Which metal joining method should I choose?
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. Here are some basic brazing considerations:
How big is the metal joining assembly?
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 dissipate 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.
How thick are the metal sections?
Thickness of base metal sections is an important consideration in selecting your metal joining method. If both sections are relatively thick—say .500” (12.7mm)—either welding or brazing can produce a strong joint. But if you want to make a T-joint, bonding a .005” (.127mm) thick sheet metal section to half-inch stock, for example, brazing is the better choice. The intense heat of welding is likely to burn through, or at least warp, the thin section. The broader heat and lower temperature of brazing allows you to join the sections without warpage or metal distortion.
What’s the joint configuration?
Is the joint a “spot” or a “line”? A spot joint made at one point can be accomplished as easily by welding as by brazing. But a linear joint—all other things being equal—is more easily brazed than welded. Brazing needs no manual tracing. The filler metal is drawn through the joint area by capillary action, which works with equal ease on any joint configuration.
What metals are you joining?
Suppose you’re planning a two section metal assembly. You want high electrical conductivity in one section, high strength and corrosion resistance in the other. You want to use copper for conductivity, and stainless for strength and corrosion resistance.
Welding this assembly will present problems. As we’ve seen, you have to melt both metals to fuse them. But stainless melts at a much higher temperature than copper. The copper would completely melt and flow off before the stainless came anywhere close to its melting temperature.
Brazing these dissimilar metals offers no such obstacle. All you have to do is select a brazing filler metal that is metallurgically compatible with both base metals and has a melting point lower than that of the two. You get a strong joint, with minimal alteration of the properties of the metals.
The point to remember is that brazing joins metals without melting them, by metallurgically bonding at their interfaces. The integrity and properties of each metal in the brazed assembly are retained with minimal change.
If you plan to join dissimilar metals—think brazing.
How many assemblies do you need?
For a single assembly, or a few assemblies, your choice between welding and brazing will depend largely on the factors discussed earlier— size of parts, thickness of sections, joint configurations, and nature of base metals. Whether you braze or weld, you’ll probably do the job manually. But when your production needs run into the hundreds, or thousands (or hundreds of thousands), production techniques and cost factors become decisive.
Which method is best for production metal joining?
Both methods can be automated. But they differ greatly in flexibility of automation. Welding tends to be an all-or-nothing proposition. You weld manually, one-at-a-time, or you install expensive, sophisticated equipment to handle very large runs of identical assemblies. There’s seldom a practical in-between.
Brazing is just the opposite. You can braze “one-at-a-time” manually, of course. But you can easily introduce simple production techniques to speed up the joining of several hundred assemblies. As an example, many assemblies, pre-fluxed and bearing preplaced lengths of filler metal, can be simultaneously heated and brazed in a furnace. When you get into larger runs, it may become practical to rig up a conveyor which can run the assemblies past banks of heating torches and brazing filler metal can be applied to the joint in a premeasured amount. And there are endless “in-between” possibilities, a good many of which you can accomplish with relatively inexpensive production devices.
The point to keep in mind is that brazing is flexible. You can automate it on a step-by-step basis, at each step matching your automation investment to your production requirements.
When to Braze
Brazing as a means to make a part
So far, we’ve been talking about brazing as a way of joining two or more metals into a permanent assembly. And we’ve limited our discussion to the situations where you have a metal assembly in mind from the outset, from initial product concept through finished piece.
Now let’s discuss brazing from a very different point of view. Think about the parts your company fabricates, and consider whether any of those parts now made as monolithic units, might not be made as brazed assemblies.
Consider this real-life story…
A company was fabricating thousands of small, closed-end metal cylinders. The part looked like this:
For years, the cylinders were machined out of solid bar stock, with considerable labor required to drill and bore the blind holes. Finally, someone suggested that the cylinder was actually two parts—a tube and a plug.
Now the cylinders are made as assemblies—bar stock cut-offs brazed into lengths of stock tubing:
The assembly is a lot less expensive to make than the machined part— and it works just as well.
Brazing Consideration Should Occur at the Beginning
Brazing consideration should occur at the beginning, when you’re first planning or designing any metal component.
Ask yourself if the part should be made as a single unit, or if it can better be made as an assembly of simple components.
The “assembly” approach may help you eliminate expensive casting, forging and machining operations.
It may save materials. It may enable you to use low-cost stock forms—sheet, tube, rod, stampings or extrusions.
It will almost invariably be lighter in weight than the monolithic part, and will probably work better as the metals in the assembly can be selected to match their functions.
Let’s look at some typical metal “parts.” First we’ll see how they’re made by conventional casting, forging and machining methods. And then we’ll see how they could be made better and more economically as brazed assemblies.
From casting to sheet metal
You’re designing a housing, with threaded holes in the flange. You could make it as a casting. But consider instead making it as a brazed assembly, joining bar stock sections to a sheet metal deep draw.
The brazed assembly works just as well as the casting. And it’s a lot cheaper to make, because you’re putting the thickness only where you really need it—in the flange and not the shell. You save weight, materials and labor.
From forging to brazing.
You’re planning a part—a hardened cam on a steel camshaft. Should you machine the unit out of a solid bar of tool steel? That’s a lot of lathe chips. Perhaps forge the piece, and then finish-machine it?
Still a lot of work. After hardening, the cam has to be drawn and the shaft ends annealed. How about making the cam and shaft separately— and then join them mechanically as an assembly?
You’re on the right track. By substituting cold rolled for tool steel in the shaft, you’re saving on material cost. But machining is still somewhat involved, and a locking device, such as a set screw, is subject to loosening under vibration.
Now try the “assembly” approach again, but this time use a brazed joint instead of a mechanical one.
Simplest of all. No keyway, no key, no set screw. Minimum material, minimum labor—and a strong, permanent, vibration-proof bond.
The awkward elbow
Extensions or projections on metal parts require excessive material (expensive!), and then a lot of work to machine away the unwanted metal (twice as expensive!). Consider what happens when you make an elbowshaped part from solid stock:
You’re paying for metal you don’t want, and the labor of getting rid of it. There’s an easier way. Make the “part” as a brazed assembly, joining together standard tubing and bar stock components:
The assembly will be just as strong as the machined part. And you’ll save materials, labor—and weight. (The more awkward and complex the extension, the more you’ll save.)
From hard to easy
You have to design a leak-tight component, with complex configuration. You can plan it as a cored casting…
It will be leak-tight, but a cored casting is an expensive one. An open casting is a lot cheaper to make. So why not make it that way?
By using brazing, you’ve replaced the complex cored casting with a simple open casting—and a metal stamping. Machining is easier, and brazing’s capillary action assures you of a leak-tight bond.
From casting to stock parts
Let’s say you’re designing a base plate with a threaded coupling. You can make it in one piece as a casting…
Material cost is low, but material choice is limited. Weight is excessive, machining extensive, and the finished part may be weak and brittle.
Consider making the “part” as a brazed assembly of stock elements…
Machining is minimal—the base plate is a stamping and the coupling a screw machine part. Weight is down to the bone, too, because the thickness is only where it’s needed, in the threaded coupling. Material can be matched to function. And the assembly will undoubtedly be stronger than the casting.
Two metals are better than one
The ability of brazing to join dissimilar metals is helpful in many applications, but in some instances it’s quite critical. A classic example is the carbide metal-cutting tool. The tool could be made entirely of carbide. But carbide is expensive. What’s more, though carbide is fine for the cutting tip, you don’t really want to use it for
the tool shank. It’s too hard and brittle to withstand shock.
Brazing solves the problem…
By brazing, you’ve reduced material cost—obviously. But even more— you’re now using metals perfectly suited to their functions. Hard carbide at the cutting edge, and shock-resistant tool steel for the shank.
Freedom for the designer
We started this section with a question: “When do you think brazing?” And we’ve indicated, through just a few of the many possible examples, that you think brazing at the beginning—at the design stage.
The fact is—brazing liberates the designer. It enables him to design for function, for light weight, for selective use of metals, and for production economy. The designer who’s fully aware of the possibilities of brazing thinks less and less in terms of castings, forgings and parts machined from solid metal. He thinks more and more in terms of brazed assemblies, which combine plate or sheet stock, standard tubing and bar, stampings and screw machine parts.
Assemblies based on the use of such elements are generally lighter in weight, less expensive to fabricate, and at least equal in performance to metal parts made as monolithic units.