Turbine blade makers like Howmet, Precision Castparts Corp. and Chromalloy (shown) are highly secretive about their processes. “Seeding is a technique where you
have a crystal in the mold that has the orientation that you want in the cast- ing,” Mikkola said. “It actually ‘seeds’ the growth of the crystal in the exact same direction. Believe it or not, there is a lot of trial and error to figure out how to seed. There are trade secrets that are beyond just the science.” Starting with a properly seeded investment casting mold, single crystal castings are produced by controlling the solidification of the molten alloy in the mold using a heated coil or some other thermal device. The coil is removed from the mold slowly in the direction grain growth is desired. Equiaxed and direc- tionally solidified parts also are produced using such controlled solidification tech- niques, but the process is much faster. According to Tom Trotter, vice president 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. 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 companies 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. “The 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 ‘per- fect,’ boundary free, single crystal com- ponent is rarely produced,” they said. To address this challenge, different
engine manufacturers have developed conventions for the definition of the
38 Metal Casting Design anD PurChasing
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).
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 sta- tionary 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. According to McCormick, it takes some stretching of the imagination to come up with those applications.
May/June 2011
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