In brazing, specific fluxes and filler materials with melting points lower than that of the parent metal are used for making a joint without melting the pieces to be joined. Brazing can be used to advantage when sections are too thin for welding, and for those assemblies having many parts which must be joined in an intricate manner. Brazing is generally lower in cost than gas or arc welding and is adaptable to mass production. Brazed joints have a smoother appearance, with well rounded fillets which often require no finishing.
Brazed joints should be carefully designed to provide for full penetration of filler metal, because its flow depends largely on capillary action and gravity. Joints should be self-jigging for easy assembly prior to brazing. Lock seams, lap fillet, and T-joints are preferred because they have greater strength than butt or scarf joints.
Three commonly used aluminum brazing methods are furnace, molten flux dip, and torch.
Furnace Brazing
Furnace brazing consists of applying a flux and filter material to the workpieces, arranging them, then heating in a furnace to a temperature that causes the filler material to melt and flow into the joint without melting the parent metal. Filler material in various forms is added to the joint. In many cases, filler material in the form of a flat shim or wire ring can be fitted into the joint. Filler material is also supplied by using clad brazing sheet, shaped to fit the joint.
Standard types of furnace heating systems include forced air circulation, direct combustion, electrical resistance, controlled atmosphere, and radiant tube. The selection of furnace type is determined by the application requirements, as furnace operation and results vary. For example, temperature is most easily controlled in electrical resistance furnaces. Although combustion furnaces are least expensive, some assemblies cannot be exposed to the gases which are always present in this type. Radiant heat furnaces are sometimes difficult to regulate, but the type of heat produced is excellent for most brazing requirements. Aluminum-coated steel or firebrick linings are preferred for all types of heating units.
Rate of production is another consideration when selecting a heating unit. In batch furnaces, brazing is accomplished by placing a tray of assemblies inside, heating for the required time, then removing the batch. Though simpler, this furnace is slower than the furnace with a continuous conveying system in which the work moves through on a belt. The continuous furnace is more conservative of heat, and the gradual heating reduces danger of warping.
Temperature for individual batches will necessarily depend on such factors as the design of the parts, size of fillets, and alloy to be brazed. However, furnaces should have operating temperature ranges from 540 to 650°C (1000 to 1200″F), with control capability within +/- 3°C (5°F). Since regulation of temperature is critical, automatic control is the rule in production jobs. If uniform rise of temperature does not occur naturally, forced circulation is essential.
Assemblies are generally placed in the furnace immediately after fluxing. When large areas have been fluxed, most of the moisture must be removed because the brazing process may be hindered if it is not removed. Preheating the parts for about 20 minutes at approximately 200°C (400°F) is usually sufficient.
Brazing time depends on the thickness of the parts. For instance, material 0.15 mm (0.006 in.) thick reaches temperature in a few minutes, while 13 mm (0.5 in.) thick material may take up to 45 minutes. After the filler material begins to melt, it takes approximately five minutes for the material to fill the joints.
Dip Brazing
Parts are assembled and dipped into a molten flux in dip brazing. This method has been very successful for the manufacture of elaborate assemblies, such as heat exchanger units. The flux application does not require a separate operation and the bath transmits heat to the interior of thin walled parts without overheating outside surfaces. Contamination is also held to a minimum.
Dip brazing is versatile. It is used in the manufacture of delicate specialty parts where tolerances up to k0.05 mm (0.002 in.) are maintained in production, or in making large parts approaching 450 kg (1000 lb).
A separate furnace is necessary to preheat the assembly to prevent undue cooling of the flux bath. A furnace used for furnace brazing operated at 280 to 300°C (540 to 565°F) is satisfactory for preheating. It should be located near the dip pot so heat loss will be held to a minimum.
Size of the dip pot will depend on the size of the assemblies to be brazed, but should be large enough to prevent the parts from cooling the flux more than 5°C (10°F) below operating temperature when they are added.
Dehydration of the flux bath is accomplished by dipping 1100 or 3003 alloy sheet into it. As the sheet is attacked, the hydrogen evolved is ignited on the surface. Residue that forms on the bottom of the pot must be removed on a regular basis.
A modification of dip brazing is the application of a flux mixture to the assembly prior to immersion in a salt bath furnace. A typical example consists of making a paste of a mixture of a dry, powdered aluminum-silicon (548°C [1018″F] flow point) brazing alloy and flux, and water, and applying as much as required to fill the joints and make fillets. Next, the assembly is placed in an oven and heated to about 540°C (1000°F) to remove the water. This leaves the brazing alloy powder firmly cemented to the aluminum surfaces, the flux serving as the cement.
When the assembly is placed in the molten brazing salt, the alloy is held firmly in place by the flux cement while it is being heated and melted. The flux cement has a higher melting point than either the brazing alloy or the brazing salt, but it is soluble in the salt bath, so the brazing alloy is held in place, even while melting, until the cement has been dissolved by the molten salt. As the flux cement is dissolved away from the molten filler metal, the alloy runs into the joint capillary spaces and also forms smooth fillets.
Torch Brazing
This method of brazing can be accomplished by using a standard torch as a heat source. Correct torch tip can best be determined through trial, and often depends on the thickness of the piece to be brazed. Filler alloys with suitable melting ranges and efficient fluxes are available for all brazeable aluminum alloys. Most work can be torch brazed with 3 mm (1/8 in.) diameter wire.
A reducing flame with an inner cone about 25 mm (1 in.) in length and a larger exterior blue flame is preferred. Oxyhydrogen, oxyacetylene, oxynatural gas, or gasoline blow torches can be used. Ample clearance space must be allowed where the filler will flow, and a path for flux to escape must be allowed.
After painting with flux paste, the entire area of the joint is heated until the filler melts when it is touched against the heated parent metal. Too hot a flame, or allowing the joint to cool repeatedly, will cause uneven results. Capillary flow tends to be toward the hottest spot, so it is important that the flow of the filler wire be controlled throughout. Heat should be applied just ahead of where flow is desired. Joints can be produced that have a final fillet that needs a minimum of finishing, if any. All flux should be removed after brazing. If joints are accessible, a fiber brush with boiling water bath can be used. Scrubbing with hot water and rinsing with cold, then drying is often effective, as is blasting with a steam jet. When possible, a chemical treatment should be used to clean the joint.
Cleaning
Clean surfaces are essential if strong brazed joints are to result. All grease should be removed. Solvent or vapor cleaning will probably be sufficient for the nonheat-treatable alloys, but the for the heat-treatable alloys, the oxide film must be removed with a chemical or by abrasion with steel wool, or stainless steel brushes. All burrs should be removed, as flux will not flow around them.
In post-brazing cleaning, it is essential to remove all the flux. A solution of nitric acid (concentrated technical grade) in equal amounts of water is effective. When a large area is to be cleaned of residual flux, however, this method is not recommended because noxious fumes are generated. An exhaust system is advisable even for small production situations. To achieve a uniform etch and remove flux in one operation, the work can be immersed in a nitrichydrofluoric acid solution, using 2 L (0.5 gal) nitric acid, 1/8 L (1/4 pint) hydrofluoric acid, and 17 L (4.5 gal) of water. The major portion of flux should be removed first by immersing in boiling water, then immersing in the acid solution for 10 to 15 minutes, depending on the desired extent of etching. Parts are then drained and rinsed in cold running water, then in hot water. To avoid staining, the hot water bath should be limited to about 3 minutes.
Because of the reaction of a hydrofluoric acid solution with aluminum, in which hydrogen gas is generated, flux removal is efficiently accomplished by this method. The solution is compounded of 600 mL (1.25 pints) of acid, (technical concentrated grade) and 19 L (5 gal) of water. Though this solution is less contaminated by flux than those containing nitric acid, the hydrofluoric acid solution does dissolve aluminum. Therefore, immersion time should be limited to 10 minutes or less. Discoloration can be removed by a quick dip in nitric acid.
When maximum corrosion resistance is important, or when parts are thin, parts can be dipped in a solution of 2 L (2.25 qts.) of nitric acid (technical concentrated grade), 1.8 kg (4 lb.) of sodium dichromate, and 17 L (4.5 gal) of water. The usual procedure is to immerse the parts in hot water, then in the dip solution at 65°C (150°F) for 7 to 10 minutes, followed with rinsing in hot water.