Numerical Modeling of Melting and Columnar Solidification with Convection in a Gradient Zone Furnace in a Centrifuge
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TRIFUGAL casting is widely used for the production of thin-walled castings being the technology of choice for applications which demand high quality cast parts. Centrifugal investment casting was proven to be capable of producing near-net-shape components from Titanium aluminide alloys, e.g., low-pressure turbine blades.[1,2] The final microstructure and properties of a centrifugally cast alloy are greatly determined by the complex interaction between the solidifying dendrites, segregated solute, and convective fluid flow in the given cooling conditions. Solidification and flow are coupled at the micro-scale in the mush and also have influence on the transport phenomena in the bulk melt. Meanwhile, as the Coriolis force is present, the system can become quite complex, even if flow from melt filling
CAN HUANG and ULRIKE HECHT are with the ACCESS e.V., Intzestrasse 5, D-52072 Aachen, Germany; Contact e-mail: [email protected] ANDREAS BU¨HRIG-POLACZEK is with the Foundry Institute, RWTH Aachen University, Intzestrasse 5, D-52072 Aachen, Germany. Manuscript submitted January 6, 2020.
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is neglected. The experiments from Friedrich et al.[3] provide a good example. The authors analyzed the melt convection and the related dopant segregation in Ga-doped Germanium during crystal growth in a centrifuge by applying different rates of centrifugation specified by the angular velocity, x. The flow pattern and the intensity of the convective flow were shown to depend on the resultant effect from centrifugal acceleration, Coriolis force, and the direction of the temperature gradient. A minimum of convection at a particular x was reported and explained to result from counter-acting effects from centrifugal acceleration and the Coriolis force. To investigate the columnar solidification and the columnar-to-equiaxed transition in two TiAl-based alloys under centrifugal condition, experiment series were performed in ESA’s Large Diameter Centrifuge (LDC) as well as in micro-gravity on-board the sounding rocket MAXUS9 by a European Research team.[4] In these experiments, a cylindrical sample from a TiAl alloy was molten and then solidified directionally in a gradient zone furnace under the same thermal processing conditions, however, under distinct values of the rotational speed. The experimental parameter was thus the angular velocity, x, which defines the resultant centrifugal force acting on the sample, expressed relative
to Earth’s gravity as multiples of g. The experimental conditions thus spanned from lg to hyper-g conditions reaching a maximum level of 18.3 g.[4] In all experiments, the solidification comprised an extended region of transient columnar growth of b-Ti dendrites under decreasing temperature gradient G and increasing isotherm velocity V followed by the columnar-to-equiaxed transition (CET) or a columnar-to-radial transition. The columnar-to-radial transition was observed in lg and in the majority of 1g reference experiments, while all hyper-g experiments lead to a columnar-to-equia
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