Where does brazing fit in?
Consider brazing when you want permanent and strong metal-to-metal joints. Mechanically-fastened joints (threaded, staked, riveted, etc.) generally don’t compare to brazed joints in strength, resistance to shock and vibration, or leak-tightness. Adhesive bonding and soldering will give you permanent bonds, but generally neither can offer the strength of a brazed joint—strength 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).
If you want metal joints that are both permanent and strong, it’s best to narrow down your consideration to welding versus brazing.
Welding and brazing both use heat. They both use filler metals. They can both be performed on a production basis. But the resemblance ends there. They work differently, and you need to understand the nature of that difference to know which method to use where.
How welding works
Welding joins metals by melting and fusing them together, usually with the addition of a welding filler metal. The joints produced are strong, usually as strong as the metals joined or even stronger.
In order to fuse the metals, a concentrated heat is applied directly to the joint area. This heat is high temperature.
It must be—in order to melt the “base” metals (the metals being joined) and the filler metals as well. So welding temperatures start at the melting point of the base metals.
Because welding heat is intense, it is impractical to apply it uniformly over a broad area. Welding heat is typically localized, pinpointed heat. This has its advantages. For example, if you want to join two small strips of metal at a single point, an electrical resistance welding setup is very practical.
This is a fast, economical way to make strong, permanent joints by the hundreds and thousands.
However, if the joint is linear, rather than pinpointed, problems arise. The localized heat of welding tends to become a disadvantage. For example, suppose you want to butt-weld two pieces of metal—start by beveling the edges of the metal pieces to allow room for the welding filler metal. Then weld, first heating one end of the joint area to melting temperature, then slowly traveling the heat along the joint line, depositing filler metal in synchronization with the heat.
This is a typical conventional welding operation. Let’s look at its characteristics.
It offers one big plus—strength. Properly made, the welded joint is at least as strong as the metals joined.
But there are minuses to consider.
The joint is made at high temperatures, high enough to melt both base metals and filler metal. High temperatures can cause problems, such as possible distortion and warping of the base metals or stresses around the weld area.
These dangers are minimal when the metals being joined are thick. But they may become problems when the base metals are thin sections.
High temperatures are expensive as well since heat is energy, and energy costs money. The more heat you need to make the joint, the more the joint will cost to produce.
Now consider the automated process. What happens when you join not one assembly, but hundreds or thousands of assemblies? Welding, by its nature, presents problems in automation. We know that a resistance weld joint made at a single point is relatively easy to automate. But once the point becomes a line—a linear joint—the line has to be traced. It’s possible to automate this tracing operation, moving the joint line, for example, past a heating station and feeding filler wire automatically from big spools. But this is a complex and exacting setup, warranted only when you have large production runs of identical parts.
Of course, welding techniques continually improve. You can weld on a production basis by electron beam, capacitor discharge, friction and other methods. But these sophisticated processes usually call for specialized and expensive equipment and complex, time-consuming setups. They’re seldom practical for shorter production runs, changes in assembly configuration or—in short—typical day-to-day metal joining requirements.
How brazing works
A brazed joint is made in a completely different way from a welded joint.
The first big difference is in temperature. Brazing doesn’t melt the base metals. So brazing temperatures are invariably lower than the melting points of the base metals. And, of course, always significantly lower than welding temperatures for the same base metals.
If brazing doesn’t fuse the base metals, how does it join them? It joins them 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, in essence, is how a brazed joint is made.
What are the advantages of a joint made this way?
Why Choose Brazing
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. Brazing temperatures generally range 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. And this “integrity” of the base metals is characteristic of all brazed joints, of thin section as well as thick-section joints. Also, the lower heat minimizes any danger of metal distortion or warping.
(Consider too, that lower temperatures need less heat which can be a significant cost-saving factor.)
An important advantage of brazing is the ease with which it joins dissimilar metals. 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. You must melt the base metals to fuse them. So if you try to weld copper (melting point 1981°F/1083°C) to steel (melting point 2500°F/1370°C), you have to 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.
Another advantage of a brazed joint is its good appearance. The comparison between the tiny, neat fillet of a brazed joint and the thick, irregular bead of a welded joint is like night and day.
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. And that too is a money-saver.
Brazing offers another significant advantage over welding in that brazing skills can usually be acquired faster than welding skills. The reason lies in the inherent difference between the two processes. A linear welded joint has to 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. (A considerable portion of the skill involved in brazing actually lies in the design and engineering of the joint.) The comparative quickness with which a brazing operator may be trained to a high degree of skill is an important cost consideration.
Finally, brazing is relatively easy to automate. The characteristics of the brazing process—broad heat applications and ease of positioning of filler metal—help eliminate the potential for problems. There are so many ways to get heat to the joint automatically, so many forms of brazing filler metal and so many ways to deposit them, that a brazing operation can easily be automated to the extent needed for almost any level of production.
- Joint strength
- Lower temperatures/lower cost
- Maintains integrity of base metals
- Dissimilar metals easily joined
- Good joint appearance
- Operator skill easily acquired
- Process easily automated