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Brazing Aluminum
Many aluminum alloys can be joined by brazing. Brazing aluminum is more difficult than brazing other metals because the melting points of the base metal and the filler are close together, and because of the oxide layer present on the surface of aluminum.
Aluminum alloys which can be joined by brazing include 3003 and the 6XXX series alloys. Alloys 2024, 2219 and 7075 cannot be brazed with commercially available fillers because they melt at a lower temperature than the currently available braze filler alloys. The BAlSi series aluminum/silicon alloys are generally used as the filler metal. All aluminum base metal alloys will be annealed at the temperatures required for brazing. Heat treatable alloys, such as 6061, can be heat treated after brazing to restore their mechanical properties.
The most common brazing processes used for aluminum include torch brazing, dip brazing and furnace brazing. Regardless of process, the brazing temperature must be tightly controlled to be successful. This limits successful torch brazing to highly skilled fabricators.
Torch brazing requires the use of flux, which is a chemically aggressive mixture of chlorides and fluorides that melts at a temperature just below the filler metal. Torch brazing very fine or very thin structures is extremely difficult and not recommended, because of the difficulty in uniformly heating the parts.
Dip brazing aluminum is carried out in a molten salt bath containing the flux. The filler metal is formed around the braze joint and the part is dipped into the bath. This method is often used for small or delicate assemblies or assemblies that require multiple joints.
Aluminum may be furnace brazed in air using flux. It is also possible to oven braze aluminum in a vacuum furnace without flux. Successfully vacuum brazing aluminum without flux requires a good vacuum (10-5 torr). The presence of magnesium, either in the base metal or the filler, seems to aid the process by acting as a getter for oxygen. Filler alloys BAlSi-6, -7 and -8 have magnesium introduced for this purpose. It is thought that thermal expansion of the aluminum base metal breaks up the oxide layer and allows the braze filler to come into contact with the base metal.
Fluxless vacuum brazing avoids the critical post braze cleaning step that must follow brazing when flux is used. Brazing flux is corrosive and may lead to failure if it is not completely removed. This is particularly true in thin wall structures such as heat exchangers. Flux removal is done by soaking and rinsing in water or by chemical cleaning.
The design of aluminum braze joints is similar to the design of braze joints for other metals, though lap rather that butt type joints are preferred. Adequate clearance must be provided to allow capillary action to draw the filler into the joint. Typical clearances are 0.004" to 0.010" for torch and furnace brazing. A slightly smaller gap is suitable for dip brazing.
Aluminum alloy sheets coated with braze filler on one or both sides are available. These sheets may be formed and then brazed to uncoated sheets.
It is possible to braze aluminum to other alloys such as stainless steel, however, the resulting joints are reported to be brittle. When it is necessary to braze aluminum to an alloy such as steel it is common to first coat the steel with aluminum.
Specializing in cryogenic, vacuum and pressure technologies, Meyer Tool’s commitment to providing our customers with the lowest total cost of ownership means our engineering staff is ready to assist you in selecting the best fabrication methods for your application. We understand the intricacies and challenges involved in aluminum fabrication. Determining the best process, whether that be brazing, welding or another selection, we work with you to achieve the optimal results.
Conclusions
Aluminum alloys which can be joined by brazing include 3003 and the 6XXX series alloys. Alloys 2024, 2219 and 7075 cannot be brazed with commercially available fillers because they melt at a lower temperature than the currently available braze filler alloys. The BAlSi series aluminum/silicon alloys are generally used as the filler metal. All aluminum base metal alloys will be annealed at the temperatures required for brazing. Heat treatable alloys, such as 6061, can be heat treated after brazing to restore their mechanical properties.
The most common brazing processes used for aluminum include torch brazing, dip brazing and furnace brazing. Regardless of process, the brazing temperature must be tightly controlled to be successful. This limits successful torch brazing to highly skilled fabricators.
Torch brazing requires the use of flux, which is a chemically aggressive mixture of chlorides and fluorides that melts at a temperature just below the filler metal. Torch brazing very fine or very thin structures is extremely difficult and not recommended, because of the difficulty in uniformly heating the parts.
Dip brazing aluminum is carried out in a molten salt bath containing the flux. The filler metal is formed around the braze joint and the part is dipped into the bath. This method is often used for small or delicate assemblies or assemblies that require multiple joints.
Aluminum may be furnace brazed in air using flux. It is also possible to oven braze aluminum in a vacuum furnace without flux. Successfully vacuum brazing aluminum without flux requires a good vacuum (10-5 torr). The presence of magnesium, either in the base metal or the filler, seems to aid the process by acting as a getter for oxygen. Filler alloys BAlSi-6, -7 and -8 have magnesium introduced for this purpose. It is thought that thermal expansion of the aluminum base metal breaks up the oxide layer and allows the braze filler to come into contact with the base metal.
Fluxless vacuum brazing avoids the critical post braze cleaning step that must follow brazing when flux is used. Brazing flux is corrosive and may lead to failure if it is not completely removed. This is particularly true in thin wall structures such as heat exchangers. Flux removal is done by soaking and rinsing in water or by chemical cleaning.
The design of aluminum braze joints is similar to the design of braze joints for other metals, though lap rather that butt type joints are preferred. Adequate clearance must be provided to allow capillary action to draw the filler into the joint. Typical clearances are 0.004" to 0.010" for torch and furnace brazing. A slightly smaller gap is suitable for dip brazing.
Aluminum alloy sheets coated with braze filler on one or both sides are available. These sheets may be formed and then brazed to uncoated sheets.
It is possible to braze aluminum to other alloys such as stainless steel, however, the resulting joints are reported to be brittle. When it is necessary to braze aluminum to an alloy such as steel it is common to first coat the steel with aluminum.
Specializing in cryogenic, vacuum and pressure technologies, Meyer Tool’s commitment to providing our customers with the lowest total cost of ownership means our engineering staff is ready to assist you in selecting the best fabrication methods for your application. We understand the intricacies and challenges involved in aluminum fabrication. Determining the best process, whether that be brazing, welding or another selection, we work with you to achieve the optimal results.
Conclusions
- Brazing has widespread application in the fabrication of aluminum heat exchangers, which require a large number of leak tight seams and joints in thin wall material.
- Brazing is particularly appropriate in situations when the aluminum parts to be joined are too small or too thin to be welded easily.
- High production applications where processes and fixturing can be optimized would also make brazing a viable method.
- Otherwise the relative ease and simplicity of welding processes over braze processes leads to the selection of welding as the preferred permanent joining method for aluminum assemblies.
- If you need assistance in determining the optimal manufacturing processes for your application contact us or call us at 708-425-9080.