Chapter 15 Introduction to Nonferrous Metals 345 Technicians who can manipulate a torch, filler rod, flux, and components to produce sound, consistent joints have mastered a fine skill. Setting up a bank of gas torches or a brazing furnace to produce uniform, consistent parts in production takes considerable experience as well. For the manufacture of radiators and other heat exchangers, a great many joints must be made to produce intricate water channels that transfer thermal energy from water to air. This can be done best with braze-clad sheet. Braze-clad sheet, commonly called brazing sheet, is made by roll bonding slabs of braze filler alloy onto a core ingot, as previously discussed. Typically, the cladding totals 5% to 20% of the total sheet thickness. This sheet is formed, assembled, and stacked into steel fixtures. The stacked parts are placed in an oven or dipped into a tank of hot liquid flux, where the stacked parts are heated so that the cladding melts but the core alloy does not. Surface tension pulls the liquid filler metal into all the joint crevasses at once, resulting in sound, leak-tight joints throughout. Many yards (meters) of joint can be made in each heat exchanger in one furnace cycle. Soldering. Soldering occurs when the filler melts below 815°F (435°C) and the parent metal does not melt. All soldering involves a filler metal, and almost all soldering requires a flux that will be active at the soldering temperature. Surface tension pulls the liquid filler into a smooth round fillet, just as with brazing. Get the Lead Out Traditionally, lead was a major component of solder filler alloys. However, a great deal of work has been done in the last 35 years to minimize lead in solder alloys. This reduces lead in the environment and lead exposure for plumbers and other workers who commonly use solder. SUSTAINABLE METALLURGY SUSTAINABLE METALLURGY 15.5.7 Machining, Polishing, and Surface Finishes The shape of most metal parts can be changed by machining. “Machining” usually me ans removing metal with a sharp blade and mechanical force, and all such blades must be harder than the workpiece metal. Very hard alloys can be cut using electric discharge machining (EDM). In this process, the workpiece is placed in a bath of nonconductive fluid, and an electrode is brought close to it while a large voltage is applied between them. When the electrode is very close to the workpiece, an electric spark jumps between them, removing a tiny amount of metal from the workpiece. Hundreds of thousands of tiny sparks every second form slots and cavities in the workpiece in a few minutes. Very tiny holes can be put into plates. Complex shapes can be made by EDM, and the surface finish can be made quite smooth without further processing. Parts made by EDM remain at the temperature of the dielectric bath, so the original microstructure is retained. However, the properties of a microscopic surface layer may be changed due to melting by the electric sparks. All metals can be polished, as demonstrated in the Chapter 4 Self-Discovery activity, “What Is a Metal?” Some metals are plated onto a substrate to provide a Copyright Goodheart-Willcox Co., Inc.