Brazing and soldering are the sibling rivals of the manufacturing world. While both joining processes are similar, brazing tends to be the more introverted process that's often overlooked in favor of more popular fusion methods.

Traditionally, soldering is used for glamorous, high-tech applications, such as printed circuit board production, while brazing is relegated to more robust, industrial applications, such as assembling automotive radiators. However, innovative brazing techniques, emerging technology and nontraditional applications are forcing manufacturing engineers to take a new look at an age-old process.

Brazing and soldering are metallurgically identical processes that involve joining components without melting the base materials. While both use heat and a filler material, operating temperature is what separates them.

The American Welding Society (Miami) defines brazing as "A group of joining processes that produces coalescence of materials by heating them to the brazing temperature in the presence of a filler metal having a liquidus above 840 F and below the solidus of the base metal. The filler metal is distributed between the closely fitted faying surfaces of the joint by capillary action."

Soldering occurs at temperatures below 840 F. Welding, a first cousin of brazing and soldering, occurs when the base metals are melted and fused together, usually with the addition of a filler metal.

Brazing is much stronger than soldering, and unlike many other production processes, it is ideal for joining dissimilar materials. Brazing is a versatile joining method that produces a permanent, strong, leak-proof joint. It is used to assemble a wide variety of products, such as air conditioners, automotive parts, aerospace components, radiators, refrigeration systems, lightbulbs, jet engines, industrial valves, gas distribution systems, turbines and other items exposed to severe temperature extremes.

"There's a lot of confusion about what's brazing and what's soldering," notes Daniel Hauser, Ph.D., principal engineer for brazing and soldering technology at the Edison Welding Institute (EWI, Columbus, OH). He says brazing should be considered whenever engineers want permanent, strong joints. However, the decision depends on numerous variables, such as the size of the parts to be joined, the thickness of the parts, joint configuration and the number of joints to be made.

"Some individuals use the terms brazing and soldering interchangeably," adds Gary DeVries, marketing analyst at Lucas-Milhaupt Inc. (Cudahy, WI). "Brazing creates a metallurgical bond between the filler metal and the surfaces of the two metals being joined."

Because the temperature of joining can be below the solidus of both metals being joined, metals and nonmetals with vastly different melting temperatures can be joined. For instance, ferrous metals can be joined to nonferrous metals. In addition, nontraditional materials, such as ceramics, can be joined together by brazing.

According to Walter Sperko, P.E., president of Sperko Engineering Services Inc. (Greensboro, NC), a consulting firm that specializes in brazing technology, "brazing is one of the most underutilized joining processes. This is generally due to lack of awareness of what it can do."

Some experts believe that the culprit may simply be a lack of widespread resources. For instance, they point out that there are few textbooks dedicated to brazing.

"Compared to welding, there are fewer resources for brazing technical information," claims Randy Brisell, president of CyberTech International (Daytona Beach, FL). "Truthfully, it is a much easier technology. When the fit up of the parts is to tolerance, the joint fill is automatic from one delivery point unlike welding, which requires full articulation around the joint."

In addition, American universities have been very slow in developing brazing programs. In fact, the topic is not included in most curriculums that cover joining processes. While some schools, such as the University of Kentucky (Lexington, KY), actively conduct brazing research, the only university that currently offers a degree in brazing is in Europe-the Aachen University of Technology (Aachen, Germany).

Joining Advantages

Brazing offers many advantages that make it an attractive joining process. For instance, a brazed joint can offer strength equal to or greater than that of the base metals themselves. Brazed joints are ductile and can withstand considerable shock and vibration. Brazing is performed at relatively low temperatures, reducing the possibility of warping, overheating or melting the metals being joined. It allows simple fit up and easy disassembly without the need for additional components like nuts, washers and O-rings.

In addition, brazing is highly adaptable to automated methods. The brazing process is also flexible. Parts can be brazed manually with a hand torch or fixtured for semiautomated and fully automated assembly.

According to Tony Straniero, district sales engineer at Fusion Inc. (Willoughby, OH), brazing is ideal when service temperatures get high and leak-free joints are required. "Stampings and tubes can be fabricated via brazing at low component cost," he points out. "Also, many metals can be readily plated or e-coated after brazing."

Many engineers automatically associate brazing with copper. "That comes from the heating, ventilating and air conditioning industry, where a lot of copper is used and joints are brazed with a Sil-Fos brazing alloy," says DeVries. "However, other metals can be easily joined by brazing, including mild steel, stainless steel, brass, aluminum and carbide. It is also used for more and more ceramic joining applications."

Indeed, brazing is ideal for joining dissimilar materials. "Metallurgically incompatible, dissimilar metals won't interact with one another," explains EWI's Hauser. "They form brittle intermetallic compounds, low-melting eutectic compositions that lead to solidification cracking.

"[With brazing], the parent metals will not interact with each other because they are separated by the filler metal," adds Hauser. "They will, however, react with the filler metal, which may form a brittle intermetallic compound. The thickness of such intermetallic compound can be controlled to some degree and deleterious effects may be mitigated."

Filler metals for brazing applications are available in numerous forms, including powders, paste, wire, rods, strips, preforms and flux-coated forms. In addition, a variety of flux chemistries is available to minimize the oxidation that may form on both the filler metal and the parts being joined.

"A trend in brazing filler metals is the use of flux-cored materials," notes DeVries. "The brazing alloy is formed around a powdered flux." According to DeVries, another popular product is designed for wire feed applications, where the flux releases at the end of the wire or from the seam. It can also be used with brazing preforms.

"This material eliminates the fluxing operation," claims DeVries. "We have seen stronger, better quality joints, fewer flux inclusions or gaps, higher productivity, less flux exposure and a reduction in post-braze cleaning."

Hauser says any metal and ceramic material, such as oxide, carbide, nitride or glass, that melts above 840 F typically works the best with brazing. Lap joints that support shear are best for brazing, like adhesive bonding. While butt joints are occasionally brazed, they are typically avoided.

"Being a higher temperature process inherently increases the cost of brazing," Hauser points out. "But, for some applications, it's the best and cheapest method. I can't imagine how one could fabricate an automobile radiator evaporator, condenser and other heat exchangers cost-effectively without brazing."

Growing Popularity

While brazing is still not as widely used as soldering or welding, it is becoming more popular today, especially as manufacturers look for alternative joining methods to avoid fumes, heat and post cleaning.

"In many fields, there are no realistic alternatives," says CyberTech's Brisell. "But, brazing offers more automation, lower temperatures, less clean up, faster cycles, no leaks or voids, and less scrap. With the improvements we've made in automation and the new alloys on the market, there is more brazing than ever, especially where aluminum is concerned."

According to Dr. Dusan Sekulic, professor of mechanical engineering at the University of Kentucky Center for Manufacturing (Lexington, KY), brazing is being used at an increasing rate. "This is because novel approaches have been developed for brazing more difficult-to-join materials, such as titanium, molybdenum and ceramics," says Sekulic, who serves as director of the brazing R&D program. "Brazing becomes popular for netshape operations, and is used whenever welding can be avoided, because a cheaper operation and mass production can often be secured.

"The art of brazing is very well-developed," Sekulic points out. "However, the science of it is, to a large extent, still in its infancy. Brazing of titanium and other high-performance materials, as well as light metals, continues to have great importance for all branches of transportation, such as aerospace and automotive."

Engineers are looking for faster joining methods that produce assemblies with fewer failures and long service lives. "They are looking for fewer field failures whether from mechanical sources, corrosion, high temperature or fatigue," says Bob Peaslee, vice president emeritus of Wall Colmonoy Corp. (Madison Heights, MI). "Improvements in all of these items will reduce the overall cost of producing a brazement.

"Brazing is a growing industry [today]," claims Peaslee, who has been preaching the benefits of the joining process since the mid-1940s, when he worked on high-temperature brazing of early jet engine assemblies. "It has seen increasing usage throughout the last five to 10 years as engineers see the benefits of brazing. When an engineer looks at manufacturing an assembly, he has many processes to consider. Brazing should be one of them.

"New brazing filler metals are being developed to handle the increasing needs of different base materials that are continually being developed," notes Peaslee. "Filler metals for the brazing of titanium, ceramics and nonweldable base metals are but a few of the areas being engineered."

For instance, engineers at EWI have been experimenting with new processes, such as active metal brazing. "In active metal brazing, an active metal such as titanium, zirconium, tantalum, alumina or vanadium is added to the filler-metal alloy," explains Hauser. "These elements will react with nonmetals such as oxides, carbides and nitrides that normally are not wettable, to drive the wetting of the base material." Active brazing has been successfully used to braze ceramics in automobile engines, hybrid microelectronic packages and medical products.

"Manufacturers worldwide are adopting brazing techniques that offer decisive advantages over methods such as welding, soldering or adhesives," adds Nigel Cotton, automotive manager at the International Copper Association (New York). Cotton's organization has developed CuproBraze, a controlled atmosphere brazing process that eliminates subsequent rinsing and water treatment steps, avoiding strong fluxes that are required in aluminum brazing. According to Cotton, it is an economical process that is suitable to mass production.

"The CuproBraze technology joins new alloys of copper and brass with a filler metal that melts at a relatively high brazing temperature," explains Cotton. "This results in joints that are much stronger than soldered joints. And, unlike welding, brazing does not melt the base metals, so the dimensions can be tightly controlled and dissimilar alloys can be joined."

In China and Russia, CuproBraze has caught on in the automotive industry. Cotton claims that manufacturers are switching from first-generation technologies, such as soft soldering, directly to third-generation technologies, such as copper-brass brazing. "In the off-road market for heat exchangers, strong brazed joints allow for the use of brazed copper-brass serpentine fins, which are more durable than their alternatives," explains Cotton. Young Touchstone (Jackson, TN) recently unveiled a dedicated CuproBraze production line for producing heat exchangers.

Automated Equipment

During the past 25 years, several developments have slowly changed the brazing market and the way engineers view the technology. For instance, "much of the manual brazing using silver-bearing alloys and the copper-phosphorus family of materials has changed to automated processing," explains Philip Roberts, president of Delphi Brazing Consultants (Congleton, UK). "There has been a big increase in the use of mechanized brazing systems and processes that demand the use of furnace brazing." As a result, Roberts claims that the number of joints made by brazing has increased dramatically.

Roberts says three trends have had a major impact on brazing over the last 15 years:

• The use of continuous conveyor furnaces operating under an atmosphere of nitrogen for the brazing of automotive heat exchangers, oil coolers and evaporators fabricated from aluminum. "This is a market segment that has grown from about zero in 1984 to a situation where now [more than] 600 furnaces produce billions of joints per day on a round the clock basis," Roberts points out.
• The use of continuous conveyor furnaces operating under a reducing atmosphere for the brazing of mild-steel tubular assemblies for automotive applications. "A recent development in this area has been the brazing of stainless steel fuel-injector rails and associated items with copper or copper-nickel filler metals. These parts will see service temperatures up to about 200 C," says Roberts.
• The use of vacuum furnaces and high-temperature nickel-base filler materials for the brazing of aerospace and automotive applications where service temperatures up to 800 C might be encountered. "Such processing is ideally suited to the manufacture of plate heat exchangers fabricated in stainless steel where internal brazing of the assembly is of fundamental importance," says Roberts.

Automation has addressed manufacturers' demands for productivity, quality and safety. The complexity of brazing machines can vary from single-station machines to multistation, fully automatic equipment.

"Machines can be as simple as dispensers for brazing paste and flux, to indexing machines, to fully automated and robotic machines that sense the brazing temperature and provide the correct brazing temperature," says Lucas-Milhaupt's DeVries.

Selecting the right type of equipment depends on several variables, such as cycle time and production volume. The heating method is also important. Engineers can heat the parts with torches or a furnace, or they can choose induction or resistance heating.

"The equipment the engineer selects must be able to produce the quality of brazement at the required production rate at the lowest cost," notes Peaslee. "It is necessary to know the types of brazing equipment and their capabilities.

"If you wish to join many materials in high-volume production, then the continuous brazing furnace may be the best choice," explains Peaslee. "If you have complex assemblies that require specialized thermal cycles, the vacuum or batch furnace is an excellent choice. Induction brazing is also a good process, and the equipment can be put in the production line for a fast brazing operation."

A new trend is the digital torch and gas saver. The gas saver unit incorporates electronically operated valves for on-off switching of the gas-oxygen mixture, and the gas control unit maintains a constant pressure of gas, oxygen and air, even if the pressure is changed on the primary supply. The control unit can adjust the torch flame from a strong flame for heating parts prior to brazing to a soft flame for post-heating after the alloy is fed into the joint. The unit can store flame settings so the conditions are repeatable from part to part and operator to operator.

"The gas saver does just as the name implies-it saves fuel gas," says DeVries. "When the operator is done brazing, he or she places the torch on the arm of the gas saver unit and it shuts off the gas to the torch. With the increase in the price of natural gas and other fuel gases, it is becoming more economical to consider such a unit."

Manufacturers are also using more and more robots to dispense filler metals. "Our focus has been on robotic brazing, but what we do is still proprietary" says CyberTech's Brisell. "The basic principle is that we use a robot arm to mimic what thousands of brazers all around the country are doing right now manually. We introduce the principles of our other brazing automation systems and apply them to an articulated arm. The key is close control of the movement of heat and alloy application."

New Applications

Spurred by rising energy prices, researchers around the world are stepping up efforts to develop cost-effective fuel cells that can be easily mass-produced. Because there are numerous ceramic and metal parts that need to withstand high temperatures, fuel cells present endless opportunities and challenges for brazing technology.

"Many specialized heat exchangers are being developed and many specialized applications of brazing metals to ceramics are being explored," says Peaslee. For example, engineers at Dana Corp. (Toledo, OH) have developed an ultra-clean nickel brazing process that offers advanced thermal management.

Organizations such as EWI and the Welding Institute (Cambridge, UK) have been researching alternative methods to assemble fuel cell components with adhesives, brazing and laser welding. Research includes work on ceramic-reinforced braze systems for ceramic-to-ceramic and ceramic-to-metal joints, and glass-ceramic systems for improved thermal expansion matching.

According to Roger McKain, president of SOFCo-EFS Holdings (Alliance, OH), fuel cells present "all sorts of joining issues, such as brazing an all-ceramic stack to stainless steel conductors."

Heat exchangers are a good example of a fuel cell component with unique assembly requirements.

"Radiators, trim heat exchangers, bipolar plates, interconnects, recuperators and fuel processors have extremely high quality requirements," says Stan Ream, EWI's automotive market leader. "Because of their requirements, they are almost always made out of thin stainless steel. Assembling thin stainless steel rapidly, cost-effectively and efficiently is critical. We need aerospace quality parts at automotive prices. One little leak will ruin the whole stack. It's a tough, tough challenge."

Renewed interest in space exploration may also provide a major boost for brazing technology. Scientists at NASA's Goddard Space Flight Center (Greenbelt, MD) have been studying how electron beam (EB) brazing can be used to cost-effectively assemble large truss structures in space. The structures would be used to support huge antennas, satellites, solar panels, telescopes and other devices that would orbit in space or be anchored on the moon.

According to Yury Flom, Ph.D., head of the materials engineering branch at Goddard Space Flight Center, EB brazing is highly suitable for assembly applications in space because it takes advantage of the vacuum of space; can be easily automated; is a noncontact process; has no moving parts; and can join ultra-thin parts of any shape or profile.

"The advantage of EB brazing is a rapid, local heating of the joining area, as opposed to a relatively slow heating of the entire assembly [that occurs] during traditional furnace brazing," says Flom. "EB brazing is a very specialized process since it requires good vacuum and sophisticated motion controllers to joint complex parts."

Flom and his colleagues have developed a process called "snap-n-braze." It involves snapping parts together and brazing them using EB brazing. "It is envisioned as the main process of robotic assembly of large trusses in space or on the moon surface," explains Flom.