Chapter 15 Introduction to Nonferrous Metals 339 Semisolid Metal (SSM) Casting A process called semisolid metal (SSM) casting injects a slurry of part ly solidified metal into the die. The resulting part has minimal embedded oxide “skin,” less microsegregation, can be heat-treated more easily to higher strength, has more uniform properties throughout the part, and causes less wear on the dies. The metal preparation and casting sequence are shown in Figure 15-19. In Chapter 10, you saw that when a metal alloy, such as magnesium (Mg) with 7% aluminum (Al), is solidified, the alloy changes from fully liquid into a liquid-solid mixture, with solid magnesium surrounded by liquid alloy, and then into a solid, as shown in Figure 15-20. In a still mold, dendrites form in the liquid. However, if A B C D Goodheart-Willcox Publisher Figure 15-19. For die casting semisolid metal slurry, the alloy is cast into rods during intense electromagnetic stirring (step A). The rod is cut into “bolts,” then reheated just enough to become 50% liquid (step B). The partially liquid bolt can be handled gently with tongs, as if it were solid. Then the bolt is placed into the shot chamber (step C), and a plunger forces it into the die (step D). When the plunger pushes the bolt, the semisolid metal flows, as thick as hair shampoo. Liquid Liquidus line Temperature of first solid Temperature of last liquid AI12Mg17 Semisolid zone (very slow cool) Semisolid zone (typical casting) Liquid & solid Intermetallic Mg (hcp) Eutectic hcp + Intermetallic Alloy composition: 7% Al Solid 1202°F 710°F 0 13 35 Percent aluminum 42 100°F Goodheart-Willcox Publisher Figure 15-20. A portion of the magnesium-aluminum (Mg-Al) phase diagram, showing the temperature range over which an Mg-7% Al alloy is a mixture of solid and liquid during cooling. As the amount of solid increases, the concentration of aluminum in the liquid increases along the liquidus line. When this alloy is stirred during solidification, small spheroidal solid pieces are formed by mechanically deforming the solidifying dendrites. The last liquid solidifies at the eutectic temperature due to the increased aluminum concentration. In the finished solid, the spheres of magnesium are spread uniformly through a eutectic matrix. Copyright Goodheart-Willcox Co., Inc.