Brazing is a joining process whereby a filler metal or alloy is heated to melting temperature above - or, by the traditional definition in the United States, above - 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 .
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.
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. 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.
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 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.
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.
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.
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.
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:
Nb Flux coating colours are manufacturer specific and do not indicate specific alloy types.
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.
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.
Alternatives to brazing include the use of a connector that does not require heat similar to Lokring connectors used by most of the auto makers and larger appliance manufacturers