Evaluating Silicon Carbide-Based Slurries and Molds for the Manufacture of Aircraft Turbine Components by the Investment Casting

Paweł Wiśniewski

doi:10.3390/cryst10060433

Abstract

Silicon carbide (SiC) is a promising material for the fabrication of ceramic shell molds due to its high mechanical strength, hardness, thermal shock resistance, and thermal conductivity compared with commonly used slurries. This article describes the test results of casting materials, i.e., SiC-based powders and aqueous binders with aluminum oxide nanoparticles, as well as the parameters of slurries used for prime coats and structural layers. Tests were also performed to evaluate the physical and mechanical properties of SiC-based shell molds for the manufacture of aircraft turbine components. Two SiC-based slurries with solid concentrations of 65 and 70 wt.% were prepared, and their viscosity, density, pH, quantity, thickness, and copper plate adhesion (plate weight test) were investigated. Fourteen days were required to prepare and evaluate the slurry parameters. The results showed that SiC-based slurries had a Zahn cup #4 outflow time of 33.1 s to fabricate the first two coats and 14.8 s to fabricate the shell mold structural layers. Three series of SiC-based shell mold samples were prepared: after dewaxing (PW1), after burnout preheating at 700 °C (PW2), and after annealing at 1200 °C (PW3). The bending mechanical strength, Young’s modulus, and Weibull’s modulus of the samples were calculated, and the roughness (Ra and Rq) and microstructures of samples were also analyzed (SEM). Inner defects were evaluated by CMT (µCT). The Ra and Rq values of the prime coat of the SiC-based shell mold did not exceed 5 µm. The fabricated SiC shell molds had bending mechanical strengths from 1.21–2.28 MPa, Young’s modulus of 102.97–207.66 MPa, and a Weibull’s modulus from 5.36–9.94. The shell molds fabricated on the technical scale met the requirements specified for industrial shell molds.

Highlights

Investment casting is the primary method for manufacturing aircraft turbine parts from nickel and cobalt superalloys because it can be used to fabricate products with intricate geometries [1,2,3,4,5].The key advantages of investment casting lie in its ability to obtain superior dimensional accuracy, smooth surfaces, and intricately shaped shell molds [6,7]
The primary objective of this work was to investigate the preparation of stable slurries for prime and second coats and structural layers for the fabrication of technical-scale, Silicon carbide (SiC)-based shell molds with colloidal Al2 O3 binder, instead of the commonly used SiO2 nanoparticle binder
Three powders were used in this study: SiC F400 meal (Stanchem, Poland) with an average particle size of 21.8 μm was used as the main slurry component, and two powders: F80 (253.9 μm, layers 1–2) and F40 (564.8 μm, layers 3–8), were used as sprinkling

Summary

Introduction

Investment casting is the primary method for manufacturing aircraft turbine parts from nickel and cobalt superalloys because it can be used to fabricate products with intricate geometries [1,2,3,4,5].The key advantages of investment casting lie in its ability to obtain superior dimensional accuracy, smooth surfaces, and intricately shaped shell molds [6,7]. Manufacturing investment shell molds with good mechanical properties strongly depends on the quality and technological properties of the shell molds [8,9,10]. Multilayer ceramic molds are used to reconstruct intricate geometrical shapes. Such molds should have appropriate physical and mechanical properties because they influence the quality of the fabricated castings [11,12,13]. The structures of ceramic shell molds may have from one to several dozen layers. The number of layers determines the characteristic properties of the molds when used to fabricate monocrystalline castings

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Conclusion