The Process

Commonly referred to as silver brazing, the process uses a silver-containing alloy with a melting temperature above 840°F (450°C) but below the melting point of the metals to be joined. In brazing, the base metals are heated, usually to a point slightly above the liquidus (flow point) of the filler metal, causing it to melt. The filler metal then flows into the parallel joint clearance between the two base materials by capillary attraction and bonds to their surfaces through atomic attraction and diffusion.

Unlike other methods of metal joining, such as braze welding and welding wherein the filler metal is applied in quantity and generally in the form of fillets, in brazing we are interested in flowing the alloy between closely fitted members.

To effectively braze with a silver-containing filler metal, one must clearly understand brazing fundamentals. Most problems associated with brazing are the result of one or more of the brazing fundamentals being violated. When brazing fundamentals are understood, problem solving becomes a simple matter of the process of elimination.

These fundamentals include the following:

• Good fit and proper clearance
• Clean base metals
• Proper fixturing
• Proper fluxing/atmosphere
• Heating the assembly
• Cleaning the brazed assembly.

Good Fit and Proper Clearance

A braze alloy relies on capillary action to distribute the brazing filler metal throughout the joint interface. Capillary action is the force that pulls a liquid through two parallel surfaces. In brazing, the clearance at which capillary action is most effective is in the 0.001- to 0.005-in. range.

Joint clearance also has a profound impact on joint strength. Figure 2 shows how the tensile strength of a stainless steel brazed joint varies with the amount of clearance between the parts being joined. Note that the strongest joint (135,000 lb/in.2 [930.8 MPa]) is achieved when the joint clearance is 0.0015 in. (0.038 mm). When the clearance is narrower, it is difficult for the filler metal to distribute itself adequately throughout the entire joint, reducing joint strength. Conversely, if the joint clearance is wider than necessary, the strength of the joint will be reduced almost to that of the filler metal itself.

Translated into everyday shop practice - an easy slip fit will give you a perfectly adequate brazed joint between two tubular parts. And, if you are joining two flat parts, you can simply rest one on top of the other. The metal-to-metal contact is all the clearance you will usually need, since the average "mill finish" of metals provides enough surface roughness to create capillary "paths" for the flow of molten filler metal. Highly polished surfaces, on the other hand, tend to restrict filler metal flow.

There are, of course, certain factors affecting clearance that need consideration, particularly as it applies to tubular members. For example, when brazing a brass bushing into a steel sleeve (Fig. 3), consider that the brass has a greater coefficient of expansion than the steel. And, as it is to be the inner member of the assembly, you must allow a greater clearance than if both pieces were steel. By the same premise, if the position of the parts were reversed - the brass becoming the outer member and the steel the inner - you probably would want to allow a little less clearance than if both parts were of the same material. In general, clearances should be considered in light of the parts at brazing temperature rather than at room temperature.

Cleaning the Metals

Capillary action will work properly only when the surfaces of the metals are clean. Contaminants, such as oil, grease, rust, scale or plain dirt, must be removed. If they remain, they will form a barrier between the base metal surfaces and the brazing materials. An oily base metal, for example, will repel the flux, leaving bare spots that oxidize under heat and result in voids. Oil and grease will carbonize when heated, forming a film over which the filler metal will not flow.

Start by getting rid of oil and grease, usually done easily by dipping the part into a suitable degreasing solvent, or by vapor degreasing, alkaline or aqueous cleaning. If the metal surfaces are coated with oxide or scale, you can remove those contaminants chemically or mechanically. For chemical removal, use an acid pickle treatment, making sure the chemicals are compatible with the base metals. Mechanical removal calls for abrasive cleaning. Particularly in repair brazing, where parts may be very dirty or heavily rusted, you can speed the cleaning process by using emery cloth, a grinding wheel, file or metallic shot blast.<> Once the parts are thoroughly clean, it is a good idea to flux and braze as soon as possible. That way, there is the least chance for recontamination of surfaces by factory dust or body oils deposited through handling.

Fluxing the Parts

Flux is a chemical compound applied to the joint surfaces before brazing. Its use is essential for brazing (with a few exceptions noted later). The reason? Heating a metal surface accelerates the formation of oxides, the result of chemical reactions between the hot metal and oxygen in the air. These oxides have to go, or they will prevent the brazing filler metal from wetting and bonding to the surfaces. A coating of flux on the joint area, however, will shield the surfaces from the air, preventing oxide formation. The flux will also dissolve and absorb any oxides that form during heating or that were not completely removed during the cleaning process.

Since flux conventionally comes in a paste, it is usually most convenient to brush it on. For large quantities, it may be more efficient to apply the flux by dipping or by using a low-viscosity dispensable flux sprayed from a gun. Flux the assembly just before brazing, if possible.

There are fluxes formulated for practically every need -fluxes for brazing at very high temperatures (i.e., 2000°F), fluxes for metals with refractory oxides, fluxes for long heating cycles, dry flux powders and fluxes for dispensing by automated machines. As a general rule, do not skimp on the flux. It is your insurance against oxidation. Think of the flux as a sort of blotter. It absorbs oxides like a blotter absorbs ink; too small an amount of flux will quickly become saturated and lose its effectiveness. A flux only mildly loaded with oxides will not only ensure a better joint than a totally saturated flux, but it is a lot easier to wash off after the brazed joint is completed.

Flux also acts as a temperature indicator, minimizing the chance of overheating. White low-temperature silver brazing flux, for example, becomes completely clear when active at 1100°F. At this temperature it looks like water and reveals the bright metal surface underneath - telling you the base metal is just about hot enough to melt the brazing filler metal.

While fluxing is usually an essential step in the brazing operation, there are a couple of exceptions to the rule. You can join copper to copper without flux by using a brazing filler metal specially formulated for the job, such as silver-copper-phosphorus alloys. The phosphorus in these alloys acts as a fluxing agent on copper. Also, you can often omit fluxing if you are going to braze the assembly in a controlled atmosphere -a gaseous mixture contained in an enclosed space, usually a brazing furnace. The atmosphere (usually hydrogen, nitrogen or argon) completely envelops the assemblies and, by excluding oxygen, prevents oxidation. Even in a controlled atmosphere, however, a small amount of flux may improve the wetting action of the brazing filler metal.

Proper Fixturing

If the shape and weight of the part permit, the simplest way to hold them together is by gravity. Or you can add additional weight. If you have a number of assemblies to braze and their configuration is too complex for self-support or clamping, it may be a good idea to use a brazing support fixture. In planning such a fixture, design it for the least possible mass;contact area between the fixture and the assembly should be at a minimum - Fig. 4. A fixture that contacts the area broadly will conduct heat away from the joint area. If the fixture is to be used in a torch application, be sure to allow clearance for the open flame to reach the joint area without restriction. Choose materials that are resilient to high temperature and thermal cycling, such as stainless steel, InconelŪ or ceramics. If you need to fixture close to the joint, where you risk brazing the assembly to the fixture, use a nonwetting material, such as titanium.

However, if planning to braze hundreds of identical assemblies, consider designing the parts themselves so they are self-supporting during the brazing process. Typical methods of self-support are crimping, interlocking, swaging, peening, riveting, pinning, dimpling or knurling - Fig. 5.

Heating the Assembly

This step brazes the joint. It involves heating the joint to brazing temperature and flowing the filler metal through the joint. Both metals in the assembly should be heated as uniformly as possible so they reach brazing temperature at the same time. Therefore, when joining a thick section to a thin section, more heat should be applied to the thick section. Or, when joining a good conductor of heat to a poor conductor, such as copper to stainless steel, more heat will have to be applied to the good conductor (in this case, the copper) simply because they dissipate the heat more rapidly.

In all cases, your best insurance against uneven heating is to keep a watchful eye on the flux. If the flux changes in appearance uniformly, the parts are being heated evenly, regardless of the difference in their mass or conductivity.

In manual brazing, when the assembly reaches brazing temperature, hold the brazing rod carefully against the joint area. Do not heat the brazing rod directly. The heated assembly will melt off a portion of the braze rod, which will instantly be drawn by capillary action throughout the entire joint area. This technique ensures the assembly is at braze temperature and helps to prevent cold joints.

Take care, however, because molten brazing filler metals tend to flow toward areas of higher temperature. In the heated assembly, the outer base metal surfaces may be slightly hotter than the interior surfaces. So take care to deposit the filler metal immediately adjacent to the joint. If you deposit it away from the joint, the filler metal tends to plate over the hot surfaces rather than flow into the joint. It is best to heat the side of the assembly opposite the point where you are going to feed the filler metal - Fig. 6.

If using preforms - slugs, washers, shims or special shapes of filler metal - preplace them in the joint before applying heat to the assembly.

Cleaning the Brazed Assembly

Postcleaning of brazed assemblies is done primarily to remove flux residue. Flux removal is a simple, but essential, operation for one reason. Flux residue is corrosive and if not removed can attack the base metal or braze filler metal, possibly weakening the joint. Since most fluxes are water soluble, the easiest way to remove them is to submerse the assembly in hot water (150°F or hotter).

You can use more elaborate methods of removing flux as well, such as an ultrasonic cleaning tank to speed the action of the hot water. Or, if the assembly is too large to submerse in hot water, hot-pressure washing or live steam may be viable options.

Quenching the assembly in hot water will crack the flux off and expedite the flux removal procedure. However, avoid quenching assemblies having base metals with large differences in coefficient of expansion (such as tungsten carbide to steel) or assemblies with vastly different cross sections; otherwise, the base metal or joint may fracture. When in doubt, allow the assembly to cool slowly to ambient temperature before submersing it in water. After removing the flux, you can remove residual oxides by acid pickling or mechanical cleaning.

Conclusion

Brazing with silver-containing filler metal is an effective means of creating strong, leaktight joints on a diverse group of base metals. By clearly understanding the process, its fundamentals and how to apply them, it can be a cost-effective and reliable method of manufacturing metal-to-metal joints.