Vacuum Brazing Technology of Tube-and-Strip Heat Exchanger and Factors Affecting Brazing Quality

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The vacuum needle brazing process is one of the key technologies in the manufacture of automotive air-conditioning tube-and-belt heat exchangers. Only by strictly controlling various factors affecting the quality of vacuum brazing can we ensure stable product quality.

The commonly used materials for tube-and-belt heat exchangers that have an important influence on the brazing quality are composite aluminum foil, composite aluminum tube and composite aluminum plate. These materials adopt a sandwich structure with a core material in the middle, generally using AA3003, double-sided composite aluminum-silicon-magnesium alloy brazing material, called skin material, as shown in Figure 3. The single-sided composite rate of the leather is as follows: the composite aluminum foil is generally 12% to 18%, the composite aluminum tube is generally 8% to 12%, and the composite aluminum plate is generally 5% to 10%.

The commonly used brazing materials (skin materials) for vacuum brazing are as follows: AA4004, AA4005, AA4104. The most commonly used brazing filler metal is AA4004, whose solidus temperature is 559 °C, liquidus temperature is 591 °C, and the chemical composition of the main alloying elements is Si: 9.0% ~ 10.5%, Mg: 1.0% ~ 2.0%. Suppliers can appropriately adjust the composition according to user requirements. Table 2 shows the chemical composition of AA4004 solder provided by several companies. Due to the evaporation of magnesium during vacuum needle welding, the solidus temperature of AA4004 will increase to 577℃.

Factors affecting the quality of vacuum brazing and their solutions:

The common defects of vacuum brazing of tube-and-belt heat exchangers are mainly as follows:

(1) the brazing angle of the brazed serpentine flat tube and the fin is not full;

(2) the brazing seam between the serpentine flat tube and the liquid inlet header or return gas header Leakage at the place;

③ leakage at the brazing seam between the liquid inlet header or the return gas main pipe and the plug;

④ the fins are bent and lodged;

⑤ the solder is lost;

Oxide film on raw material surface

According to relevant information, in order to make the liquid solder have good wettability and fluidity, the thickness of the oxide film on the surface of the aluminum alloy should not exceed 3x 10-5mm. In general, the thickness of the oxide film on the surface of the raw material is less than 1×10-5mm, which has no adverse effect on the quality of brazing. If the storage is not good, the thickness of the oxide film on the surface of the raw material will still affect the brazing quality.

cleaning

Under normal circumstances, if the production site is clean and volatile light oil is used in the processing of parts, the flat tube and fins can be brazed without cleaning, and the degreasing system of the vacuum brazing furnace can be used for vacuum degreasing. In order to ensure the quality of brazing, the vacuum brazing of tube-and-belt heat exchangers generally uses organic solvent vapors such as trichloroethane and trichloroethylene for cleaning and degreasing, and other cleaning methods are rarely used. However, when brazing the serpentine flat tube and the composite aluminum tube, the composite aluminum tube and the plug, it is easy to cause leakage. It is recommended to wipe it with ethanol or acetone before entering the furnace for assembly.

Parts Processing

When machining parts, the critical dimensions and assembly clearances of the brazed parts of the following parts must be guaranteed. ①The size of the elbow after shaping the serpentine flat tube; ②The size of the shrinkage at the brazing point of the serpentine flat tube and the composite aluminum tube; The punching size of the weld; ⑤ the plug forming size (the assembly gap between the plug and the composite aluminum pipe).

assembly

It is beneficial to obtain good vacuum brazing quality by ensuring the brazing gap at the brazing part during assembly. Since the composite layer (skin material) on the surface of the fins plays the role of clearance during brazing, the fins and the flat tube are brazed with no gaps. (It may cause local erosion of fins, too large solder joints at plugs, etc.). The brazing gap of other parts is mainly guaranteed by the processing size of the parts, generally not greater than 0.05 mm.

Regardless of whether it is clamped with a clamp or bundled with steel strips during assembly, thermal expansion during brazing should be considered. When the assembly clamping force is too large, it is easy to cause defects such as bending and lodging of the fin after brazing and the size exceeds the negative deviation; if the assembly clamping force is too small, it is difficult to ensure the assembly gap between the fin and the flat tube, which is easy to cause the fin at the brazing part. Defects such as virtual welding and loosening.

Skin material (solder) compound rate

Composite aluminum foil, composite aluminum plate, and composite aluminum tube are respectively used to make important parts such as fins, plugs, intake headers, and return gas headers of tube-and-belt heat exchangers. Because the composite layer acts as a gap during brazing, the unevenness of the composite layer is one of the main reasons for defects such as virtual welding, burning loss, discontinuous brazing, and local leakage. The uneven phenomenon of the composite layer of domestic composite materials is relatively serious, the appearance quality is poor, and the market price is close to the CIF price of imported materials, so the companies mainly use imported composite materials for mass production.

Bending resistance of composite aluminum foil

The bending resistance of composite aluminum foil at brazing temperature is a very important performance index to measure the quality of aluminum foil. Our company has used a certain imported composite foil, but when it is close to the lower limit of brazing temperature (590℃, 3 min), the phenomenon of fin bending and lodging still occurs, and the brazing performance is very poor. According to relevant research, this is due to the erosion (dissolution) phenomenon caused by the small diameter of the core material of the composite aluminum foil and the diffusion of silicon atoms in the skin material to the center of the core material along the grain boundary. Therefore, when the composite aluminum foil is rolled, the cold work rolling rate and annealing process are strictly controlled, which is beneficial to obtain a core material with coarse grains, so that the composite aluminum foil has excellent brazing performance.

Solder (leather) chemical composition

Generally speaking, no matter it is imported material or domestic material, the manufacturer can guarantee the chemical composition of the solder within a certain range. However, for the user, the smaller the deviation of the chemical composition of the brazing filler metal, the more favorable it is to control the brazing process parameters. Therefore, foreign manufacturers generally adjust the chemical composition of the solder according to the user’s usage, and the composition deviation is small. Relatively speaking, it is more difficult for domestic manufacturers to do so.

At present, the brazing filler metal widely used in vacuum brazing is aluminum-silicon-magnesium alloy, with a silicon content of 9.0% to 13% and a Mg content of 0.2% to 2.0%. Studies have shown that when the silicon content of the aluminum-silicon solder exceeds the eutectic composition (the silicon content is 11.7%). The base metal is more prone to corrosion defects; when the aluminum-silicon brazing filler metal is eutectic and hypoeutectic, the corrosion tendency of the base metal is relatively small. Therefore, most of the Si content in the brazing filler metal for vacuum brazing is hypoeutectic.

Adding an appropriate amount of Mg to Al-Si solder is beneficial to reduce the melting temperature of the solder, improve the flow coefficient of the solder, and reduce the requirement for vacuum degree; but with the increase of the magnesium content, the corrosion tendency of the solder to the base metal increases. , the flow of solder is difficult to control.

degree of vacuum

Vacuum degree is one of the key factors affecting the quality of brazing. With the increase of the content of Si, Mg and Ba in the brazing filler metal, it is beneficial to reduce the requirement for vacuum degree during brazing and improve the brazing performance of the brazing filler metal. However, since these data are all test samples, they are quite different from the brazing of heat exchangers in the actual production process. Therefore, in order to ensure the qualified rate of workpieces during vacuum brazing, when the temperature in the furnace reaches above 500℃, the vacuum degree in the furnace should be guaranteed to be within the range of 1.33×10-3Pa. Otherwise, the workpiece is prone to defects such as leakage of the brazing seam, discontinuity of the brazing seam, desoldering, and dark color of the workpiece.

Brazing temperature and holding time

Formulating suitable brazing temperature and holding time is the key to ensure brazing quality. The degree of brazing explosion is too high. The holding time is too long, which is easy to cause defects such as material loss, corrosion, and fin bending: the needle welding temperature is too low, the holding time is too short, and the workpiece has not reached the necessary brazing temperature. The temperature of each part is not balanced , it is easy to cause defects such as virtual welding, leakage, and discontinuous brazing.

The heat transfer method of the tube-and-belt heat exchanger during vacuum brazing is mainly cadmium jet heat transfer, and the phenomenon that the temperature of the workpiece lags behind the temperature near the heating body (controlling the measured temperature of the thermocouple) is obvious. Due to the different heat capacity of each part of the workpiece, the thermal conductivity of the stainless steel fixture is poor, so that the time for each part of the workpiece to reach the brazing temperature is inconsistent; the brazing heating program or the production cycle of the semi-continuous furnace caused by the long-term operation of the equipment will also make the various parts in the furnace. The temperature is inconsistent. Therefore, the brazing heating program of a single-chamber furnace or the production cycle of a semi-continuous furnace should be adjusted according to the actual temperature curve measured. At the same time, the temperature uniformity of the heating system should be calibrated regularly, and the placement of the workpiece in the furnace should be properly adjusted.

cooling rate

The aluminum-silicon solder commonly used in vacuum needle welding is hypoeutectic solder. Due to the large liquid-solid phase interval, the cooling rate should be fast rather than slow. Otherwise, defects such as the simultaneous solidification and dissolution of the brazing filler metal are prone to occur, which will affect the compactness of the seam.

Placement

The return gas header and liquid inlet header of the evaporator are made of composite aluminum tubes. If they are placed in the direction shown in the figure, the header brazing material is easily lost under the action of gravity, which will cause the brazing seam to be less dense and the fins near the brazing seam to corrode. and other defects; if the composite aluminum tube is placed downward, the loss of solder can be avoided.

product design

The product design must make the brazing joint form have good manufacturability, and the structural form of the product should also be conducive to vacuum brazing. Otherwise, it will bring unnecessary trouble in the process, which is not conducive to ensuring the quality of brazing. As shown in the figure, when the return gas main pipe is brazed, no matter how it is placed, it is difficult to control the flow of the brazing filler metal of the composite aluminum pipe, which often easily causes defects such as brazing filler metal loss, welding lumps, and corrosion.

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