Brazing vs Welding

Brazing is an economical method for making strong, permanent metal joints.

Steve Marek
Brazing Application Specialist
Lucas-Milhaupt Cudahy, Wis.

There are a number of options when it comes to joining metal parts, including adhesive bonding, nuts and bolts, and many other types of mechanical fasteners. But for strong and permanent metal joints, the choice usually comes down to either welding or brazing.

Welding joins metals by melting and fusing them, usually adding a filler material. Fusion requires concentrated heat directly at the joint, and temperatures must exceed the melting point of the metals and filler. Welded joints are usually as strong or stronger than the base materials.

Brazing differs from welding in that the temperature is considerably lower and does not melt the base metals. Rather, the heat source melts a filler metal and draws it into the joint by capillary action. It creates a metallurgical bond between the filler metal and part surfaces.

Like welding, joint strength often exceeds that of the individual parts. For instance, the tensile strength of stainlesssteeljoints can exceed 130,000 psi. But because brazing temperatures are lower, generally 1,150 to 1,600F, most physical properties remain unaffected. Distortion and warping are minimal, and it minimizes stresses in the joint area. Lower temperatures also require less energy, which can result in significant cost savings.

WELDING VERSUS BRAZING
Both methods produce strong, permanent joints, so the obvious question is which is best for a given application. Let's look at several key considerations:

Assembly size. Welding is usually more suited for joining large assemblies. Brazing applies heat to a broad area, often the entire assembly. Larger assemblies tend to dissipate heat and can make it difficult to reach the flow point of the filler metal. Welding's intense localized heat overcomes this drawback, as does the ability to trace a joint.

Thickness. If both metal sections are relatively thick say 0.5 in. or greater either method works well. But thin sections tip the scales in favor of brazing. For instance, brazing is the better option on a T-joint with 0.005-in. sheet metal bonded to 0.5-in. stock. The intense heat of welding will likely burn through, or at least warp, the thin section. Brazing's broader heating and lower temperature joins the sections without distortion.

Joint configuration. Welding and brazing both readily produce spot joints. Welding heat is typically localized, which has certain advantages. For instance, if joining two metal strips at a single point, electrical-resistance welding provides a fast, economical way to make strong, permanent joints by the thousands.

But linear joints are usually easier to braze than weld. Welding requires heating one end of the interface to melting temperature, then slowly traveling along the joint line and depositing filler metal in sync with the heat. Brazing requires no manual tracing, and filler metal is drawn equally well into straight, curved, or irregular joint configurations.

Types of materials. Brazing holds a significant advantage when joining dissimilar metals. These can form a strong joint with minimal alteration of basemetal properties, provided the filler material is metallurgically compatible with both base metals and has a melting point lower than the two.

Because welding melts the base metals, attempting to join copper (1,981F melting point) to steel (2,500F melting point), for instance, would require sophisticated and expensive welding techniques. And more likely than not, the copper would melt before the steel even approached welding temperature.

Brazing's ability to join dissimilar metals lets users select metals best suited for an application's functional requirements, regardless of differences in melting temperatures.

Production volume. Jobs requiring just a few assemblies will most likely be done manually. The choice between welding and brazing then comes down to size, thickness, configuration, and material considerations.

But when part volumes run into the hundreds or thousands, production techniques and cost become decisive. Both methods can be automated, but they differ in terms of flexibility. Welding tends to be an all-or-nothing proposition. Either weld manually, one at a time, or install expensive, sophisticated equipment to handle large runs of identical assemblies. There is seldom a practical in-between.

Brazing lends itself to various degrees of automation. For instance, for moderate production runs, simple automation techniques such as prefluxed assemblies and preplaced lengths of filler metal can speed production. For larger runs, conveying assemblies past banks of heating torches and robots apply premeasured amounts of filler metal.

Appearance. Brazing typically produces a tiny, neat fillet, versus the irregular bead of a welded joint. This is especially important on consumer products-where appearance is critical. Brazed joints can almost always be used as is, without additional finishing operations.

 

SOURCE

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