Best Practices for Effective Brazing

Posted on Feb 23, 2025 in Telecommunication Sciences and Technologies

The Importance of Proper Brazing Procedures

Brazing is ideally suited for joining dissimilar metals and is performed at a relatively low temperature. A brazed joint essentially “makes itself”—capillary action, more than operator skill, ensures the distribution of the filler metal into the joint. However, even a properly designed joint can turn out imperfectly if the correct brazing process steps are not followed.

Brazing Process Step 1: Good Fit and Proper Clearances

Brazing uses the principle of capillary action to distribute the molten filler metal between the surfaces of the base metals. Therefore, during the brazing process, maintain a clearance between the base metals to allow capillary action to work most effectively. This means, in almost all cases, a close clearance.

The following chart is based on brazing butt joints of stainless steel, using filler metal. It shows how the tensile strength of the brazed joint varies with the amount of clearance between the parts being joined. Note that the strongest joint (135,000 psi/930.8 MPa) is achieved when the joint clearance is 0.0015” (0.038mm). When the clearance is narrower, it’s harder for the filler metal to distribute itself adequately throughout the entire joint, and joint strength is reduced.

Conversely, if the gap is wider than necessary, the strength of the joint will be reduced almost to that of the filler metal itself. Also, capillary action is reduced, so the filler metal may fail to fill the joint completely, again lowering joint strength. The ideal clearance for a brazed joint, in the example above, is in the neighborhood of 0.0015” (0.038mm). But in ordinary day-to-day brazing, you don’t have to be this precise to get a sufficiently strong joint.

Capillary action operates over a range of clearances, so you get a certain amount of leeway. Clearances ranging from 0.001” to 0.005” (0.025 mm to 0.127 mm) still produce joints of 100,000 psi (689.5 MPa) tensile strength. If you’re joining two flat parts, you can simply rest one on top of the other. The metal-to-metal contact is all the clearance you’ll 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.)

However, there’s a special factor you should consider carefully in planning your joint clearances: Brazed joints are made at brazing temperatures, not at room temperature. You must take into account the “coefficient of thermal expansion” of the metals being joined. This is particularly true of tubular assemblies in which dissimilar metals are joined.

For example, if you’re brazing a brass bushing into a steel sleeve, brass expands more than steel when heated. So if you machine the parts to have a room temperature clearance of 0.051 mm-0.076 mm, by the time you’ve heated the parts to brazing temperatures the gap may have closed completely! The answer? Allow a greater initial clearance, so that the gap at brazing temperature will be about 0.051 mm-0.076 mm. The same principle holds in reverse. If the outer part is brass and the inner part steel, you can start with virtually a light force fit at room temperature. By the time you reach brazing temperature, the more rapid expansion of the brass creates a suitable clearance.

How much allowance should you make for expansion and contraction? It depends on the nature and sizes of the metals being joined and the configuration of the joint itself. Although there are many variables involved in pinpointing exact clearance tolerances for each situation, keep in mind the principle involved—different metals expand at different rates when heated.

Proper Joint Clearance

There are only two basic types – the butt and the lap. The rest are essentially modifications of these two. Let’s look first at the butt joint, both for flat and tubular parts.

Designing Butt Joints

The butt joint gives you the advantage of a single thickness at the joint. Preparation of this type of joint is usually simple, and the joint will have sufficient tensile strength for most applications. However, the strength of the butt joint does have limitations. It depends on the amount of bonding surface, and in a butt joint, the bonding area can’t be any larger than the cross-section of the thinner member.

For a given thickness of base metals, the bonding area of the lap joint can be larger than that of the butt joint and usually is. With larger bonding areas, lap joints can usually carry larger loads. The lap joint doubles the thickness at the joint, but in many applications (plumbing connections, for example) this reinforcement of the joint is preferable. Resting one flat member on the other is usually enough to maintain a uniform joint clearance. And, in tubular joints, nesting one tube inside the other holds them in proper alignment for brazing. However, suppose you want a joint that has the advantages of both types; single thickness at the joint combined with maximum tensile strength. You can get this combination by designing the joint as a butt-lap joint.

Figuring the Proper Length of Lap

You don’t have to calculate the bonding area of a butt joint. It will be the cross-section of the thinner member. A good rule of thumb is to design the lap joint to be three times as long as the thickness of the thinner joint member.

A longer lap may waste brazing filler metal without a corresponding increase in joint strength. And a shorter lap will lower the strength of the joint. For most applications, you’re on safe ground with the “rule of three.”

Designing to Distribute Stress

When you design a brazed joint, you aim to provide at least minimum adequate strength for the given application. But in some joints, maximum mechanical strength may be your overriding concern. You can help ensure this degree of strength by designing the joint to prevent concentration of stress from weakening the joint.

Spread the stress: Figure out where the greatest stress falls, then impart flexibility to the heavier member at this point, or add strength to the weaker member. When you’re designing a joint for maximum strength, use a lap to increase joint area rather than a butt, and design the parts to prevent stress from being concentrated at a single point. There is one other technique for increasing the strength of a brazed joint, frequently effective in brazing small-part assemblies. You can create a stress- distribution fillet, simply by using a little more brazing filler metal than you normally would, or by using a more “sluggish” alloy. Usually, you don’t want or need a fillet in a brazed joint, as it doesn’t add materially to joint strength. It pays to create the fillet when contributing to spreading joint stresses.

Designing for Service Conditions

In many brazed joints, the chief requirement is strength. But there are frequently other service requirements which may influence the joint design or filler metal selection. For example, you may be designing a brazed assembly that needs to be electrically conductive. A silver brazing filler metal, by virtue of its silver content, has very little tendency to increase electrical resistance across a properly-brazed joint. But you can further ensure minimum resistance by using a close joint clearance, to keep the layer of filler metal as thin as possible. In addition, if strength is not a prime consideration, you can reduce the length of lap. Instead of the customary “rule of three,” you can reduce lap length to about 1-1/2 times the cross-section of the thinner member.

If the brazed assembly has to be pressure-tight against gas or liquid, a lap joint is almost a must since it withstands greater pressure than a butt joint. And its broader bonding area reduces any chance of leakage. Another consideration in designing a joint to be leak proof is to vent the assembly. Providing a vent during the brazing process allows expanding air or gases to escape as the molten filler metal flows into the joint. Venting the assembly also prevents entrapment of flux in the joint. Avoiding entrapped gases or flux reduces the potential for leak paths. If possible, the assembly should be self-venting. Since flux is designed to be displaced by molten filler metal entering a joint, there should be no sharp corners or blind holes to cause flux entrapment. The joint should be designed so that the flux is pushed completely out of the joint by the filler metal. Where this is not possible, small holes may be drilled into the blind spots to allow flux escape. The joint is completed when molten filler metal appears at the outside surface of these drilled holes. To maximize corrosion-resistance of a joint, select a brazing filler metal containing such elements as silver, gold, or palladium, which are inherently corrosion-resistant. Keep joint clearances close and use a minimum amount of filler metal, so that the finished joint will expose only a fine line of brazing filler metal to the atmosphere. These are but a few examples of service requirements that may be demanded of your brazed assembly. As you can see, both the joint design and filler metal selection must be considered.

Brazing Process STEP 2: Cleaning the Metals

Capillary action will work properly only when the surfaces of the metals are clean. If they are “contaminated”—coated with oil, grease, rust, scale, or just plain dirt—those contaminants have to 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. And brazing filler metal won’t bond to a rusty surface. Cleaning the metal parts is seldom a complicated job, but it has to be done in the right sequence. Oil and grease should be removed first, because an acid pickle solution aimed to remove rust and scale won’t work on a greasy surface. (If you try to remove rust or scale by abrasive cleaning before getting rid of the oil, you’ll wind up scrubbing the oil, as well as fine abrasive powder, more deeply into the surface.) Start by getting rid of oil and grease. In most cases, you can do it very easily either by dipping the parts into a suitable degreasing solvent, by vapor degreasing, or by 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 that the chemicals are compatible with the base metals being cleaned, and that no acid traces remain in crevices or blind holes. 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 a grinding wheel, or file or metallic grit blast, followed by a rinsing operation. Once the parts are thoroughly clean, it’s a good idea to flux and braze as soon as possible. That way, there’s the least chance for recontamination of surfaces by factory dust or body oils deposited through handling.

Brazing Process STEP 3: Fluxing the Parts

Why Brazing Requires Flux

Flux is a chemical compound applied to the joint surfaces before brazing. Its use, with a few exceptions, is crucial in the atmospheric brazing process. Heating a metal surface accelerates the formation of oxides, the result of chemical combination between the hot metal and oxygen in the air. If you don’t stop these oxides from forming, they’ll inhibit the brazing filler metal from wetting and bonding to the surfaces. A coating of flux on the joint area guards the surfaces from the air, preventing oxide formation. It also dissolves and absorbs any oxides that form throughout heating or that were not completely removed in the cleaning process. Understanding the functions and stages of flux will help you achieve strong, quality joints in your operation.

Flux Application

You can apply flux as long as you cover the joint surfaces completely. Flux traditionally is made in a paste, so it’s usually most convenient to brush it on. But as production quantities increase, it may be more effective to apply the flux by dipping or dispensing a pre-measured deposit of high-viscosity dispensable flux from an applicator gun. Many companies find the repeatable deposit size improves joint consistency, and because typically less flux is used, the amount of residue entering the waste stream is also reduced.

When do you flux and how do you choose?

Typically just before brazing, if possible. That way the flux has the least chance to dry out and flake off, or get knocked off the parts in handling. Which flux do you use? Choose the one formulated for the specific metals, temperatures, and conditions of your brazing application. There are fluxes formulated for practically every need; for example – fluxes for brazing at very high temperatures (in the 2000°F/1093°C area), fluxes for metals with refractory oxides, fluxes for long heating cycles, and fluxes for dispensing by automated machines.

The addition of metallic boron changes white flux to black. Black flux is beneficial for fast induction heating, may provide better protection in a high-temperature brazing operation, and can be helpful with high-liquidus filler metals.

How much flux do you use?

Enough to last throughout the entire heating cycle. Keep in mind that the larger and heavier the pieces brazed, the longer the heating cycle will take – so use more flux. (Lighter pieces, of course, heat up faster and require less flux.) As a general rule, don’t skimp on the flux. It’s your insurance against oxidation. Think of the flux as a sort of blotter. It absorbs oxides like a sponge absorbs water. An insufficient amount of flux will quickly become saturated and lose its effectiveness. A flux that absorbs fewer oxides not only ensures a better joint than a totally saturated flux, but it is a lot easier to wash off after the brazed joint is completed.

Fluxing is an essential step in the brazing operation, aside from a few exceptions. You can join copper to copper without flux, by using a brazing filler metal specially formulated for the job. You can also omit fluxing if brazing occurs in a controlled atmosphere (i.e. a gaseous mixture contained in an enclosed space, usually a brazing furnace). The atmosphere (such as hydrogen, nitrogen, or dissociated ammonia) completely envelops the assemblies and, by excluding oxygen, prevents oxidation. Even in controlled atmosphere brazing you may find that a small amount of flux improves the wetting action of the brazing filler metal.

Brazing Process Step 4: Assembly for Brazing

The parts of the assembly are cleaned and fluxed. Now you have to hold them in position for brazing. Be sure they remain in correct alignment during the heating and cooling cycles, so that capillary action can do its job. If the shape and weight of the parts permit, the simplest way to hold them together is by gravity. Or give gravity a helping hand by adding additional weight.

If you have a number of assemblies to braze and their configuration is too complex for self-support or clamping, use a brazing support fixture. Design it for the least possible mass, and the least contact with the parts of the assembly. (A cumbersome fixture that contacts the assembly broadly will conduct heat away from the joint area.) Use pinpoint and knife-edge design to reduce contact to the minimum.

Try to use materials in your fixture that are poor heat conductors, such as stainless steel, Inconel, or ceramics. Since these are poor conductors, they draw the least heat away from the joint. Choose materials with compatible expansion rates so you won’t get alterations in assembly alignment during the heating cycle

If you’re planning to braze hundreds of identical assemblies, think in terms of designing the parts themselves for self-support during the brazing process. At the initial planning stage, design mechanical devices that will accomplish this purpose, and that can be incorporated in the fabricating operation. Typical devices include crimping, interlocking seams, swaging, peening, riveting, pinning, dimpling, or knurling.

Fixture Facts

A fixture is built to cradle, hold, or secure an assembly to be joined. Proper fixtures should meet these criteria:

Allow easy insertion of assembly components and easy removal of the brazed assembly
Support assembly components to permit expansion and contraction during heating and cooling
Support the assembly at points away from the heat zone (preventing the fixture from becoming a heat sink)
Permit the flame to be directed around the entire joint area, so the alloy can flow throughout the joint
Use gravity to assist capillary action
Maintain alignment and dimensional stability until the alloy solidifies
Be sufficiently flexible to accommodate other similar assemblies.

Better Joint Design

An axiom of metal joining is that proper joint design is the path to efficient fixturing. These tips for improving your joint design:

Make component pieces of the assembly self-locating, so the fixture only supports and cradles the components.
Allow space for the filler metal to flow and for flux to be forced out of the joint.

Example Problem: Where a tube enters a fitting or casting, some manufacturers use a press fit to keep externally applied alloy from reaching the bottom of the joint, where it might plug a hole in the fitting. Unfortunately, molten flux reaches the bottom of the blind hole and is trapped there, as alloy melts and tries to enter the joint. The alloy cannot displace the flux, so heavy flux inclusions and poor joint quality result.

Example Solution: Use a slip fit and a buried preform in the hole. The alloy is induced by heat to flow to the top of the joint, pushing the flux out. This leaves the hole open and results in a sound joint. Take into account the expansion and contraction characteristics of the metals being joined. If possible, design the joint so the higher- expansion material is the outer member of the joint. (It will expand more than the inner member, providing clearance where the filler metal will flow.) Silver brazing is a cost-effective method for joining metals, especially when joints are designed for maximum brazing efficiency and fixtures are designed as described. Many products manufactured today could be redesigned for brazing to reduce manufacturing costs. Even though silver is expensive, it represents a small percentage of total assembly costs.

Fixturing Tips

These ideas for improving manufacturing efficiency with fixturing:

Construct fixtures from 300-series, non-magnetic stainless steel for better corrosion resistance and dimensional stability (other ideas include Inconel and some ceramics).
When fixturing for furnace brazing, hold the mass of the fixture to a minimum. (Payoff is dependent on lb/hr through the furnace; increased weight = reduced fuel efficiency.)
When fixturing for induction heating, keep fixtures away from the work coil, so they will not act as heat sinks or interfere with the magnetic field.
Avoid use of springs for bringing parts back to dimensional alignment. If they are required, keep them out of the heat zone, so they will not be affected by flux residue or oxidation.

Fixture design is a key factor in achieving quality flame-brazed or soldered joints. In addition, proper joint design is the path to efficient fixturing. Combining improvements to both these areas can result in better-quality joints, increase operating efficiency and reduce costs.

Brazing Process Step 5: Brazing the Assembly

There are four main types of brazing heating methods: torch or manual brazing, induction brazing, resistance brazing, and vacuum brazing. The heating method most commonly used in brazing a single assembly is the hand-held torch. First, apply heat broadly to the base metals. If you’re brazing a small assembly, you may heat the entire assembly to the flow point of the brazing filler metal. If you’re brazing a large assembly, you heat a broad area around the joint. Both metals in the assembly should be heated as uniformly as possible so they reach brazing temperature at the same time. When joining a heavy section to a thin section, the “splash-off” of the flame may be sufficient to heat the thin part. Keep the torch moving at all times. When joining heavy sections, the flux may become transparent—which is at 1100°F (593°C)— before the full assembly is hot enough to receive the filler metal.

In torch brazing, a variety of fuels are available—natural gas, acetylene, propane, propylene, etc., combusted with either oxygen or air. The most popular is still the oxy/acetylene mixture. When it comes to safely brazing with oxy-acetylene torches, let’s look at two important aspects: safety equipment, plus procedures for safe operation. This is serious business: arc rays and sparks can result in loss of sight, fume inhalation can lead to lung damage, and other accidents can cause burns, fires, or explosions.

Oxy-acetylene Torch Safety Equipment

In addition to gloves, eye protection, and related safety gear, these are the important elements:

Cylinders: Oxygen and acetylene are kept in separate cylinders and not combined until the torch tip is ignited.
Regulators: Installed on the cylinder, regulators control both the pressure of the cylinder and the outlet pressure leading to the torch.
Check Valves: From the regulator, the two gases travel through check valves, which ensure that the gases can be shut off in the event of backflow.
Flashback Arrestors: Flashback arrestors prevent fire from flowing back into the cylinders, which will otherwise lead to an explosion.

Oxy-acetylene Torch Safe Operation

In addition to using safety equipment, workers should practice safe operation to prevent flashbacks. Keep acetylene and oxygen separate until the torch is ignited. When starting a torch, the acetylene valve should be opened first. Next, the torch should be ignited, and then oxygen can be introduced. Please note that opening both gas valves prior to ignition can cause gas backflow into either gas hose, leaving the system vulnerable to flashback. After use, it is critical that both gas lines be emptied separately-one at a time-through the torch. Flashbacks can also be caused by brazing with multiple torches, simultaneously, on one part. If using dual torches to heat both sides of a part, do not aim the torches at each other, but rather, angle each torch toward the part. If one torch should cause flashback in the other, operators will hear a loud hissing sound and should immediately turn off the gas by closing first the acetylene valve and then the oxygen valve.

Flux Changes During the Brazing Process

Some metals are good conductors—and consequently carry off heat faster into cooler areas. Others are poor conductors and tend to retain heat and overheat readily. The good conductors will need more heat than the poor conductors, simply because they dissipate the heat more rapidly.

Depositing Filler Metals

In manual brazing, all this involves is carefully holding the rod or wire against the joint area. The heated assembly will melt off a portion of the filler metal, which will instantly be drawn by capillary action throughout the entire joint area. You may want to add some flux to the end of the filler metal rod— about 2” to 3” (51 mm to 76 mm)— to improve the flow. This can be accomplished by either brushing on or dipping the rod in flux. On larger parts requiring longer heating time, or where the flux has become saturated with much oxide, the addition of fresh flux on the filler metal will improve the flow and penetration of the filler metal into the joint area.

General Safety in Brazing

Ventilate confined areas.
Clean base metals thoroughly.
Use sufficient flux.
Heat metals broadly.
Know your base metals.
Know your filler metals.

Brazing Process Step 6: Cleaning the Brazed Joint

After you’ve brazed the assembly, you have to clean it. And cleaning is usually a two-step operation: 1: removal of the flux residues. 2: pickling to remove any oxide scale formed during the brazing process.

Flux Removal

Flux removal is a simple, but essential operation. (Flux residues are chemically corrosive and, if not removed, could weaken certain joints.) Since most brazing fluxes are water-soluble, the easiest way to remove them is to quench the assembly in hot water (120°F/50°C or hotter). Best bet is to immerse them while they’re still hot, just making sure that the filler metal has solidified completely before quenching. The glass-like flux residues will usually crack and flake off. If they’re a little stubborn, brush them lightly with a wire brush while the assembly is still in the hot water.

Reasons to Remove Flux

Let’s examine five reasons why post-braze flux removal is important:

You cannot inspect a joint that is covered with flux.
Flux can act as a bonding agent and may be holding the joint together, without successful brazing. This joint would fail during service.
In pressure service, flux may mask pinholes in a braze joint, even though it withstands a pressure test. The joint would leak soon after being placed into service.
Flux is hygroscopic, so residual flux attracts available water from the environment. This leads to corrosion.
Paint or other coatings do not stick to areas covered with residual flux.

Methods for Flux Removal

After brazing, flux forms a hard, glass-like surface and can be difficult to remove. You can remove excess flux by various means; the most cost-effective approaches involve water.

The most common methods for post-braze flux removal are:

Soaking/wetting – Use hot water with agitation in a soak tank to remove excess flux immediately following the braze operation, and then dry the assembly. When soaking is not possible, use a wire brush along with a spray bottle or wet towel. When using a soak bath of any kind, change the solution periodically to avoid saturating the cleaning solution.
Quenching – This process induces a thermal shock that cracks off residual flux.
Steam lance cleaning: This process employs superheated steam under pressure to dissolve and blast away flux residue.
Chemical cleaning: You can use an acidic or basic solution, generally with short soak times to avoid deteriorating the base materials. For chemical soaks, monitor the pH level to determine when to change the solution.
Mechanical cleaning: Clean residue from brazed joints with a wire brush or by sandblasting. Be advised that soft metals-including aluminum-require extra care, as they are vulnerable to the embedding of particles.

Post-Cleaning Inspection of Brazed Joints

Depending on your brazing process, you may need to perform post-braze joint cleaning to remove residual flux. Common discontinuities of brazed joints, identified through nondestructive examination, include:

Voids or porosity – an incomplete flow of brazing filler metal which can decrease joint strength and allow leakage-often caused by improper cleaning, incorrect joint clearance, insufficient filler metal, entrapped gas or thermal expansion.
Flux entrapment – resulting from insufficient vents in the joint design-preventing the flow of filler metal and reducing joint strength as well as service life
Discontinuous fillets – areas on the joint surface where the fillet is interrupted-usually discovered by visual inspection
Base metal erosion (or alloying) – when the filler metal alloys with the base metal during brazing- movement of the alloy away from the fillet may cause erosion and reduce joint strength
Unsatisfactory surface condition or appearance – excessive filler metal or rough surfaces-may act as corrosion sites and stress concentrators, also interfering with further testing
Cracks – reducing strength and service life of the joint-may also be caused by liquid metal embrittlement.

Brazed Joint Examination Methods: Nondestructive Testing

Visual examination – with or without magnification-for evaluating voids, porosity, surface cracks, fillet size and shape, discontinuous fillets plus base metal erosion (not internal issues such as porosity and lack of fill)
Leak testing – for determining gas- or liquid-tightness of a brazement. Pressure (or bubble leak) testing involves the application of air at greater-than-service pressures. Vacuum testing is useful for refrigeration equipment and detection of minute leaks, employing a mass spectrometer and a helium atmosphere.
Radiographic examination – useful in detecting internal flaws, large cracks and braze voids, if thickness and X-ray absorption ratios permit delineation of the brazing filler metal-cannot verify a proper metallurgical bond
Proof testing – subjecting a brazed joint to a one-time load greater than the service level- applied by hydrostatic methods, tensile loading or spin testing
Ultrasonic examination – a comparative method for evaluating joint quality, in immersion mode or contract mode-involves reflection of sound waves by surfaces, using a transducer to emit a pulse and receive echoes
Liquid penetrant examination – dye and fluorescent penetrants may detect cracks open to the surface of joints-not suitable for inspection of fillets, where some porosity is always present.
Acoustic emission testing – evaluating the extent of discontinuity-using the premise that acoustic signals undergo a frequency or amplitude change when traveling across discontinuities
Thermal transfer examination – detects changes in thermal transfer rates due to discontinuities or unbrazed areas-images show brazed areas as light spots and void areas as dark spots.

Brazed Joint Examination Methods: Destructive Testing

There are also several destructive and mechanical testing methods, often used in random or lot testing:

Peel testing: useful for evaluating lap joints and production quality control for general quality of the bond plus presence of voids and flux inclusions-where one member is held rigid while the other is peeled away from the joint.
Metallographic examination: testing the general quality of joints-detecting porosity, poor filler metal flow, base metal erosion and improper fit
Tension and shear testing – determines the strength of a joint in tension or in shear-used during qualification or development rather than production.
Fatigue testing: testing the base metal plus the brazed joint-a time-consuming and costly method.
Impact testing: determines the basic properties of brazed joints-generally used in a lab setting.
Torsion testing: used on brazed joints in production quality control-for example, studs or screws brazed to thick sections