Brazing methods for aluminum and aluminum alloys
Vacuum brazing of aluminium has been widely used in industrial production. How is the brazing of aluminium and aluminium alloy carried out? Next, I will introduce the brazing methods for aluminum and aluminum alloy.
Vacuum brazing of aluminum alloy is carried out in high altitude air. It is difficult to form a thick oxide film on the surface of aluminium alloy after careful cleaning under high temperature and vacuum. The solder can wet the surface of the base metal without brazing flux to achieve the purpose of brazing. When the vacuum brazing temperature of aluminum alloy is higher than that of solder liquidus and lower than that of base metal solidus, the solder melts into liquid and the base metal remains solid.
Compared with other metal vacuum brazing, aluminum vacuum brazing has its particularity. Magnesium is often used as activator in vacuum brazing of aluminum and aluminum alloy. Among the metal activators which can accelerate aluminum brazing, Mg has high vapor pressure and evaporates easily in vacuum, which is helpful to remove Al2O3 and has a low price. Therefore, Mg has become a widely used activator in vacuum brazing of aluminum alloy. Some elements, such as antimony, bismuth and magnesium, which have higher vapor pressure and higher affinity for oxygen than aluminum, are used as metal activators.
Magnesium as an activator can be directly placed in the form of particles on the workpiece, or in the form of steam into the brazing zone aluminum profile manufacturers, magnesium as an alloy element can be added to the aluminum silicon filler metal.
The amount of magnesium in the filler metal has a significant effect on the wettability of the filler metal. With the increase of magnesium content, the flow coefficient of the filler metal increases. However, with the increase of Mg content, the corrosion of aluminium is also intensified, which is due to the formation of Al-Mg-Si ternary eutectic; and the high content of Mg, the solder is easy to lose and damage the surface of the weldment. Considering the aluminum profiles manufacturers, the Mg of filler metal is 1.0%-1.5%. The results show that adding 0.1% bismuth to Al-Si brazing filler metal can reduce the amount of Mg added to the filler metal, reduce the surface tension of the filler metal, improve the wettability and reduce the requirement of vacuum.
Vacuum aluminum brazing is suitable for butt joints, T-joints and similar joints because these joints are open and the oxide film in the gap is easily removed. The oxide film in lap joint is harder to discharge, so it is not recommended.
The brazing capacity of solder is much worse than that of dip brazing when vacuum brazing, so larger brazing clearance should be used.
The vacuum brazing process of aluminum is basically the same as that of vacuum brazing of other metals. Aluminum profile manufacturers, however, depend on magnesium activator for its film removal. In order to ensure the full effect of magnesium vapor from the base metal, local shielding supplementary process measures are often adopted, i.e. first placing the weldments in stainless steel boxes (commonly known as process boxes) and then heating and brazing in a vacuum furnace. To improve the quality of brazing. If necessary, a small amount of pure magnesium granules can also be used to strengthen the box. The vacuum brazed aluminum surface is clean and smooth, and the brazing seam is compact.
Vacuum brazing opens a way for non-flux brazing of aluminium and improves the quality of brazing products, but it also has some shortcomings, such as complicated equipment, high production cost, difficult maintenance technology of vacuum system, magnesium vapor deposition in furnace wall, heat shield and vacuum system, which affects the workability of equipment. Yes, it needs regular cleaning and maintenance; relying on radiation heating, the speed is slower and the uniformity is poor, especially for large complex weldments, this phenomenon is more significant, so it is suitable for small size, simple structure weldments.