When brazing beats welding
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 1,600°F, 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 (1,981°F melting point) to steel (2,500°F 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.
While there are many types of joints, all are essentially variations of either butt or lap joints.
Butt joints have the advantage of a single thickness at the joint. Preparation is usually simple and they have sufficient tensile strength for many applications. However, strength depends on the bonding surface area, which cannot be larger than the cross section of the thinner member.
Lap joints make a joint twice as thick as the stock, but bonding area can be much greater than with butt joints. For applications that demand the advantages of both single thickness with maximum strength consider the butt-lap joint. It usually requires more preparation work but delivers higher strength with minimum thickness.
Obviously, part geometry limits bonding area of butt joints. But lap joints are often variable. Too long a lap wastes filler metal and may use more base material than necessary, without a corresponding increase in joint strength. And too short a lap may compromise joint strength.
A good rule of thumb for most applications is to make the lap three times as long as the thickness of the thinner member. However, when joint strength is critical, or when brazing a substantial number of identical assemblies, it helps to calculate the lap length more precisely. This maximizes strength while minimizing brazing materials.
For flat joints, X = TW/CL where X = length of lap; T = tensile strength of the weakest member; W = thickness of the weakest member; C = joint integrity factor, 0.8; and L = shear strength of brazed filler material.
For the lap length in tubular joints, X = WT(D --- W)/CLD where W = thickness of the weakest member; D = diameter of the lap area; T = tensile strength of the weakest member; C = joint integrity factor, 0.8; and L = shear strength of brazed filler material.
When to think brazing
Often, parts manufactured as a single unit can be made better and more economically as an assembly. The latter approach may eliminate expensive castings, forgings, and machining operations and save on materials. It often permits the use of lowcost stock forms sheet, tube, rod, stampings, and extrusions. This can reduce weight and improve performance,-if each part's material is matched to its specific function.
Consider the classic case where a company was fabricating thousands of small, closed-end metal cylinders. For years, the parts were machined out of solid bar stock, with considerable labor to drill and bore the blind holes. When it was suggested the cylinders were actually just tubes and plugs, the company began making assemblies of round bar-stock cutoffs (the plug) brazed into lengths of stock tubing. The process is much less expensive and the parts work just as well.
A hardened cam on a steel shaft is another example. It could be machined from a solid bar of tool steel, but that is expensive. It could also be forged and then machined, an option that is also labor intensive. After hardening, the cam must be drawn and the shaft ends annealed. The cam and shaft could be manufactured separately and mechanically fastened with a setscrew. This lets the shaft be made of lessexpensive cold-rolled steel, but some machining is still involved and the locking device could loosen under vibration. A better idea brazes the two components to produce a strong, permanent, vibrationproof bond with minimum material and labor.
In another example, consider a base plate with a threaded coupling. It could be made as a one-piece casting. However, the coupling requires face, drill, and tap operations, material choices are limited, and weight may be excessive. Brazing a threaded coupling to a stock plate minimizes weight and lets each material match the part's function.
Finally, there are many cases where two metals are better than one. The ability to join dissimilar metals is critical in some applications. A classic example is the carbide metalcutting tool. It could be made entirely of carbide, but that is expensive. And though carbide is fine for cutting, it is too hard and brittle to withstand shock in the tool shank. Brazing reduces material cost and permits hard carbide on the cutting edge and shock-resistant tool steel for the shank.
(414) 769-6000, lucas-milhaupt.com