340 Section 4 Nonferrous Metallurgy the alloy is stirred during cooling, small solid spheroids form in the liquid, Figure 15-21. This liquid-solid mixture behaves like a liquid as long as it is stirred, until less than 40% liquid remains. This sudden change from solid to liquid behavior is a material marvel called thixotropic behavior. The prepared, heated material can be handled with tongs, because surface tension keeps the solid globules together, similar to packed wet sand. When a ram forces the rod into the die cavity, the rod becomes fluid and flows easily into the die cavity, forming the part. This is the functional definition of thixotropic. The same phenomenon explains why a tailings dump of mud can suddenly shift from solid mud on a hillside to a flowing sheet of mud, causing great damage. The preparation of the liquid-solid condition can be developed at the point of injection into the die, such as illustrated in Figure 15-22. This makes the process more economical and easily controlled. Thixotropic casting below the fully liquid temperature of the alloy means the dies wear less and last longer. The microstructure is more uniform than casting a liquid, so the properties of the part are more uniform. The parts have less oxide entrapped in them, because the fluid metal splashes less during injection. Preheating time for later heat treatment is shorter. Semisolid casting requires tight temperature control during reheating. Temperatures and stirring rates must be closely monitored and controlled, and operators must be knowledgeable and very alert to achieve good production rates with low scrap losses. 15.5.2 Bulk Deformation Processing of Nonferrous Metals Rolling, forging, extrusion, and drawing for nonfer rous metals are all similar to steel production. All ingot processing requires hot-working temperatures for the metal being worked. Metals that react with air at these temperatures must be protected by inert gas, a vacuum, or another method. A B Goodheart-Willcox Publisher Figure 15-21. Alloys cooled in a still mold develop dendrites (A). The dendrites interlock, so the part behaves like a weak solid even when the interdendritic material is still liquid. Stirring alloys during cooling breaks the dendrites into spherical particles (B). This semisolid material can flow like a liquid even though only half the material is liquid. Band heaters Die cavity Die fixed half Nozzle Barrel Auger Auger drive & injection motors Auger drive motion Argon atmosphere Mg chips feeder Mg chips feedstock hopper Die movable half Goodheart-Willcox Publisher Figure 15-22. This is a schematic drawing of the machine designed and patented by Thixomat to develop semisolid, thixotropic metal for continual die casting. Copyright Goodheart-Willcox Co., Inc.