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On Brazing Aluminum: An Aluminum Joining Process
- May 21, 2017 -

Joining processes for metals (such as iron, aluminum, steel) include fasteners , soldering, friction stir-welding, adhesives , welding , ultrasonic welding , brazing, and others Joining processes stem from two types of joining. Mechanical and metallurgical joining. Mechanical joining, including processes threaded fittings, soldering, stir-welding, adhesives, hook and loop, join materials through surface attachment, relying on friction. Metallurgical joining includes processes such as direct diffusion, welding, sintering, brazing, change the solutions of joining materials to bond at a chemical level, mixing at a molecular level, and requiring large amounts of energy.

Brazing aluminum occurs above 427°C (800°F) but below the melting point of 660°C (1220°F). Brazing is used to join materials without melting them. Typically used for heat-treatable aluminum alloys. Brazing requires less heat than welding. Filler metal must be liquid above 450 degrees Celsius .There are 6 major methods of brazing: dip brazing, furnace brazing, induction brazing, infrared brazing, resistance brazing, and torch brazing . Brazing is typically better for the strength of different materials and their expansion characteristics. Modern brazing allows for strong, leak-proof joints made quickly and inexpensively  The liquidus of the filler metal must be lower than the solidus of the base materials. Bonds from brazing metal as thin as 12.5 micrometers typically exhibit the same or better physical strengths of the aluminum parent material as well as the same levels of corrosion resistance

Brazing can be used for hard to reach joints in complex assemblies, such as the undulating surfaces of heat transfer devices . Quenching after brazing is standard practice and makes heat treatment generally unnecessary . A traditional brazing filler material for aluminum 6061 is Al-12Si. Parts must be cleaned and protected by fluxing or a protective non-oxygen atmosphere to prevent oxidation. Vacuum furnace brazing often eliminates the need for flux as vacuum prevents oxidation .

Vacuum brazing has found adoption in industrial areas where reactive materials, such as aluminum and copper, are joined. For brazing, fluxes prevent refractory oxide formation on the brazing material and the joining material. Flux also encourages the flow of braze throughout the accessible surface of the part. Fluxes can also regulate braze flow at a desired rate depending on application and can be applied in different ways. Fluxes may be brushed on or sprayed prior to brazing or may sit upon the brazing solution in a dip-braze process. Detergents 14 can remove unwanted flux from the joined part. In small parts, fluxes can be difficult to remove. In microfluidic applications, fluxes can affect the flow through the narrow channels. Flux is difficult to remove from devices with complex shapes and small openings, such as heat exchangers .

 Honeycomb parts are as difficult to remove flux from in the same way that heat exchangers are due to small openings and difficult shapes . Honeycomb structures without perforations are impossible to braze. Some claim that it is not economically feasible to fabricate honeycomb panels by coating both sides with an interlayer metal such as copper, silver, gold, tin, etc.

 Eutectic Brazing A form of brazing called eutectic brazing, uses two metal alloys that produce refractory oxides solids, to break up the oxide coatings and reduce them into the structure of the joint. Eutectic brazing may be used to create joints that are nearly as strong as the parent metals 

Most refractory oxides, such as aluminum oxide, have high hardness and high melting temperatures. While two separate metals may have two high melting points, their contact and heat applied can motivate the generation of eutectic alloys. Using a thin layer of silver, copper, gold, tin, or zinc as an interlayer , a eutectic reaction can dissolve the oxides and lead to diffusion in the weld seam. Films may be applied using vacuum and electrolytic deposition methods. Additionally, vapor deposition may be used.