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jeudi 31 mai 2012
Brazing vs. Welding
What's the Difference?
Welding joins base 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 as, 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 is broadly applied to the base metal to melt the filler metal and draws it into the joint by capillary action. This 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 stainless-steel joints can exceed 130,000 psi. However, because brazing temperatures are lower, generally 1,150°F to 1,600°F (620°C to 870°C), many physical properties may remain unaffected. Distortion and warping are minimal, and stress is minimized in the joint area. Lower temperatures also require less energy, which can result in significant cost savings.
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:
- 1. Assembly size. Welding is usually more suited to 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.
- 2. Thickness. If both metal sections are relatively thick-0.5" (12.7 mm) or greater-either method works well. However, thin sections tip the scales in favor of brazing. For instance, brazing is the better option on a T-joint with 0.005" (0.127 mm) sheet metal bonded to 0.5" (12.7 mm) 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.
- 3. Joint configuration. Both welding and brazing readily produce spot joints. Welding heat is typically localized, which has certain advantages. For instance, in joining two metal strips at a single point, electrical-resistance welding provides a fast, economical way to make strong, permanent joints by the thousands. However, linear joints are usually easier to braze than to 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.
- 4. 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, such as components being plated. Examples of products requiring cosmetic joints include jewelry, eyeglass frames, and plumbing hardware/faucets.
- 5. Types of materials. Brazing holds a significant advantage when joining dissimilar base metals. Brazing can form a strong joint with minimal alteration of base-metal properties, provided the filler material is metallurgically compatible with both base metals and has a melting point lower than the two. For example, because welding melts the base metals, attempting to join copper (1,981°F/1,083°C melting point) to steel (2,500°F/1,370°C melting point) would require sophisticated and expensive welding techniques. Also, the copper would likely 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. In addition, there are many non-metals, such as tungsten carbide, alumina, graphite, diamond, etc., which also can be bonded to various base materials using a brazing technique.
- 6. Production volume. Jobs requiring just a few assemblies will most likely be done manually. The choice between welding and brazing is determined by size, thickness, configuration, and material considerations. However, when part volumes run into the hundreds or thousands, production techniques and cost become vital.
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. By contrast, brazing lends itself to various degrees of automation. For example, with 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 applies pre-measured amounts of filler metal.
When considering brazing vs. welding for your operation, consider the size of your assembly, the thickness of materials, the type of joints, the appearance required, plus types of materials and production volume.
Questions? Lucas-Milhaupt's experts can help you navigate the challenges of joining metals. For more information on processes and applications, please contact us.