Alternate Vacuum Brazing Process for Aluminum Alloy Materials

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With the development of avionics systems, the requirements for airborne electronic equipment are getting higher and higher, and the chassis of airborne electronic equipment is required to have a complex and compact structure, strong electrical and thermal conductivity, high heat conduction efficiency, and can adapt to complex electromagnetic conditions, high temperature, humidity and other environments. condition. Under this requirement, aluminum alloy vacuum brazing chassis and modules have been widely used. After the aluminum alloy chassis is vacuum brazed, the welding precision is high, and the precision welding parts can be processed with less allowance to meet the precision requirements of the product.

Alternate vacuum brazing of 6061 and 3A21 materials

Due to limited production resources, the same vacuum brazing furnace will weld boxes of different materials in actual production. When a vacuum brazing furnace continuously welds 6061 material, and then uses the vacuum brazing furnace to weld 3A21 material, it is found that the one-time welding success rate of 3A21 material is only 75%, and the unwelded box has different degrees of weld cracking. After finishing the box body, cracks were found in the weld seam of the box body. These defects basically exist inside the brazing seam, and will be exposed on the surface of the brazing seam after machining. The presence of these brazing defects will reduce the air tightness, water tightness and joint strength of the box. There are many reasons for the defect, which may be that the surface of the brazing parts is not cleaned in place, the gap of the brazing joint is not suitable, the melting temperature of the brazing filler metal and the base metal does not match, and the base metal or the brazing filler metal releases gas during the brazing process.

Vacuum brazing of 6061, 3A21 material

The process flow of vacuum brazing of 6061 material and 3A21 material is the same. First, the trial assembly is carried out. After the trial assembly is no problem, it is disassembled, and then each component is subjected to alkali cleaning, pickling and other processes. The parts are reassembled after cleaning is complete, and the solder is assembled during the assembly process. After the box body is assembled, it is put into the furnace for vacuum brazing. The process diagram of vacuum brazing is as follows.

The solder used for vacuum brazing of 3A21 material is 4004. The solder used for 6061 material is different from that of 3A21 material. The solid liquidus temperature of the solder used in the 6061 material is relatively low, and the solder with the composition of Al-SiCuMg is selected.

The welding temperature of vacuum brazing is related to the solid and liquidus temperature of the brazing filler metal and the base metal. In actual production, the temperature of vacuum brazing of 3A21 material is about 610℃, and the temperature of vacuum brazing of 6061 material is about 590℃, and the difference of vacuum brazing temperature of the two materials is 20℃. After processing according to the general vacuum brazing process, the 3A21 material case and the 6061 material case have no external defects, the welding seam surface is good, the size of the brazing angle is suitable, and the transition is smooth.

Analysis of the reasons for the brazing seam after finishing

The process flow of vacuum brazing of the two material chassis is the same, the vacuum brazing temperature is different, and the brazing material is different. The biggest difference in the element content of the two brazing filler metals is Mg. The Mg content in the AlSiCuMg used in the 6061 material is much larger than that in the 4004 brazing filler metal. The elements in the alloy are volatilized during vacuum heating. As the temperature increases, the saturated vapor pressure of the elements increases. When the saturated vapor pressure of the elements is greater than the gas pressure in the vacuum chamber, the metal elements begin to volatilize strongly. The volatilization of metal elements causes serious pollution to the vacuum furnace. First weld the 6061 material chassis, then weld the 3A21 material chassis, and use the same vacuum brazing furnace for the two material chassis. When welding 3A21 material chassis, since the temperature is higher than the welding temperature of 6061 chassis, the Mg element attached to the vacuum brazing furnace begins to volatilize as the temperature rises. As an activator of aluminum alloy, Mg reduces and destroys the oxide film on the surface of aluminum alloy, and accelerates the flow of solder around the chassis. Because the metal surface inside the brazing gap cannot be absolutely flush, and the cleanliness is also different, plus the physical and chemical effects of the liquid brazing filler metal passing through the aluminum alloy surface and other factors, the brazing filler metal moves toward the gap at an uneven speed when filling the gap. advance forward. The temperature around the solder joint tends to be higher than the temperature in the gap, in which case the liquid solder will flow faster. When the flow velocity of the peripheral solder is greater than that of the inner solder, a surrounding phenomenon will be formed, resulting in internal defects of vacuum brazing. During the welding process, the base metal and the brazing filler metal will release gas in the process, and these gases will not be able to completely remove the brazing seam before the brazing filler metal solidifies, which will strengthen the formation of the surrounding phenomenon.

After vacuum brazing, no surrounding defects can be found from the appearance of the box. The surrounding defects belong to the internal defects of welding, which will appear after finishing. From the above analysis, a guess can be drawn that the cause of the cracking of the weld seam after finishing the box body is the existence of surrounding defects.

Vacuum brazing process optimization

This defect occurs when the 6061 material and the 3A21 material chassis are alternately welded. The two materials use the same vacuum brazing furnace, and the 6061 material is vacuum brazed first, and then the 3A21 material chassis is vacuum brazed. The existing technological process is optimized, and pretreatment is carried out before vacuum brazing of 3A21 material. The pretreatment is to design the vacuum furnace air-fired heating program, and set a higher temperature, which is higher than the brazing temperature of 3A21 material, so that the Mg element attached to the vacuum brazing furnace is volatilized and pulled out of the vacuum furnace.

After adding pretreatment, the one-time welding success rate of 3A21 material has increased from the original 75% to more than 99%, which saves the cost of repair welding for joint cracking, and ensures that the box brazing process of this material is completed in advance or according to the node. .

Equipment selection: The RHVB-H vacuum brazing furnace provided by SIMUWU is an excellent product for processing the vacuum brazing process. It has the characteristics of good temperature uniformity and high temperature control accuracy. SIMUWU provides a professional team of engineers who can solve various problems encountered in the production process and are committed to giving customers the most convenient and efficient experience.