Brazing

Overview

Brazing is a joining process whereby a filler metal or alloy is heated to melting temperature above 450°C (842°F), or, by the traditional definition that has been used in the United States, above 800°F (425°C) 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 a strong, sealed joint. By definition the melting temperature of the braze alloy is lower (sometimes substantially) than the melting temperature of the materials being joined. 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 parent materials due either to the inherent lower yield strength of the braze alloy or to the low fracture toughness of intermetallic components. To create high-strength brazes, a brazement can be annealed to homogenize the grain structure and composition (by diffusion) with that of the parent material.

Variations in Definition

Brazing has historically been defined in many ways and is often confused with soldering. A defining characteristic is that the braze melts while the material(s) being joined do not. The distinction between brazing and soldering is largely semantic; brazing occurs at a higher temperature than soldering. One definition of brazing is “joining of two materials using a third, dissimilar material at higher temperatures than soldering.” While the exact temperature difference between brazing and soldering is often disputed, there are definite metallurgical reasons to use the 840°F figure. This is the official American Welding Society definition.

Braze alloy is often used to define an alloy that flows in thin joints while braze filler metal is used for thicker joints and for gap filling.

Common techniques

Furnace brazing

The furnace brazing method is accomplished by assembling the material to be brazed and the filler metal in the appropriate configurations and then placing the assembly in a furnace where it is heated uniformly.

File:Furnacebrazing.png

furnace brazing schematic.

Furnace brazing is practical when the brazing material can be in contact with the joint, and the part can survive uniform heating. This process is generally used for applications that need high volume production. When it is an applicable process, it offers the benefits of a controlled heat cycle, no post braze cleaning, and no skilled labor needed. The type of furnace used depends on whether batch or continuous operation is desired and can be designed to have a protective atmosphere to eliminate the need of protective flux in the filler metal. The type of atmosphere depends on the filler metal and the material being brazed. Common atmospheres used include hydrogen based and vacuum. In a hydrogen atmosphere, the gas cleans braze components and eliminates the need for flux. It is often mixed with inert gasses such as nitrogen, argon, or helium to lower the overall percentage of hydrogen in the furnace atmosphere. When a vacuum furnace is used, heat treating processes can be combined with the brazing process. Vacuum furnaces typically require a larger capital investment but also produce products of typically higher quality.

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 (842 °F), or, as traditionally defined in the United States, having a melting point below 800°F or 425°C. 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. Silver brazing requires a gap not greater than a few micrometres or mils for proper capillary action during joining of parts. (Soldering also uses capillary action to fill small spaces, although the need for small gap distances may be less critical than in brazing.) This often requires parts to be silver brazed to be machined to close tolerances.

Brazing is widely used in the tool industry to fasten hardmetal (carbide, ceramics, cermet, and similar) tips to tools such as saw blades. “Pretinning” is often done: the braze alloy is melted onto the hardmetal tip, which is placed next to the steel and remelted. Pretinning gets around the problem that hardmetals are hard to wet.

Brazed hardmetal joints are typically two thousandths to seven thousandths of an inch thick. The braze alloy joins the materials and compensates for the difference in their expansion rates. In addition it provides a cushion between the hard carbide tip and the hard steel which softens impact and prevents tip loss and damage, much as the suspension on a vehicle helps prevent damage to both the tires and the vehicle. Finally the braze alloy joins the other two materials to create a composite structure, much as layers of wood and glue create plywood.

The standard for braze joint strength in many industries is a joint that is stronger than either base material, so that when under stress, one or other of the base materials fails before the joint fails.

One special silver brazing method is called Pinbrazing or Pin Brazing. It has been developed especially for connecting cables to railway track or for cathodic protection installations.

The method uses a silver and flux containing brazing pin which is melted down in the eye of a cable lug. The equipments are normally powered from batteries.

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., 870 °C to 980 °C or 1600 °F to 1800 °F for bronze alloys) which is often considerably lower than the melting point of the base material (e.g., 1600 °C (2900 °F) for mild steel).

File:Brazing welding tee joint.png

In Braze Welding or Fillet Brazing, a bead of filler material reinforces the joint. A braze-welded tee joint is shown here.

Braze welding has many advantages over fusion welding. It allows you to join dissimilar metals, to minimize heat distortion, and to reduce extensive pre- heating. Another side effect of braze welding is the elimination of stored-up stresses that are often present in fusion welding. This is extremely important in the repair of large castings. The disadvantages are the loss of strength when subjected to high temperatures and the inability to withstand high stresses.

The equipment needed for braze welding is basically identical to the equipment used in brazing. Since braze welding usually requires more heat than brazing, an oxyacetylene or oxy-mapp torch is recommended.

‘Braze welding’ is also used to mean the joining of plated parts to another material. Carbide, cermet and ceramic tips are plated and then joined to steel to make tipped band saws. The plating acts as a braze alloy.

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 a materials joining technique that offers significant advantages: extremely clean, superior, flux-free braze joints of high integrity and strength. The process can be expensive because it must be performed inside a vacuum chamber vessel. Temperature uniformity is maintained on the work piece when heating in a vacuum, greatly reducing residual stresses due to slow heating and cooling cycles. This, in turn, can significantly improve the thermal and mechanical properties of the material, thus providing unique heat treatment capabilities. One such capability is heat-treating or age-hardening the workpiece while performing a metal-joining process, all in a single furnace thermal cycle.

Vacuum brazing is often conducted in a furnace; this means that several joints can be made at once because the whole workpiece reaches the brazing temperature. The heat is transferred using radiation, as many other methods cannot be used in a vacuum.

Brazing fundamentals

In order to attain the highest strengths for brazed joints, parts must be closely fitted and the base metals must be exceptionally clean and free of oxides. For capillary action to be effective joint clearances of 50 to 150 µm (0.002 to 0.006 inch) are recommended. In braze-welding, where a thick bead is deposited, tolerances may be relaxed to 0.5 mm (0.020 inch). 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.

Using an abrasive to clean oil or grease physically removes some of it just as any wiping would. However to get the parts clean it is necessary to use a saponifier that will change the oils and greases to soap. Oven cleaners and detergents work well.

Flux

In most cases, flux is required to prevent oxides from forming while the metal is heated and also helps to spread out the metal that is used to seal the joint. 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.

The flux chars and adheres to the workpiece when it is used up and / or overheated. Warm flux can be extremely tenacious. Once the flux has cooled to room temperature it is much easier to remove. The goal is to use enough flux and a proper heating cycle so that the flux is not all used up.

The flux does not interact with the materials being brazed but serves as a barrier and oxygen interceptor. It often has some cleaning properties including the ability to remove oxides but should not be counted on for this.

When hot quenching remember that you are in effect, heat treating the materials. Quenching will change material properties.

Many types of brazing flux contain toxic chemicals, sometimes very toxic. Silver brazing flux often contains Cadmium, which can cause very fast onset of metal fume fever (within minutes in extreme cases), especially if brazing fumes are inhaled due to inadequate ventilation. Due care must be taken with these materials to protect persons working, and also the environment.

Brazing strength and joint geometry

Brazing is different from welding, where 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 although a properly designed and executed brazed joint can be stronger than the parent metal. Careful matching of joint geometry to the forces acting on the joint and properly maintained clearance between two mating parts can 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 braze joints roughly equivalent in strength 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.

However Titanium can be prepared to be successfully brazed if the tendency for oxidation is allowed for. If the material is deoxidized and protected by plating, vacuum or other means then you have a chemically active surface that can make for very strong joints. This is not true with unprepared Titanium and the braze joint is a chemical join that is not dependent on the metal solubility.

Brazing filler material is commonly available as flux-coated rods, very similar to stick-welding electrodes. Typical sizes are 3 mm (1/8") diameter. Some widely available filler materials are:

• Nickel-Silver: Usually with blue flux coating. 600 MPa (85,000 psi) tensile strength, 680 - 950°C (1250-1750°F) working temperature. Used for carbon and alloy steels and most metals not including aluminum.
• Bronze: Available with white borax flux coating. 420 MPa (60,000 psi) tensile strength. 870°C (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.

Nb Flux coating colours are manufacturer specific and do not indicate specific alloy types.

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 870°C (1600 °F). When a large (more than 50 kg (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

• alternatives to brazing include the use of a connector that does not require heat similar toLokring connectors used by most of the auto makers and larger appliance manufacturers

See also

• Amorphous brazing foil
• Braze-on

Further reading

• M.J.Fletcher, “Vacuum Brazing”. Mills and Boon Limited: London, 1971.
• P.M.Roberts, "Industrial Brazing Practice" CRC Press, Boca Raton, Florida, 2004.

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.

http://www.tpub.com/content/construction/14250/css/14250_126.htm