how to aluminum brazing

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Many aluminum alloys can be joined by brazing. The brazing of aluminum is more difficult than that of other metals, because the melting points of metal and filler are close, and because of the oxide layer on the surface of aluminum.

Aluminum alloys that can be brazed include 3003 and 60 series alloys. Alloys 2024, 2219 and 7075 cannot be brazed with commercially available solders because they melt at a lower temperature than currently available solder alloys. Aluminum / silicon alloys are commonly used as filler metals. All Al based metal alloys will be annealed at the temperature required for brazing. The heat treatable alloy can be heat treated after brazing to recover its mechanical properties.

The most common brazing processes for aluminum include torch welding, immersion welding and furnace brazing. No matter which process is used, the brazing temperature must be strictly controlled in order to succeed. This requirement is undoubtedly very high for the temperature control of vacuum brazing furnace.

Torch brazing requires the use of flux, which is a chemically aggressive mixture of chloride and fluoride that melts just below the temperature of the filler metal. Because it is difficult to heat the parts evenly, it is extremely difficult or not recommended to use torch brazed very fine or very thin structures.

The immersion welding of aluminum is carried out in a molten salt bath containing flux. A filler metal is formed around the brazed joint and the part is immersed in the bath. This method is usually used for small or precision assembly or assembly requiring multiple joints.

Aluminum can be brazed in air with flux. Aluminum can also be brazed in a vacuum furnace without flux. Successful vacuum brazing of aluminum without flux requires a good vacuum degree (10 – 5 Torr). The presence of magnesium in the metal or filler seems to assist the process by acting as a getter for oxygen. The filling alloys balsi-6, – 7 and – 8 introduce magnesium for this purpose. It is considered that the thermal expansion of the aluminum base metal destroys the oxide layer and makes the solder contact with the base metal.

Vacuum brazing without flux avoids the key post brazing cleaning step that must be carried out after brazing when flux is used. The flux is corrosive and may cause failure if not completely removed. This is especially true in thin-walled structures such as heat exchangers. Remove flux by immersion and washing in water or chemical cleaning.

The design of aluminum brazed joint is similar to that of brazed joint of other metals, but lap joint is better than butt joint. Sufficient clearance must be provided to allow capillary action to draw the packing into the joint. For cutting torch and furnace brazing, the typical gap is 0.004 “to 0.010”. The smaller gap is suitable for immersion welding.

Aluminum alloy plates coated with solder on one or both sides are available. A sheet can be formed and then brazed into an uncoated sheet.

Aluminum can be brazed to other alloys, such as stainless steel, but the joint is brittle. When it is necessary to braze aluminum to an alloy such as steel, the steel is usually coated with aluminum first.

SIMUWU Vacuum furnace manufacturers specialize in low temperature, vacuum and pressure technology, and are committed to providing customers with the right perfect equipment, which means that our engineers are ready to help you choose the best manufacturing method for your application. To determine the best process, whether it is brazing, welding or other options, we work with you to achieve the best results.

Brazing has been widely used in the manufacture of aluminum heat exchanger. Aluminum heat exchanger needs a lot of sealing joints and joints of thin-walled materials.

Brazing is particularly suitable when the aluminum parts to be joined are too small or too thin to be easily welded.

High yield applications that can optimize processes and fixtures will also make brazing a viable method.

Otherwise, the welding process is relatively easy and simple than the brazing process, which leads to the selection of welding as the preferred permanent connection method for aluminum components.

If you need help to determine the best manufacturing process, please contact us.