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Vacuum brazing of aluminum alloys and filler metals

Because aluminum alloy has the advantages of light weight, good corrosion resistance, and good thermal and electrical conductivity, it is used in various sectors, and its usage is increasing year by year. It often appears in aerospace, construction, automobiles, ships and other fields, so it has always been people’s choice. Recognized structural material for making heat exchangers and some complex components. The reason why it is so important is mainly to adopt a vacuum brazing process that does not require flux. The so-called vacuum welding process does not require complex cleaning work before and after welding, has high productivity, simplified operations, and also incorporates the concept of environmental protection.

1 Overview of aluminum alloy vacuum brazing technology

1.1 Filler metal for vacuum brazing of aluminum alloys

Aluminum alloys are a series of materials that are excellent in terms of brazeability, strength, color consistency of the base metal, plating properties and corrosion resistance. Especially after modification treatment, the toughness and toughness of the brazed joints are greatly increased. Bending strength has become a commonly used standard material in production. It is suitable for a variety of aluminum alloys with relatively high melting points. However, remember that it cannot be used for elements with low boiling points, otherwise it will cause extreme damage to the vacuum brazing furnace. Big pollution. Most current data show that brazed composite panels are coated with a layer of brazing alloy on one or both sides of the structural parent metal, because this composite panel is more convenient to use and can be used in a wider range of fields. If the melting point can be further reduced, the solder of this system will have good application prospects. At present, solder alloys that truly have certain corrosion resistance and good mechanical properties have not yet reached the requirements suitable for industrial production. Therefore, when selecting materials, be sure to choose materials with good fluidity and excellent spreadability of the solder, as well as corrosion-resistant properties.

1.2 Vacuum aluminum brazing equipment

With the development of vacuum brazing technology, some aerospace precision components have put forward more stringent requirements on heating rate, brazing temperature, and brazing time when brazing them. Vacuum brazing equipment mainly consists of a vacuum furnace and a vacuum system. This technology has high requirements for vacuum equipment. It requires a sufficient heating rate and a huge pumping speed, and it also needs an advanced and reliable electrical control system for protection. , at the same time, it also needs to meet the forced cooling function to ensure the heat treatment of materials and shorten the brazing cycle. Ordinary containers are evacuated and then sent to the furnace for heating and brazing. Since there are no heating elements and insulation materials, the operation is more complicated. Therefore, vacuuming and heating can be performed at the same time. After brazing, the container exits the furnace and is air-cooled, which can achieve To shorten the production cycle, due to the different operating temperatures of the furnace, different reflective screen materials need to be used. The purpose is to prevent heat from radiating outward, so that the heating efficiency can be improved. At present, semi-continuous furnaces are widely used, and they can greatly improve the quality of aluminum alloy vacuum brazing.

1.3 Aluminum alloy vacuum brazing process

The vacuum brazing process for aluminum alloys is carried out in an ordinary heating furnace. Therefore, a vacuum furnace can be selected based on factors such as production cost and the size and shape of the weldment, and the weldment must be carefully cleaned before brazing. In order to better To remove the thick oxide film generated during heat treatment, it must also be pickled. Because the difference between the solid phase line of aluminum alloys and the brazing temperature is often not large, the thinner parts of the workpiece will easily overheat, causing surface melting. In order to prevent this phenomenon, larger workpieces can be preheated to ensure welding. The parts are heated evenly before reaching the brazing temperature. The most difficult and critical process parameter during vacuum brazing is the degree of vacuum, so the leakage rate requirements for vacuum equipment are very low. Under normal circumstances, when the heating furnace is turned on, it absorbs water vapor in the air at room temperature, which will cause Extend the vacuuming time before the next brazing temperature rise. If the brazing equipment has not been used for a long time, it is necessary to let the vacuum equipment run empty for several hours before using it again. Especially when the production volume is concentrated, the time interval between two uses should be as short as possible to ensure that the required requirements are met quickly. In order to achieve efficient production in the end, it is also necessary to thoroughly clean the surface of the brazed aluminum alloy parts, control the temperature, and shorten the time as much as possible while ensuring penetration.

2 Development of aluminum alloy vacuum brazing technology

2.1 Principle of aluminum alloy vacuum brazing

Since there is a dense and very stable oxide film on the surface of the aluminum alloy, it becomes the main obstacle for the molten solder to wet the base metal. However, it is difficult to remove the oxide film just by relying on simple vacuum conditions, so the activation of some metals is difficult. Came in handy. Earlier, some people believed that the purpose of film removal is achieved through the action of magnesium activator. In this mechanism, magnesium plays a great role, which shows that magnesium can be beneficial to vacuum brazing, mainly by destroying surface oxidation. The film is combined with the base material so that the molten solder can wet the base material and spread on the base material under the film to achieve the effect of floating the surface film and removing it. Although this technology has been widely used, it is In actual production, many scrapped brazed joints will appear. Since aluminum alloy vacuum brazing is very sensitive to subtle changes in parameters, even if the compositions of the brazing alloy coatings prefabricated by different manufacturers are within the regulations, the brazing joints will still be significantly different, which requires careful attention in real life. Accumulation of experience.

2.2 Development of aluminum alloy vacuum brazing

Vacuum welding technology refers to a method of brazing under vacuum conditions without using flux. Compared with traditional methods, it has the characteristics of improving joint quality and reducing scrap rate, and has excellent environmental protection and energy saving advantages.

Vacuum brazing has opened up a new situation for the processing and assembly of aluminum alloys, which will become a huge driving force for the development of the aluminum alloy industry. With the development of this technology, different problems have also emerged, such as relatively high production costs and complex equipment. As well as difficulty in maintenance, etc., in actual production, it also appeared that the vapor of elements was deposited in the furnace heat shield and vacuum system, which hindered the working performance of the equipment, thereby affecting the quality of the brazed products, and This technology relies on radiation heating, which inevitably results in slow speed and poor uniformity. In addition, the solidus lines of existing aluminum solders are relatively close to those of most aluminum alloys, making the temperature and holding time during brazing difficult. Achieve precise control.