Aluminum vacuum brazing technology introduction

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Speaking of aluminum vacuum brazing, we must first know the mechanism and process characteristics of vacuum aluminum brazing:

During vacuum brazing of aluminum alloy, the oxide film on the surface of the product affects the wetting and flow of the liquid solder. During the vacuum brazing process, if the oxide film on the surface of the parts cannot be effectively removed, it is difficult to form a high-quality brazing joint. In order to remove the oxide film and prevent the aluminum from forming an oxide film in the vacuum brazing process, a small amount of magnesium is added as an activator to 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°C, the thermal stress is sufficient to partially crack the aluminum oxide film, 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 of the vacuum brazing furnace to protect the exposed aluminum surface from oxidation.

On the other hand, it penetrates the surface of the aluminum alloy through the gaps in the aluminum oxide film to form an alloy, which is partially melted, and further penetrates 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 fillet under the action of capillary force. From the above description, it can be seen that the vacuum brazing of aluminum alloy must be carried out in a high vacuum state, generally in the order of 10-3Pa. If the vacuum degree is lower than 10-3Pa, the magnesium will be combined with the trace oxygen and moisture in the vacuum brazing furnace immediately after evaporation, and a protective magnesium atmosphere cannot be formed, so that the fresh aluminum surface that has just been exposed will be oxidized immediately. Failure to remove the oxide film will cause the liquid solder to be unable to wet and flow. For radiator products, due to the relatively large surface area of ​​the radiator product, the amount of adsorbed gas is also large, and a large amount of gas will be released during the heating process, which will reduce the vacuum degree. In order to maintain the vacuum degree of the vacuum brazing furnace, it is necessary to choose a vacuum unit with a higher pumping speed (the pumping rate: atmospheric to 5.0×10-4Pa not greater than 30min.) The melting point of Al-Si-Mg solder is close to that of aluminum Therefore, the 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. Therefore, the temperature uniformity of the heating zone of the vacuum brazing furnace is very strict, and generally does not exceed ±3℃ (empty furnace, nine-point temperature measurement), so it is necessary to control the temperature of the heating zone of the vacuum brazing furnace. An oven at 200°C can be added on rainy days. Keep it warm for 60 minutes. The vacuum degree is less than 5*10-2Pa. SIMUWU aluminum brazing vacuum furnace has good temperature uniformity and high vacuum degree and serves many well-known auto parts manufacturing companies at home and abroad, such as Valeo, UFI, SWEP, etc. contact SIMUWU via whatsapp:+86-13916614261 or send email to contact@vacfurnace.com

Vacuum brazing and brazing process: 20 minutes from room temperature to 360°C or 8-10°C/S heating rate, heat preservation 40-60 minutes (depending on the number of products into the furnace), the temperature difference does not exceed ± 25°C and the vacuum degree is less than 7.5* 10-3, 25 minutes heating up to 470℃ or 6-8℃/S heating rate. When the temperature of the core body of the workpiece is higher than 330℃, keep it for 60-100 minutes, and the temperature difference should not exceed ± 15℃. The vacuum degree is less than 6.5*10-3. The temperature is raised to 520℃ in 20 minutes, the temperature difference of the workpiece core body temperature is higher than 510℃ for 60 minutes, the temperature difference does not exceed ± 10℃, and the vacuum degree is less than 5.0*10-3. The temperature is raised to 570℃ in 15 minutes , Or at a heating rate of 15-20℃/S, when the core body temperature of the workpiece is higher than 560℃, keep it for 50 minutes, and the temperature difference does not exceed ±5℃. The temperature is increased to 620℃ in 10 minutes, or the temperature is increased by 10-15℃/S Rate The core body temperature of the workpiece is kept at 598℃-605℃ for 20-50 minutes, the temperature is reduced to 603℃ in 10 minutes, and the temperature is kept for 10-30 minutes. The temperature difference does not exceed ±2℃. It is cooled to below 500℃ and out of the oven.
The heating and brazing procedure can only be opened when the vacuum degree reaches 5×10-3Pa, and the aluminum vacuum brazing section must ensure that the vacuum degree reaches 3×10-3Pa.

Film removal mechanism of aluminum alloy material in vacuum brazing process

When the brazing temperature is 598-605℃, the base metal expands sharply, causing the surface oxide film to crack, and the liquid solder flows from the cracked place under the oxide film layer. The oxide film on the surface of the aluminum alloy material is lifted and further crushed. Al2O3 film with a dense, stable and high melting point (about 2050℃) can easily form on the surface of Al, which hinders the wetting and bonding of the brazing filler metal and the base metal during vacuum brazing. The vacuum brazing of Al and aluminum alloy does not use flux, but under vacuum conditions (3-5.0×10-3 Pa), with the help of M g, which has a higher vapor pressure and a greater affinity for O2 than Al The metal activator removes the film, and Mg is added as an alloying element to the aluminum-silicon solder. The content of Mg in the aluminum-silicon solder for vacuum brazing sold in the domestic market is generally controlled at 1% to 1.5%.

The mechanism of aluminum vacuum brazing to remove the film is as follows：
1. The thermal expansion coefficient of Al2O3 is only 1/3 of that of aluminum, and aluminum expands rapidly, causing the oxide film to crack.

2. The volatilization of Mg during heating destroys the oxide film on the surface of the solder.

3. The volatile Mg reacts with the residual CO2, O2 and water vapor in the vacuum:

2Mg + O2 = 2MgO
Mg + H2O = MgO + H2
Mg + CO2 = MgO + C
3Mg + Al2O3 = 3MgO + 2Al

4. The volatilized Mg vapor in the material penetrates into the surface layer of the base material under the film, and together with the diffused Si, the surface layer forms a low melting point Al-Si-Mg alloy and melts, and the surface oxide film floats and removes.

5. The absorption of the liquid solder reduces the strength of the oxide film, breaks and disperses and dissolves into the solder, the liquid solder can wet the base material.

6. The working vacuum can provide a clean surface for the good wetting and flow of the solder, and at the same time affect the solid-gas, liquid-gas interfacial tension, and improve the wettability of the solder.

Vacuum brazing is more sensitive to the cleanliness, roughness, assembly quality, fit tolerances, etc. of the surface of the parts before brazing. It has high requirements on the working environment and the theoretical level of workers. During vacuum brazing, in order to obtain high-quality brazing seams, The key condition is that the liquid brazing filler metal can fully flow into and densely fill all brazing gaps, and interact with the base metal and base metal to form a new alloy, which meets the requirements after condensation and crystallization. Brazed joints. Vacuum brazing is to assemble under the conditions of oil-free, moisture-free, clean and dust-free. At present, the working environment of the general workshop is dusty, and the failure rate of the vacuum brazing furnace is relatively high. The surface color of the product after the vacuum brazing is not ideal, and there is slight oxidation. The vacuum brazing process has strict requirements on the degreasing of the surface of the parts. Even if there is oil stains on the surface of the vacuum brazing, the oil stains will remain on the surface of the product parts after heating, which will act as a brazing inhibitor and destroy the liquid state. The brazing filler metal wets the base material and spreads and the brazing seam is formed well. At present, many companies use trichloroethylene or trichloroethylene substitute SKIESOL-bromoclonal steam to remove oil, that is, to put the workpiece to be degreasing in trichloroethylene or trichloroethylene substitute SKIESOL-bromoclonal steam, and use the steam The liquid condensed when it comes into contact with the cold workpiece dissolves the oil on the workpiece (practice has proved to be better than the traditional cleaning process, and many units of the original acid-base cleaning process are replaced with vapor phase cleaning equipment).

Principle of ultrasonic cleaning
The human ear generally responds to a frequency range of approximately 20 to 18,000 vibrations per second (18KHZ). Sound waves greater than 20KHZ are called ultrasonic waves. Ultrasound is a kind of alternating sound pressure. When the ultrasonic wave propagates in the liquid, it can make the liquid appear sparse and dense. In the dense state area, the liquid bears positive pressure, while in the sparse state area, it bears tension. Form holes. The cavity is constantly generated, closed and disappeared, and a huge pressure of up to hundreds of atmospheres immediately appears around the cavity. Along with a strong shock wave, it has a cleaning effect on the surface of the workpiece, and the dirt is peeled off the surface of the workpiece. This repeated process of the generation and disappearance of holes is called the cavitation effect. Thousands of vacuum bubbles quickly rupture thousands of times within 1 second. The cavitation effect is very huge, and it has a strong cleaning effect on the surface of the workpiece, especially for blind holes, gaps and places that are generally difficult to clean. It’s very clean. We analyze from the energy point of view that the generation of a large number of holes increases the surface area of ​​the liquid, so energy must be absorbed. This energy comes from sound energy converted from electrical energy. When the cavity ruptures and disappears, the surface area is reduced, and the energy must be released and transformed into mechanical energy and thermal energy.

When the ultrasonic frequency of the gas-phase cleaning machine is increased to several hundred kilohertz high-frequency ultrasonic, the cavitation phenomenon almost disappears, and the ultrasonic energy appears as acceleration energy. For example, at 950KHZ, the energy will reach 105 times the acceleration of gravity, which is very effective in removing submicron particles.

The process flow of ultrasonic cleaning vacuum brazing parts:

Pass the product parts that need vacuum brazing → hot dip cleaning → cold dip ultrasonic rinsing → vapor phase drying cleaning → cooling and drying → blanking
Ultrasonic cleaning equipment operation process: according to the set cleaning process, the workpiece basket is manually put into the feeding and discharging device, and the whole cleaning process is automatically completed by two workpiece moving vehicles, and finally the workpiece basket is raised.

Working principle of ultrasonic cleaner pipeline:

1. Add liquid from the liquid storage tank to the cold rinse tank; overflow to the hot dip ultrasonic tank and distillation machine; display when it is added to the liquid level in the working state;

2. Circulation: hot-dip ultrasonic tank pump filtration circulation; pump, filter, pipeline, etc.;

3. Distillation machine circulation: after the dirty liquid reaches the working liquid level of the distillation machine, new liquid will be distilled to the cold rinse tank and cleaning agent storage tank;

4. Internal circulation of the equipment: the cleaning liquid is vaporized and cooled to the liquid return tank (vapor state—liquid) and flows into the steam washing tank after water separation;

5. Rehydration: automatic rehydration when the fluid is insufficient
The parts to be cleaned are preheated and soaked in the tank ①, and then cleaned in the ultrasonic cleaning tank ②. The cavitation effect of ultrasonic is used to accelerate the dissolution and peeling of pollutants, and then rinsed in the ultrasonic cold rinsing tank ③. Remove residual dirt and cool the workpiece. The greater the temperature difference between the workpiece and the solvent vapor, the more conducive to condensation, and the better the vapor phase cleaning effect. Finally, the vapor phase rinsing in the solvent vapor layer (bath washing tank), the hot solvent vapor hits the cold workpiece to produce condensation, the solvent vapor becomes liquid, wetting and dissolving the contaminants on the surface of the workpiece, and dripping like rain , To take away pollutants. When the temperature of the workpiece reaches the temperature of the solvent vapor, the condensation will no longer proceed, and the surface of the parts will no longer drip, indicating the end of the vapor phase cleaning. Finally, the workpiece is lifted out of the vapor layer to the freezing zone, and the workpiece is completely dry after a while. Because the steam is clean and there is no secondary pollution during the cleaning process, this is a completely clean cleaning process that can achieve a high degree of cleanliness.

In the process of vacuum brazing parts cleaning, the cyclic regeneration process of the cleaning solvent: boiling tank-vapor layer-condensation-water-liquid separator-flowing into the rinse tank-overflowing to the ultrasonic tank-overflowing to the hot dip tank-flowing back To the boiling tank. Equipment composition: The equipment consists of two ultrasonic cleaning tanks, a hot dipping tank, a steam bath washing tank, a freeze drying tank, a condensation recovery coil, a water-liquid separator, etc. The equipment is equipped with filter circulation device, automatic liquid supplement and discharge valve, feed and discharge trolley, automatic manipulator, industrial chiller, refrigeration unit, machine monitoring system, air purification device, ultrasonic system, electrical control part, and the cleaning part of the whole machine is fully enclosed The structure is equipped with a detachable observation and inspection door. The specific structure is shown in the following description. The power distribution facility of the equipment is an independent electric control cabinet. All measuring instruments on the equipment use metric units.
Working process: First, the operator puts the workpieces to be cleaned in the tooling basket one by one, then places them on the feeding trolley and pushes them to the loading position, and is sent to the first station by the fully automatic manipulator through the induction switch linkage for cleaning. After rinsing, bath washing, and drying, the automatic manipulator puts it on the discharging trolley, and then the operator takes out the workpiece and transfers it to the next process. Equipment composition: The equipment consists of two ultrasonic cleaning tanks, a hot dipping tank, a steam bath washing tank, a freeze drying tank, a condensation recovery coil, a water-liquid separator, etc. The equipment is equipped with filter circulation device, automatic liquid supplement and discharge valve, feed and discharge trolley, automatic manipulator, industrial chiller, refrigeration unit, machine monitoring system, air purification device, ultrasonic system, electrical control part, and the cleaning part of the whole machine is fully enclosed The structure is equipped with a detachable observation and inspection door. The specific structure is shown in the drawings and the following description. The power distribution facility of the equipment is an independent electric control cabinet. All measuring instruments on the equipment use metric units.

Working principle: First, the operator puts the workpieces to be cleaned in the tooling basket one by one, then places them on the feeding trolley and pushes them to the loading position, and is sent to the first station by the automatic manipulator through the induction switch linkage for cleaning. After rinsing, bath washing, and drying, the automatic manipulator puts it on the discharging trolley, and then the operator takes out the workpiece and transfers it to the next process.

The picture below is a test of the product after normal ultrasonic cleaning operation with the Dying pen: the product will not stretch evenly after 2 seconds of reaching the liquid.

When the vacuum brazing furnace is working: first vacuumize to remove the gas and impurities in the vacuum brazing furnace. Gas and impurities contain many kinds of dust, including metal dust particles, oil particles, and cotton yarn particles. Each type of dust particles contains moisture, and has viscosity and adsorption properties. The critical vapor pressures of metal dust particles, oil particles, and water vapor particles in the air are different, and their evaporation temperature and time in the furnace are also different. Due to the different viscosity and adsorption properties of different kinds of dust, it will affect the vacuuming time of the equipment and the ability to increase the vacuum degree. The pollution in the vacuum brazing furnace directly affects the acquisition and maintenance of vacuum, and affects product quality. If it is dirty, it can become a source of a large amount of gas and steam, extend the pumping time, reduce the vacuum degree, and also deteriorate the components themselves. Contamination in the vacuum furnace also affects the correct execution of the process. The vacuum brazing furnace used for brazing the plate-fin aluminum radiator must be kept clean and dry. The radiator to be welded and fixtures to be sent into the vacuum brazing furnace must be kept clean, oil-free and dry. Many units generally use lubricating oil to lubricate the screws and nuts of the fixtures during installation and removal. At this time, it is necessary to use volatile oil, because the volatile oil has a lower volatile temperature and will not pollute the vacuum brazing furnace. The radiator assembly work site must have sufficient isolation work area, no dust, oil pollution, and relative humidity not more than 55%. The copper and aluminum production lines must be separated. The personnel in the assembly workshop must be clean and free from oil stains. Clean white yarn gloves must be worn when assembling the radiator product. The radiator assembler is not allowed to wipe sweat with gloves, the radiator assembler has to directly touch the cleaned parts with gloves, and the radiator assembler must not lean against the installed product. The parts to be assembled, brazing filler metals and other process auxiliary materials must be placed in a special frame and covered with a clean, oil-free plastic sheet to prevent dust and other pollutants from pollution. For radiator parts that cannot be assembled for some reason, the assembled parts must be put into the furnace or covered with clean, oil-free plastic cloth. Before assembly, the spring must be inspected and used in pairs. The fixtures that have been idle for more than three weeks and newly processed fixtures must be sent to the vacuum brazing furnace for purification before use. When assembling the product, the various parts of each layer must be naturally flat, without arching, local unevenness and overlapping and jamming. When the fins are spliced, the upper and lower layers of the splicing seam should be staggered. The inspector must pass the inspection before entering the vacuum brazing furnace. The heat exchanger product department should be brazed in time after installation. If you cannot enter the vacuum brazing furnace in time, you must take measures to prevent dust, oil, and pollution. The time outside the vacuum brazing furnace is not allowed to exceed 24 h. Now we want to increase the yield. The following points must be done, and the details are as follows.

(1) Strengthen the purification of the air in the working environment of the workshop, so that the air in the workshop does not contain measurable mass points (dust, dust), nor is it contaminated by acid or other substances. The walls and ceiling should be painted with waterproof paint and cleaned regularly to prevent mechanical particles from falling into the device. The ground must be smooth and free of cracks to avoid accumulation of dust and maintain a certain amount of dry humidity. There should be a clean environment around the radiator production room. In general vacuum equipment assembly site parts storage, packaging, etc., strict environmental hygiene should be maintained.

(2) For the storage and packaging of radiator parts, paper and cotton fabrics should not be used. The better anti-pollution materials are cellophane and plastic film.

(3) The personal hygiene of the operators must always be maintained, which is an important link in production. Workers’ work clothes should be clean and their hands should be clean. Even clean hands are not allowed to hold parts. Otherwise, grease and perspiration will be left on the parts, making it difficult to exhaust. The maintenance work site should be separated from the maintenance room and the assembly room. The maintenance personnel should have two sets of non-cotton fabric work clothes, dark and white, so as to ensure the quality of product assembly.

(4) The inside and outside of the vacuum brazing furnace should be kept clean as required. To ensure the conduction capability of the vacuum system pipes, deposits should be removed at any time. Regularly replace the rotary vane vacuum pump, Roots pump, diffusion pump oil.

Vacuum brazing process parameters of plate-fin aluminum alloy radiator

Plate-fin aluminum alloy radiator vacuum and brazing mainly rely on the accumulation of production experience to formulate the process. The product qualification rate is generally 75%-85%. The current vacuum brazing process of plate-fin aluminum alloy radiator in my country is: Preparation → parts processing → parts cleaning → parts assembly → vacuum brazing → product inspection after brazing → product drying and painting. The vacuum brazing of aluminum alloy radiators is carried out in a vacuum furnace, and a suitable vacuum brazing furnace can be selected according to the production scale, productivity, size of the weldment and process requirements.

The process parameters of vacuum brazing mainly include: vacuum degree, heating rate, stable temperature and time, brazing temperature, brazing holding time, cooling rate, temperature and so on. They all directly affect the process of filler filling and the interaction between the filler metal and the base metal, and have a decisive effect on the quality of brazing joints. Therefore, it must be studied carefully. The brazing heating rate should be able to ensure that the gas precipitated by the parts is fully extracted, and at the same time, the components should be heated uniformly to reduce or prevent the components from being deformed by the stress caused by sudden heating. The main factors that should be considered to determine the heating rate are:
The material, shape, structure and size of the component
For radiators with complex shapes and large assembly prestress, heating should be slow; for thick product parts, the heating rate should not be too fast. The stable temperature and holding time refer to heating to a temperature close to the solid phase line of the solder during brazing, and at this time, the heating is suspended and the temperature is maintained for a period of time. The purpose is to reduce the temperature gradient of product parts and make the temperature of each part of the product parts uniform. Reduce the large temperature difference between the inner and outer layers.

Vacuum brazing temperature is one of the most important process parameters in the brazing process. At the brazing temperature, on the one hand, the brazing filler metal will melt and fill the joint gap under the capillary action to improve the performance of the brazing joint. The main basis for determining the brazing temperature，the first is the melting point of the selected solder. The brazing temperature should be appropriately higher than the melting point of the solder to reduce the surface tension of the liquid solder, improve wetting and caulking, and enable the solder and the base metal to fully interact, which is beneficial to improve the joint strength . However, too high brazing temperature is harmful. It may cause the components with lower boiling points in the solder to evaporate, the growth of the crystal grains of the parent metal, and the excessive interaction between the solder and the parent metal, which leads to corrosion and intergranular infiltration. Etc., so that the joint strength is reduced. Generally, it is better to choose the brazing temperature between 30 and 100°C higher than the liquidus temperature line of the solder. However, different solders require different temperature ranges above their melting point. The larger the crystallization range of the solder, the more the soldering temperature should be higher than the melting point of the solder. For single element solder, as long as the melting point is 30-70℃ higher, the fluidity of the solder can be in the best state; the solder can be in the best flow state. The holding time of brazing is an important stage of filling the gap and controlling the alloying effect of the brazing filler metal. The effect on the strength of the joint has similar characteristics to the brazing temperature. A certain holding time is necessary for the mutual diffusion of the solder and the base metal to form a firm bond. But too long holding time will also lead to the excessive development of certain processes and go to the opposite side. The choice of brazing holding time is mainly determined by the interaction between the brazing filler metal and the base metal. When the brazing filler metal and the base metal have strong dissolution, formation of brittle phases, causing intergranular penetration and other undesirable interactions, the brazing holding time should be shortened as much as possible. On the contrary, if the brittle phase or structure in the brazing joint can be eliminated through the interaction of the two, it should be suitable, and the holding time of brazing should be extended. Under normal circumstances, under the premise of ensuring good filling, the brazing holding time should be as short as possible to prevent the occurrence of undesirable phenomena such as overheating, corrosion or softening of the base material. The holding time of brazing is also related to the size, thickness and gap of the weldment. For large and thick workpieces, the holding time should be longer than that of small and thin workpieces to ensure uniform heating. When the brazing gap is large, in order to ensure the necessary interaction between the brazing filler metal and the base metal, there should be a longer holding time. Some components are not too thick, but the brazed joints are partially or completely shielded and cannot be directly heated by radiation. The holding time should be extended appropriately to minimize the temperature difference between the surface and the inside of the heated workpiece. The brazing temperature and brazing holding time should not be determined in isolation. There is a certain complementary relationship between them, which can be changed within a certain range accordingly. Although the cooling of the weldment is carried out after the brazing and heat preservation, its cooling rate also has a great influence on the quality of the joint. An excessively slow cooling rate may cause the grains of the base metal to grow, and the strengthening phase may precipitate. Too high a cooling rate may cause cracks in the weldment due to excessive thermal stress, or the rapid solidification of the brazing joints may make it too late for gas to escape Form pores. Generally, the furnace temperature of aluminum and aluminum alloy is below 500℃.

The inside and outside of the vacuum brazing furnace should be kept clean as required. To ensure the conduction capability of the vacuum system pipes, deposits should be removed at any time. Regularly replace the rotary vane vacuum pump, Roots pump, diffusion pump oil.

The vacuum brazing of radiators realized industrial production abroad in the 1970s. In terms of vacuum brazing of radiators, there is still a big gap between my country and foreign advanced level. In order to speed up the development of brazing technology in our country, the following suggestions are made: (1) The internal and external quality of domestic composite aluminum foil and composite plate needs to be improved. At present, domestically produced composite materials have neither price advantage nor quality advantage. Companies mostly use imported composite materials for mass production.

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