Sometimes it is the smallest of details that actually makes or breaks a project. This is certainly true when you are brazing aluminum as there are a wide variety of factors that you have to keep in mind at each step in the process.
One of the least factored considerations for many people when they are working with aluminum brazing techniques is the type of flux they are using. Without choosing the right type of flux you will find that your brazed joint is not as durable, strong and solid as it could be with just a bit of thought.
What Is Flux?
Flux is used in a wide variety of applications including when brazing aluminum and other metals. Flux is basically cleaner that is also used to promote the flow or movement of the liquid filler along the surface of a metal.
However, when it comes to hi-temp brazing aluminum, flux also plays another very important role. It prevents any oxidation by exposure to heat of the base metal and the filler alloy metals. Without this oxidation would begin to occur immediately because of the heat applied to the filler alloy, which would weaken the joint and the braze and lead to less structurally sound joints.
The oxide formation would actually prevent the liquid filler from being able to move into the joint or to prevent capillary action from distributing the filler along the joint. Flux, with its oxidative prevention, removes this complication from the brazing process.
How to Use Flux
Ideally apply flux only to a completely clean surface an only right before the brazing process. Check the flux to determine what type of temperatures, metals and processes it is recommended for. Apply a generous and uniform layer of flux with a brush and heat, watching for the color change to indicate that it is active. Some flux will change color while some will become clear.
Once the change occurs in the flux you are ready to apply the brazing filler metal and have it melt and flow into the joint, creating a solid, strong joint in the metals you are brazing.