In our lives, many families are established through the matchmaking of a "matchmaker" who is good at observing words and emotions. Sometimes, metals also need to have such a "matchmaking" similar to the "matchmaker" of a marriage agency to be able to join together.
In daily life, if something goes wrong with an electrical appliance and needs to be soldered, we often encounter situations where the solder is not easy to hang on. Especially when welding aluminum products and stainless steel, the solder often forms a ball that does not stick to it, making it very difficult to weld. What if the two don't "combine"?
To answer such a question and find a solution, we must first analyze the reasons for their occurrence.
It turns out that the surface of aluminum products as we have known before generally has an extremely fine and uniform protective film of aluminum oxide (Al2O3). Others, such as stainless steel, are alloys containing metals such as nickel. Because there is an oxide film on the metal surface, it often hinders the "combination" of the two. It is precisely because there is no such strong oxide on the surface of copper that copper is easier to weld. Therefore, many conductors in electrical appliances are made of copper.
When we want to braze a metal, if the solder used can be firmly bonded to the metal, it can be considered that a strong solid solution or intermetallic compound is formed between the solder and the metal, and a strong metal bond is formed. However, when there are oxides on the surface of the metal, the oxide will hinder the bonding between the solder and the metal aluminum, that is, the oxide (Al2O3) and the Sn-Pb alloy can only be a heterogeneous combination, mainly forming a van der Waals bond with weak binding force, so the phenomenon seen is that the solder can not be well attached to the metal. Of course, there are other more complicated reasons for this phenomenon.
In the past, soldering masters usually used a hydrochloric acid solution containing zinc when welding barrels or pots. At this time, it seemed that brazing was very simple. But it also brings people such a question, "Why do we have to use hydrochloric acid, what is the role of hydrochloric acid?" Now we know that hydrochloric acid has the same effect as the solder paste usually used in brazing. They are always called fluxes when brazing, and they usually change because of the difference between the solder and the weldment. Do you see whether these fluxes are like matchmakers who "combine" metals? However, zinc hydrochloric acid aqueous solution or solder paste and other flux, if not used properly, welding will not be solid. However, as long as you can choose flux, that is, "matchmaker", even aluminum can be brazed. Therefore, understanding the role of flux will be the key for us to understand the "bonding" between metals.
The functions of flux are roughly as follows:
(1) Remove the oxide film on the surface of the weldment (the metal to be welded) through chemical reaction to form a clean metal surface.
(2) A certain chemical reaction occurs with the solder to promote wetting of the solder on the metal surface (expansion force).
(3) After completing the functions of (1) and (2), it does not directly participate in the combination between metal and solder and exits the "reaction".
The most common metal welding parts used for brazing are copper. Commonly used fluxes include organic acids (stearic acid, etc.), organic salts (aniline hydrochloride), inorganic acids and salts (zinc - chloride hydrochloride).
Stearic Acid Product List
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Aniline Hydrochloride Product List
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When brazing aluminum products, fluorine-containing compounds such as fluoroboric acid and sodium fluoride can be used as fluxes because these substances have the ability to remove oxides that are firmly attached to the metal surface. If the solder selected at this time is also zinc-containing "aluminum solder", not only can it react with aluminum, but it can also be soldered firmly. Therefore, the larger the solder spreads on the metal plate, the better the soldering effect will be. Next, let's talk about the important role played by the flux that dominates the expansion [promoting wetting in (2)] during soldering.
When we heat a certain amount of solder on a copper plate in air, the solder melts into a ball. If the flux (stearic acid) is used, the spherical solder can be seen expanding on the copper plate.
If we observe this diffusion phenomenon carefully, we can see that a green compound is formed where the flux, copper and air come into contact with each other on the copper plate, which spreads to the molten solder and becomes colorless and transparent when it comes into contact with the solder. At the same time, the solder is wetted on the copper plate.
The reaction between solder and copper stearate is carried out according to the three-step procedure mentioned above. Only when the two react, can the wetting area be increased, and the acceleration of wetting is beneficial to the reaction.
The wetting between copper and solder is based on the reaction between Cu and Sn to form Cu3Sn5, and the flux can accelerate the wetting, which can be considered as the reaction between Cu and Sn as in formula (1).
Using an X-ray microanalyzer to perform ray analysis on the cross-section of the wetted sample, we can find that the Cu-Sn reaction occurs at the interface between copper and solder and on the surface of the solder, and the Cu brought by the flux is dissolved in the solder. Other fluxes, such as hydrochloric acid, perform the reaction of formula (2) through the generated copper chloride. At this time, the reaction between Cu and Sn also occurs, which also accelerates wetting.
To sum up, in the process of brazing, the promotion and expansion of wetting is due to the reaction between flux, solder, and solder metal. Therefore, if the oxide film on the metal surface is removed and a flux that can promote wetting is used, then no matter what kind of metal it is, it is easier to braze.
The metal aluminum itself can accept solder at any time, but because it does not create this state without surface film, such as the previous use of zinc-containing hydrochloric acid solution, it is to create such a state, so aluminum products often repel the solder.
