What is Tube Brazing and How Does it Work?
At Axenics, we perform a variety of fabrication processes for our clients, to meet the needs of all types of industries. We work with many different types of metals and plastics, and offer solutions for joining, cutting, bending, and otherwise forming materials to fit specific project needs and challenges. While we specialize in GTAW welding (both manual and orbital), our technicians also sometimes use tube brazing as a solution for joining metal materials.
What is Tube Brazing?
Tube brazing is a common fabrication process used for joining two or more metals. It is similar to the process of soldering, but it is done at higher temperatures. For best results, tube brazing must be performed with the appropriate brazing rod material based on the metals being brazed together.
How Does Tube Brazing Work?
When metals are joined together through tube brazing, a filler metal is melted and flowed into the joint between the metals. When brazing, the base metals are not melted. Instead, the based metals are joined together through the creation of a metallurgical bond between the surfaces of the metals being joined and a filler metal. The process used to draw the filler metal through the joint and ultimately create the metallurgical bond is called “capillary action.” This process ensures that the filler metal is properly distributed throughout the joint.
The following steps should be taken whenever tube brazing is performed:
- 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 to occur.
- Clean the Metals.
For capillary action to occur properly, 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 cleaned, a barrier will be created between the filler materials and base materials, which will prevent a proper bond from forming.
- Flux the Parts.
Prior to tube brazing, it is crucial to apply a coating of flux to the joint area. By doing this, the formation of oxides can be prevented, which would otherwise keep the brazing filler metal from bonding the metal surfaces. This should be done just before brazing for best results.
- Assemble Parts for Brazing.
Once the metals have been cleaned and fluxed, you will 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 to use 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 are heated first, and then the filler metal is heated automatically upon coming in contact with the heated base metal products (due to its lower melting point). For the process to work properly, the filler metal must have a lower melting point than that of 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. Begin by removing any flux residue, which can be done by immersing the assembly in hot water after the filler metal has solidified completely. The next step is called “pickling.” A pickling solution is used to remove 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 as well.
Some of the above steps require more attention and time than others. Brazing requires a skilled and experienced technician to perform the process properly. The time each step takes will depend on the materials being brazed, the filler metals being used, and the experience of the technicians.