Effects of casting and process designs on shrinkage porosity in nozzle segments investment casting of Rene 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 investigated. The casting under investigation is a 6-vane turbine multi-nozzle segment with item dimensions of 230mmL * 180mmH * 51mmW. The casting conditions under consideration include gate position, blade trailing edge direction, outer shroud direction, shell thickness, insulation wrap method, chill thickness, and pouring temperature. The Design of Experiment (DOE) method is also employed to design the combinations of the various casting variables to effectively relate the casting conditions and the shrinkage porosity of the casting. The casting conditions investigated include seven gating designs, two blade trailing edge directions, two outer shroud directions, three shell thicknesses, four insulation wrap methods, five chiller thicknesses, and three pouring temperatures. Porosity content and distribution of the castings are measured using several Nondestructive Inspection (NDI) methods; namely visual examination, Fluorescent Penetrant Inspection (FPI), and X-ray inspection. The extent and location of macroporosity are then examined and recorded. Acceptable castings, i.e. those without macroporosity, are then dissected into 21 specimens. The specimens are grounded, polished, and observed under an optical microscope. The worst area for porosity formation is selected and the extent of microporosity in that area is quantified by an image analyzer. A microporosity map is then drawn for each and every casting. The study shows that placing outer shroud upward can reduce the size of macroporosity, but placing blade trailing edge direction in making shell of 6 1/2 coats, and pouring 1440℃ of temperature do not have obvious effects on macroporosity. At inner shroud under 3 mm thicknesses and no steel chiller, regardless of gate location or type, both liner shrinkage and through porosity can be found in the inner shroud of the castings by using visual examination, FF1, and X-Ray analysis. It also shows that liner shrinkage and through porosity are both eliminated if a chiller is employed in the inner shroud area or wall thickness is increased from 3 mm to 4.5 mm and 5.5 mm through casting redesign which combined with a suitable wrap method. Increasing inner shroud wall thickness from 3mm to 4.5 and 5.5mm combined with the fourth wrap method can eliminate both linear shrinkage and through porosity. However, the worst microporosity formation is 8.3 % and 10.8 % in the longitudinal blade face, individually. It is also found that by using room temperature chiller not only can remove the macroporosity but also can eliminate the microporosity. Insulation wrap methods, chiller thickness and temperature, and trailing edge direction, which under chiller used, are also found to have a direct effect on the content and distribution of microporosity. From results of experiment, the best casting and process designs were used steel chiller on the inner shroud area under room temperature, TE direction inward, outer shroud direction upward, 6 1/2 shell coats, 1440℃ pouring temperature, and wrapping the mold from the bottom of airfoils to the pour cup by 6.35 mm thick insulation. The result of microporosity from best conditions showed the worst microporosity formation which are occurred in the tip-section of the third longitudinal blade, inner shroud and outer shroud are only 4%.