The next time you put on your $200 aviator sunglasses, thank the Mesopotamians. That's right, the metalwork used to join the small, delicate pieces of your Ray-Bans can trace its roots back to ancient Sumerian blacksmiths.
Here's how the basic history goes: First, humans discovered metals, including the seven metals of antiquity -- gold, copper, silver, lead, tin, iron and mercury. The first two to be widely used were gold and copper, and the Mesopotamians, Egyptians, Greeks and the Romans all produced artisans who hammered the soft metals into sheets and fashioned them into everything from utensils and cups to jewelry and weapons.
It didn't take long for these craftsmen to move from simple objects made from a single piece of metal to complex objects made by joining several pieces together. For example, during the first dynasty of Ur, circa 2650 to 2500 B.C.E., metalworkers created some remarkable architectural decorations fashioned from copper. One such decoration, which adorned the temple at al-'Ubaid, features an eagle with a lion's head, holding two stags by their tails. The stags' antlers were fashioned separately and then joined to the larger piece.
The oldest method of joining metal involved rivets -- short pins driven into holes punched in metal sheets. Unfortunately, rivets (and their modern counterpart, nuts and bolts) can become loose over time and don't always lead to the strongest joint. This led craftsmen to look for other ways to fuse their bits and pieces of metal. As iron became more prevalent, they relied on forge welding, which required preformed metal pieces to be superheated and then hammered -- or pressed -- together.
Today, other techniques have replaced forge welding, although they all need heat to work. These include soldering, welding and brazing. Each of these methods uses heat to melt a filler metal into the gap formed between two pieces of metal that need to become one. What's different is the intensity of the heat, the nature of the filler and, as a result, the strength and durability of the joint. In brazing, temperatures must be hot enough to melt the filler metal but cool enough not to melt the metals being joined. Often, that filler is a silver alloy, which leads to brazing's other common name, "silver brazing."
On the next few pages, we'll take a closer look at brazing to understand how it works and why someone might choose the technique over welding. Let's begin by taking a microscopic look at the boundary formed when two pieces of metal come together.
Capillary Action: The Science Behind Brazing
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.
Braze Filler Metals
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]:
- Powder -- The filler takes the form of dry, spherical particles. Each particle contains the elements making up the alloy in the proper proportion.
- Paste -- A paste combines alloy powders with a neutral binder to create a filler that can be extruded into a joint. Binders may be water or an organic compound and can affect the drying time of brazed joints.
- Tape -- Manufacturers use binders to attach thin ribbons of braze metal to an adhesive backing, which can be rolled up and shipped. They often cut the tape so its dimensions (width and thickness) conform to a metalworker's exact specifications.
- Foil -- Braze foil is a thin, but pure, sheet of metal alloy. It can be cut into pieces and shapes and stacked to produce filler of varying thickness.
- Rod -- Brazing rods come in diameters ranging from 0.3125 to 0.375 inches (0.7938 to 0.9525 centimeters). Like foil, rods contain just the elements making up the alloy, with no binders added.
Heat Sources for Brazing
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.
- A fantastic fit: As we discussed earlier, brazing relies on capillary action, and capillary action works best if the space between the metals to be joined falls in a certain range -- between 0.001 and 0.005 inches (0.0025 and 0.0127 centimeters) [source: Belohlav]. Before a metalworker enters the shop, she must spend some time drawing up engineering specs. She must understand the structural requirements of the project and then design a joint to make sure the final assembly performs properly. She may choose a lap joint (where two metal pieces overlap), a butt joint (where two metal pieces fit end-to-end) or a tee joint (where two metal pieces connect at a right angle). Then she must account for the properties of the metal being used. All metals expand when they're heated, so the brazing procedure must allow for this. If not, the joint may be too tight or too wide and, as a result, weaker than necessary.
- A clean slate: Contaminants in the joint can interfere with good capillary action. For example, heat can carbonize oil and grease, which can form a film that impedes the flow of the filler metal. To avoid this, surfaces should be free of dust, grease, oil or rust. A steel brush can remove dirt and oxide contaminants, while solvents can dissolve oil.
- Flux before flame: Heating a metal surface initiates a chemical reaction in which metal atoms combine with oxygen. This produces oxides, which can prevent the filler metal from wetting the joint surfaces. Applying a coating of certain chemicals can block or neutralize the oxidation process. These chemicals are known as flux, and they can vary in chemical composition depending on the brazing conditions. Many fluxes come in paste form and can be applied manually by brush or dipping. In automated production environments, spray guns may be used to apply dry flux powder to surfaces.
- Clamp and support: If you're joining two pieces of metal, you need them to remain aligned until the brazing process can be completed. For most projects, gravity provides enough force to hold parts together until the brazed joint cools. Otherwise, clamps and vises may be helpful. A complex assembly may require a support fixture -- a device that supports several pieces of metal in a precise configuration until brazing is completed. Metalworkers usually look for stainless steel or ceramic fixtures because, as poor conductors of heat, they don't pull as much heat away from the base metals.
- Braze away! After fluxing the joint and clamping the piece, it's time to heat things up. In brazing, a metalworker doesn't apply heat directly to the filler. Instead, he increases the temperature of the base metals until they reach the melting point of the filler. If it's small, the entire assembly can be heated. If it's large, a broad area of metal surrounding the joint can be heated. Uniform heating is critical, so metalworkers must be aware of the piece's basic structure. For example, thick sections of metal will require more heating than thin sections. Similarly, metals with different heat conductivities must be warmed at different rates. When the assembly reaches the brazing temperature, the metalworker can remove the heat source and introduce the filler. The most basic technique calls for him to touch a rod or wire to the joint surface. The intense heat melts the rod, and capillary action pulls the molten metal into the gap between the base metals. He must be careful not to apply the filler too far from the joint, as the liquefied filler may run over the metal surfaces without flowing into the joint.
- A clean slate, part 2: As the temperature of the assembly falls, the filler metal will solidify, securing the individual pieces as it does. The final step is to clean away the flux material, which can, if not removed, corrode and weaken the joint. A common technique involves submerging the entire structure in a hot water bath. This causes the flux material, glass-like after the heating process, to crack and flake off. Rubbing the joint with a brush or steel wool can remove any flux that clings to the metal surface.
Brazing Applications and Advantages
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.
Author's Note: How Brazing Works
When I was a kid, my next-door neighbor's father worked as a welder on the Washington Metro rapid transit system, so I learned a bit about the process from him. And I saw my father use a soldering iron in the garage. But until I began working on this article, I hadn't heard of brazing or thought much about the complex manufacturing required to produce something as outwardly simple as a pair of sunglasses.
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