Brazing of aluminium and aluminium alloy
The solderability of aluminum and its alloys is poor. The main reason is that the oxide film on their surface is difficult to remove. Aluminum has a great affinity for oxygen. On the surface, it is very simple to form a fine and stable oxide film Al2O3 with high melting point. Together with aluminum alloy containing magnesium, it will also form a very stable oxide film MgO. They will severely prevent the solder from wetting and spreading. And it's hard to get rid of. In brazing, only choosing appropriate flux can make the brazing process go on.
Secondly, the operation of aluminum and aluminum alloy brazing is very difficult. The melting point of aluminum and aluminum alloy is not much different from that of the used brazing filler metal. The temperature scale that can be selected during brazing is very narrow. If the temperature control is a little improper, it will simply cause the base metal to overheat or even melt, making the brazing process difficult to carry out. Some aluminum alloys strengthened by heat treatment will be softened by over aging or annealing due to brazing heating, resulting in the degradation of the properties of brazed joints. In the process of flame brazing, it is not easy to distinguish the concavity and convexity of temperature because the color of vacuum brazing aluminum alloy does not change during heating, which also increases the demand for the operator's operation level.
Moreover, the corrosion resistance of aluminum and aluminum alloy brazed joints is easily affected by the filler metal and flux. The electrode potential of aluminum and its alloy is quite different from that of solder, which makes the corrosion resistance of the joint decrease, especially the effect on the soft solder joint. In addition, most of the brazing fluxes used in the brazing of aluminum and aluminum alloys are highly corrosive. Even if they are cleaned after brazing, the effect of the fluxes on the corrosion resistance of the joints will not be completely eliminated.
2. Brazing data
(1) The soldering of solder aluminum and aluminum alloy is not often used. Because of the difference of composition and electrode potential between solder and base metal in soldering, the joint is prone to electrochemical corrosion. According to the application temperature scale, it can be divided into low temperature solder (150-260 ℃), medium temperature solder (260-370 ℃) and high temperature solder (370-430 ℃). When tin lead solder is selected and copper or nickel plating is applied on the surface of aluminum for brazing, corrosion can be prevented at the interface of the joint, and then the corrosion resistance of the joint can be improved.
Aluminum and aluminum alloy brazing methods are widely used, such as filter guide, evaporator, radiator and many other parts choose brazing methods. The brazing of aluminum and aluminum alloy can only use aluminum base solder, of which aluminum silicon solder is the most widely used. The detailed selection scale and the shear strength of the brazed joint are shown in Table 8 and table 9. However, the melting point of the solder is close to that of the base metal, so the heating temperature should be controlled strictly and accurately to avoid overheating or melting of the base metal.