Effects of Casting and Process Designs on Shrinkage Porosity in Nozzle Segments Investment Casting of Ren6 77 Alloy

摘要

In this study, the effects of casting and process designs on the occurrence of shrinkage porosity in nozzle segments investment casting of Rene 77 alloy are systematically investigated by using the Design of Experiment (DOE) method. The ultimate goal is to reach an optimal design to produce castings which are free from porosity defects. The casting under investigation is a 6-vane turbine muki-nozzle segment with item dimensions of 230mmL 180mmH 51mmW. The casting conditions under consideration include seven gate positions, two blade trailing edge directions, two outer shroud directions, three ceramic shell thicknesses, four insulation wrap methods, five chiller thicknesses, and three pouring temperatures. Macro-porosity content and distribution of the castings are measured using visual examination, Fluorescent Penetrant Inspection (FPI), and X-ray inspection. Those castings without macro-porosity are then dissected into 21 specimens. The specimens are grounded, polished, and observed under an optical microscope. The worst area for microporosity formation is identified. The extent of micro-porosity in that area is quantified by an image analyzer. A microporosity map is then drawn for each and every casting. The results show that for the original casting design, where the inner shroud is 3 mm thick, without the employment of steel chiller born linear shrinkage and through porosity can be found in the inner shroud of the castings regardless of the casting conditions. By the employment of room temperature chiller, not only the macro-porosity can be eliminated but also the micro-porosity can be significantly reduced. The optimal casting and process designs are to stay with the design of 3 mm thick for the inner shroud and to use steel chiller of room temperature on the inner shroud area, inward TE direction, upward outer shroud direction, 6 1/2 shell coats, pouring temperature of 1440 deg C, and wrapping the mold from the bottom of airfoils to the pouring cup by 6.35 mm thick insulation. For this optimal design, the worst micro-porosity formation is around 4 percent for the casting.