A brazed joint is made in a completely different way from a welded joint. The first big difference is n 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?
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, the 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.) And 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 vase 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 auto- mated to the extent needed for almost any level of production.
As we've indicated, when you want to make strong and permanent metal joints, your choice will generally narrow down to welding or brazing. So, which method is best? It depends entirely on the circumstances. The key factors in making a decision will boil down to the size of the parts to be joined, the thickness of the metal sections, configuration of the joint, nature of the base metals, and the number of joints to be made. Let's consider each of them.
Welding is usually more suited to the joining of large assemblies than brazing. Why? Because in brazing the heat must be applied to a broad area, often to the entire assembly. And if the assembly is a large one, it's often hard to heat it to the flow point of the filler metal as the heat tends to dessipate faster than you build it up. You don't meet this limitation in welding. The intense localized heat of welding, sometimes a drawback, becomes an advantage in joining, a large assembly. So does welding's ability to trace a joint. There's no way to establish exactly the point at which size of assembly makes one metal joining method more practical than another. There are too many factors involved. For example, if the assembly is unable to be brazed in open air (torch, induction, etc.) due to size, a furnace or dip brazing process may eliminate the size consideration. However, you can still use this rule-of-thumb as a starting point: Large assembly-weld, if the nature of the metals permits. Small assembly-braze. Medium-sized assembly-experiment.
 |
 |
 |