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Aluminum alloy finned radiator vacuum brazing
Plate-fin heat exchangers have the advantages of small size, light weight, and can handle more than two kinds of media. Widely used in petroleum, chemical, natural gas processing and other industries. The characteristics of plate-fin heat exchangers are:
(1) The heat transfer efficiency is high. Due to the continuous rupture of the boundary layer due to the disturbance of the fin to the fluid, it has a large heat transfer coefficient. The heat exchanger can achieve high efficiency.
(2) Compact, because the plate-fin heat exchanger has an expanded secondary surface, its specific surface area can reach 1000㎡/m3.
(3) Lightweight, because it is compact and mostly made of aluminum alloys, and now steel, copper, composite materials, etc. have also been mass-produced.
(4) Strong adaptability, the plate-fin heat exchanger can be applied to the heat exchange between various fluids and the phase change heat that occurs with the state change. The arrangement and combination of flow channels can adapt to different heat exchange conditions. The heat exchange needs of large-scale equipment can be met through the series-parallel combination between units. In industry, it can be stereotyped and mass-produced to reduce costs, and expand interchangeability through building block combinations.
But at the same time, the manufacturing process of plate-fin heat exchanger is strict and the process is complicated. The high-quality aluminum plate-fin heat exchanger has high compactness and low weight. The core components of the product are welded by vacuum brazing at one time. However, its structural characteristics also lead to the disadvantages of high difficulty in vacuum brazing and low yield. In order to improve the qualification rate of vacuum welding, it is necessary to strictly control the pretreatment process, assembly process and brazing process of parts.
Structural characteristics of plate-fin heat exchanger:
In order to ensure a compact structure, the width of the brazing surface is 2.5mm, the welding seal is trapezoidal, and the thickness of the fin is only 0.1mm. The specific situation is shown in the following figure:
Because the width of the seal is smaller than the width of the brazing surface of the conventional seal. The reduction of the width of the brazing surface directly reduces the width of the brazing seam, which is more demanding on the brazing process. The thickness of the fins is only 0.1mm, which not only increases the difficulty of punching the fins, but also increases the difficulty of brazing. More precise requirements for brazing temperature control accuracy and brazing time. A slightly higher brazing temperature or a slightly longer holding time will cause the fins to corrode, while a slightly lower brazing temperature or a slightly shorter holding time will cause internal defects in the brazing seam (porosity, lack of penetration, etc.).
1.Parts cleaning process
The principle of ultrasonic cleaning is that the high-frequency oscillation signal sent by the ultrasonic generator is converted into high-frequency mechanical oscillation by the transducer and propagated into the medium (cleaning solution). The liquid flows to produce tens of thousands of tiny bubbles, which form and grow in the negative pressure area where ultrasonic waves propagate longitudinally, and close quickly in the positive pressure area. In the process of this so-called “cavitation” effect, the closing of the bubble can form an instantaneous high pressure exceeding 100MPa atmospheric pressure, and the instantaneous high pressure is continuously generated like a series of small “explosions” that continuously impact the surface of the object, making the object’s The dirt on the surface and in the gap is quickly peeled off, so as to achieve the purpose of cleaning the surface of the object. The characteristics of ultrasonic cleaning are:
(1) Thorough cleaning The cleaning liquid easily penetrates into all parts of the metal workpiece to be cleaned, even the gaps, holes, back grooves and other parts can be cleaned.
(2) Environmental protection and pollution-free The cleaning liquid can be recovered, filtered and reused, and will not cause harm to the human body and the environment like acid-base cleaning.
(3) Stable product cleaning quality During the cleaning process of parts, automatic control is realized throughout the whole process. Only need to input relevant parameters, and the equipment will automatically complete the entire cleaning process without being affected by human factors.
Parts cleaning process parameters:
(1) The pH value of the steam inside the ultrasonic gas-phase steam bath is strictly controlled between 5 and 8.
(2) The steam temperature inside the vapor-phase steam bath is strictly controlled between 90 and 93 °C.
(3) The cleaning time for fins is set at 3 minutes, and the cleaning time for partitions and seals is set at 2 minutes.
2.Parts assembly process
(1) Assembly should be carried out in a dust-free assembly room, and the assembly fixture (special fixture), backing plate and counterweight should be descaled.
(2) The relative humidity of the air in the assembly room is 60%~70%, and the temperature is 16~18℃ in winter and (20±5)℃ in summer.
(3) Assemblers must wear spun gauze gloves and must not touch parts with bare hands.
(4) After the assembly height reaches the theoretical value, correct the flatness and diagonal deviation, and then tighten the upper and lower fixture plates with a torque wrench, and the torque value is controlled within the range of 105~110N*m.
3.Vacuum brazing process
1) Principle of vacuum brazing
In a vacuum environment, when the brazing temperature is lower than the melting point of the base material and higher than the melting point of the solder, the solder melts, and then enters and spreads into the gap between the parts through capillary wetting. When the brazing temperature is lowered, the solder solidifies and the parts are brazed together by the solidified solder.
2) The composition of the solder
The brazing filler metal is a cladding layer attached to both sides of the metal plate. The thickness of the cladding layer should be controlled between 0.1 and 0.4mm, and its composition is Al-Si-Mg alloy.
3) Vacuum brazing process parameters
Vacuum brazing process parameters mainly include vacuum degree, heating rate, brazing temperature, holding time and cooling rate. The vacuum brazing temperature-time process curve of this product is shown in the figure.
(1) Degree of vacuum
Before brazing and heating, vacuumize first, and the vacuum degree shall not be lower than 4×10~2, and then start heating.
(2) Heating method
Using the step heating method, when the temperature rises to about 500 ℃, keep the heat preservation and ensure that the workpiece is heated evenly (furnace temperature uniformity ≤ ± 3 ℃), and then continue to heat to the brazing temperature for thermal brazing.
(3) Brazing temperature
Mainly based on the brazing situation, generally higher than the liquidus temperature line 25-50 ℃, below this temperature, although the solder has melted, the solid crystal has not been completely disintegrated, and the fluidity difference temperature is too high, because the 3A21 liquidus temperature is At 654°C, over-burning or even melting will occur. In addition, the base metal and brazing material may also be corroded.
(4) Holding time
The holding time is mainly determined by the complexity of the structure. It is necessary to ensure that the brazing filler metal can be fully wetted in the brazing seam and that defects such as corrosion will not occur.
(5) Cooling speed
The cooling rate also affects the quality of brazing. Too fast cooling is easy to generate stress, and it may even cause cracks in the joints. Too slow cooling will cause the grains to grow. Therefore, in general, cooling is performed with the furnace.
When the furnace temperature drops to 200°C, open the furnace door, take out the workpiece, and air-cool to room temperature.
(7) Product inspection: The surface of the product is bright, without blackening, the brazing seam is full, the corners of the brazing seam are round, and there are no defects such as surface pores and de-soldering. The air of 2.6 MPa (gauge pressure) was introduced into the inner cavity of the product to conduct an air tightness test in water, and no air leakage was found for 5 minutes.
Vacuum brazing equipment selection: After mastering the corresponding process, it is necessary to select a suitable vacuum furnace for operation to ensure sufficient yield. At present, the most suitable product series of SIMUWU is RVB-L low temperature vacuum brazing furnace. It has the characteristics of high temperature accuracy, high vacuum degree, and good temperature uniformity in the heating area. It is an ideal equipment for vacuum brazing of aluminum alloy fin plate radiators.
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