Aluminum Brazing

Aluminum and its alloys are brazed using almost similar procedures applied in other metal brazing. The only difference appears in the usage of filler metals, fluxes along with lower brazing temperatures in this case. The brazing procedure is easily executed employing standard brazing processes as torch, dip & furnace processes. While others such as induction & resistance processes are applied for specific purposes. Aluminum brazing requires close control of the process parameters for its successful production.



Generally, aluminum brazing alloys are divided into two main areas which are :

• Flux based alloys
• Fluxless based alloys

Materials
Brazing of aluminum alloys demand certain materials which can facilitate the brazing procedure. These are :

• Base Metal
  Base Metal in an aluminum alloy brazing includes non heat treatable & heat treatable wrought alloys & casting alloys. In case of non heat treatable alloys, high purity aluminums & aluminums with low addition of alloy elements are required while heat treatable alloys contain magnesium & silicon. These are brazed at comparatively lower temperatures than the non heat treatable alloys. Broadly, not all aluminum alloys are capable of being brazed, since, certain high strength wrought aluminum alloys & casting alloys consists of high amount of alloying ingredients. Such ingredients prevent adequate wetting by filler metal simply because of their unique oxide film combination.
  
  
• Filler Metal
  Filler Metals are based on the aluminum-silicon alloy system for aluminum brazing. They are available in the form of a powder, paste, wire or thin-gauge shim stock which can be either face-fed or preplaced in the joint area. Filler metal can as well be supplied using a much convenient method which allows the usage of brazing sheet consisting of a core of aluminum clad with a lower melting filler metal where the cladding is of an aluminum-silicon alloy.

Flux
Except fluxless inert gas or fluxless vaccum brazing processes, brazing flux is a must to be applied for every brazing process. In case of aluminum brazing fluxes, there is a fine mixture of inorganic chloride & fluoride salts supplied in a powder form. Applied either in dry or after with distilled or deionized water, or alcohol using a brush, spray or through dipping. Lot of caution is required while handling brazing flux, since, flux mixtures are strong chemical agents which can result in injurious repercussions when in contact with the body & can as well be corrosive to equipment. The brazing flux is generally used in a slurry form for brush application & a thinner mixture for dipping or spraying application.

Joint Types & Fixtures
While going for brazing aluminum alloys, lap joints are preferred than butt joints. Press fit or tightly fitting joints are not preferred when assembling aluminum parts which facilitates fluxing & the smooth flow of filler metal, thus, minimizing the flux entrapment.

Brazing Processes
The varied brazing processes applied for aluminum alloys have been dealt with in detail in this section. Let's take a look at these processes to understand their importance.

• Furnace Brazing
  Making use of flux, aluminum may be furnace brazed which incurs maximum benefits when brazed in a controlled atmosphere. Requirement of flux is comparatively less, since, the dehumidification process reduces the moisture content of the air.
  
  
• Atmosphere Furnace Brazing
  The equipment required for this process is almost the same which is used while their heat treating procedure. The furnace may be either oil or gas fired or it can be electrically heated. Proper circulation of the furnace atmosphere is essential to achieve uniformity of temperature by making use of improved heating conduction rates.
  
  
• Inert Gas Brazing
  Inert gas brazing requires furnace chambers of varied designs. As in fluxless brazing, the base metal surface is made rid of water molecules by using a combination of heat & vacuum before flushing the system with inert gas. In flux brazing, sufficient amount of inert gas is all that is required.
  
  
• Vacuum Furnace Brazing
  Vacuum brazing of aluminum alloys without any flux is one successful brazing process. Furnaces are used along with magnesium acting as a getter of gaseous oxygen as well modifier of the oxide film or aluminum.
  
  
• Torch Brazing
  Used primarily for small parts, short production runs & the attachment of fittings to previously welded or brazed aluminum assemblies, this brazing process is even applied for joining tubes to headers, return bends on exchangers & varied other similar joint configurations. Generally, air fuel gas or oxyfuel gas torches are used which are adjusted in a manner which allows the flame to slightly reduce.

Dissimilar Metal Brazing
Aluminum alloys can be directly brazed to certain other metals apart. These are :

• Titanium
• Nickel
• Cobalt
• Beryllium

With some metals, it is not possible for aluminum to be directly brazed such as in the case of aluminum & magnesium, since, they both have mutual solubility & posses the capability of forming an extremely brittle aluminum-magnesium intermetallic. Therefore, in case of bimetal joints, there is a requirement for special treatments to ensure adequate corrosion resistance simply because of the dissimilarity in electrochemical potential. Such a resistance power can be enhanced either by :

• Painting
• Encapsulation
• Coating with moisture impervious material