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A wax pattern containing the 3D-printed Silicore ceramic core.

Ceramic Cores Raise Design Standards for Investment Casting

March 15, 2021
Turbine blades are increasingly complex structures, in line with performance standards, and SLA additive manufacturing is making it possible for foundries to keep up with the changes.

Investment casting is a complex, multi-stage procedure, but the role that coremaking plays in effective investment casting is somewhat overlooked. In particular for high-profile, high-value investment cast parts like turbine blades, reliable and precision cores are critical to quality finished products.

Turbine blades draw a lot of attention for their part in engine performance, efficiency, and safety. All these factors raise the significance of the cores. To perform as expected, turbine blades have to dissipate heat effectively, which allows a turbine to operate at higher temperatures safely, with increased power and efficiency. To that end, the blades are designed with integral cooling channels – which is the reason that investment casting is the production process of choice. The ceramic core used in the process is responsible for shaping those cooling channels, which improve the blades’ heat tolerance and thus the engine’s reliability.

Newer, more complex core designs are difficult to produce using ceramic injection molding, the usual process, because that requires forming the core in several pieces to be assembled manually, a complex and time-consuming effort that also can result in high rates of wasted products. A further factor is the lead-time involved in developing and testing the blades, which means design changes are difficult to apply and test.

3DCeram – a French firm that develops and provides 3D printing processes and materials for technical ceramics – addressed this issue with a process for printing ceramic cores using laser stereolithography (SLA), an additive manufacturing (3DP) technology.

SLA is a process involving layer-by-layer deposition of a photopolymerizable ceramic suspension in a pattern drawn from the CAD version of the finished core; followed by selective curing of the ceramic material using a computer-controlled, UV laser beam, until the layers form a 3D version of the CAD image.

According to 3DCeram, SLA technology saves time and increases productivity compared to ceramic injection molding, but it also offers greater flexibility for product design, meaning greater complexity for ceramic cores. In addition, it allows faster design and redesign of parts    including no need to design and create injection molds. More than these, 3DCeram maintains that the process achieves better responsiveness and increased profitability for the core producer, with lower fixed costs, no tooling maintenance requirements, and lower storage costs.

The choice of the ceramic used to produce foundry cores depends on the type of alloy to be cast, but the following criteria are offered: no chemical reaction between the core and the metal during the casting phase; heat and mechanical resistance to the cast metal; good leachability after the metal cooling; and low coefficient of thermal expansion (CTE.)

To supply this market 3DCeram developed a silica­based composition Silicore. Silica-based compositions are extensively used in investment casting of Ni-based turbine blades, and fused silica ceramic cores have good thermal stability due to a low CTE (about 0.6×l0-6 K-1), as well as excellent thermal shock resistance and high leachability. Silica cores also are easily removed in solutions of soda or potash, not harmful to alloys.

Finally, the sintering of a silica core leads to the formation of cristobalite by a devitrification process, which ensures temperature resistance of the core.

In 2019, 3DCeram began a development project with a Ukrainian research body, Zaporozhye Machine-Building Design Bureau Progress State Enterprise, to validate the ceramic formulation. Cores were printed using a Ceramaker 900 SLA system, and according to 3DCeram, the high precision of the SLA technology resulted in ceramic cores that were used to obtain the wax forms needed for the first stage of single-crystal investment-cast turbine blades.

X-ray control applied to all the wax molds produced showed no cracks in the ceramic cores, 3DCeram reports. The Silicore material has now been qualified as compliant for industrial use for producing foundry cores.