Vacuum brazing technology for aluminum alloy
Careful cleaning is required before vacuum brazing. Usually, oil treatment is sufficient for welded parts, but cleaning agents are sometimes required to remove the thick oxide film formed during heat treatment. The temperature cycle depends largely on the shape of the welds due to various factors, as heat is transmitted in the form of radiation in a vacuum and parts of different working thicknesses are heated inhomogenously. Because the difference between the solid and brazing temperatures of aluminum alloys is usually small, the thinner parts of the workpiece tend to overheat, leading to surface melting. So there are
The workpiece must be preheated, especially the larger workpiece, to ensure uniform heating of the component before reaching the brazing temperature. As the removal of the oxide film on the surface of vacuum brazing aluminum alloy mainly depends on Mg activator, in order to ensure that the base metal is fully affected by Mg vapor, some domestic research institutions often adopt the auxiliary technical measures of partial shielding and achieve good results. The most common method is to place the welded parts in a stainless steel box (called a process box) and then place them in a vacuum furnace for brazing, which can significantly improve the brazing quality, in which the formation of magnesium vapor can be used. Solid Mg particles are added to the filler metal or directly introduced into Mg vapor. This can greatly reduce the pollution of magnesium and other elements in the furnace during charging and discharging, reduce the cleaning times of the vacuum brazing furnace, and improve the service life of the vacuum brazing furnace.
Vacuum brazing is the most important and difficult process parameter to control. In order to obtain excellent joint, the vacuum degree of vacuum brazing furnace depends on very low leakage rate of vacuum equipment to a great extent. During the heating process, the furnace is pumped to a certain vacuum degree. The requirements for aluminum and its alloys are 0.0013Pa to 0.o133pa. It is generally believed that when the vacuum brazing furnace is opened, water vapor in the ambient temperature air is absorbed, which greatly extends the vacuum time before the next vacuum brazing temperature rise. According to experience, if the vacuum brazing furnace is not used for a long time, the vacuum brazing furnace should be used after baking. When in use, especially in batch production, the time interval between the two entrances and exits should be as short as possible, so that the vacuum degree of the vacuum brazing furnace can easily and quickly meet the requirements, thus improving the production efficiency. The heating rate, stable temperature and holding time of the vacuum brazing furnace temperature depend on the material, shape, structure, size, solder morphology and solder crystallization temperature range.