BRAZING & SOLDERING TODAY Fig. 2 — Batch vacuum aluminum brazing furnace. Fig. 3 — Typical vacuum aluminum brazing cycle. WELDING JOURNAL 51 bath (dip brazing), and controlled atmosphere (CAB). Brazing performed in a vacuum furnace is considered fluxless brazing because it does not use flux to create the joint. Fluxless brazing processes can be performed using inert gas atmospheres or in vacuum furnaces. Such processes include, but are not limited to, semiconductor manufacturing and ceramic-tocopper brazing; due to the cleanliness of the vacuum environment, flux is not needed. Magnesium is used as an additive, or getter, in the vacuum aluminum brazing process. Vacuum Aluminum Brazing Benefits Brazing has many advantages when compared to other metal-joining processes. Given that brazing does not melt the base metal of the joint, it allows for more precise control of tolerances and provides a clean joint with no need for additional finishing. The meniscus (crescent shape) formed by the filler metal in the brazed joint is ideally shaped for reducing stress concentrations and improving fatigue properties. Ideal situations for brazing include the following: • Joining parts of thin or thick cross sections • Compact components containing many junctions to be sealed (e.g., heat exchangers) or deep joints with restricted access • Joining dissimilar metals such as copper and stainless steel • Assemblies with a large number of joints. Specifically, vacuum aluminum brazing minimizes distortion of the part due to uniform heating and cooling as compared to a localized joining process. This type of brazing creates a continuous hermetically sealed bond. Components with large surface areas and numerous joints can be successfully brazed. Hardening can also be accomplished in the same furnace cycle if hardenable alloys are utilized and the furnace system is integrated with a forced cooling system, reducing cycle time. Vacuum furnace brazing offers extremely repeatable results due to the critical furnace parameters that are attained with every load, that is, vacuum levels and temperature uniformities. Capillary joint paths (even long paths) are effectively purged of entrapped gas during the initial evacuation of the furnace chamber, resulting in more complete wetting of the joint. Vacuum aluminum brazing is ideal for oxidation-sensitive materials; vacuum brazing is considered a flux-free process that eliminates corrosive flux residue. Postbrazed parts are clean with a matte gray finish. The process is relatively nonpolluting and no postbraze cleaning is necessary. Parts Examples of vacuum-aluminumbrazed parts, as shown in Fig. 1, often include heat exchangers, condensers, and evaporators used in automotive, aerospace, nuclear, and energy industries. Types of Furnaces Typical vacuum aluminum brazing furnaces are either single chamber (batch type) or multiple chamber (semicontinuous). Batch-type furnaces are usually loaded horizontally, but they can be designed for a vertical loading operation. Semicontinuous furnaces are horizontally loaded and typically automated using load carriers and external conveyor systems. Batch furnaces tend to be simpler in design (one loading/unloading door) than semicontinuous furnaces, less expensive, and easier to maintain. Semicontinuous furnaces have higher production rates because of the multichamber design and operate more efficiently by not having to cool heating zones or heat cooling zones. Examples of batch type vacuum furnaces are shown in Fig. 2. Vacuum Aluminum Brazing Process The vacuum aluminum brazing
Welding Journal | February 2014
To see the actual publication please follow the link above