① Aluminum has a great affinity for oxygen. It is easy to form a layer of dense, chemically stable and high melting point alumina on the surface, which is difficult to remove. Thus, it hinders the wetting and combination of needle material and base metal, which has become one of the main obstacles of brazing.
② When brazing with brazing flux, the brazing temperature must be strictly controlled because there is little difference between the melting point of solder and that of aluminum. During flame brazing, because the color of aluminum remains unchanged during heating, it is difficult to judge the temperature, so the technical level of the operator is required to be high.
③ When using soft flux, the great difference of electrode potential between solder and base metal brings adverse factors to the corrosion resistance of brazed joints.
④ The flux used to remove the alumina film is highly corrosive. If the needles are not cleaned immediately after welding, there is a risk of corrosion and rapid damage to the connectors.
In short, the main difficulty in realizing the brazing connection between aluminum pipe profile and steel pipe and boiler pipe lies in; It is necessary to select the film removal measures that can meet the oxide requirements on the surface of two different base metals; The intermetallic compound layer is often formed in the brazed joint, which makes the joint brittle; The thermal expansion coefficient and electrode potential of aluminum are too different from other metals. For the former two problems, it can be solved by plating a metal layer on the surface of aluminum or another base metal; The latter can be solved by vacuum brazing and lillock ring welding.
From the function of the plating layer on the surface of the base metal, it can be divided into three categories; That is, process coating, barrier mirror layer and solder mirror layer. The application conditions and specific functions of these three types of coatings are different.
The process key layer is mainly used to improve or simplify the brazing process conditions. Therefore, it can be used for the base metal of aluminum pipe profile with strong oxidation to protect it from oxidation, so that it can obtain high-quality joints under low process conditions (such as using shielding gas with low purity, low vacuum and flux with poor activity). The coating on the base metal is in the brazing process, It should be able to dissolve the solder as a whole to obtain better bonding strength; The function of barrier coating is to inhibit some harmful reactions that may occur in the brazing process (such as self cracking of base metal under the action of solder, brittle compounds produced by the reaction between solder and base metal, etc.).
In order to play a blocking role, it is required that the coating can be wetted with liquid solder without being dissolved. The direct use of solder coating is as solder, but the more important role is to reduce the defects of brazing joint and improve the compactness. The solder coating is generally a full-component needle material, and sometimes it may also be a component of the solder. The solder is formed by reacting with the base metal during heating. When the aluminum pipe profile is brazed with copper pipe and steel pipe, the aluminum surface can be galvanized and then plated and forged. Then, tin lead solder (hl604) can be used for brazing according to the general brazing process.
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