348 Section 4 Nonferrous Metallurgy • Cold-chamber die casting and hot-chamber die casting are two pressure die casting methods used for nonferrous metals. • In the hot isostatic pressing (HIP) process, the casting is compressed in an isostatic chamber at high pressure and temperature, so that pores in the metal are closed and healed. • For semisolid metal (SSM) casting, a slurry of partly solidified metal is injected into the die. The resulting part has minimal embedded oxide “skin” and less microsegregation, it can be heat-treated more easily to a higher strength and has more uniform properties throughout the part, and it causes less wear on the dies. • Rolling, forging, extrusion, and drawing for nonferrous metals are all similar to those processes for steel production, with the primary differences being the melting points and chemical reactivity of the metals. • During heat treatment, some alloys that develop significant segregation during casting may suffer partial melting, or liquation, if processed incorrectly. • Nonferrous metal parts can be joined by forge welding or fusion welding. • Two common procedures of joining are brazing and soldering. 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. • Metal parts can be machined to achieve almost any desired shape. Electric discharge machining (EDM), a machining method used for hard alloys, uses electric sparks to remove tiny amounts of metal from the workpiece. Review Questions Answer the following questions using the information provided in this chapter. Know and Understand 1. Which of the following metals has the highest conductivity? A. Magnesium (Mg) B. Uranium (U) C. Gold (Au) D. Cobalt (Co) Summary • Different types of unit cells occur in metals. Most nonferrous metals have unit cells of body-centered cubic (bcc), face-centered cubic (fcc), hexagonal close-packed (hcp), or body-centered tetragonal (bct) structure. • Dislocations form in metals when the atoms in a crystal grain slide past one another. Metals become stronger as dislocation tangles build up. • Dislocation tangles are removed by heating a nonferrous metal at about 55% of its melting point temperature, measured on the kelvin temperature scale. • Hot-working on nonferrous metals is done while the metal is hot enough to recrystallize, which creates more uniform microstructures with finer grains, and thus higher strength. • Cold-working on nonferrous metals is done when the metal is deformed and the dislocation tangles remain. The dislocation tangles increase the strength and reduce the ductility of the metal during cold-working. • At temperatures between hot and cold work, dislocation tangles develop, and recrystallization removes only the most severely tangled portions, allowing for some dislocation motion. Work at this temperature is referred to as warm work, and the change in the microstructure is called recovery. • Annealing can restore ductility in a cold-worked metal by annealing the metal at 55% to 65% of the melting temperature. The resulting metal is recrystallized with new, strain-free, “clean” grains. • Nonferrous alloys can be strengthened by cold work, by the addition of a solid solution alloy or a refractory alloy, or through precipitation hardening. • During precipitation hardening, the metal is solutionized and then cooled by water quenching. Finally, the metal forms different precipitate sizes depending on the temperature at which it is aged. The alloys can be naturally aged at room temperature, artificially aged at a high temperature, or overaged. • Galvanic corrosion occurs in a metal piece that is more electronegative than a less electronegative piece in a metal part. The corroded metal protects the second, less electronegative metal. • The processing of nonferrous metals seeks to maximize electrical conductivity, low density, formability, or corrosion resistance in metal parts. CHAPTER REVIEW Copyright Goodheart-Willcox Co., Inc.