and general manager of Chromalloy’s Tampa casting facility, the solidification of equiaxed parts is measured in seconds and minutes, where it can take hours to produce a single crystal casting. “It doesn’t solidify, it grows with almost perfect equilibrium cooling,” Mikkola said. “Te grain moves in the direction the heat is taken out. For single crystal, you always take the heat out in one direction. In equiaxed [cast- ing], you take it out in one direction, but you take it out a lot faster.”
The Cost of Single Crystal Chromalloy has spent tens of mil-
lions of dollars to develop its casting techniques, and it’s the value of the secrets and processes such compa- nies develop—not the raw material prices—that result in the elevated cost of single crystal components, Mikkola said. Plus, the costs of making bad single crystal parts are exorbitant. “Te scrap rates are higher than they
would like,” Mikkola said. “People that are really good can manage less than 10%. But when you scrap a single crystal part, you throw the mold away, the wax pattern and the time it took to grow it.” Mikkola also noted the inspection
costs for single crystal products can be high, and according to Clay and Quested, what’s being inspected can be an area of debate. “Single crystal orientation is both an
engineering control requirement and a key manufacturing process control parameter for this type of product… [but] limitations in the casting process, especially for larger and more complex shaped components, mean that a ‘perfect,’ boundary-free, single crystal component is rarely produced,” they wrote in a paper on evaluating single crystal parts. To address this challenge, different engine manufacturers have developed conventions for the definition of the quality of single crystal components, using reflection techniques to assess the surface and sometimes interior of the parts after they are produced. Component-specific quality acceptance standards for grain boundaries are then applied that test whether the parts meet prescribed limits on the grain location, size and shape, as well as R-values (a measure of thermal resistance).
Single crystal turbine blades take much longer to solidify than equiaxed blades.
Can the Process Grow? In a jet engine, where hotter means
more efficient, single crystal parts are uniquely able to pull heat away from the engine, where another set of stationary castings holds the blades in place. Ceramic blades could also do the job, according to Mikkola, but they would be more susceptible to breakage in unforeseen events like bird strikes. While the cost and availability of
single crystal parts have made them scarce in non-turbine applications thus far, some additional end-use markets could justify the heightened price tag.
Single crystal
castings are the Cadillacs of the group because of their boundary free grain orientation.
According to McCormick, it takes some stretching of the imagination to come up with those applications. “You may see applications in either
nuclear materials, where you may need limited grain boundary failure due to alpha 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.” If all of the challenges to produc-
tion and steep price tag can be over- come, Mikkola said single crystal parts eventually could put another great tool in casting designers’ toolboxes. “Single crystal parts offer the ulti- mate in engineered properties,” he said.
August 2011 MODERN CASTING | 35
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