Vacuum brazing of aluminum has been widely used in industrial production. How is brazing of aluminum and aluminum alloys carried out? The following Shanghai Nonferrous Network will introduce you to the brazing methods of aluminum and aluminum alloys.
Vacuum brazing of aluminum alloys is carried out in high vacuum. After careful cleaning, the surface of aluminum alloy is not easy to form a thick oxide film under vacuum and high temperature conditions. The brazing material can wet the surface of the base metal without brazing agent to achieve the purpose of brazing. The temperature of vacuum brazing of aluminum alloy is higher than the liquidus line of the brazing material and lower than the solidus line of the parent material. During brazing, the brazing material melts into liquid state while the parent material remains in solid state.
Vacuum brazing of aluminum has certain particularities compared with vacuum brazing of other metals. Magnesium metal is often used as an activator for vacuum brazing of aluminum and aluminum alloys. Among the metal activators that can accelerate aluminum brazing, Mg has a high vapor pressure and is easy to evaporate under vacuum, which helps to remove Al2O3. It is also relatively cheap, so it has become a commonly used activator in vacuum brazing of aluminum alloys. The metal activators are some elements with higher vapor pressure and greater affinity for oxygen than aluminum, such as antimony, bismuth, magnesium, etc.
Magnesium can be used as an activator directly on the workpiece in the form of particles, or introduced into the brazing area in the form of vapor, or added to the aluminum silicon brazing filler metal as an alloy element.
The amount of magnesium added to the brazing filler metal has a significant effect on the wettability of the brazing filler metal. As the amount of magnesium increases, the flow coefficient of the brazing filler metal increases. However, as the magnesium content increases, the brazing filler metal also intensifies the dissolution of aluminum, which is due to the formation of the Al-Mg-Si ternary eutectic; and if the magnesium content is too high, the brazing filler metal is easy to lose and damage the surface of the weldment. Considering the aluminum profile manufacturer, the ωMg of the brazing filler metal is preferably 1.0%-1.5%. Studies have shown that when adding bismuth with a mass fraction of about 0.1% while adding magnesium to the aluminum silicon brazing filler metal, the amount of magnesium added to the brazing filler metal can be reduced, the surface tension of the brazing filler metal can be reduced, the wettability can be improved, and the requirements for vacuum can be reduced.
Vacuum aluminum brazing is suitable for butt joints, T-type joints and similar joints, because these joints are more open and the oxide film in the gap is easy to remove. The oxide film in the lap joint is more difficult to remove, so it is not recommended.
The spreading ability of the brazing material during vacuum brazing is worse than that during dip brazing, so a larger brazing gap should be used.
The process of vacuum brazing of aluminum is basically the same as that of vacuum brazing of other metals. However, since its film removal depends on the action of magnesium activator, for welds with complex structures, in order to ensure that the parent material obtains the full action of magnesium vapor, local shielding supplementary process measures are often taken, that is, the weld is first placed in a stainless steel box (collectively referred to as a process box), and then placed in a vacuum furnace for heating and brazing, which can significantly improve the brazing quality. If necessary, a small amount of pure magnesium particles can be added to the box to enhance the effect. The surface of vacuum brazed aluminum parts is smooth, the brazing seam is dense, and no cleaning is required after brazing.
Vacuum brazing has opened up a new path for flux-free brazing of aluminum and improved the quality of brazing products, but it also has certain disadvantages, mainly: complex equipment, high production cost, and difficult maintenance technology of the vacuum system; magnesium vapor is deposited on the furnace wall, heat insulation screen and vacuum system, affecting the working performance of the equipment, requiring frequent cleaning and maintenance; it relies on radiation heating, with slow speed and poor uniformity, especially for large and complex weldments, this phenomenon is more significant, so it is suitable for weldments with smaller size and simpler structure.
