation; the second stage (b) energy storage, constant temperature and heat preservation; and the third stage (c) brazing, constant temperature and heat preservation; power failure. After the furnace temperature drops to the specified temperature, it is released. It can be seen from the curve that the key parameters of vacuum brazing include vacuum degree, heating rate, holding time, brazing temperature, brazing time and cooling rate.

Fig 1  Furnace temperature of vacuum brazing furnace

According to the collection of vacuum brazing related data, the product structure and the analysis of historical test data, the welding parameter table 1 is formulated, and the product is placed in the vacuum brazing furnace. Adjust the final heating parameters and holding time, adjust the holding temperature from 602°C to 596°C, and extend the holding time from 28min to 30min. After welding, it was found that there was no external leakage, and there were individual pores in the dissection and metallographic analysis.

Heat up/1℃	T1/min	Keep warm/2℃	T2/min	Heat up/3℃	T3/min	Keep warm/4℃	T4/min	Heat up/5℃	T5/min	Keep warm/6℃	T6/min
Room temperature/-400	40	400	40	400-500	20	560	40	560-602	20	602	28

Table 1  Welding parameters

3. Measures to improve generalization

In order to ensure the welding quality of the product, the above work needs to be done:

1). Confirm the correctness and rationality of the selection of parts and materials;

2). Ensure the dimensional tolerance requirements of the parts, thereby reducing the assembly gap;

3). Strictly follow the process procedures for material preparation, surface treatment and assembly;

4). Optimize and strictly control the brazing temperature, holding time, vacuum degree and other process systems in practice.

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