Comparative Investigation of the Downward and Upward Directionally Solidified Single-Crystal Blades of Superalloy CMSX-4
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SINGLE-crystal Ni-based superalloys have found increasing applications for turbine blades and vanes for aero-engines as well as industrial gas turbines (IGTs).[1–5] The increasing operating temperature requirements for high-efficiency turbines demand that the blades possess increased high-temperature creep and fatigue strengths. The improved high-temperature properties may be achieved via development of alloys[6–11] and the directional solidification (DS) process. Innovations in processing focus on increasing the thermal gradient (GL) at the solidification front because, during solidification, high thermal gradients assure sequential solidification along the axial direction and prevent equiaxed grains from initiating in constitutional undercooling zones within the melt. The high thermal gradients also reduce segregation and allow the operating temperature of the alloy’s components to be increased.[12] Since the Bridgman directional solidification process was developed in the 1920s,[13] a number of directional solidification processes have to date been developed. In the 1970s, a few researchers[14–16] presented the high-rate solidification (HRS) process based on the Bridgman process in which the concept of mold translation was utilized. HRS can employ various types of radiation baffles to sharpen the thermal gradient between the hot and cool zones of the furnace. Since the original
FU WANG, Scientific Staff, DEXIN MA, Senior Scientist, SAMUEL BOGNER, Ph.D. Student, and ANDREAS BU¨HRIG-POLACZEK, Professor, are with the Foundry Institute, RWTH Aachen University, Intzestraße 5, Aachen 52072, Germany. Contact e-mail: darrel0112038@ hotmail.com Manuscript submitted November 12, 2015. Article published online March 8, 2016 2376—VOLUME 47A, MAY 2016
development of HRS, the process has been highly optimized for the production of aero-engine scale components. However, due to the lower rate of radiative heat exchange, the open baffle insulation between the heating and cooling zones, and the large thermal resistance of the thick ceramic molds (especially for IGTs’ blades[17]), this process results in ineffective heating in the heating zone and inferior heat extraction in the cooling zone. These inefficiencies lead to lower thermal gradients and the occurrence of process problems, such as mold warping and cracking, mold–metal reaction and low yield.[18] In addition to this, at the beginning of HRS process the heat primarily conducts through the casting to the chill. At increasing distances from the chill, heat extraction thus quickly becomes inefficient owing to the low thermal conductivity of the superalloys. When this occurs, mold radiation to the cooling chamber becomes the dominant method of heat extraction resulting in low thermal gradients ahead of the solidification front.[19] The experimental results[20,21] show that the thermal gradients and cooling rates decrease with increasing distance from the chill. Therefore, in order to keep the solid/liquid (S/L) interface stable, the withdrawal rate must be reduced; otherwise casting defects
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