Improved Castability of Directionally Solidified, Ni-Based Superalloy by the Liquid Metal Cooling Process
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ed superalloys with a columnar grain structure, the so-called directionally solidified (DS) superalloys, have been used widely as turbine blade material because of their improved mechanical performance when compared with conventionally cast alloys.[1] The DS process assisted by liquid metal cooling (LMC) has attracted more attention recently as it is compared with the traditional high-rate-solidification (HRS) technology. Refined microstructures,[2] improved mechanical properties,[3] and reduced freckle tendency[2] in single-crystal alloys have been reported in previous work. The current paper reports our recent observations, which revealed another advantage of this process, i.e., a reduced hot-cracking sensitivity of the columnargrained castings. The cracking of DS superalloys along grain boundaries, which is known as hot tearing, during cooling from the casting temperature is a common problem in the casting industry. It is generally agreed that hot tearing occurs in the so-called ‘‘film stage’’ at the last stages of solidification.[4] Feeding in this stage is difficult. Only small cohesion forces are developed along the broad faces of columnar grains in this film stage,
CHANG LIU and JIAN SHEN, Doctors, KAIWEN LI, Master, and LANGHONG LOU, Professor, are with the Superalloys Division, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, P.R. China. JIAN ZHANG, Professor, is with the Superalloys Division, Institute of Metal Research, Chinese Academy of Sciences, and is also with the Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences. Contact e-mail: [email protected] Manuscript submitted June 30, 2011. Article published online December 21, 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A
essentially by bridging of the secondary dendrite arms.[5–7] If this small resistance is overcome by tensile forces and feeding is impeded, then hot tearing will occur. The tensile forces are generated by the volume contraction associated with the liquid-to-solid phase transformation in the mushy zone and thermal contraction in the solid.[8,9] A critical temperature range DTCTR describing this film stage has been defined a previous publication.[10] It corresponds to the lower end of the freezing range, where the solid fraction is high enough to prevent feeding and bonds begin to form between the secondary dendrite arms to allow the development of stress.[11] In that work, it was observed that a high hot-tearing sensitivity was generally associated with large DTCTR.[5,10,12] The castability of an experimental DS Ni-based superalloy was studied in the current work. The nominal composition is listed in Table I. Tube-like castings with a wall thickness of 1 mm and a length of 150 mm (Figure 1) were designed to evaluate the castability. A ceramic mold with a cluster of three tubes was used. The alloy was solidified directionally by both the LMC and HRS processes. Liquid Sn was selected as the cooling medium in the LMC furnace and a floating baffle of approximately 20 mm
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