Battery cooler vacuum brazing

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Vacuum brazing is to heat and heat the structural parts in a vacuum state, so that the brazing filler metal melts within a suitable temperature and time range, and fully infiltrate, dissolve, diffuse and weld with the solid metal under the action of capillary force, so as to achieve An advanced welding method for welding purposes.

Mechanism and process characteristics of vacuum brazing

When the 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, adding a small amount of magnesium as an activator to the solder can effectively promote the breakage and dispersion of the oxide film on the surface of the parts, and the wetting and spreading of the liquid solder.

Vacuum brazing of aluminum alloys is a complex process that includes both physical and chemical changes. Because the linear expansion coefficient of aluminum oxide film is only about one third of that of aluminum. According to calculations, after the temperature is higher than 400 °C, the thermal stress is enough to locally crack the aluminum oxide film, exposing the pure aluminum surface. The magnesium in the solder begins to volatilize a lot 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, it penetrates into the surface of the aluminum alloy through the gap on the aluminum oxide film to form an alloy, which melts locally and further excavates the oxide film. The liquid solder creates wetting and flow on the fresh surface without oxide film, and fills the weld to form a fillet under the action of capillary force.

It can be seen from the above analysis that the brazing of aluminum alloys 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 be combined with oxygen and water in the environment immediately after filtration, 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, resulting in the liquid solder not being wet. and flow.

For heat exchanger products, the surface area of ​​the workpiece is large, and the amount of gas adsorbed is also large. During the heating process, a large amount of gas will be released, which will reduce the vacuum degree. In order to maintain a positive vacuum, it is necessary to choose a vacuum unit with a larger pumping speed.

The melting point of Al-Si-Mg solder 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 size is generally large and the spatial distribution is very unbalanced, and most of them are thin-walled materials, so the temperature uniformity in the hot zone is very strict, generally not exceeding

±5 °C, so Zoned temperature control is required.

Vacuum Brazing Process

Selection of parts material

The workpiece to be brazed is a battery cooler. The parts are divided into main board, side board and pressing block. The main part is the main board, as shown in the figure.

The alloy material of the brazing layer of vacuum brazing contains 1%-2% of Mg, and adding Bi element can improve the wettability of the brazing filler metal and reduce the requirement for the vacuum degree of brazing, so the alloy material of the brazing layer is 4104, the mother Material selection 3003mod.

Cleaning of workpieces before brazing

Before brazing, the oil stains on the surface of the brazing filler metal and aluminum must be removed, but the caustic sodium aqueous solution cannot be used for degreasing. Because the magnesium in the solder is easy to interact with the carbon, it affects the vacuum degree, increases the consumption of the getter metal, and causes the flowability of the solder to decrease. Therefore, ultrasonic cleaning is used, and the cleaning process is shown in the figure:

Assembly positioning of workpieces

In order to ensure the relative position of the workpiece and the good contact of the joint during the vacuum heating process, a stainless steel fixture should be used for positioning. This fixture has a certain high temperature elasticity and can also ensure the close contact of the joint at the temperature of brazing. When assembling the positioning fixture, pay attention to controlling the assembly gap at the joint. The size of the gap will directly affect the capillary action of the caulking process and is an important factor that must be considered. Assembly clearance is generally 0.025-0.635mm.

We have adopted bolt connection clamping structure and clip clamping structure. In order to prevent the contamination of the product and the vacuum furnace by the fixture, we polished and cleaned the fixture before clamping. After assembly, measure the height of the four corners of the core body, and adjust the nut pre-tightening force to prevent the core body from being deformed after being released.

Vacuum brazing process technology

In order to ensure that the workpiece is heated evenly, stage heating is adopted. After the vacuum brazing furnace is evacuated, three stages of heating and heat preservation are carried out, and the process curve is shown in the figure. Namely: the first stage (a) preheating to set temperature and heat preservation; the second stage (b) energy storage to set temperature and heat preservation and the third stage (c) brazing to set temperature and heat preservation; power failure. Wait for the furnace temperature to drop to the specified temperature.

Among them, the first heating rate was 40°C/min, the temperature was kept at 400°C for 90 minutes, and the second heating rate was 40°C/min, and the temperature was kept at 56°C for 60 minutes. The last temperature was raised at 20°C/min, and the temperature was kept at 596°C for 20min.

Welding Quality Assurance

Surface Pretreatment of Structural Parts All structural parts of the heat exchanger must undergo surface treatment, acid-base washing or ultrasonic cleaning before assembly to remove surface dirt, grease, oxide film, etc. Dirt will hinder the effective contact between components; oil stains will decompose and gasify at high vacuum temperatures; reduce the vacuum degree in the vacuum brazing furnace; due to the dense oxide film on the surface of the aluminum alloy, its melting temperature is much higher than that of the base material.

In particular, the oxide film of the brazing filler metal layer of the composite plate is not fully melted during brazing, resulting in the inability to fully fuse with the metal to be welded, thereby affecting the quality of brazing. To this end, the surface preparation of the raw materials must be strictly controlled, including the necessary mechanical cleaning, while reducing the assembly time before brazing.

Structural Dimension Tolerance

Each structural part must not only ensure its own size, but also ensure the assembly clearance. Because the main board and side board are mold parts, the design and debugging requirements for the mold are strict. Otherwise, the assembly gap is large, and it is easy to cause the phenomenon of virtual welding, discontinuous brazing or non-welding.

Grip force

Since the heat exchanger is clamped with stainless steel clamps after assembly, and the thermal expansion coefficient of the stainless steel clamps is smaller than that of aluminum alloy products, the clamping force is too large, which is easy to cause deformation of the core body after brazing. Therefore, the appropriate clamping force should be set according to the structural design of the specific heat exchanger.

In addition, the vacuum degree of the vacuum furnace, the material of the parts, and the process parameters all have an important impact on the welding quality of the product.

Improvements

In order to ensure the welding quality of the product, it is necessary to do the above work:

1.Confirm the correctness and rationality of the material selection of the parts;

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

3.Carry out material preparation, surface treatment and assembly in strict accordance with the process procedures;

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

Equipment selection: The RHVB series vacuum brazing furnace produced by SIMUWU is a high-quality product for the vacuum brazing process of tools and molds. The good temperature control accuracy and temperature control uniformity ensure the effective progress of the vacuum brazing process. SIMUWU specializes in the manufacture of vacuum furnaces, has more than ten years of relevant experience, and has a good reputation in the field of vacuum furnace manufacturing. The product line includes vacuum air quenching furnace, vacuum oil quenching furnace, vacuum brazing furnace, etc., which are widely sold in developed and developing countries.