Development of WZ Series Vacuum Aluminum Brazing Furnace

Development of equipment WZ series vacuum aluminum brazing furnace Zhou Youchen, Dai Fang, Liu Wenying (Beijing Institute of Mechatronics, Beijing 100083): Briefly introduced the technical parameters of WZ series vacuum aluminum brazing furnace models and specifications, and to WZLQH-30 The vacuum aluminum brazing furnace is taken as an example to introduce the structure and characteristics of the vacuum aluminum brazing furnace, and some experiences in the design and research of the vacuum aluminum brazing furnace are summarized.

The brazing methods for aluminum and aluminum alloys include dry air chloride flux brazing, salt bath brazing, nitrogen protection non-corrosive flux brazing, and vacuum brazing-free brazing. Dry air brazing and salt bath brazing require the use of chloride flux. After welding, it is necessary to remove the corrosive solder residue. The cleaning process consumes a large amount of fresh water, which inevitably causes environmental pollution. This led people to explore and research the fluxless brazing technology and promoted the development of vacuum aluminum brazing technology.

Vacuum aluminum brazing is based on the melting of a third metal placed between the workpieces to be bonded under vacuum conditions and is joined at a temperature lower than the melting point of the workpiece or the metal to be welded. During the joining process, the filler can be melted to form a good mechanical bond with the substrate, even atomic energy levels. In order for the filler material to form a good metallurgical bond, any non-metallic barrier layer on the base metal must be removed and its cleaning is done by the atmosphere of the furnace. Under vacuum brazing conditions, the formation of a dense barrier layer is inhibited so that the metal or filler can be completely melted. This industrial method of joining metals without the need of flux compared to conventional brazed joints has the advantages of cleanliness and strength of the joints and low corrosiveness. In addition, it can eliminate residual gases and make the surface of the workpiece bright. At the same time, the welded metal flows quickly under vacuum, shortening the welding cycle and increasing the efficiency.

1 Basic requirements for vacuum aluminum brazing equipment Since the advent of vacuum aluminum brazing technology, great progress has been made. It is mainly applied to various heat exchanger cores, waveguide antennas of space satellites and other brazing processes, and how to satisfy vacuum aluminum. The research and manufacture of the vacuum aluminum brazing furnace required by the brazing process is the main work we will carry out.

The requirement for vacuum aluminum brazing is that the molten liquid metal surface is wet and can flow. Due to its active chemical properties, aluminum can easily form an aluminum oxide film with oxygen or water vapor in the surrounding environment. This layer of aluminum oxide film is compact and chemically stable. It adheres firmly to the surface of the substrate, and it has atmospheric conditions. Self-healing ability is protective. The melting point of aluminum oxide membrane is as high as 2000C, and the vapor pressure is very high. - Male, Xunyang, Shaanxi, Researcher, Senior Engineer, Deputy Director of Vacuum Heat Treatment Technology Research Institute, Beijing Institute of Mechanical and Electrical Technology, mainly engaged in the development, design, and research of vacuum heat treatment equipment. He has won 2 second prizes for scientific and technological progress, 1 third prize, and published more than 10 papers. The contact telephone is low, and almost does not evaporate at a soldering temperature of about 600C. When brazing, neither melting nor evaporation, will become the resistance of vacuum aluminum brazing. In order to remove the aluminum oxide film and prevent the aluminum oxide film from being formed again, adding a small amount of magnesium as an activator in the solder can effectively promote the crushing and dispersion of the aluminum oxide film on the surface of the component and make the surface of the base metal wet. Magnesium in the filler metal mainly evaporates above the 551C Al-Si-Mg ternary eutectic temperature to the brazing temperature and breaks the alumina film. When the temperature of the brazing filler metal is close to the liquidus temperature of the aluminum-silicon alloy, the welding angle is formed under the action of the capillary force.

According to the characteristics of aluminum alloy vacuum aluminum brazing, in order to successfully carry out brazing of aluminum, vacuum aluminum brazing equipment must be designed with the following characteristics: High vacuum aluminum and aluminum alloy brazing must be in a high vacuum state Under the general requirements in the order of 1a3Pa. When the vacuum degree is too low, magnesium oxide will not form a protective magnesium atmosphere immediately after evaporation and oxygen and water vapor in the environment, so that the surface of the exposed fresh aluminum alloy will be immediately oxidized, and the aluminum oxide film cannot be removed, resulting in that the liquid solder cannot be wetted and flow.

High pumping rate For aluminum brazed products, most parts have a large surface area and a complex shape. The amount of gas adsorbed on the surface is large, and a large amount of gas is released during the heating process. In order to maintain the degree of vacuum in the furnace, a vacuum unit with a high pumping speed must be selected.

The pressure rise rate is small Although the equipment with large pressure rise rate can use the unit with large pumping speed to increase the ultimate vacuum, the pressure rise rate can be different, and the composition of the residual gas in the furnace is different. The residual gas in the furnace with a small pressure rise rate is mainly water vapor, and the residual gas in the furnace with a large pressure rise rate is mainly oxygen, nitrogen, and steam. At the same extreme vacuum degree, the surface of the brazed workpiece with a large pressure rising rate is not bright and the color is dark, while the brazing working surface with a small pressure rising rate is bright and white in color.

2WZ series vacuum aluminum brazing furnace technical parameters WZ series vacuum aluminum brazing furnace is the Beijing Institute of Electrical and Mechanical Services from the beginning of the 20th century, started in the 1990s, in the vacuum aluminum brazing process and equipment based on extensive research, based on market demand Development and design of vacuum aluminum brazing furnace products. At present, it has been widely used in aviation, aerospace, electronics, petrochemical, vehicles, ships, refrigeration and other industrial sectors. Table 1 is the main technical parameters and indicators of the WZ series vacuum aluminum brazing furnace, which is specially designed according to the use of the unit's process characteristics and product development needs.

Technical indicator effective heating zone (length X width X height) /mm rated capacity /kg maximum temperature /C heating power /kW; furnace temperature uniformity / C disabilities 3 ultimate vacuum /Pa working vacuum degree (400C ~ pressure rise Rate / Pa-h-1 inflation pressure / MPa cooling time (650C ~ 3WZLQH-30 vacuum aluminum brazing furnace performance and structure W7LQ vomiting 30 vacuum aluminum brazing furnace is Beijing Institute of Electrical and Mechanical Services in 1999 for the Second Institute of Aerospace It was specially developed for the waveguide antenna of the space radar in Beijing No. 699 Factory and the First Institute of Aerospace Beijing 703. Since the design temperature is 1300Q, it is also suitable for the annealing of soft magnetic alloys such as 150, 195, 112 DY, 121. Treatments, such as solid solution and annealing treatment of maraging steels such as 333 and titanium alloys such as TC4R, and high-temperature brazing of other precision machine parts, etc. The furnace was completed and put into production and became the Capital Aerospace Machinery Plant. Guihang Huayang Electric Power Plant and 29 of Chengdu, the Ministry of Information Industry, each produced one.Table 2 shows the design specifications and actual measurement results of the vacuum aluminum brazing furnace of Model WZLQH-30.

Table 2 WZLQH-30 Vacuum Aluminum Brazing Furnace Design Index and Actual Test Results Item Name Design Index Measured Result Effective Heating Area (Length X Width Height)/mm Rated Capacity/kg Max Temperature/C Heating Function/kW Furnace Temperature Uniformity / C 士3 ultimate vacuum / Pa working vacuum / Pa (400C ~ pressure rate / Pa-h-1 inflation pressure / MPa W7LQ 吐-30 type vacuum aluminum brazing furnace is the main characteristics of the heating element using high-temperature molybdenum Belt, heat shield adopts combined molybdenum sheet structure, the ultimate vacuum degree of the air furnace is up to 104Pa working vacuum degree (about 650C) up to 3.0X10-3Pa; the pressure rise rate WZLQHr30 type single chamber vacuum aluminum brazing furnace is mainly composed of furnace body , Furnace, air-cooled device, vacuum system, inflation system, water cooling system, charging trolley, power supply system and electric control system, etc. Its structure is shown in the diagram, and its main structure is introduced as follows.

The furnace body consists of the furnace door, furnace shell, furnace door clasp and furnace body bracket, etc. The furnace shell is a double cylinder structure of hGGammX1300mm, and the furnace door is a double head structure, in order to facilitate a higher degree of vacuum, the furnace The inside cylinder and door inner head of the shell adopt a well-polished stainless steel structure. The front door and the furnace body are hinged and equipped with a door clamping device.

Furnace gallbladder is composed of heat shield, heating element, material table and so on. The heater is one of the key parts of the equipment. Due to the high uniformity of the furnace temperature, the belt heaters are densely arranged, have a large radiation area, and easy to achieve a high uniformity of the furnace temperature; a small surface load, its use Relatively long life. Since the vacuum aluminum brazing furnace requires a rapid heating rate in the temperature range of 400600C, the heating power is about 50% larger than that of other types of vacuum furnaces in the same specification. With different sizes, the current density of vacuum aluminum brazing furnace with more partitions is also different. Generally, the current density is in the range of 2.5~3.0WVcm2. The bottom and sides of the material basket are slightly larger, and the upper part and front and back of the material basket are slightly smaller.

The heat shield is cylindrical in shape with two layers of molybdenum sheets and four stainless steel plates.

The heating element consists of 12 molybdenum strips 1.0 mm in width and 35 mm in width, and each group is connected in series of 4 groups. Each group has a power of 25 kW and a total power of 3 X. The thermal element and the heat shield are fixed with insulating porcelain sleeves with Si-Mg deposition resistance. , The power supply is connected to the heating magnetization transformer by the heating element through a molybdenum rod, a copper soft crucible, and a water-cooled electrode. Furnace front cover is provided with five molybdenum nozzles that point to the center. When cooling, the cold air is injected into the workpiece at a high speed. There are five molybdenum pipes in the back wall of the furnace bile. There are two molybdenum pipes in the furnace with two vertical molybdenum columns fixed on the molybdenum column. The material table is used to support the basket (workpiece). There are 4 rollers on the bottom of the furnace, which can be moved back and forth on the guide rail at the bottom of the furnace body. It is easy to pull out from the vacuum chamber for assembly and maintenance.

The forced cooling system of air-cooled vacuum aluminum brazing furnace is the same as other types of vacuum air-cooled furnace. It can be divided into furnace circulation system and furnace circulation system. Generally, the outer circulation system covers a larger area and the air duct is longer. Large wind pressure loss, internal circulation system can make the structure simple and compact.

The air cooling system of W7LQHh30 Vacuum Aluminum Brazing Furnace adopts the internal circulation system. It is composed of 22kW motor, high-capacity high-pressure fan, high-efficiency all-copper tube heat exchanger and air guide device. The fan and motor are directly connected. And after being assembled with the furnace body's rear head, when the workpiece is heated and air cooled, the furnace is first filled with an inert gas (basically balanced with the atmosphere) of about 0.1 MPa to protect the motor from starting, and the motor continues to operate normally. The furnace is inflated to the required pressure, the air flow from the fan to the furnace air nozzle and rapidly blows to the workpiece, the hot gas flow from the workpiece passes through the gas return nozzle to the heat exchanger for cooling, and the gas cooled by the heat exchanger The fan is pressed against the air nozzle and the workpiece to form a circulation loop (see). At the same time, the water cooling interlayer of the furnace shell can be used to further fully cool the inert gas of the convection flow to accelerate the cooling rate of the workpiece. According to the different quenching process, the air cooling pressure is different. It can be arbitrarily selected within the range of 0.1 to 0.2 MPa.

Vacuum and Inflate System Aluminium brazing requires high vacuum and large pumping speed. Choose a relatively large main pump. The vacuum system consists of 2X-70 rotary vane pump, Z-300 roots pump, KT-500 diffusion pump, SD500 cold trap, GFQ500 High-vacuum pneumatic flap valve, high vacuum pneumatic butterfly valve, differential pressure valve and connecting pipeline and other components, see for the realization of quenching when the rapid inflation is equipped with a gas pressure tank including inflation pressure up to 09MPa, inflatable valve and pipeline components The inflatable system. The gas pipeline and gas storage tank of the gas filling system must also be evacuated. The gas purity is 99999%. Otherwise, the finished workpiece will be discolored after brazing. In addition, the pressure will increase or decrease with the temperature change after the cooling gas is filled in the furnace. , so the system is equipped with automatic air replenishment and automatic deflation device. When the temperature of the furnace rises, the pressure in the furnace increases, and the automatic deflation device opens. When the temperature of the furnace decreases, the pressure in the furnace decreases, and the automatic replenishment device opens, allowing the furnace to always maintain a relatively stable pressure.

Magnesium capturing device aluminum alloy will evaporate a large amount of S-Mg vapor in the brazing process. The most obvious evaporation occurs in the range of 380C~500C. According to experimental observations, Mg is less deposited on a very cool surface, and more deposited on a surface with a certain temperature (usually at a temperature of 100 ~ 250C; the same principle with the vacuum coating), such as the furnace electrode, external heat shield, electricity Insulators and other parts. At the same time, a part of the Mg vapor will flow into the diffusion pump of the vacuum system, which will deteriorate the diffusion pump oil and affect its pumping speed and ultimate vacuum degree. Therefore, the structure of the electric insulator must adopt a special anti-deposition structure. In the interface between the furnace body and the vacuum system, a magnesium capturing baffle plate is required, and the baffle plate adopts a shutter structure capable of cooling water. Because the baffle will affect the effective pumping speed of the vacuum system, in order to reduce its impact, the nominal diameter of the baffle should be greater than the nominal diameter of the diffusion pump, see.

4 Equipment debugging experience and test results W7LQ Zhuang 30 vacuum aluminum brazing furnace has some problems in the first cold and hot debugging process. It is worth learning lessons from us. It is necessary to carry out some discussions here.

Even if the vacuum pump with a small pumping speed can reach the ultimate vacuum degree, due to the large amount of deflation of the workpiece and the furnace material in the heating process, once the heating speed is too fast, it is difficult to maintain a high working vacuum degree, thereby affecting the welding quality. For an effective 1.5m3 container, the pumping rate is 500017s. Considering that the baffle of the magnesium capturing device will affect the effective pumping speed of the vacuum system, we chose the KT-400 diffusion pump (extraction rate of 750017s). It is easier to reach the ultimate vacuum of 6.0XW4Pa, and it is difficult to maintain the working vacuum of 50XKT3Pa or more at about 650C. Even if it can be achieved, it requires a longer holding time, which is absolutely not acceptable for the aluminum alloy brazing process. of. Later, the KT-500 diffusion pump (withdrawing speed of 1,200,017 s) can be used to pump from the atmospheric pressure to the ultimate vacuum in less than 1 h. The working vacuum pumped to 5X 103 Pa requires only 40 minutes. It is recommended that imports be made under economic conditions. Diffusion pumps, such as the use of the German company Leybold's products, can further improve the equipment's grade and service life.

(2) The heating power of the heating transformer is one of the main indicators to measure the performance of a vacuum furnace. If the power is too high, it will result in waste, power consumption and water consumption. When the power is too low, the temperature of the furnace will not rise or the heating rate will increase. Very slow, can not meet the vacuum heat treatment process needs, low productivity. Vacuum aluminum brazing furnace in the 400C ~ 600C requires a heating rate as fast as possible, requires high precision temperature control within the low temperature range, no-load voltage as small as possible, which will increase the cost and production difficulty of magnetic transformers, optional thyristor regulator And its idle power is equal to zero.

The temperature-sensing thermocouple vacuum aluminum brazing furnace requires a relatively strict temperature control thermocouple for its main sensing element. The reaction is sensitive and rapid, the operation is stable, the measurement consistency is good, and there is no hysteresis. It is best to use a metal sheathed thermocouple that can be bent freely and easily vacuum sealed. The working monitoring thermocouples are preferably led to the cooled locations for connection to avoid contact resistance at the plug sites affecting the zero temperature difference of the thermocouples and also to prevent evaporation of Si-Mg vapors from depositing on the surface causing short circuits.

Furnace Temperature Uniformity Test There are two types of thermocouple lead-out devices for measuring furnace temperature uniformity. The first is that the twin wire is connected directly to the printer through a vacuum vacuum plate that is clamped in the flange; The wire is mated with a terminal block located on the furnace body and then connected to the printer via a compensation wire. The first method is reliable and reliable, and there is no contact resistance at the plug-in location that affects the zero-point temperature difference of the thermocouple. The second method is to connect the thermocouple to a relatively cool location for connection. This measured temperature can reflect the true temperature of the furnace.

Oven processing Before the first furnace is opened, oven treatment is required. According to past experience, it is believed that as long as the vacuum degree is higher or higher than the working vacuum level at a working temperature of 30 to 50C. Actually, after many tests, we learned that high temperature drying (that is, the degree of vacuum reaching the working vacuum level) does not guarantee that the low temperature temperature ranges can meet the process requirements, and that the amount of gas released in the furnace at about 400C is relatively large. The temperature point should be given a longer oven treatment.

In addition, when the environment is relatively humid, a large amount of condensable gas will be adsorbed on the workpiece, which brings great difficulties to vacuuming. Some people use the method of pre-baking degassing in the oven to dry the work. We use the vacuum directly. The furnace itself is baked, the furnace temperature is controlled at about 200C, and the middle vacuum system (opening the mechanical pump and the Roots pump, and the main valve of the diffusion pump is not turned on) is used for exhaust. When the vacuum degree in the furnace reaches a certain level, Then use a diffusion pump to vacuum the furnace so that a good working vacuum can be obtained. The baking temperature should be controlled below 250°C to prevent the oxidation of the workpiece and the loss of S-Mg from affecting the brazing quality.

5 Conclusion After several years of process testing and production applications, we have listened to a lot of feedback from users, and on the basis of continuous improvement and continuous improvement, we have gradually formed a series of WZ vacuum aluminum brazing furnace products. The technical indicators can not only meet the production needs, but also provide good conditions for the development and development of new products for all use units. Production practice proves that its operating mechanism is reliable and easy to operate. After the brazing treatment of various types of product components, its performance is significantly higher than the standard requirements, product quality and product qualification rate has increased significantly.

The China Thermal Assessors reviewed the first two batches of “China's Heat Treatment Industry Outstanding Achievements and Products - BH Catalysts and Catering Technology Small Induction Heating Heat Treatment Centers. These two projects after years of research and development and production applications, advanced technology, social and economic benefits Significantly, it has been praised by users and has broad application prospects, and will play a role in promoting the technical progress and production development of heat treatment. In 2001, it had passed the technical appraisal of experts chaired by China Association of Heat Services and Henan Science and Technology Department respectively. "Infiltration Enhancer and Permeation Enhancement Technology" is regarded as "inherently created at home and abroad" and "having the leading international level in the application effect and application stability of small induction heat treatment processing centers". It is recognized as "reaching domestic leading level" and "close to international advanced According to the assessment standards of China's Heat Treatment Industry Outstanding Achievements and Products and the China Machinery Industry Science and Technology Award, the two products declared for the project were rated as outstanding new products in the Chinese heat treatment industry by the jury on August 27th. And was recommended as China Machinery Industry Science and Technology Award - Second Prize. September 10th National Association of thermal heat in Nanjing, China plant manager meetings were awarded medals and certificates. After a review once a year.

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Small Induction Heat Treatment Processing Center This project was developed by Luoyang Hi-Tech Sublimation Induction Heating Co., Ltd. The mission is to meet the needs of users and markets. The induction heat treatment machine tool introduced by the Luoyang Beiyi Company from Nippon Electric Industrial Co., Ltd. discovered problems such as poor flexibility and inconvenient operation during its use, and therefore proposed to the company the task of developing this project. After many years of development, we have developed outstanding technical achievements and products, expanded our users, and developed markets. In 1999, we achieved results that meet the requirements of users. The product is based on the special needs of induction heat treatment process, multi-station rotary structure and numerical control system, heating power supply, quenching transformer, inductor, quenching liquid and coolant circulation system are integrated in the industrial controller to develop induction heat treatment. The numerical control system has a breakthrough in technology. The product runs smoothly and reliably, has high positioning accuracy, is versatile, has comprehensive performance, and has a high degree of integration. It has recently proposed and realized the compactness and flexibility of induction heat treatment machines. Multi-axis, on-line and computerized control, innovative, practical, innovative, reaching the domestic advanced level, close to the international advanced level u The project has technical innovation, solved the key technology of induction heat treatment technology in the development of heat treatment, promote heat treatment Technological progress has a significant role. In 1999, it was identified as a high-tech product in Henan Province. In 2000, it was identified as the Technology Innovation Fund for Science and Technology Small and Medium-sized Enterprises of the Ministry of Science and Technology. It won unpaid funding. In September 2001, it passed the appraisal of scientific and technological achievements hosted by the Department of Science and Technology of Henan Province. In July 2002, the project was approved by the supervision organization Luoyang Hi-tech Development Zone entrusted by the Technology Innovation Fund Management Center of the Ministry of Science and Technology. The performance was excellent and stable, and the project's mission objectives and production application indexes were completed. After rigorous review by the jury, the product was evaluated as an outstanding new product in China's heat treatment industry, and was recommended as the second prize for China's machinery industry science and technology award (China federation Wang Dewen)