Effects of micro‐sized mullite on cristobalite crystallization and properties of silica‐based ceramic cores

In this work, silica-based ceramic cores with alumina as a mineralizer were prepared via an injection molding method, and the effects of alumina on the microstructural evolution and properties at 1450 • C (simulating the process of equiaxed castings) and 1550 • C (simulating the process of columnar/single crystal castings) were investigated. It was found that alumina promoted the cristobalite crystallization of fused silica refractory during sintering but inhibited the devitrification rate in the subsequent heating. The flexural strength of silica-based ceramic cores at an ambient temperature and 1450 • C improved with an increasing alumina content, whereas the opposite trend appeared at 1550 • C. The creep resistances of silica-based cores were improved significantly and then slightly deteriorated with an increasing alumina content from 5% to 20%, depending on the competition effects of alumina hindering the viscous flow of liquid silica (favorable), but suppressing the devitrification rate (unfavorable). The results of this work show that silica-based cores need to follow different compositional design principles for equiaxed and columnar/single-crystal turbine blade castings.

Section: Resultsmentioning

confidence: 99%

Section: Influence Of Alumina On the Properties Of Silica-based Ceram...mentioning

confidence: 99%

Investigation of microstructure evolution and properties in silica‐based ceramic cores with alumina as a mineralizer

Pan

Guo

Xiong

2022

“…Meanwhile, it was found that adding a certain amount of substances with crystalline structure to the amorphous material powder could impede its fluidity and and hinder sintering, thus increasing the porosity of silicabased ceramic cores. 1,[21][22][23] There are two crystallographic forms of cristobalite, namely tetragonal β-cristobalite at low temperature and stable cubic α-cristobalite in the temperature range of 1470-1713 • C. The α-β phase transition in cristobalite is accompanied by the volume effect, and micro stress is generated, which leads to microcracks in the ceramic cores. [24][25][26] Thus it can be seen that if a large amount of cristobalite crystal is added to fused SiO 2 powder for sintering silica-based ceramic cores, on the one hand, crystallization can be induced and the increase of crystalline content can ensure good high-temperature stability.…”

Section: Introductionmentioning

confidence: 99%

“…The amount of fused silica crystallized into cristobalite played a decisive role in the thermal properties of silica‐based ceramic cores. Meanwhile, it was found that adding a certain amount of substances with crystalline structure to the amorphous material powder could impede its fluidity and and hinder sintering, thus increasing the porosity of silica‐based ceramic cores 1,21–23 . There are two crystallographic forms of cristobalite, namely tetragonal β‐cristobalite at low temperature and stable cubic α‐cristobalite in the temperature range of 1470–1713°C.…”

Section: Introductionmentioning

confidence: 99%

Improved collapse properties of silica‐based ceramic cores via flexibly regulating cristobalite content

Qin

Cheng

Li³

et al. 2023

Although silica‐based ceramic cores have important applications in the precision casting of metallic devices, their high‐temperature stability and removal performances are seriously affected by the liquid phase sintered fused silica. Herein, we develop a manufacturing strategy of high‐collapse silica‐based ceramic core via using cristobalite crystals as the sintering inhibitor, waterglass as the binder, and injection moulding at 100°C and 80 MPa, followed by heat treatment simulating the casting process for sintering at 1200°C and 1500°C. The results demonstrated that the addition of cristobalite crystals could effectively form the core skeleton to ensure high‐temperature performance. Meanwhile, it inhibited the liquid flow during sintering and induced the crytsallization from fused SiO2 glass into cristobalite crystals, and the resulting plenty of micropores and microcracks within the microstructure effectively improve the removal performance. Especially, the porosity was highest up to 35.36% and the flexural strength was only 6.74 MPa when the addition of cristobalite reached 45%, realizing a 100% removing by high‐frequency and fast‐speed specific mechanical vibration. And, the casting is guaranteed to be flat and free of defects. This work provides a simple and flexible strategy to manufacture high‐collapse silica‐based ceramic cores, which can be removed by specific mechanical vibration without immersion in acid or alkali solutions after casting.

“…The purpose of the hollow spaces inside the blades is to reduce the weight of the blade and supply cooling ducts during turbine operation, which provides the possibility of a temperature increase in the entrance gases 1–3 . The ceramic cores' properties, such as high mechanical strength, low thermal expansion coefficient, excellent thermal shock resistance, and chemical resistivity against molten metal, are essential for high performance under severe conditions 4,5 . Due to its low thermal expansion coefficient (.55 × 10 −6 K) from 25–1000°C and excellent chemical resistivity against molten metal, fused silica is widely used as the primary raw material to produce ceramic cores.…”

Section: Introductionmentioning

confidence: 99%

The influence of graphite on the physical, thermal, and mechanical properties of silica‐based ceramic cores

Naghibi,

Kazemi‐nafchi,

Bavand‐vandchali

et al. 2023

Due to their excellent thermal stability and chemical resistivity against molten metals, silica‐based ceramic cores have been prepared for use in nickel‐based superalloy investment casting via injection molding under 12 kPa pressure and 115°C temperature. This paper evaluated the effect of graphite as an additive on the thermal and mechanical properties of silica‐based ceramic cores. The results show that adding graphite increases the core's modulus of rupture (MOR) and hot MOR (at 1200°C) by 6% and 35% to 10.65 and 18 MPa, respectively. According to phase analysis by X‐ray diffraction, the presence of graphite reduced the crystallization behavior of fused silica and increased the open and closed porosity amounts by 2.5%–33.6%. Additionally, the formation of the in situ SiC phase due to reduction reactions decreases the shrinkage and enhances the strength of graphite‐containing bodies. No difference in the thermal expansion coefficient of samples in the presence of graphite has been detected.