Good part and process design pave the way to strong, aesthetically pleasing assemblies.

In many ways, we live in a soldered and welded world. When people think of automotive manufacture, an image of weld sparks flying during chassis assembly quickly comes to mind. The electronics industry is in many ways synonymous with soldering.

However, as assemblers confront the manufacturing challenges inherent in many of today's sophisticated products, brazing is often an attractive option. This is especially true for manufacturers creating components or products made up of dissimilar metals, such as carbide and steel. Brazing is also a viable candidate for creating strong, reliable leak-proof joints that are aesthetically appealing. As an added bonus, brazing doesn't require a great deal of skill on the part of operators.

At it's most basic, brazing involves the use of a filler metal-traditionally some kind of silver alloy, although a variety of other fillers are available-with a melting point that is above 840 F, but below the melting temperature of the components being joined. To create a brazed joint, an operator first positions the components so that they are in the correct final configuration. Then he heats the joint area with a gas torch-generally propane, propylene or acetylene-until it surpasses the melting temperature of the filler material. At this point, the operator applies the filler material to the joint area, where it is quickly liquefied and drawn into the joint via capillary action.

Because the strength of a brazed joint depends on the filler bonding with the base metal of the parts being joined, fluxes are used to remove any oxidized metal and ensure the base materials don't oxidize as they are heated. The flux can also serve to help "wet out" a joint, or ensure that the molten filler material is quickly and efficiently drawn into the entire joint area.

Fluxes come in a number of different forms, including wire, powders, preforms and pastes. Fluxes can also be combined with the filler material as part of an all-in-one paste or within the filler material. Once the brazing is complete, the resulting flux residue is generally easy to remove with a hot water bath, spray or wire brush. Fluxing gasses like hydrogen or a vacuum can also be used, when the brazing is performed inside special controlled atmosphere brazing (CAB) ovens.

Whatever the brazing environment or filler material used, brazing offers a distinct advantage over soldering in that the resulting joints are much stronger. (Soldering is very similar to brazing, but involves a filler alloy with a liquidus of less than 840 F.) This is because the filler materials that are used in brazing are more robust than those used in soldering. In addition, the higher heat used in brazing creates a stronger intermetallic layer between the base metal and filler material. "A properly brazed joint will actually be stronger than the base metals themselves," says Sue Urban, research and development chemist at Fusion Inc. (Willoughby, OH). "We like to say you solder for seal and braze for strength."

Brazing also offers an advantage over welding in that it does not involve melting the base materials of the components being joined. This means all the difference in the world when joining dissimilar metals with different melting points and conductivity characteristics, not to mention exotic assemblies that combine metals and ceramics. Because brazing does not require melting the base materials, it is also the technique of choice for assemblies made up of fragile, thin-walled components.

For all these reasons, brazing has long been a workhorse in the manufacture of plumbing components, in the HVAC industry and in the assembly of automotive radiators and fuel rail systems-applications that put strong, leakproof joints at a premium. According to Daniel Hauser, Ph.D., principal engineer for brazing and soldering at the Edison Welding Institute (EWI, Columbus, OH), manufacturers also use brazing in hundreds of different niche applications, for the production of everything from bicycles, jewelry and eyeglass frames to electrical contacts, carbide-and-steel cutting tools and fuel cells. Other manufacturers taking advantage of brazing can be found in the aerospace, industrial valve and gas distribution equipment industries.

While the above step-by-step outline may seem cumbersome, in practice, the brazing process is both flexible and easy to automate. It can also be an effective way of creating complex assemblies, because multiple joints can be heated and brazed simultaneously in a single oven or by a coordinated battery of gas torches.

Many manufacturers use indexing rotary tables to carry fixtured assemblies through the requisite series of heating, brazing and cleaning stations. The brazing process also lends itself to applications in which manually fixtured and loaded assemblies are carried through a brazing oven by a conveyor as part of an in-line process, or brazed in batches. Ultimately, the best approach for any given application is largely a function of the materials involved and production volumes.

"The reasons for choosing brazing are the usual, lower cost, producibility, cosmetic appearance and simplicity," Hauser says. "[For example] an item like a heat exchanger, having thousands of joints, couldn't readily be made by any other processes."

Getting It Right

No matter what the assembly or the exact materials used, one of the keys to successful brazing is good joint and process design. Again, a well-brazed joint can be stronger than the base material from which it is made. But, this is only possible when the joint includes the right amount of filler material, and that filler material has completely wetted the joint surface with minimal voids.

To ensure this happens, engineers need to provide the correct joint clearance, or space between parts: too much space and the filler will not bridge the gap, making for a weakened assembly (and wasting often expensive filler material); too little space and the capillary action will not be able to effectively and reliably draw in the molten filler material.

As a general rule, joint clearances should fall between 0.002 and 0.004 or 0.005 inch. However, depending on the application and the specific flux-filler-base metal combination, the ideal clearance might fall somewhere outside that range. For example, according to brazing material supplier Bellman-Melcor Inc. (Tinley Park, IL), the ideal joint clearance for some types of CAB furnace brazing should actually be less than 0.002 inch.

When thinking about clearances, engineers also have to bear in mind that the base materials of the parts being joined will also expand as they are heated. This is a situation that becomes especially critical when joining different materials possessing markedly different rates of thermal expansion and contraction.

Let's say an application calls for creating a tubular lap joint in which the outer tube is brass and the inner lap is steel. Because brass expands more when heated than steel, the space between the inner and outer surfaces will be greater when the two are heated. Obviously, the situation would be the reverse if brass were used for the inner tube and steel on the outside.

"Joint clearances must be controlled at the brazing temperature for dissimilar metal or metal ceramic combinations that have different coefficients of thermal expansion," emphasizes Hauser.

Of course, in the real world, ensuring an effective manufacturing process means not just creating the correct design, but making sure that parts are in spec. "Part components have to be consistent, especially if you are going to automatically feed parts via vibratory bowls," says Fusion Inc. Sales Manager, Tony Straniero.

Another important step in establishing an effective brazing process is ensuring that the parts making up the joint are sufficiently clean. There's something almost magical about watching molten filler material instantly wet out a joint, creating a strong, aesthetically appealing assembly. However, the presence of contaminants will reduce the capillary action of the joint area, creating voids, inhibiting the bonding of the filler material to the base metal, and ultimately, reducing joint strength.

Cleaning the joint can involve the use of an emery pad, grinder or wire brush, or require the use of a host of different solvents, sprays or pickling solutions. For example, Bellman-Melcor Inc. lists a wide range of cleaners to accommodate different filler materials and base metals, including petroleum or chlorinated solvents, acid pickling solutions and phosphate-type acids. However, the company warns that manufacturers should be careful of any burnishing or sandblast media that might embed themselves in the base materials.

Good Fixturing

As is the case with most assembly processes, good fixturing is essential to effective brazing. At its most basic, effective fixturing not only ensures that the final assembly will be to the correct dimensions, but that the joint clearances remain at the correct values.

Then there are those considerations that don't come into play when, say, driving threaded fasteners, but do make a difference when brazing. For example, an effective fixture will not conduct an undue amount of heat from the joint area as it is being processed. To accomplish this, it should make as little contact as possible with the assembly, and where it does, this contact should be via pinpoint or knife-edge contacts. Engineers should also build their fixtures from poorly conducting materials like ceramics or stainless steel.

Another way to minimize the influence of a process's fixturing is to implement elements in the basic component design that will hold the parts in place without the need for any additional mechanical support or clamping. This is sometimes called self-fixturing. For example, brazing materials company Handy & Harman Canada Ltd. (Rexdale, Ontario), which is also the publisher of a comprehensive guide to the process, suggests a variety of different approaches, including crimping, interlocking seams, swaging, peening, riveting, dimpling or knurling. Handy & Harman also recommends that engineers minimize the use of sharp corners in their joint assemblies, because corners can impede the flow of filler material, whereas slightly rounded edges facilitate the effectiveness of a joint's capillary action.

As a kind of corollary to the fixturing process, assemblers should bear in mind that when brazing, the entire joint area needs to be heated as uniformly as possible. This includes those portions of the base material that are below the surface, deeper within the joint. The reason for this is that it is the heat of the base material that is actually melting the filler, not the flame itself. Therefore any portion of the joint area that is not hot enough will fail to be wetted, creating voids and reducing the strength of the joint.

For this reason, assemblers often do not heat the joint area directly, but instead apply the heat to the underside of an assembly and the broader area surrounding the joint. Automated systems will often include multiple heating stations, each with multiple torches to ensure that the entire joint area is brought up to temperature before filler material begins to flow.

Bellman-Melcor notes that mass differences and conductivity of the base metals will affect the amount of heat and time required to heat the entire joint area to the correct temperature. In some cases, the "splash off" of the flame may be sufficient to heat a thin part being joined to a heavier component. In other cases, engineers may want to consider other heating options, like induction, resistance and gradiation.

Getting it Right

Of course, these recommendations represent just a part of the equation when it comes to implementing effective brazing into an assembly process. For example, engineers have to decide on the specific types of flux or brazing gas for a given application; the amount of flux and filler material; and the heating rate or profile, which can often be a multistep process.

Nonetheless, for those jobs requiring strong, sealed joints, brazing is often the answer. As is the case with most assembly processes, it's just a matter of doing the homework and maintaining well-controlled processes.

"Generally, brazing is relatively easy to automate," Hauser says. "As for all manufacturing processes, the details control the selection of most of the manufacturing process operations, such as filler metal and form, joint design, atmosphere vs. flux, time-temperature profile, etc. My experience is that fit up and cleanliness are the most common causes of unacceptable brazement quality."