Se-lection and Matching of Fluxes with Filler Metals


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  • The effective wetting of the molten solder to the parent metal during brazing depends mainly on the action of the flux. The strength and corrosion resistance of the brazed joint depends primarily on the solder and its interaction with the parent metal.
  • The direct wetting ability of the brazing filler metal to the base metal has a considerable relationship with the nature of the brazing filler metal, but the brazing is much less than the effect of the flux. The concept of flux should be extensive, including molten salt, organic matter, reactive gas, gold vapor, etc., that is, in addition to the base metal and brazing filler metal, it refers to the third material used to reduce the interfacial tension between the base metal and the brazing filler metal. . The molten lead on the molybdenum base material is difficult to wet on the phase diagram or in practice, but it can be completely wetted by the action of a suitable deodorant, but the strength and corrosion resistance of the joint are very poor. .

Selection of Fluxes

  • The function of the flux can be divided into three parts: the matrix, the second is the remover, and the third is the surfactant. Some fluxes can clearly distinguish these three functional parts, such as aluminum flux. The functional part of most fluxes is not clearly defined, but these three functions do exist.
  • The matrix is ​​the main component of the brazing process and controls the melting strength of the flux. It is shy on the surface of the solder joint and acts as a barrier to air. It is also the solvent for other functional components in Xuan Jing. In order to match the entropy point of the brazing filler metal, the deuteration temperature of the flux should be 10~30«C lower than that of the brazing filler metal. In special applications, the flux of the flux is slightly higher than the temperature at which the solder is supplied. If the soldering temperature of the flux is too low, the premature melting will cause the flux component to evaporate and react with the base material, especially the mass transfer rate between the flux and the base material, the flux and the solder. For other reasons, the flux has lost its activity when the solder is supplied. It can be seen that it is important to adjust the composition of the substrate so that the culture temperature matches the melting & degree of the solder.
  • The role of the remover is to remove, break or loosen the surface of the base material by a physicochemical process, so that the molten solder can be moistened.
  • Wet fresh base material surface. The role of the surfactant is to further reduce the interfacial tension between the molten solder and the base metal, so that the molten solder can be spread on the surface of the base material.
  • The base of the aluminum flux is composed of an alkali metal or an alkaline earth gold nitride, and the remover is acted upon by fluorine ions. Since the removal process of the aluminum oxide film is only broken and loosened, the formation of the new oxide film is still strong despite the existence of a certain covering and fluoride ion, and it is necessary to replace the reaction between some heavy metal ions and the base metal. To reduce the wetting of the molten solder to the parent metal by the tension iW. This heavy metal ion acts as a surfactant. When brazing copper alloys and iron alloys at high temperatures, borax (N% B407 • 10H20) is commonly used as a flux. Its melting point is 742. 51C, sometimes too hot and B203, become a binary flux, can get different melting temperatures, as shown in Figure 1-25 [30]. This kind of flux has the function of three and one. The viscous liquid melted by borax becomes the matrix. The oxide film on the surface of the base metal is dissolved in B203 and is removed by the formation of metaborate. The dissolution process of the oxide film is a mass transfer process and acts to reduce the interfacial tension. Sometimes in order to prolong this mass transfer process, even an oxidizing agent such as potassium permanganate [31] 381 is added to the borax, so that the surface valence of the base material is raised to increase the dissolution into the flux.The choice of flux depends on the nature of the oxide film. An alkaline oxide film, for example, an oxide such as Fe, Ni, or Cu, usually using an acidic boronic anhydride (B203)-based flux; an acidic oxide film, for example, an oxide film containing lanthanum SiO 2 in cast iron, A flux containing an alkaline MCO is usually used to form a fusible Na^iO and enter a slag.
  • When brazing some alloy steels or heat-resistant steels containing elements such as &, Ti, Mo, W, etc., since the oxides of these elements are acidic, the oxides of the matrix element Fe are alkaline, The boronic anhydride (B203) is added with a partially basic alkali metal or alkaline earth metal fluoride, so that the flux has a certain double-living character, which improves the activity of the flux against such alloys. In order to simultaneously adjust the melting temperature of the enamel agent, 丨iF(mp 845^), NaF (mp 9951C) or KF( rn_ p. 8511C ) is often added below 850T, and CaF2 (nLp. 1423弋) is often added above 850t: .
  • When brazing some structural steel, corrosion resistant steel and heat resistant steel, and alloys such as copper, silver, gold, etc., if it is desired to braze at a lower temperature, the above fluoride is not used and replaced with potassium fluoroborate (KBF4). Or sodium borohydride (NaBF4). The molten salt listed in Table 1-2 is often used to form a variety of fluxes.Some gasification gases are also often used as fluxes, which have the advantage of uniform reaction and no residue after welding. Bf3 is often used in combination with n2 to braze + stainless steel at high temperatures. The BF3 is available in steel cans. Another liquid trimethyl borate is mixed with BF3 as a gas flux for flame brazing. When a combustible gas such as acetylene or natural gas flows through the above-mentioned mixed liquid in a special tank, the combustible gas is vaporized by the bubbling gas containing the flux. The trimethyl borate is hydrolyzed to b203 in the flame. Wherever the flame is, the workpiece is … that is, a very thin layer of B2o3 melted film, which acts as a enamel agent and protects the surface of the workpiece from oxidative discoloration due to cauterization.
  • Aluminum and magnesium bismuth are relatively difficult to braze, especially magnesium with the highest alloy. Their fluxes are mainly composed of vapors, fluorides and some heavy metal ions. This type of flux is more difficult to braze after brazing, and a little + caution can cause corrosion. Large-area lap joints, slag inclusions in the weld are almost inevitable, forming ant nest-like defects
  • There is often no hole n in the atmosphere of the Unicom. If the surface is machined to form a hole, the Xuan agent will expand due to the gradual absorption of moisture and will damage the joint. The so-called Nwolok (no-corrosive look) flux developed in the past 20 years has become a “corrosion-free flux” because it does not dissolve in water, does not absorb moisture. This is mainly composed of two intermediate compounds K3A1F6 and KA1F4 eutectic molten salts in the A1F3 and KF systems. In recent years, A1F5-CSF and A1F3-CSF-KF fluxes have been developed. The flux containing CsF is particularly suitable for brazing aluminum alloys and magnesium alloys containing Mg piers, which have special effects on removing magnesium oxide films.
  • The titanium alloy flux and the aluminum flux are substantially the same, and are composed of an alkali metal, a base L metal vapor and a fluoride, but since the oxide film of titanium is more difficult to break, the surfactant is generally more active in AgCl and SnCl2.
  • Sometimes it is necessary to braze at a lower temperature. The flux for brazing below 4501 is called a solder. There are two kinds of softeners: one is water-soluble, usually composed of a plurality of aqueous solutions of hydrochloric acid, ZnCl2, SnCl2, NH4C1, H3P03, etc. or a single salt, which has high activity and strong corrosivity, and needs to be cleaned after welding. Brazing of steel, stainless steel, copper alloy, nickel alloy, etc.; another kind of solder is a water-insoluble organic flux, usually based on rosin or artificial resin, adding organic acid, organic amine or its HC1 or HBr The salt is used to improve the ability and activity to dissolve the membrane. In order to adjust the consistency, anhydrous alcohol or isopropanol is often used as a solvent. Such a flux is often brazed in the electronics industry due to its low corrosivity. Aluminum alloy solders are often made from organic heavy hydroxyl fluorides or fluoroborates in organic hydroxylamines, and aluminum brazing avoids the use of aqueous fluxes. Other heat-resistant alloys, such as Cr, Mo, W alloys, Mg alloys, Ti alloys, etc., cannot be brazed with solder under normal conditions.