Brazing is the joining of metals through the use of heat and a filler metal – one whose melting temperature is above 840°F(450°C) but below the melting point of the metals being joined. (A more exact name for the brazing process discussed in this book may be "silver brazing," since in most cases the filler metal used is a silver alloy. To remain brief, we'll use the term "brazing" throughout this book, with the understanding that we are referring to a torch brazing process with a silver-bearing filler metal. Where exceptions occur, it will be noted.) Brazing is probably the most versatile method of metal joining today, for a number of reasons. Brazed joints are strong. On non- ferrous metals and steels, the tensile strength of a properly made joint will often exceed that of the metals joined. On stainless steels, it is possible to develop a joint whose tensile strength is 130,000 pounds per square inch. ( 896.3 megapascal [MPa] ). Brazed joints are ductile, able to withstand considerable shock and vibration. Brazed joints are usually easy and rapidly made, with operator skill readily acquired. Brazing is ideally suited to the joining of dissimilar metals. You can easily join assemblies that combine ferrous with nonferrous metals, and metals with widely varying melting points. Brazing is essentially a one-operation process. There is seldom any need for grinding, filing or mechanical finishing after the joint is completed. Brazing is performed at relatively low temperatures, reducing the possibility of warping, overheating or melting the metals being joined. Brazing is economical. The cost- per-joint compares quite favorably with joints made by other metal joining methods. Brazing is highly adaptable to automated methods. The flexibility of the brazing process enables you to match your production techniques very closely to your production requirements. With all its advantages, brazing is still only one of the ways in which you can join metals. To use brazing properly, you must understand its relationship to other metal jointing methods. What are some of those methods and which should you use where?
Brazing, as we've noted, relies on heat and a filler metal to join metals. There is nothing unique about this. Welding and soldering are similar in these respects. And metals can also be joined efficiently and economically without the need for heat or a filler metal at all, by mechanical fastening or adhesive bonding. When would you use brazing, rather than one of these other methods? It depends on the circumstances. Let’s start our evaluation of brazing as a metal joining method by eliminating those situations were brazing is generally unsuitable. The first of these situations is the non-permanent joint. This is the joint that’s made with future disassembly in mind. (For example, a pump connected to a piping assembly.)
The pipes won't wear out, but some day the pump will. It's easier to disassemble a threaded or bolted pump connection than a brazed con- nekton. (You can "de-braze" a brazed joint if you have to, but why plan on it?) For the typical non-permanent joint, mechanical fastening is usually the most practical method. There's another kind of joint where brazing will likely be your last, rather than your first, consideration. And that is the permanent, but low-strength joint. If you're joining metal assemblies that won't be subjected to much stress or strain, there are frequently more economical ways to join them than by brazing. (Mechanical fastening, for example, or soft soldering or adhesive bonding.) If you are selecting a method to seal the seams of tin cans, there is nothing to stop you from brazing. Yet soft-soldering would be perfectly adequate for this low-stress type of bond. And soft-soldering is generally less expensive than brazing. In these two areas – the non-permanent joint and the permanent but low-strength joint – other joining methods are adequate for the job and usually more economical than bronzing.
Consider brazing hen 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 and 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.
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 ore minuses to consider. The joints 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.