When you want to join metal parts, you can choose from several options--including adhesive bonding and mechanical fasteners--but when you want to create robust, permanent metal joints, you typically have only two choices: welding or brazing. While welding creates metal joints by applying concentrated heat at the joint to melt and fuse metals together, brazing involves significantly lower temperatures and does not entail the melting of base metals. Instead, a filler metal is melted and forced to flow into the joint through capillary action. (Welding usually adds a filler material as well.)

In welding and brazing, the strength of the joint often surpasses that of the base materials. But because the heat used in brazing is less intense, this process does not alter most physical properties and minimizes distortion, warping and stresses in the joint area. Additionally, brazing's lower temperatures translate to less energy requirements.

But of course, the decision to weld or to braze comes down to the specifics of the application. Here are the factors to consider:

Size of the assembly: If you want to join large assemblies, welding is the more appropriate method. The tendency of larger assemblies to disperse heat can be a stumbling block for brazing, making it hard to reach the temperatures required for the filler metal to flow. In contrast, the concentrated heat of welding and its ability to trace a joint allow it to excel in joining big assemblies.

Thickness: Both methods are sound choices when metal sections are on the thick side, around 0.5 inches or more. Brazing has the edge, however, when it comes to thin sections. The high heat of welding can warp or burn through such sections. In contrast, brazing can help you avoid distortion.

Shape of joint: While both welding and brazing can create spot joints, the localized heat of welding offers the advantage of speed and low cost. However, when it comes to linear joints, the manual tracing required by welding makes it the less convenient choice. Additionally, brazing can just as easily draw the filler metal into straight, curved or irregular joint configurations.

Types of materials: Brazing soundly beats welding when joining dissimilar metals. As long as the filler material is metallurgically compatible with both base metals and melts at a lower temperature, brazing can create strong joints with barely any alteration of the base metals' properties. In contrast, base materials are melted during welding so joining two dissimilar metals using this method can entail complex and costly techniques.

Production volume: In manual jobs, the above factors--size, thickness, shape of joint and materials--will guide your decisions, but when part volumes are in the hundreds or thousands, production techniques and cost will become the most important considerations. While both brazing and welding can be automated, brazing allows for more degrees of automation. With welding, you're usually forced to choose between two extremes--either weld manually, one by one, or use pricey, cutting-edge equipment for large runs. In contrast, brazing can support medium-sized runs.

Appearance: With a neat and discreet strip, brazed joints are generally more attractive than welded ones, which have non-uniform beads. The vast majority of the time, you won't need additional finishing operations after brazing.

Source:

When Brazing Beats Welding
Steve Marek
Machine Design, December 9, 2004
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