How Brazing Works

If you ran your finger over a metal ingot, or piece of metal that's been worked into a specific shape for storing, you might describe its surface as smooth to the touch. If you viewed the same piece under a microscope, you would see the truth -- that the metal's surface is riddled with nooks and crannies. When you bring two pieces of metal together, these imperfections and irregularities create channels along which a liquid can move. In brazing, that liquid is a molten filler metal, and the force that pulls it through the microscopic "pores" is something called capillary action.

You can create a model of capillary action by placing a length of glass tubing into a dish of water. When you do this, you'll see the liquid climb up the tube, higher than the water level in the dish. This occurs because water molecules are sticky: They like to stay close to each other, and they also like to stick to the surfaces of other materials. Scientists refer to the former phenomenon as cohesion, the latter as adhesion. When adhesion is stronger than cohesion, capillary action commences. In other words, the water molecules sticking to the surface of the glass tubing pull up on the water molecules below.

Capillary action depends on the diameter of the glass tubing. As the diameter decreases, the water column rises higher. As the diameter increases, the water column falls to a lower position. This happens because the attractive forces between water molecules exert a stronger pull over shorter distances.

Brazing also depends on capillary action, which means the metal being joined must remain solid so the filler metal, once liquefied, can pull itself along the closely fitting adjacent surfaces. To meet this requirement, the filler metal must have a melting temperature above 840 degrees Fahrenheit (450 degrees Celsius) but below the melting point of the metals being joined [source:Sulzer Metco]. The filler metal can be placed into the joint before heating or fed into the joint as it's heated. But in either situation, the temperature must remain in a certain range to promote good capillary action. A craftsman must also be mindful of the gap between the abutting metal pieces. Brazing works best if the joint clearance falls between 0.001 and 0.005 inches (0.0025 and 0.0127 centimeters) [source: Belohlav]. If the gap is narrower, the movement of the filler may be impeded. If it's wider, the capillary forces will be reduced and the resulting joint may not be as strong.

If you're thinking brazing sounds a lot like welding, you're right. Welding usually uses a filler to join two pieces of metal, but it requires a much higher temperature. In fact, welding actually fuses the two pieces of metal together by melting the "base" metals and, if it's used, the filler. Capillary action can't occur during welding because there is no solid surface along which the filler can move.

In addition to the metals being joined, brazing has two important requirements -- a filler metal and a source of heat to melt the filler. We're going to look at each of these in greater detail, starting with the filler metal.

A brazing filler begins with one of several common primary metals: silver, aluminum, gold, copper, cobalt or nickel. These primary metals are then mixed, or alloyed, with other metals to improve or tweak their properties. For example, manganese acts as melting point suppressant, meaning it lowers the temperature at which the primary metal will melt. Chromium increases the strength of a brazed joint. And boron can help increase resistance to corrosion. The accompanying table shows a few common alloys used in brazing [source: Aufhauser Corporation].

Ultimately, metalworkers must balance a number of considerations when choosing a filler metal. Alloys behave differently than pure metals. The latter melt at a single temperature; they're a solid before this temperature and then, snap, they're a liquid. In contrast, alloys don't go directly from a solid to a liquid. Instead, they pass through a "mushy" stage during which they're both solid and liquid at the same time. A metalworker can use this to his advantage. If he's brazing a joint with a narrow clearance, he may choose a filler with a small brazing range, which results in a "lively" flow of filler metal in the gap. If he's brazing a joint with a wider clearance, he may choose a filler with a larger brazing range, which results in a "sluggish" flow that can adequately fill the gap. And, of course, he must be acutely aware of any hazards associated with the alloys used in the filler. Cadmium fumes, for example, are poisonous, so brazing with a cadmium filler must be done with adequate ventilation.

Finally, a metalworker must choose the form of the filler appropriate for the project. Some of this depends on how the filler will be applied. In some cases, he may place the filler in the joint before heating. Other times, he may manually feed the filler as he's heating. Either way, he has a wide selection [source:Sulzer Metco]:

When you think of industrial metalwork, you probably picture torches, full-face visors and sparks showering to the ground. This is a fairly accurate image of gas welding, which uses an acetylene torch to produce the heat necessary to fuse two pieces of metal. In many cases, pure oxygen is mixed with the gas to make the flame more intense. These oxyacetylene torches can produce a flame that's almost twice as hot as a flame resulting from an air-gas mixture.

Brazing can occur at lower temperatures than welding, though that doesn't eliminate the gas torch as an option. In fact, torch brazing is still common in certain applications, such as joining a tube into a fitting using copper or silver brazing filler metals. Gases include acetylene, hydrogen or propane, and metalworkers must exercise some care in choosing a heat source appropriate for their project.

Say, for example, a plumber wants to join two pieces of copper tubing. He would know that copper begins to anneal, or soften, at 700 degrees Fahrenheit (371 degrees Celsius) and that annealing can weaken the metal. All of which creates an interesting dilemma. Brazing, by definition, won't occur until 840 degrees Fahrenheit, so clearly the plumber must balance two key factors -- the strength of the joint and the strength of the overall assembly -- as he selects the best torch for the job. An oxyacetylene flame burns at 6,330 degrees Fahrenheit (3,499 degrees Celsius), which means it would anneal the copper to a greater degree. A propane flame, mixed with air, burns at just 3,630 degrees Fahrenheit (1,999 degrees Celsius), making it a better choice for this application.

Luckily, torch brazing isn't the only option. Induction brazing, which generates heat by passing electricity through a coil, is another way to join metal reliably. Using this technique, a metalworker holds the assembly between a set of induction coils and then initiates flow of a high-frequency current. As the current flows through the coil, electrical resistance generates heat, which rapidly raises the temperature of the metal part and the brazing filler. When the filler melts, he can turn off the current and allow the whole assembly to cool. A high-quality induction system can heat very small areas within narrow production tolerances. And because the heat can be precisely controlled, the process doesn't change the characteristics of the base metals being brazed.

Furnaces offer a final option as long as it's acceptable to heat the entire assembly. In this case, filler metal must be applied before the heating process. Then, a conveyor belt transports the piece into the furnace, where brazing occurs, and out the other side for cooling. Silver- and copper-based fillers are most commonly used in standard furnaces, although vacuum furnaces, which can pump oxygen out of the heating environment, extend the flexibility of the process, making it possible to braze with alloys that are sensitive to oxidation at high temperatures. Furnaces are also well-suited to automation as multiple pieces can pass through the preheating, heating and cooling phases in a continuous operation.

Brazed joints are incredibly strong -- stronger than the metals being joined in most cases -- but only if a metalworker follows good brazing procedure. Like welders, tradesmen who practice brazing techniques often receive training in certified programs. These programs help participants understand which variables affect the quality of brazing and how to evaluate solutions based on cost and performance.

Almost all courses cover the "Six Fundamentals of Brazing" -- the six essential steps that, if followed correctly, consistently produce high-quality joints. Let's review those steps now to see how a metalworker executes a brazed joint.

When it comes to joining two pieces of metal, tradespeople have a wide selection: mechanical fasteners, adhesives, soldering, welding and brazing. The first three options produce weaker joints, which are preferable in certain circumstances. Consider an assembly in which a pump must be connected to pipes. Because the pump has a finite lifetime and will eventually need replacement, it makes little sense to use a permanent joining technique. Instead, metalworkers would opt for a mechanical fastener, which could be easily disassembled when the pump failed.

If the goal, however, is to create a strong, permanent joint -- one that has superior resistance to shocks, vibrations and leaking -- the best candidates are welding and brazing. Knowing which technique to choose depends on the requirements of the project. One important consideration is the overall size of the finished piece. Metalworkers often choose welding if they're producing large assemblies, brazing if they're producing smaller assemblies. Why? Because brazing can only be achieved by heating all or most of the base assembly to the temperature at which the filler metal flows. If the assembly gets too large, heat dissipates more quickly than it builds up. Welding, on the other hand, doesn't rely on large-scale heating. In fact, a strong welded joint can be accomplished with intense, localized heating only.

Next, metalworkers must be concerned with the types of metals being joined. Thin sections, for example, are more likely to warp than thick sections. And the composition of the metals is just as important. Welding works better if someone is trying to join similar types of metal. That's because the welding process melts both the base metals and the filler. If a project calls for joining two wildly different metals -- say copper and stainless steel -- welding would melt one metal long before the other. Brazing, however, can readily join dissimilar metals because it's possible to find a filler that's compatible with both base metals and has a melting point lower than the two.

Finally, a metalworker must consider production requirements. Does he need 10 finished pieces or 10,000? Both welding and brazing can be done manually, but brazing is far more suitable for automation. Automated welding can be accomplished, but it takes sophisticated equipment and a highly repeatable fabrication process. Brazing offers much more flexibility and, as a result, can be set up quickly and cost-effectively.

All of which brings us back to those $200 aviator sunglasses -- and why companies such as Ray-Ban choose brazing in their manufacturing plants. Sunglasses possess all of the hallmarks of a braze-friendly assembly. Strong joints? Check. Thin, delicate pieces of metal? Check. Millions of units produced in an automated environment? Check. Now you can impress your friends with your cool shades and wow them with your knowledge of ancient metallurgical techniques.