How Brazing Works
A brazed joint is made in a completely different manner from a welded joint. The first big difference is in temperature – brazing does not melt the base metals. This means that brazing temperatures are invariably lower than the melting points of the base metals. Brazing temperatures are also significantly lower than welding temperatures for the same base metals, using less energy.
If brazing doesn’t fuse the base metals, how does it join them? It works by creating a metallurgical bond between the filler metal and the surfaces of the two metals being joined. The principle by which the filler metal is drawn through the joint to create this bond is capillary action. In a brazing operation, you apply heat broadly to the base metals. The filler metal is then brought into contact with the heated parts. It is melted instantly by the heat in the base metals and drawn by capillary action completely through the joint. This is how a brazed joint is made.
Brazing applications include electronics/electrical, aerospace, automotive, HVAC/R, construction and more. Examples range from air conditioning systems for automobiles to highly sensitive jet turbine blades to satellite components to fine jewelry. Brazing offers a significant advantage in applications that require joining of dissimilar base metals, including copper and steel as well as non-metals such as tungsten carbide, alumina, graphite and diamond.
Comparative Advantages. First, a brazed joint is a strong joint. A properly made brazed joint (like a welded joint) will in many cases be as strong or stronger than the metals being joined. Second, the joint is made at relatively low temperatures, ranging from about 1150°F to 1600°F (620°C to 870°C).
Most significant, the base metals are never melted. Since the base metals are not melted, they can typically retain most of their physical properties. This base metal integrity is characteristic of all brazed joints, including both thin- and thick-section joints. Also, the lower heat minimizes danger of metal distortion or warping. Consider too, that lower temperatures require less heat – a significant cost-saving factor.
Another important advantage of brazing is the ease of joining dissimilar metals using flux or flux-cored/coated alloys. If you don’t have to melt the base metals to join them, it doesn’t matter if they have widely different melting points. You can braze steel to copper as easily as steel to steel. Welding is a different story because you must melt the base metals to fuse them. This means that if you try to weld copper (melting point 1981°F/1083°C) to steel (melting point 2500°F/1370°C), you must employ rather sophisticated and expensive welding techniques. The total ease of joining dissimilar metals through conventional brazing procedures means you can select whatever metals are best suited to the function of the assembly, knowing you’ll have no problem joining them no matter how widely they vary in melting temperatures.
Also, a brazed joint has a smooth, favorable appearance. There is a night-and-day comparison between the tiny, neat fillet of a brazed joint and the thick, irregular bead of a welded joint. This characteristic is especially important for joints on consumer products, where appearance is critical. A brazed joint can almost always be used “as is,” without any finishing operations needed – another cost savings.
Brazing offers another significant advantage over welding in that operators can usually acquire brazing skills faster than welding skills. The reason lies in the inherent difference between the two processes. A linear welded joint must be traced with precise synchronization of heat application and deposition of filler metal. A brazed joint, on the other hand, tends to “make itself” through capillary action. In fact, a considerable portion of the skill involved in brazing is rooted in the design and engineering of the joint. The comparative speed of highly skilled operator training is an important cost factor.
Finally, brazing is relatively easy to automate. The characteristics of the brazing process – broad heat applications and ease of filler metal positioning – help eliminate the potential for problems. There are many ways to heat the joint automatically, many forms of brazing filler metal and many ways to deposit them so that a brazing operation can easily be automated for almost any level of production.
How Soldering Works
Soldering joins materials, usually metals, together by melting and placing a filler metal – solder – into the joint, with the filler metal having a lower melting point than the adjoining metal. Today’s solders use lead-free alloys for applications in the electronics and plumbing industries using metals including gold, silver, copper, brass and iron.
What is the difference between brazing and soldering? The American Welding Society (AWS) defines brazing as a group of joining processes that produce coalescence of materials by heating them to the brazing temperature and by using a filler metal (solder) having a liquidus above 840°F (450°C) and below the solidus of the base metals.
Soldering has the same AWS definition as brazing, except that the filler metal used has a liquidus below 840°F (450°C) and below the solidus of the base metals. Soldering can be considered the low-temperature cousin to brazing.
Comparative Advantages. Although there are similarities between brazing and soldering, the temperature difference between the processes yields different behavior. Base metals involved in soldering are typically stronger than the solder itself; under the stress and fatigue of service, failure may occur through the solder joint. This means a that a soldered assembly may exhibit less joint strength and lower fatigue resistance than a brazed assembly
Should you braze or solder?
There are many factors that impact this decision including the service loading and temperature, to name two. Many substrates are damaged by the high temperatures required by brazing. Wettability of the substrate by either the solder or brazing filler metal is another key consideration in selecting the appropriate process. The ability to remove flux residue can be an important factor such as in certain HVAC and other fluid transport systems; closed loop systems which cannot be readily cleaned after joining must often be brazed or soldered in vacuum or under a protective atmosphere, or a self-fluxing filler metal such as the Lucas Milhaupt Sil-Fos alloys (BCuP-5) in copper-based assemblies must be used.
Other options? Mechanically fastened joints (threaded, staked or riveted) generally don’t compare to brazed joints in strength, resistance to shock and vibration, or leak-tightness. Adhesive bonding and soldering will provide permanent bonds, but generally, neither can offer the strength of a brazed joint –equal to or greater than that of the base metals themselves. Nor can they, as a rule, produce joints that offer resistance to temperatures above 200°F (93°C). When you need permanent, robust metal-to-metal joints, brazing is a strong contender.