“You may see applications in either
nuclear materials, where you may need limited grain boundary failure due to al- pha decay, or deep well oil drilling where temperatures could get very hot, he said.” Mikkola suggests automotive hot-
end turbocharger wheels and high- performance engine valves could also benefit from the parts. He said single crystal parts would bring the mass of a turbocharger assembly down, which limits its delay upon activation. “Current turbocharger rotors are
heavy, and it takes a lot of torque to move them,” Mikkola said. “If you could go to single crystal, you could avoid the delay. But they are almost 10 times more expensive than the current technology.” If all of the challenges to production
and steep price tag can be overcome, Mikkola said single crystal parts even- tually could put another great tool in casting designers’ toolboxes. “Single crystal parts offer the ultimate in engineered properties,” he said. METAL
Explaining Cast
Microstructures All metal components are com-
posed of an underlying structure. In cast components (as in other engi- neered metal parts, such as wrought structures), this microstructure plays a crucial role in determining the proper- ties of the solidified metal. In microstructural terms, a crystal
is a solid composed of atoms, ions or molecules arranged in a pattern that occurs periodically through a metal structure in three dimensions. A grain is an individual crystal in a polycrystalline metal or alloy. The directional solidification of
an alloy implies the manipulation of grain growth in the orientation pre- ferred by the casting (or metal com- ponent) designer. Elongated grains, running parallel to one another, are produced in a preferred orientation using this method. Single crystal components are de-
signed to have no grain boundaries (the interface separating two grains at which the orientation of the lattice changes from that of one grain to that of the other). The grain structure of equiaxed components is such that all the grains have approximately the
same dimensions in all directions. METAL
The information in this sidebar was adapted from the ASM Hand- books Volume 9 Metallography and Microstructures and Volume 15 Casting.
May/June 2011 Metal Casting Design anD PurChasing 39
Turbine blades are produced via the investment casting process, as shown here. Single crystal blades take much longer to solidify than equiaxed blades.
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