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Metal Casting Technologies : March 2010
36 www.metals.rala.com.au TECHNICAL FEATURE The fourteen intricately-shaped blades make an impeller difficult to produce in a metal mold, and so a special tooling approach was developed to semi-solid cast the impellers. As shown in Figure 2, this consists of fourteen pie-shaped die inserts and a two-piece split-plate, held together by a clamp ring. This die design allows the blades to be essentially cast net-shape. The hardened steel tool cartridge shown in Figure 2 is ejected from the die along with the cast impeller, and a special station is utilized outside of the casting machine to disassemble the tool. Fig 2. Schematic drawing of the hardened steel tool cartridge used to semi-solid cast impellers A Buhler die casting machine is used to cast the impellers. The Buhler machine provides the injection control necessary to ensure that the semi-solid metal fills the cavity in a non-turbulent manner. It is crucial that air pre-existing in the cavity is not mixed with the metal, but instead is pushed ahead of the metal front and eliminated from the die cavity through strategically placed vents. Once the die is filled, the plunger continues to pressurize the semi-solid metal, to feed solidification shrinkage. After solidification is complete, the die opens and the semi-solid casting and the steel cartridge is ejected. Properties and performance The impellers are semi-solid cast using an Al-Si-Cu-Mg alloy. A number of tests are used to evaluate the quality of the impellers, including penetrant testing, ultrasonic examination, metallography, hardness testing and measurement of mechanical properties. Figure 3a shows a machined surface through an impeller, and no porosity or other defects are visible. However, the critical pore size at which fatigue life is reduced (about 50 μm) is too small to be detected on a machined surface. To detect defects of this size, it is necessary to perform penetrant testing on the machined surface (Figure 3b), which also demonstrates the lack of porosity in the semi-solid cast impellers. To optimize strength and fatigue resistance, the semi-solid cast impellers are fully heat treated to the T6 temper. Careful choice of die design, casting parameters and die and plunger lubricants avoids blistering during T6 heat treating. Samples for tensile testing are machined from the center of the impellers and measurement of mechanical properties is performed to monitor quality. The data in Table 1 show typical mechanical properties. 0.2% YS (MPa) UTS (MPa) Elongation (%) 340 400 7.7 Table 1. Average mechanical properties of commercial SSM impellers produced in January 2009 Fig 3: Sections through impellers showing lack of porosity CAREFUL CHOICE OF DIE DESIGN, CASTING PARAMETERS AND DIE AND PLUNGER LUBRICANTS AVOIDS BLISTERING DURING T6 HEAT TREATING. a) Machine surface b) Machined and penetrant tested (viewed under ultraviolet light)†
Media Kit 2010