From Academic Kids

Brazing, not to be confused with Braising, is a joining process whereby a non-ferrous filler metal and an alloy are heated to melting temperature (above 450°C / 800 °F) and distributed between two or more close-fitting parts by capillary action. At its liquidus temperature, the molten filler metal interacts with a thin layer of the base metal, cooling to form an exceptionally strong, sealed joint due to grain structure interaction. The brazed joint becomes a sandwich of different layers, each metallurgically linked to each other. If silver alloy is used, brazing can be referred to as Silver Brazing. Colloquially, the inaccurate terms "Silver Soldering" or "Hard Soldering" are used.

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In Braze Welding or Fillet Brazing, a bead of filler material reinforces the joint. A braze-welded tee joint is shown here.
In the more common, more specific usage, brazing is the use of a bronze or brass filler rod coated with flux, together with an oxyacetylene torch, to join pieces of steel. The American Welding Society prefers to use the term "Braze Welding" for this process, as capillary attraction is not involved. Braze welding takes place at the melting temperature of the filler (e.g., 1600-1800 F for Bronze alloys) which is often considerably lower than the melting point of the base material (e.g., 2900 F for mild steel) and therefore less likely to distort the work piece or induce thermal stresses. The lower heat input associated with brazing vs. welding can increase joining speed and reduce fuel gas consumption. Brazing can be easier for beginners to learn than welding. For thin workpieces (e.g., sheet metal or thin-walled pipe) brazing is less likely to result in burn through. Brazing can also be a cheap and effective technique for mass production. Components can be assembled with preformed plugs of filler material positioned at joints and then heated in a furnace or passed through heating stations on an assembly line. The heated filler then flows into the joints by capilliary action. Braze-welded joints generally have smooth attractive beads that do not require additional grinding or finishing. The most common filler materials are gold in colour, but fillers that more closely match the color of the base materials can be used if appearance is important.

A variety of alloys of metals, including Silver, Tin, Zinc, Copper and others are used as filler for both processes. There are specific brazing alloys and fluxes recommended, depending on which metals are to be joined. Metals such as aluminum can be brazed though aluminum requires more skill. It conducts heat much better than steel and is more prone to oxidation. Some metals, such as Titanium cannot be brazed.

In order to work properly, parts must be closely fitted and the base metals must be exceptionally clean and free of oxides. For capilliary action to be effective, joint clearances of 0.002 to 0.006 inches are recommended. In braze-welding where a thick bead is deposited, tolerances may be relaxed to 0.02 inches or about 0.5 mm. Cleaning of surfaces can be done in a couple of ways. It is vitally important to remove all grease, oils, and paint. For custom jobs and part work, this can often be done with fine sand paper or steel wool. In pure brazing it is important to use sufficiently fine abrasive. Coarse abrasive can lead to deep scoring that interferes with capiliary action and final bonding strength. Residual particulate from sanding should be thoroughly cleaned from pieces. In assembly line work, a "pickling bath" is often used to chemically dissolve oxides. Dilute sulfuric acid is often used. Pickling is also often employed on metals like aluminum that are particularly prone to oxidation.

In most cases, flux is required to prevent oxides from forming while the metal is heated. The most common fluxes for bronze brazing are borax-based. The flux can be applied in a number of ways. It can be applied as a paste with a brush directly to the parts to be brazed. Commerical pastes can be purchased or made up from powder combined with water (or in some cases, alcohol). Alternatively, brazing rods can be heated and then dipped into dry flux powder to coat them in flux. Brazing rods can also be purchased with a coating of flux. In either case, the flux flows into the joint when the rod is applied to the heated joint. Using a special torch head, special flux powders can be blown onto the workpiece using the torch flame itself. Excess flux should be removed when the joint is completed. Flux left in the joint can lead to corrosion. During the brazing process, flux may char and adhere to the work piece. Often this is removed by quenching the still hot workpiece in water (to loosen the flux scale) and then wire brushing the remainder.

Brazing is similar to soldering but higher temperatures are used and the filler metal has a significantly different composition and higher melting point than solder.

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The lap joint has a large surface area and resists tensile forces by shearing

Brazing is different from welding, where even higher temperatures are used, the base material melts and the filler material (if used at all) has the same composition as the base material. Given two joints with the same geometry, brazed joints are generally not as strong as welded joints. Careful matching of joint geometry to the forces acting on the joint, however, can often lead to very strong brazed joints. The butt joint is the weakest geometry for tensile forces. The lap joint is much stronger, as it resists through shearing action rather than tensile pull and its surface area is much larger. To get joints roughly equivalent to a weld, a general rule of thumb is to make the overlap equal to 3 times the width of the pieces of metal being joined.

The "welding" of cast iron is usually a brazing operation, with a filler rod made chiefly of nickel being used although true welding with cast iron rods is also available.

Brazing processes

  • Block Brazing
  • Diffusion Brazing
  • Dip Brazing
  • Exothermic Brazing
  • Flow Brazing
  • Furnace Brazing
  • Induction Brazing
  • Infrared Brazing
  • Resistance Brazing
  • Torch Brazing
  • Twin Carbon Arc Brazingda:Slaglodning



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