Mechanical and kinetic studies on the refractory fused silica of integrally cored ceramic mold fabricated by additive manufacturing

Abstract

A refractory fused silica based integrally cored ceramic mold, the ceramic core with a ceramic mold shell in a single patternless construction, is fabricated by ceramic stereolithography of additive manufacturing. Refractory ceramic molds should satisfy the following restrictions such as similar strength to that of cast metal during solidification, thermal stability for dimensional accuracy, and easy removal of core after casting. Here, we report mechanical and transformation kinetic studies on the refractory fused silica of integrally cored ceramic mold. The flexural strength of sintered silica continually increases with higher density of better densification up to 11.4 MPa at 1300 °C, while it decreases from 11.3 MPa at 1350 °C to 4.6 MPa at 1500 °C. The degradation of the flexural strength is related to the larger amount of the cristobalite and microcracks generated by the abrupt contraction induced during the transformation of beta to alpha cristobalite. Given the quantitative x-ray diffraction study on transformation kinetics, an apparent activation energy Q is 674 ± 53 kJ/mol and the average time exponent 1.85, suggesting that the transformation kinetic is controlled by 1-dimensional interfacial growth.

Introduction

A fused silica based integrally cored ceramic mold (ICCM), the ceramic core with a ceramic mold shell in a single patternless construction shown in Fig. 1, is fabricated by ceramic stereolithography (CerSLA) [1,2]. Core and shell structures in ICCM have been developed to make interior passages of a turbine airfoil which are used to conduct cooling air through one or more passageways [3,4]. The most important step producing the hollow cooling passages in a turbine airfoil is the selection of refractory ceramic materials since ceramic core and shell molds have been exposed on the severe environmental conditions of creep and dimensional change resulting from the secondary sintering due to high casting temperature of cast metal as high as 1600 °C [5]. Therefore, requirements on material properties considered to be ceramic core and shell molds are not only the quality and accuracy but also thermal stability when pouring high melting casts.

Fused silica, among the refractory ceramic, is extensively used for ceramic core to produce complex, internal cooling passage in investment-cast, superalloy turbine airfoils [6]. It has thermal stability at high temperature resulting from a low thermal expansion coefficient (about 0.6 × 10⁻⁶/^oC) and excellent thermal shock resistance, fulfilling all requirements not to react with alloys and considering the best candidate of refractory ceramics used for investment casting. Furthermore, a fused silica core is easily removed and completely leaches out with aqueous chemical solutions such as NaOH and KOH, where the solutions are non-deleterious to the nickel-base superalloys [7].

The devitrication of fused silica has been investigated in detail as an example of a prototypical single component oxide glass. Wagstaff reported the kinetics of the nucleation growth of internally nucleated beta-cristobalite in bulk fused silica glass purposes [8,9]. For sintered fused silica ceramics, the transformation to cristobalite plays a critical role in the thermal expansion, sintering, and mechanical properties. The behavior of sintered fused silica-based ceramics is dominated by crystallization in fused silica at elevated temperature [10]. The high temperature creep of silica-based investment casting refractory cores is suppressed by formation of beta-cristobalite, but the room temperature strength after transformation is degraded when the beta-cristobalite transforms to alpha-cristobalite during cooling. An abrupt contraction of about 4 vol% due to the α-β transition generates microcracks, which decrease the room temperature strength [11]. Huseby et al. studied the high temperature properties and phase compositions of fused silica based ceramic cores [12]. To show the behavior of fused silica cores during heating and cooling, the dilatometric tests carried out from room temperature to 1600 °C. After low rate dilatometry to 1600 °C, X-ray diffraction indicated that most of fused silica in the ceramic cores transformed into cristobalite. In the dilatometric test, the abrupt contraction at ≈200 °C during cooling occurred from the cristobalite transformation from beta- to alpha-cristobalite phase. However, the flexural strength and transformation kinetics of refractory fused silica was not reported.

This paper reports the flexural strength of sintered fused silica powders used to fabricate integrally cored ceramic mold as a function of sintering temperature and discusses the dependence of flexural strength on the two parameters: relative density and transformed cristobalite. Furthermore, the transformation kinetic of fused silica is investigated such that how much apparent activation energy is required for the transformation and what kind of mechanism is dominant to control transformation.