The Effect of Shell Thickness, Insulation and Casting Temperature on Defects Formation During Investment Casting of Ni-base Turbine Blades

Abstract

Turbine blades have complex geometries with free form surface. Blades have different thickness at the trailing and leading edges as well as sharp bends at the chord-tip shroud junction and sharp fins at the tip shroud. In investment casting of blades, shrinkage at the tip-shroud and cord junction is a common casting problem. Because of high temperature applications, grain structure is also critical in these castings in order to avoid creep. The aim of this work is to evaluate the effect of different process parameters, such as, shell thickness, insulation and casting temperature on shrinkage porosity and grain size. The test geometry used in this study was a thin-walled air-foil structure which is representative of a typical hot-gas-path rotating turbine component. It was observed that, in thin sections, increased shell thickness helps to increase the feeding distance and thus avoid interdendritic shrinkage. It was also observed that grain size is not significantly affected by shell thickness in thin sections. Slower cooling rate due to the added insulation and steeper thermal gradient at metal mold interface induced by the thicker shell not only helps to avoid shrinkage porosity but also increases fill-ability in thinner sections.

Effects of Process Related Variations on Fillablity Simulation of Thin-Walled IN718 Structures

“…23 The results are in agreement with a previous work. 24 In general the cavity located at the 12 o'clock position in the bottom-gated set-up was filled to a greater extent than the cavity in the 6 o'clock position, as can been seen Figure 5a. However, a significant difference in the castings in the 12 o'clock and 6 o'clock positions is observed in setup 4 where misalignment in hanging was adjusted, as described in the footnotes of Table 1.…”

“…The material properties for the mould material were taken from a previous study 15 as shown in Figure 3.…”

“…14 In order to obtain reliable simulation results thermo-physical properties of the cast alloy and the mould material 15 were introduced into the software.…”

“…15 The moulds were insulated using a fiber blanket consisting of 48% Al 2 O 3 and 52% SiO 2 , with a density of 96 kg/m 3 and thermal conductivity of 0.62 W/(mK) at 1000°C. 16 Aerospace grade IN718 according to AMS5383 17 was used in the experimental work.…”

Effect of Grain Refining on the Shrinkage Porosity in Thin-Walled Plates Produced by Investment Casting Integrating Controlled Solidification Technique

“…Modification of high silicon (Si) alloys is performed to refine Si particles morphology in the eutectic phase. At very thin wall thickness level, cooling rate tends to be high and leads to high shrinkage velocities, as the speed goes above the threshold shrinkage velocity then pores will form, and very limited time given for diffusion to take place hence porosity is formed whether you have a grain refiner or not [5]. Even though thickness level below 2 mm possess high driving force nucleation, which may lead to finer grains [6] that should have been in assistance to promote good feeding at interdimeric regions [7], you also have high shrinkage velocities which leads to shrinkage porosity.…”