Lynch Metals has been a provider of aluminum brazing sheets for almost 30 years and is currently North America’s preeminent distributor of the sheet metal. Lynch is unique in the industry because of our relationships with aluminum mills, specialized processing machinery, vast inventory, and our skilled sales staff.
This page will examine the brazing process to familiarize readers with the steps involved and how it compares to welding.
Six Fundamental Steps to Brazing Metals
Brazing, like welding, joins two metal pieces together. Unlike welding, brazing uses a filler metal to form a robust joint, and a flux material to protect the filler. Brazing involves a simple six-step process:
- Create a good fit with the right clearances. The filler metal gets into the joint through capillary action, so it is critical that the gap between the two base pieces at the joint is correctly spaced. The typical range for this gap is 0.001–0.005” and the optimal value is 0.0015” for most filler metals.
- Pre-clean the joint. The metal surfaces forming the joint must be free of impurities for the capillary action to work properly. Common impurities on metal surfaces include dirt, grease, oil, and rust. These contaminants can be removed with acidic pickling agents, degreasing solvents, and polishing.
- Coat the joint with flux. Flux protects the joint from oxidizing at the high temperatures required for brazing. It will dissolve any metallic oxides that form during brazing, so it is important to fully coat the joint with flux using a brush or an applicator gun.
- Hold the base pieces steady during brazing. The joint gap created in the first step above must be held steady during brazing and until the parts have cooled to form a strong joint. Often times it is possible to do this with gravity if the base pieces are of the right shape and size, but sometimes a jig will be necessary.
- Braze at the right temperature. Brazing requires melting the filler metal without also melting the base metal, so the temperature must be carefully controlled to achieve this. Additionally, the heat application must be uniform along the joint. This can be judged by watching how the flux’s appearance changes under heat. Once the right temperature is reached, the filler rod or wire is pressed against the joint so that it melts and is sucked into the joint by capillary action, which is improved by the aforementioned flux coating. Be sure that the molten filler flows into the joint and doesn’t stay at the surface of the base pieces.
- Post-clean the joint. Since flux is corrosive, post-cleaning is just as important as pre-cleaning. Flux is easily washed away by hot water at 120° F or higher. Once the filler has cooled enough, the whole assembly can be quenched in hot water while it is still hot. If the residual flux is green or black—indicating that it is saturated with oxides—a mildly acidic agent will likely be necessary to remove it. If there is any metal oxide residue, it must be removed using a pickling solution.
Brazing vs. Welding
Even though brazing and welding are similar in several ways, there are significant differences between the two techniques:
- The operating temperature for brazing is lower than welding.
- In brazing, the melting point of the filler metal is lower than that of the base metals. Welding is the other way around.
- Brazing works through capillary action, whereas fusion is the working principle of welding.
- Brazing can join different metal types, where welding cannot.
Brazing is the preferred method of joining metals when the base pieces are made from different metal materials. It is also a superior technique when cleaner joints, less thermal distortion, and automation are called for.
Brazing Services From Lynch Metals
Lynch Metals is a one-stop source for aluminum brazing materials and services for a wide variety of industries ranging from aerospace to telecommunications. We are able to provide quick turnaround times thanks to our large and diverse stock of materials and the advanced equipment we use in our processing centers.
Contact us for all your brazing requirements or with questions about our other products and services.