Brazing

For the cooking technique, see braising.

Brazing is a joining process whereby a non-ferrous filler metal or alloy is heated to melting temperature (above 450°C; 800°F) and distributed between two or more close-fitting parts by capillary action. At its liquid temperature, the molten filler metal and flux 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 the adjacent layers. Common brazements are about 1/3 as strong as the materials they join because the metals partially dissolve each other at the interface and usually the grain structure and joint alloy is uncontrolled. To create high-strength brazes, sometimes a brazement can be annealed, or cooled at a controlled rate, so that the joint's grain structure and alloying is controlled.

Techniques

Silver brazing

If silver alloy is used, brazing can be referred to as 'silver brazing'. Colloquially, the inaccurate terms "silver soldering" or "hard soldering" are used, to distinguish from the process of low temperature soldering that is done with solder having a melting point below 450 °C (800 °F). Silver 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. Likewise, silver brazing often requires the prior machining of parts to be joined to very close tolerances prior to joining them, to establish a joint gap distance of a few mils for proper capillary action during joining of parts, whereas soldering does not require gap distances that are nearly this small for successful joining of parts. Silver brazing works especially well for joining tubular thick-walled metal pipes, provided the proper fit-up is done prior to joining the parts..

Braze welding

In another similar 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, unlike the prior silver brazing example. Braze welding takes place at the melting temperature of the filler (e.g., 1600 °F to 1800 °F or 870 °C to 980 °C for bronze alloys) which is often considerably lower than the melting point of the base material (e.g., 2900 °F (1600 °C) for mild steel).

Requirements

In order to work properly, parts must be closely fitted and the base metals must be exceptionally clean and free of oxides for achieving the highest strengths for brazed joints. For capillary action to be effective, joint clearances of 0.002 to 0.006 inch (50 to 150 µm) are recommended. In braze-welding, where a thick bead is deposited, tolerances may be relaxed to 0.020 inch (0.5 mm). Cleaning of surfaces can be done in several ways. Whichever method is selected, 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 (not braze welding), it is vitally important to use sufficiently fine abrasive. Coarse abrasive can lead to deep scoring that interferes with capillary action and final bond strength. Residual particulates from sanding should be thoroughly cleaned from pieces. In assembly line work, a "pickling bath" is often used to dissolve oxides chemically. Diluted sulfuric acid is often used. Pickling is also often employed on metals like aluminum that are particularly prone to oxidation.

Usage of flux

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. Commercial pastes can be purchased or made up from powder combined with water (or in some cases, alcohol). Brazing pastes are also commercially available, combining filler metal powder, flux powder, and a non-reacting vehicle binder. 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, or a flux core. 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), followed by wire brushing the remainder.

Brazing strength/Joint geometry

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 but if properly designed & executed, a brazed joint is stronger than the parent metal. Careful matching of joint geometry to the forces acting on the joint & properly maintained clearance between two mating parts however, can lead to very strong brazed joints, too. 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 braze joints roughly equivalent to a weld, a general rule of thumb is to make the overlap equal to 3 times the thickness of the pieces of metal being joined.

Filler materials

A variety of alloys of metals, including silver, tin, zinc, copper and others are used as filler for brazing processes. There are specific brazing alloys and fluxes recommended, depending on which metals are to be joined. Metals such as aluminum can be brazed, although aluminum requires more skill and special fluxes. It conducts heat much better than steel and is more prone to oxidation. Some metals, such as titanium cannot be brazed because they are insoluble with other metals, or have an oxide layer that forms too quickly at high temperatures.

Brazing filler material is commonly available as flux-coated rods, very similar to stick-welding electrodes. Typical sizes are 1/8" diameter. Some widely available filler materials are:

Nickel-Silver: Usually with blue flux coating. 85,000 psi tensile strength, 1250-1750°F working temperature. Used for carbon and alloy steels and most metals not including aluminum.
Bronze: Available with white borax flux coating. 60,000 psi tensile strength. 1600°F working temperature. Used for copper, steel, galvanized metal, and other metals not including aluminum.
Brass: Uncoated plain brass brazing rod is often used, but requires the use of some type of additional flux.

Cast iron "welding"

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.

Vacuum brazing

Vacuum brazing is another materials joining technique, one that offers extremely clean, superior, flux-free braze joints while providing high integrity and strength. The process can be expensive because it is performed inside a vacuum chamber vessel; however, the advantages are significant. For example, furnace operating temperatures, when using specialized vacuum vessels, can reach temperatures of 2400 °C. Other high temperature vacuum furnaces are available ranging from 1500 °C and up at a much lesser cost. Temperature uniformity is maintained on the work piece when heating in a vacuum, greatly reducing residual stresses because of slow heating and cooling cycles. This, in turn, can have a significant impact on the thermal and mechanical properties of the material, thus providing unique heat treatment capabilities. One such capability is heat treating or age hardening the work piece while performing a metal-joining process, all in a single furnace thermal cycle.

Advantages of brazing

Although there is a popular belief that brazing is an inferior substitute for welding, it has advantages over welding in many situations. For example, brazing brass has a strength and hardness near that of mild steel and is much more corrosion-resistant. In some applications, brazing is highly preferred. For example, silver brazing is the customary method of joining high-reliability, controlled-strength corrosion-resistant piping such as a nuclear submarine's seawater coolant pipes. Silver brazed parts can also be precisely machined after joining, to hide the presence of the joint to all but the most discerning observers, whereas it is nearly impossible to machine welds having any residual slag present and still hide joints.

The lower temperature of brazing and brass-welding is less likely to distort the work piece, significantly change the crystalline structure (create a heat affected zone) or induce thermal stresses. For example, when large iron castings crack, it is almost always impractical to repair them with welding. In order to weld cast-iron without recracking it from thermal stress, the work piece must be hot-soaked to 1600 °F. When a large (more than fifty kilograms (100 lb)) casting cracks in an industrial setting, heat-soaking it for welding is almost always impractical. Often the casting only needs to be watertight, or take mild mechanical stress. Brazing is the preferred repair method in these cases.

The lower temperature 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 capillary 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.

Possible problems

A brazing operation may cause defects in the base metal, especially if it is in stress. This can be due either to the material not being properly annealed before brazing, or to thermal expansion stress during heating.

An example of this is the silver brazing of copper-nickel alloys, where even moderate stress in the base material causes intergranular penetration by molten filler material during brazing, resulting in cracking at the joint.

Any flux residues left after brazing (inside or out) must be thoroughly removed; otherwise, severe corrosion may eventually occur.

Brazing processes

Pinbrazing
Block Brazing
Diffusion Brazing
Dip Brazing
Exothermic Brazing
Flow Brazing
Furnace Brazing
Induction Brazing
Infrared Brazing
Resistance Brazing
Torch Brazing
Twin Carbon Arc Brazing
Vacuum Brazing

External links

The Brazing Guide - Information on industrial brazing procedures, atmospheres, alloys and equipment
The Brazing Book - A manual also available in print
American Welding Society, publishers of industry standards on brazing
AWS Brazing and soldering forum
European Association for Brazing and Soldering - A detailed technical library and information about brazing services.

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