Brazing Processes

Nov. 15, 2002
Brazing joins parts by heating them to more than 840°F and applying a filler metal that has a melting temperature below that of the base metal.

Brazing joins parts by heating them to more than 840°F and applying a filler metal that has a melting temperature below that of the base metal. Filler metal flows into the joint by capillary attraction.

Brazing has several advantages. Dissimilar metals can be joined. Assemblies can be brazed in a stress-free condition, and complex assemblies can be brazed in several steps by using filler metals with progressively lower melting temperatures. Materials of different thicknesses can be joined, as can cast and wrought metals. Nonmetals can be joined to metals when the nonmetal is coated. Metallurgical properties of base materials are not seriously disturbed, and brazed joints require little or no finishing.

Brazing is typically done with a torch or in a furnace. Other methods include dip, resistance, and induction brazing.

Torch brazing joins relatively small assemblies made from materials that do not oxidize at the brazing temperature or can be protected from oxidation with a flux. The most commonly used filler metals include aluminum-silicon alloys, silver-base alloys, and copper-zinc alloys. Flux is required with these filler metals unless protective atmosphere is used. Self-fluxing copper-phosphorus alloys are also used. Torch brazing is done in air and is the most common brazing process.

Normally, torch brazing is done with handheld oxyfuel gas torches using various fuels. However, there are automated machines that use preplaced fluxes as well as preplaced filler metal in paste, wire, or shim form. Torch and machine brazing are generally used to make lap joints in sections from 0.01 to 0.25 in. thick. Joints can be brazed rapidly, but speed decreases as material thickness increases.

Furnace brazing is practical if the product is self-jigging or can be preassembled and placed in a jig; if brazing material can be placed in contact with the joint; and if the part can survive uniform heating. Furnace brazing is suited for fabricating complete brazements, and does not require a highly skilled operator. Prefluxed or precleaned parts with filler metal preplaced at the joints are heated in furnaces. Brazing can be done in an air furnace with a flux, though a protective atmosphere usually is needed. The type of atmosphere required depends on the materials being brazed and the filler metals being used.

Base metals with readily reducible oxides can be brazed in an atmosphere of combusted natural gas or cracked ammonia. Dry hydrogen, a powerful reducing agent, can be used for brazing most stainless steels and many nickel, cobalt, and iron-base alloys.

Heat-resistant, high-strength alloys that contain appreciable amounts of aluminum or titanium are frequently brazed in a vacuum to prevent formation of oxides that inhibit wetting and flow of the filler metal. Parts made from such base metals can be plated to prevent oxidation during brazing. Plated parts can be brazed in a vacuum or in a controlled atmosphere.

Dip brazing is used on aluminum assemblies because the temperature of the molten flux bath can be controlled. The molten bath serves as both heating medium and fluxing agent. Uniform heating to brazing temperature is achieved rapidly. Parts are cleaned, assembled, and held together in fixtures during brazing. Parts are normally preheated before immersion, and residues must be removed after brazing in order to prevent corrosion.

Production rates and efficiency are good; heating rates are very fast, and many joints can be brazed at once. However, molten-metal baths are limited to use on small (0.005 to 0.200-in.) wires, sheet, and fittings that can be dipped into small heated pots.

Resistance brazing is used when small areas are brazed and the electrical conductivity of the joint members is high, as in the brazing of electrical contacts to contact holders. Heat is produced by the resistance of the joint members to electric current. Conventional resistance-welding machines are often used. Resistance brazing is best used for special joints where heat must be restricted to a localized area without overheating surrounding parts.

Induction brazing heats the workpiece by inducing a high-frequency current in the metal. The technique is used when the entire assembly must be heated or when part of the assembly would be adversely affected by heat. Because the workpiece is heated selectively by the coil, induction brazing reduces unwanted part distortion or annealing. Induction heating brings the joint rapidly to brazing temperature.

Brazeable metals: Low-carbon and low-alloy steels are brazed readily with silver or copper-base filler metals. Nickel-base alloys are sometimes used for applications requiring greater corrosion resistance or higher joint strength.

For some hardenable low-alloy steels, the recommended heat treatment must be considered in selecting the filler metal. Sometimes filler metal can be selected so that brazing and heat treating are combined. Decarburization and grain growth may occur if low-alloy steels are overheated during brazing. Therefore, short brazing cycles should be used.

Stainless steels can be brazed in a dry hydrogen atmosphere or in a vacuum. Atmosphere dewpoint must be maintained at ­60°F or lower to prevent formation of chromium oxide. At moderate brazing temperatures, stainless steels can be torch brazed in air if flux is used.

Austenitic alloys (nonhardenable AISI 200 and 300-series stainless steels) are well suited for brazing. Type 200 stainless steels can be torch or furnace brazed with silver, copper, or nickel-base filler metals. Furnace brazing above 1,400°F is usually done in a dry hydrogen atmosphere. In brazing unstabilized grades such as Types 302 and 304, carbide precipitation in the grain boundaries may occur if the assembly is held too long at temperatures between 800 and 1,500°F. This problem is not present when stabilized grades (Types 321 and 347) or low-carbon 300-series varieties such as 304L and 316L are brazed.

Type 200 stainless steels can be brazed in a hydrogen atmosphere, if a very low dewpoint is maintained to prevent formation of manganese oxide, which reduces wettability. Chromium-nickel steels must not be stressed during brazing, to avoid cracking.

Ferritic stainless steels may present a problem with interfacial corrosion when torch brazed with some silver-base alloys and flux. This corrosion can be prevented by using silver-base filler metals containing small amounts of nickel.

Martensitic stainless steels are easily brazed. Filler metal must be selected so the brazing cycle is compatible with the required heat treatment. These steels must be heat treated either after brazing or as part of the brazing cycle.

Precipitation-hardening stainless steels can be brazed in a dry hydrogen atmosphere if the alloys do not contain appreciable amounts of aluminum or titanium. Otherwise, they should be brazed in a vacuum, or the surfaces of the joint members should be electroplated to permit controlled-temperature brazing.

Aluminum alloys usually can be brazed. Care must be exercised because melting temperatures are only slightly higher than those of the brazing alloys. Furnace, induction, or dip-brazing equipment should have a control accurate to within 10°F. Torch brazing can be used but considerable operator skill is required.

Wrought aluminum alloys that can be brazed with aluminum-silicon filler metals include EC, 1100, 3003, 3004, 5005, 5050, 6053, 6061, 6063, and 6951. Cast alloys 43, 356, 406, A612, and C612 can also be brazed with these fillers.

Most 2000 and 7000-series wrought alloys and many cast alloys cannot be brazed because their melting temperatures are below those of commercial filler metals.

Aluminum alloys with high manganese content (5086, 5154, and 5456) are difficult to braze because they do not wet well. Aluminum alloys for die casting cannot be brazed because they blister when subjected to brazing-cycle heat.

Magnesium-base alloys can be brazed successfully if the precautions given for aluminum are applied. Joint clearance can be as high as 0.010 in., depending on the amount of overlap in the joint, but 0.002 in. is most desirable.

Alloy BMg-1 is recommended for torch or dip brazing. Alloys BMg-1 and BMg-2a are used to braze AZ10A, K1A, and H1A alloys; AZ31B and ZE10A magnesium-base alloys can be brazed with the BMg-2a filler metal. In all cases, a flux must be used and residues removed after brazing to prevent corrosion.

Copper and its alloys are brazed readily with many filler metals by most conventional brazing processes. Filler metals include copper-phosphorus, copper-zinc, and silver-base alloys. Copper alloys are usually fluxed when brazing is done in uncontrolled-atmosphere furnaces. However, copper-phosphorus filler metals are self-fluxing when used to braze copper. Joint clearances for brazing copper range from about 0.001 to 0.005 in., depending on the type of joint and flow properties of the filler metals.

Pure copper, particularly oxygen-free grades, is relatively simple to braze. Hydrogen embrittlement may occur if oxygen-bearing copper is brazed in atmospheres that contain even small amounts of hydrogen. For this reason, oxygen-bearing coppers should be brazed rapidly with low-melting BAg or BCuP filler metals.

Silicon, phosphor, and aluminum bronzes should be brazed in a stress-free condition. Special fluxes may be needed with these alloys to prevent formation of oxides that inhibit wetting and flow of the filler metal. Copper-nickel alloys can be readily brazed, but should be stress relieved before brazing. Copper-phosphorus filler metals are not generally recommended for brazing these base metals, but can be used for 90-10 Cu-Ni. Copper-zinc alloys can be brazed without difficulty. However, brasses are subject to stress cracking and should be heated gradually. Beryllium-copper alloys can be brazed with some silver-base filler metals. Brazing should be done during heat treatment with a filler metal whose brazing range fits the heat-treating cycle.

Nickel alloys can be brazed with most filler metals suitable for brazing ferrous metals, if the filler metal has the same oxidation and corrosion-resistant properties as the base metal.

High-nickel alloys are subject to stress-corrosion cracking in the presence of molten filler metals. Parts should be stress relieved before brazing and should be assembled in a stress-free condition.

Superalloys with a cobalt base are easiest to braze. The process is carried out in a dry hydrogen atmosphere (or in a vacuum) using nickel-base filler metals or those based on silver, gold, or palladium. Filler metal and brazing cycle must be carefully selected because the properties of cobalt-base alloys (and those of the iron and nickel-base superalloys) can be adversely affected by high brazing temperatures and long brazing cycles.

Iron and nickel-base superalloys are most successfully brazed in a vacuum using nickel-base filler metals or filler metals based on the noble metals. These alloys also can be nickel plated and brazed in a dry hydrogen, but joint strength is usually less than that of vacuum-brazed joints. Filler metals and cycles used to braze these superalloys must be selected to avoid excessive metallurgical reactions between the base and filler metals.

Brazing of dissimilar metals whose physical properties differ markedly often can be minimized by selecting the proper joint design, filler metal, and brazing cycle. For example, refractory metals can be brazed to steel using a ductile filler metal and a design that keeps the joint in compression rather than tension.

Problems associated with metallurgical compatibility of dissimilar metals are more serious, and thorough understanding of the reactions that occur during brazing is essential. Aluminum or titanium, for example, are not metallurgically compatible with steel. Reactions that occur during brazing may produce undesirable intermetallic compounds.

Joint clearances should be generous (0.006 to 0.025 in.) to permit filler metals to flow into the joint and prevent flux entrapment.

Fluxless aluminum brazing, invented by the General Electric Co., involves exposing the joint area to the action of magnesium in a nonoxidizing atmosphere. A vacuum furnace is usually employed, but brazing can also be done in nitrogen or argon atmospheres.

Special cladding alloys incorporating magnesium eliminate the necessity of adding magnesium in other forms. Certain aluminum alloys containing magnesium braze well without modification. Since no flux is used, air and water pollution problems are eliminated, and water is conserved by completely eliminating rinsing cycles.

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