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Non-metallic inclusions and zyglo indications frequently occur in the superalloy castings produced through the traditional vacuum gravity investment casting process, particularly in components with thin-walled and complex structural features. The vacuum-assisted low-pressure casting (VLC), a type of counter-gravity casting (CGC) method, has been developed to minimize non-metallic inclusions and zyglo indications in superalloy castings. Rectifying frames for gas turbines made from K446 alloy were produced semi-continuously using the VLC process and subsequently evaluated through tensile testing, chemical composition analysis, X-ray diffraction, and zyglo penetrant inspection. The results indicate a roughly 10% improvement in tensile strength at 800 ℃ compared to gravity casting. Moreover, no significant changes are observed in the chemical composition of the alloys from the beginning to the end of a casting campaign, indicating that the developed VLC process is viable for the engineering-scale production of superalloy castings. Compared to traditional vacuum gravity casting (GC) method, the application of VLC can reduce the numbers of non-metallic inclusions and Zyglo indications in the castings by over 80%. At the same time, it significantly shortens the production time by 3 to 5 days. [ABSTRACT FROM AUTHOR]

In this work, some important factors such as ceramic shell strength, heat preservation temperature, standing time and withdrawal rate, which influence the formability of directionally solidified large-size blades of heavy-duty gas turbine with the liquid metal cooling (LMC) process, were studied through the method of microstructure analysis combining. The results show that the ceramic shell with medium strength (the high temperature flexural strength is 8 MPa, the flexural strength after thermal shock resistance is 12 MPa and the residual flexural strength is 20 MPa) can prevent the rupture and runout of the blade. The appropriate temperature (1,520 °C for upper region and 1,500 °C for lower region) of the heating furnace can eliminate the wide-angle grain boundary, the deviation of grain and the run-out caused by the shell crack. The holding time after pouring (3-5 min) can promote the growth of competitive grains and avoid a great deviation of columnar grains along the crystal orientation <001>, resulting in a straight and uniform grain structure. In addition, to avoid the formation of wrinkles and to ensure a smooth blade surface, the withdrawal rate should be no greater than the growth rate of grain. It is also found that the dendritic space of the blade decreases with the rise of solidification rate, and increases with the enlarging distance between the solidification position and the chill plate. [ABSTRACT FROM AUTHOR]