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Tube brazing

by Nov 3, 2021Component Manufacturing

Estimated Reading Time: 4 minutes

While we specialize in TIG aka GTAW welding (manual and orbital), our technicians sometimes use tube brazing when joining metals. Brazing is similar to soldering but done at higher temperatures. For best results, it must be performed with the appropriate brazing rod material based on the metals used. Axenics’ tube brazing process is also known as torch brazing.

How does tube brazing work?

Tube brazing uses a torch instead of a tungsten gas, adding silver or another filler metal for brazing two pieces together, creating a leak-free joint. Tube brazing is also used to remove parts from a system that can be reused elsewhere.

When metals join together through brazing, a filler metal melts and flows into the joint between the metals. When brazing, the base metals don’t melt. Instead, the base metals join together through the creation of a metallurgical bond between the surfaces of the joined metals and a filler metal.

The process for drawing the filler metal through the joint and creating the metallurgical bond is called “capillary action.” This process ensures that the filler metal properly distributes throughout the joint.

Why use tube brazing instead of welding?

There are advantages to each type of joining process, depending on your goals and materials you’re working with, such as:

Assembly size: TIG welding is usually a better choice when larger assemblies join, as brazing works by applying heat to a broader area. That can make it more difficult for the heat to reach the right point of the filler metal.

Thickness of metals: For thinner base metals, tube brazing is typically the better option. However, to join thicker base metals (.5 inches or more), either welding or brazing is appropriate.

Joint shape: For linear joints, tube brazing is preferred over welding, as it typically involves less manual work than welding. For fusing spot joints, however, welding is the easier and more economical choice, which also results in a durable joint.

Material types: When joining two different types of metals, such as brazing copper tube to stainless steel, brazing is often the better choice. It’s generally more expensive and more complex to join different metal types through welding.

Appearance: If it matters how the materials look once joined, brazing is your best bet. Joining two metals through brazing typically produces a neat bead versus welding, which results in an irregular bead.

Tube brazing filler metals

  • Copper
  • Copper-silver
  • Brass
  • Gold-silver
  • Silver
  • Nickel alloy
  • Aluminum-silicon

Tube brazing considerations

Plan for expansion and contraction: Before brazing two or more materials, it’s important to determine the appropriate clearance between the base metals, in order to allow for capillary action to do its job. When brazing two flat parts, you can typically rest one on top of the other and still allow for proper capillary action.

Clean the metals: To achieve good capillary action, the surfaces of the metals must be cleaned of any contamination, such as oils, grease, rust or dirt. If the base metal surfaces are not clean, a barrier will  develop between the filler materials and base materials, which will prevent a proper bond from forming.

Flux the parts: Prior to tube brazing, apply a coating of flux to the joint area. This prevents the formation of oxides which would otherwise keep the brazing filler metal from bonding the metal surfaces. Do this just before brazing for best results.

Assemble parts for brazing: Once the metals are cleaned and fluxed, you need to securely assemble the parts so that they remain in place during the brazing process. Otherwise, capillary action will not occur properly. The best way to keep the parts assembled in the correct alignment is through use of simple weights and clamps. Stainless steel, Inconel and ceramics are the best materials for maintaining parts assembly, as they are poor heat conductors, and therefore will draw less heat from the joint.

Braze the parts: The base metals heat first, then the filler metal heats automatically through contact with the heated base metal products (due to its lower melting point). The filler metal must have a lower melting point than the metals being joined. Brazing filler metals should have a melting point above 450˚C, but below the melting point of the metals being joined.

Clean the brazed joint: Cleaning the final brazed assembly or joint is typically a two-step process. Remove any flux residue by immersing the assembly in hot water after the filler metal has solidified completely. The next step is called “pickling.” A pickling solution removes remaining oxides that may not have had flux applied prior to the brazing process. If an exceptionally clean finish is required, the assembly can also be polished with a fine emery cloth.

Brazing requires a skilled and experienced technician to properly perform the process. The time each step takes depends on the materials being brazed, the filler metals and the experience of the technicians.

Contact our expert technicians to see if tube brazing or TIG welding is the best option for your project.

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