Brazing is a method of joining two metals together using a molten filler metal. Brazing is different from welding, as none of the base metals undergo thermal melting. Instead, the joint is formed by the bond between the filler metal and the base metal pieces.
In brazing, the melting point of the filler metal or the braze alloy is usually above 450°C (840°F) but is much lower than the melting point of the base metals. Once the braze alloy is in a molten state, capillary action distributes it between the joints. Then, as the braze alloy cools down, a strong and nonporous joint is formed between the two metal pieces.
It is possible to braze a range of metals. However, each metal will require specific filler materials, fluxes, and temperatures to form a strong bond. Some of the metals that are commonly brazed are:
- Cast Iron
- Stainless Steel
- Zinc-Coated Steel
The melting point of copper is 1981°F (liquidus) and 1949°F (solidus). Copper brazing is usually performed at temperatures between 1100°F and 1500°F. The fluxes used in copper brazing are usually water-based and dissolve quickly; they also help in removing any residual oxides from the metal surface, prevent oxidation during heating, and help in wetting the surfaces that are to be joined.
Copper-silver-phosphorus and copper-phosphorus metals are considered to be self-fluxing when joining copper base metals. In order to develop maximum joint strength with these materials, it is recommended to leave a joint clearance of 0.001 to 0.005 inches.
The melting point of red brass is between 1810-1880°F, and for yellow brass, the melting point is 1660-1710°F. Unlike when brazing copper, flux is required for all brass brazing applications. White flux is generally used for most brass brazing applications. The melting point of the flux is between 1050-1600°F.
Brazing Cast Iron
Cast iron is an alloy made from iron, silicon, and carbon, with a melting point between 2060°F and 2200°F. Cast iron can be both welded and brazed. However, brazing is typically considered the best method. The ideal filler rod for brazing cast iron is the Vulcan Systems 320, which is flux-coated, zinc-controlled, and low-fuming. When brazing cast iron, the clearance must be around 0.003 inches for maximum shear and tensile strength.
Brazing Stainless Steel
Different grades of stainless steel are available on the market, and their melting points range from 2500°F to 2750°F. The clearance required for brazing stainless steel is 0.002-0.005 inches. Stainless steel is a widely used alloy due to its special properties, and a variety of filler materials are available, depending on its properties and applications. Some of them are nickel-bearing alloys like Silvaloy 505, Silvaloy 404, Easy-Flo 3, cadmium-free alloys, and silver base alloys. The most commonly used flux when brazing stainless steel is FB3-C paste flux.
The melting point of aluminum is 1221°F, and the clearance required for aluminum brazing is 0.001 to 0.005 inches. Copper and silicon are usually added to provide good joining strength and a lower melting range. Magnesium is also added to facilitate oxide dispersion when brazing in a vacuum. Both corrosive and non-corrosive fluxes are used in aluminum brazing.
Metals You Should Not Braze
While it is possible to braze almost all metals if you are careful about the temperatures and the filler metal to be used in the process, metals like gold and silver, which are considered heating metals, require extreme care and precision while brazing. These two metals are therefore soldered more often than brazed. Although soldering will not be as strong as brazing, it can still produce an excellent bond to hold the pieces together.
Brazing Two Metal Pieces: Things to Keep in Mind
Safety is one of the most critical factors while carrying out processes that involve high temperatures and the use of chemicals. For this reason, brazing is a process that should be performed only by skilled professionals. Here are some of the useful tips for safe brazing.
- Make sure that you are wearing protective gloves and a shield to cover your eyes.
- Ventilate the area using fans and exhaust hoods, as brazing may involve releasing dangerous fumes and gases.
- Clean the base metals thoroughly, as any contamination will not only cause improper alloy bonding but may also add to the hazardous fumes released.
- Supply adequate flux while brazing, as it protects the filler metal and the base metals. This practice also helps in suppressing the gases emitted by volatile elements.
- Ensure that the base metals are heated uniformly. Intense or localized heating can again cause dangerous fumes to release.
- Know your base metals and filler metals. It is essential to know the properties of both to avoid unwanted fuming, improve efficiency, and guarantee solid joining of the metal pieces.
- Once brazing is complete, make sure to give the metal piece sufficient time to cool down and form a proper joint.
When to Choose Brazing Over Welding?
Fundamentally, there are three different processes for joining metals: brazing, welding, and soldering. But when should you choose one over the other?
The reason why some applications require brazing rather than welding is that brazing offers tighter control over the joining process compared with welding. Also, brazing does not melt the base metal, preventing any unwanted damage to the metal.
Another advantage of brazing is that it can join two different metals since the bonding comes from the filler material. Brazing also requires less power than welding.
However, brazing falls short in contrast to welding when it comes to bond strength. The ability of welding to melt the base metal creates much stronger bonds between two metal pieces.
If you compare brazing and soldering, the brazing process can be considered a more powerful version of soldering. Both methods use molten filler material to join similar or dissimilar metals. However, with brazing, the bonds are much stronger.
Selecting the correct metal joining technique also depends on the type of base metals that you need to join. Brazing and soldering are suitable for only a selected few metals, while welding is possible for a range of metals.