344 Section 4 Nonferrous Metallurgy Fusion Joining The American Welding Society (AWS) describes the different methods for joining metal parts. The workpieces being joined are called parent metals. Metal added by a rod or insert, if any, is called the filler. When the parent metals are melted, with or without a filler, the process is defined as fusion welding. When the parent metals do not melt, and the filler melts above 815°F (435°C), the process is called brazing. When the filler melts below 815°F (435°C), it is called soldering. Alloys Compatible for Joining by Fusion. The metals being joined, and any filler metal between them, must all be metallurgical ly compatible, or very special procedures must be used. Compatible alloys do not form damaging intermetallic compounds easily. If the different metals are not compatible, then brittle intermetallic compounds usually develop, and the joint will fail easily. To meet the compatibility requirement, the two parent alloys being joined by fusion usually have the same major metal. For example, many aluminum alloys can be welded to other aluminum alloys easily, but copper cannot be welded to aluminum by usual fusion arc welding methods. When copper and aluminum are melted together, brittle intermetallic compounds form and fracture as the joint solidifies and cools. Fusion Welding. Fusion welding of most nonferrous metals can be done easily. Most nonferrous metals must be shielded from oxyg en. Some nonferrous metals react with the carbon monoxide (CO) and carbon dioxide (CO2) in a gas torch flame, so electric arcs and inert cover gases must be used. When compatibility and shielding are suitable for the parent metals, then electric arc or gas torch welding, resistance welding, friction welding, and other procedures that melt the parent metals, with or without filler, can be used. For example, the small electrical wires in electronic blasting caps for use in oil fields, and other electronic devices for medical personnel, absolutely must be connected properly and never short out or fail. Precision resistance welding, Figure 15-27, assures connection integrity. Brazing. Brazing occurs above 815°F (435°C), and the parent metal does not melt. A filler alloy melts and penetrates the parent metal oxide. Surface tension pulls liquid filler into the joint area, where it penetrates into crevices that may not be visible from outside. Surface tension also pulls the liquid into a smooth, round fillet, Figure 15-28. Usually a flux is used to protect the filler and disrupt the oxide. Electrodes Wire lead Mohawk Electrical Systems, Inc. Figure 15-27. With precision resistance welding, very small wires are resistance welded in precise alignment to provide extremely high reliability of the device they are part of. Sealed plate- to-boss joint Sealed plate- to-plate joint Sealed plate- to-plate joint Rounded fillet Jay Warner Figure 15-28. Surface tension of the liquid in a braze or solder joint tends to form a rounded fillet surface, and also to wick the liquid filler into the crevasse of a joint, ensuring joint integrity and strength. In this chill plate for cooling electronics, a bottom plate, spacer, top plate, and threaded boss are all brazed simultaneously into a watertight chill plate. Note that the filler alloy has filled the entire contact region between the plates and spacer, and has also formed a smooth fillet at the bottom of the boss. Copyright Goodheart-Willcox Co., Inc.