Properties of flux for aluminium

【Summary Description】 Aluminum is active, and its surface is easy to form a layer of dense and chemically stable oxides, which is one of the main obstacles to brazing aluminum and aluminum alloys. In order to obtain a high quality joint, the surface oxide must be removed. When brazing aluminum and its alloys, the use of brazing filler metal can remove the oxide film on the aluminum surface and reduce the interfacial tension between the brazing filler metal and the base metal. The brazing agent for aluminum is divided into soft brazing agent and hard brazing agent. Usually, the brazing flux used when the brazing temperature is higher than 450 ℃ is a brazing flux, and the brazing flux used when the brazing temperature is lower than 450 ℃ is a brazing flux. The following describes the rapid development of aluminum flux, especially Nocolok flux. The traditional aluminum flux is mainly chloride flux, generally based on lic 1-KCl or lic 1-KCl-na C1 system. This kind of flux has the advantages of high activity, stable heating and not easy to fail. Moreover, various heat sources can be used, which is convenient and inexpensive. The disadvantage of this kind of flux is that due to the presence of Cl ions, it has strong electrochemical corrosion to the base material, strong hygroscopicity, and the flux is not easy to store. Therefore, it is important to clean the residue when brazing is performed using such a flux. In the late 1970 s, a non-corrosive, insoluble flux developed rapidly. The flux is composed of α-KF eutectic and has little solubility in water. It avoids the shortcomings of chloride flux easy to absorb moisture, corrosion is also very small, so it is also called Nocolok flux. Characteristics of 4.1 fluxes Nokolok solder is a fine white powder whose main component is a mixture of potassium fluoroaluminate, which may contain crystal water. The molten solder will dissolve the oxide on the aluminum surface, preventing it from being oxidized again. Under the action of the flux, the solder can freely penetrate into the bonding surface by capillary action. After cooling, the flux forms a paste-like film with strong adhesion on the surface of the element. The flux residue layer is non-hygroscopic, non-corrosive, and insoluble in water-based solvents. Although the solubility of potassium fluoroaluminate flux in water is very small, but the thermal stability is not good, heating in the air will occur chemical reaction. Improvement and new progress of 4.2 noco lok solders In recent years, a large number of literatures have reported the improved Nocolok method, mainly including two aspects: one is to add a third or more salts to potassium fluoroaluminate solder to increase the activity of the solder and other properties; the other is to develop a new application method of potassium fluoroaluminate solder. Si can improve the activity of potassium fluoroaluminate solder, ideally in the form of K2SiF6, but to calculate the excess KF. When W (Si) > 2%, self-brazing is possible. K2GeF6, SnF2, znF2, and the like are added. In particular, K2GeF6 is preferred for improving the activity of the flux. In a Nocolok improvement, solder powder was mixed with this solder. There is also a method of brazing KA1F4 as a gas phase: one is to directly mix KA1F4 vapor brazing aluminum alloy under a low pressure oxygen-free atmosphere; the other is to vacuum deposit a layer of KA1F4 on the outside of the aluminum piece, and then assemble and braze as needed. The composite solder formed by depositing a layer of KA1F4 solder on the surface of the Al-Si eutectic solder powder can be mixed with an organic solvent to form a solder paste.

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Introduction of several new and commonly used aluminum alloy brazing filler metals

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There are many welding methods used in brazing of aluminum and aluminum alloys.