Q: What is the difference between aluminum vacuum brazing and aluminum dip brazing?
A: Aluminum dip brazing uses a salt bath flux which breaks down metal oxides and enables a filler metal to diffuse in to the base metal. Aluminum vacuum brazing uses a clean vacuum environment without flux. There are some cases where the design of the part dictates which process should be used. Aluminum vacuum brazing is limited to flat, horizontal surfaces, to allow distributed pressure to be applied to the part. Aluminum dip brazing can be used when creating joints that don’t allow for pressure by design.
With aluminum dip brazing parts are assembled with the filler metal placed outside the joint. The filler metal enters the joint when heated to molten temperature and pushes the flux out of the joint. Often, flux is entrapped in the joint and leads to premature failures, leaks, and corrosion. However, there are times that the design of the assembly requires dip brazing versus vacuum brazing which requires the use of pressure. An example of this would be an assembly where the outside diameter of a tube needs to be attached to a plate.
Ideally, components with large surface areas and multiple joints use the vacuum brazing technique. Vacuum brazing uses pressure applied to the parts to break down the oxide layer in the aluminum so the filler metal will diffuse pass the oxide layer into the base metal. Unlike dip brazing, vacuum brazing is free of contaminants and is non-polluting because it does not use flux to break down the oxide layer. Brazing occurs in vacuum furnaces, which eliminates post-braze cleaning. As a result vacuum brazing is generally used for very high reliability brazing applications. The filler material is preplaced in the joint which allows a more precise bonding location. Additionally, parts with different masses or thin or thick part cross-sections can be easily joined together. For all these reasons, vacuum furnace brazing offers very repeatable results.
As noted, there are substantial benefits of aluminum vacuum brazing. At the same time, there are some fundamental process requirements to aluminum vacuum brazing. It requires the equipment necessary to perform excellent heating control and temperature uniformity. Temperature uniformity during a brazing cycle is +/- 5 degrees Fahrenheit. This is essential when brazing at the edge of the base material solidus temperature. A hard vacuum (less than 1×10-4 torr) to prevent oxidation is necessary. There is also more precision required in machining parts in preparation for vacuum brazing than with flux dip brazing.