Al2O3–Y3Al5O12(YAG)–ZrO2 ternary composite rapidly solidified from the eutectic melt

In situ fabrication of new ceramic eutectic composites by rapid solidification of eutectic drops is a cheap and quick method compared to fabrication of directional solidification or multi-step fabrication methods of fiber reinforced/layered composites for high temperature use. This study reports the fabrication of ceramic composites during rapid solification of eutectics melts in the ternary oxide alumina–yttria–zirconia system. Layered ternary eutectics are obtained in the alumina–YAG–zirconia subsystem. The microstructure of Al₂O₃–Y₃Al₅O₁₂–ZrO₂ composites rapidly solidified from melts is presented.     

Additive manufacturing of low-shrinkage alumina cores for single-crystal nickel-based superalloy turbine blade casting

We prepare bimodal particle size photo-curable ceramic pastes with high solid loadings (up to 65 vol %) and fabricate porous alumina ceramic cores with complex shapes via ceramic stereolithography (Cer-SLA) 3D printing technique. The sintering temperature is carefully selected, ranging from 1500 °C to 1650 °C, and a high holding time (>4 h) is applied to guarantee that the materials can withstand the subsequent high temperature (>1500 °C) casting process for single-crystal nickel-based superalloy hollow turbine blades. Herein, the originally spherical fine particles are found to become platelet-like after sintering, and the forming mechanism is discussed in detail. In addition, we explore the influence of platelet-like particles, coarse particles and sintering process on the microstructural evolution of alumina particles, and reveal the relationship between microstructure and properties of ceramic cores. These results illustrate that the proposed materials for SLA 3D printing exhibit a great potential in the fabrication of complex-shaped alumina ceramic cores for high-precision investment casting, e.g., manufacturing single-crystal nickel-based superalloy hollow turbine blades for an advanced aircraft engine.     

Additive manufacturing of Al2O3 ceramic core with applicable microstructure and mechanical properties via digital light processing of high solid loading slurry

Ceramic cores are essential intermediate mediums in casting superalloy hollow turbine blades. The developing of additive manufacturing (AM) technology provides a new approach for the preparation of ceramic cores with complex structure. In this study, alumina oxide (Al₂O₃) ceramic cores with fine complex geometric shapes were fabricated by digital light processing (DLP) in high resolution. The maximum solid content of 70 vol% of ceramic slurry was adopted in the printing process, which is important for the regulation of deformations and mechanical properties. The effects of the printing parameters, including exposure intensity, printing layer thickness and sintering temperature on the microstructures and mechanical properties of printed samples were investigated. The decrease of residual stress and similar shrinkage in X, Y, and Z directions could be obtained by adjusting the printing parameters, which are crucial to prepare complex ceramic cores with high quality. Besides, the flexure strength and open porosity of ceramic cores reached 34.84 MPa and 26.94%, respectively, which were supposed to meet the requirement of ceramic cores for the fabrication of superalloy blades.     

Properties of alumina coatings prepared on silica-based ceramic substrate by plasma spraying and sol-gel dipping methods

Silica-based ceramic cores are widely used in the manufacturing of hollow, nickel-based, superalloy turbine blades. However, elemental Hf, Ti, Al, and other active metals in the superalloy can react with silica-based ceramic cores during casting, resulting in a reduction in the quality of the turbine blades. In this study, both plasma spraying and sol-gel dipping methods were used to prepare alumina coatings on silica-based ceramic substrates to prevent the interfacial reaction. The performance of the alumina coatings prepared by both methods was evaluated by comparative analysis of the surface roughness, bonding interface morphologies, and the adhesive characteristics of the coating. The plasma-sprayed alumina coating has a roughness greater than 5 μm and peeled away from the substrate due to the difference in thermal expansion between SiO₂ and Al₂O₃ at temperatures above 1500 °C, rendering the silica-based substrate with the plasma-sprayed alumina coating unfit for the application requirements of the casting process. The alumina coating prepared by the sol-gel dipping method improved the roughness of the substrate from Ra 2.39 μm to Ra 1.83 μm, and no peeling was observed when heated to 1550 °C for 30 min due to the pinning characteristics of the coating on the substrate. Furthermore, the interfacial reaction between the DZ125 superalloy melt and the silica-based substrate coated with alumina by sol-gel dipping method were investigated. The alumina coating effectively inhibited the interfacial reaction and no reaction products were detected during the directional solidification with pouring temperature of 1550 °C and withdraw rate of 5 mm/min. While a uniform, 4–5 μm thick HfO₂ reaction layer formed between the uncoated substrate and the DZ125 alloy melt. Two dipping-drying cycles were required to ensure the alumina sol completely covered the surface of the substrate.     

陶瓷胶态注射成型中压力的影响

陶瓷胶态注射成型是一种结合了凝胶注模成型和注射成型优点的新成型工艺。不同于凝胶注模成型中通过温度诱发反应,在胶态注射成型中是利用温度与压力共同作用引发陶瓷浆料固化。采用自行研制的凝胶点测试装置,研究了不同温度条件下压力对氧化铝浆料固化过程的影响,发现提高压力能促进浆料固化,不仅能缩短其聚合诱导期,而且能加速固化过程。此外,探讨了胶态成型的作用机制,为胶态注射成型的自动化设计提供了相关工艺参数。     

α-Al2O3悬浮体的流变性及凝胶注模成型工艺的研究

陶瓷凝胶注模型工艺是一种新颖的成型工艺,具有很好的应用前景。它将高分子化学单体聚合的思路引入到陶瓷的成型工艺中,可制备高强度、高均匀性的陶瓷坯体。     

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

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.     

Enhanced thermal properties of silica-based ceramic cores prepared by coating alumina/mullite on the surface of fused silica powders

In this study, an alumina/mullite coating was synthesized on the surface of fused silica powders to form an alumina/mullite-silica core-shell structure. The effects of the alumina/mullite coating on the cristobalite crystallization, thermal properties, and leachability of the silica-based ceramic cores were investigated using the simulated casting process. The X-ray diffraction results indicated that the crystallization of cristobalite was significant at the simulated casting temperature of approximately 1400 °C. An increase in the cristobalite content during this stage resulted in a large thermal expansion because of its higher coefficient of thermal expansion compared with that for fused silica. The addition of optimum amounts of the alumina/mullite powders resulted in an increase in the initial shrinkage temperature and a decrease in the shrinkage of the specimens. When the coating powders were added at 43 wt%, the initial shrinkage temperature increased from 1092 °C to 1200 °C and the shrinkage decreased sharply. Leaching tests showed that the silica-based ceramic cores were removed in the form of stripped layers. The washing and shaking process accelerated the disintegration of the ceramic core and improved its leachability.     

Stabilization of zirconia lamellae in rapidly solidified alumina–zirconia eutectic composites

In-situ fabrication of ceramic eutectic composites by rapid solidification of eutectic drops is a cheap and quick method compared to directional solidification or to multi-step fabrication methods of fiber reinforced/layered materials for high temperature use. Binary eutectic composites with a homogeneous periodic microstructure have been obtained by directional solidification of eutectic melts for many years, but typical solidification velocities used in directional solidification are limited to the range of cm/hour or, more recently, up to 15 mm/min. The present study aims to determine the effects of faster solidification rates on the structure of the alumina–zirconia binary composites obtained at higher growth rates by rapid solidification from eutectic melts in air or vacuum. A binary composite with zirconia stabilized in the high-temperature tetragonal form is presented. The stabilization of the tetragonal phase has not been observed before in bulk crystalline pellets of binary Al₂O₃–ZrO₂ eutectic composites.     

2-carboxyethyl acrylate as a new monomer preventing negative effect of oxygen inhibition in gelcasting of alumina

The paper presents the application of 2-carboxyethyl acrylate as a new monomer in gelcasting of alumina. The research showed that the use of 2-carboxyethyl has a positive influence on the properties of green bodies, limiting the negative effect of oxygen inhibition. Oxygen inhibition which hampers polymerization reaction is a well known negative phenomenon in gelcasting but still not fully overcome for samples formed in air atmosphere. As a reference monomer 2-hydroxyethyl acrylate was used in the research. Rheological behavior of alumina suspensions containing both monomers has been studied, as well as the properties of green and sintered bodies obtained by gelcasting have been measured. The differences in the rheological properties and thus ceramic-monomer interactions in the slurries has been analyzed. The high values of densities and mechanical strength, accompanied with the pictures of microstructures of sintered ceramic parts have been presented, as a result of a successfully completed gelcasting process of alumina powder.     

A Synergistic Low-Toxicity Gelcasting System by Using HEMA and PVP

A new low-toxicity gelcasting system with a 2-hydroxyethyl methacrylate (HEMA) monomer was applied for casting of alumina ceramics. Polyvinyl pyrrolidone (PVP) was adopted for modifying the homogeneity of the PHEMA (poly-HEMA) gel. The rheological properties of alumina suspension in the HEMA–PVP premix solution were studied. After preparation of a concentrated alumina suspension, homogenous alumina green body with a relatively high strength of about 19 MPa could be formed through the PVP-modified HEMA system. Dense complex-shaped ceramic parts can be successfully produced through the system. Besides, the surface exfoliation phenomenon that seems inherent to the acrylamide gelcasting system could also be eliminated by using the PVP-modified HEMA system. Analysis of the interaction of HEMA and PVP suggested that the improved microstructure and strength homogeneity, as well as the elimination of surface exfoliation in the new system, should be attributed to the intermolecular hydrogen bonding formed between PHEMA and PVP molecules.     

Impulse Excitation Technique IET as a non-destructive method for determining changes during the gelcasting process

Gelcasting is a good method for obtaining ceramic components with a pre-defined shape. From the point of view of the course of the ceramic casting process, a very important issue is the so-called idle time or gelation time whose measurement is usually based on determining changes in ceramic slurry viscosity by means of rheometers. However, searches are being continued to find methods which would have the least impact on the interactions between ceramic powder particles and the forming polymeric structure and would allow more time range to observe changes in gelcasting process compared to classic measurements of viscosity.In the article, based on measurements on Al₂O₃, ZrO₂ and SiC-based gelcasting systems with various water-soluble acrylic monomers, it has been demonstrated that such requirements may be fulfilled by IET (Impulse Excitation Technique).     

Development of alumina primary coat for single crystal investment casting ceramic mould

Abstract

The use of an alumina primary coat to eliminate the interaction between aluminium in NiAl single crystal alloy and silica in the ceramic investment casting mould has been examined in this study. The properties of the alumina slurry were characterised and the main factors controlling stability identified. It was found that the pH of the slurry must be carefully controlled to avoid polycondensation reactions. The most stable slurry was produced using filler with lower mean particle size and alkalinity. Alumina primary coat produced with low filler loading does not survive completely during wax removal. The surface condition can be improved by increasing the plate weight, which corresponds to a higher filler loading in the slurry. Moulds with a consistent alumina primary coat exhibit slightly higher green and sintered strength than moulds with the standard zircon-silica primary coat.     

Fabrication of textured ferroelectric ceramics by magnetic alignment via gelcasting

Grain-oriented ferroelectric ceramics have attracted more interest recently because they may provide near single crystal properties. In the present study, a novel process combining magnetic alignment and gelcasting was explored to prepare grain-oriented ferroelectric ceramics with different crystal structures. In a strong magnetic field, ceramic particles in slurry were aligned by the magnetic force and then locked in situ by polymerization via a gelcasting technique. This process was found effective for ferroelectric ceramics with a bismuth layer structure (Bi₄Ti₃O₁₂) and tungsten bronze structure (Sr_0.5Ba_0.5Nb₂O₆). The sintered samples show highly anisotropic structure and enhanced physical properties. However for perovskite structured ferroelectric ceramics (BaTiO₃), the green compact shows grain orientation, while after sintering the sample become random again.Thus for certain materials using the conventional ceramic processes, i.e., using conventional starting powders, gelcasting under strong magnetic fields (10 T) and pressure-less sintering, the preparation of dense grain-oriented ceramic materials is possible.     

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

Silica-based ceramic cores are extensively used in investment casting process, during which they must exhibit sufficient flexural strength and deformation resistance. In this study, micro-sized mullite was used as an additive to silica-based ceramic cores to optimize their high temperature properties. To investigate the effects of micro-sized mullite on cristobalite crystallization, mechanical and thermal properties of silica-based ceramic cores, ceramic cores with different amounts of micro-sized mullite were fabricated. The XRD results showed that additional micro-sized mullite diminished the crystallization of cristobalite at high temperatures, primarily caused by the mullite related compressive stresses on the surface regions of fused silica particles. Three-point bending tests and SEM results showed that micro-sized mullite had a more significant effect on the flexural strength of ceramic cores compared with conventional additives. Particularly, the fracture mechanism of silica-based ceramic cores had been changed from intergranular fracture into a mixed fracture consisting of both intergranular and transgranular fracture. The mechanical and thermal properties of ceramic cores were all reduced slightly as the mullite content exceed 4.6 wt%. Hence, to optimize the properties of silica-based ceramic cores, the micro-sized mullite content should not exceed 4.6 wt%.     

Silica-based ceramic core for aviation applications: Facile pore filling and flexural strength improvement

The effect of alumina addition on the pore filling process, crystallization behavior and mechanical properties of silica-based ceramic cores was investigated. The sintered samples at 1250°C were treated at 1550°C for 0.5 hour in order to simulate a casting process condition. The microstructure analysis indicated that an appropriate alumina addition enhanced the pore filling process by supplying a substantial liquid viscous flow. With an increase in the liquid viscous flow, smaller pores were filled first, and larger pores were filled later. The results also indicated that the pore filling process could be enhanced by adding small powder additives to decrease the pore size between the particles in the ceramic material. However, an excessive alumina addition in the silica-based ceramic cores had a negative effect on the flexural strength and leaching rate. As the alumina content increased from 8 to 12 wt%, the flexural strength of the ceramic core decreased from 16.37 to 4.60 MPa, respectively, and the leaching rate also decreased obviously. These results were explained by an acceleration in crystallization trend of the fused silica particle surface and the merging of connected pores in the sintered body.     

Researches on the pyrolyzing strength of gelcasting Al2O3-based ceramic molds for double-wall blade

An approach using AM-MBAM and Na-alginate double-gel was proposed to avoid the cracking of ceramic impingement holes cores (CIHCs) during pyrolysis processing. Finite element analysis (FEA) was used to calculate the thermal stress that caused CIHCs cracking. For ceramic slurry with a double-gel containing various proportions of alginate, the addition of 15.0 wt.% alginate was the most beneficial for the rheological and mechanical properties of Al₂O₃-based ceramic obtained by aqueous gelcasting based on the double-gel network had a dramatically pyrolyzing strength of 14.1 MPa, which was 72.0 % higher than that of the ceramics based on the single AM-MBAM system. The double-gel network was responsible for the improved strength. By comparing the thermal stress with pyrolyzing strength, it was predicted that the available diameter of CIHCs was greater than 0.7 mm. Finally, CIHCs with a minimum diameter of 0.7 mm were successfully fabricated, consistent with the FEA prediction.     

Effect of fine alumina in improving refractoriness of ceramic shell moulds used for aeronautical grade Ni-base superalloy castings

This paper discusses an improvement in shell refractoriness and dimensional stability of columnar grained (CG) low pressure turbine blade castings made using Ni base superalloy by directional solidification process (DS). Two ceramic shell systems were adopted, namely shell system I and II. Shell moulds were prepared by using ceramic slurries containing zircon flour as a filler material and colloidal silica as a binder. As compared to shell system II (zircon filler with colloidal silica binder and fine alumina), shell system I (zircon filler with colloidal silica binder) showed lower refractoriness. Shell system II showed an increase in the flexural strength both in the green as well as in fired conditions. Shells made from shell system II showed about 13% higher green strength and 55% higher fired strength as compared to shell system I. Shell system II also exhibited superior self sag resistance up to 1625?°C. Moulds prepared from this shell system yielded aeronautical grade casting with high dimensional accuracy even at a metal pouring temperature of 1550?°C. Moulds from shell system I, on the other hand, underwent sagging even at metal pouring temperature of 1500?°C, leading to dimensionally unacceptable castings. The superior performance of shells prepared from shell system II can be ascribed to the presence of fine alumina in the shell.     

Silica strengthened alumina ceramic cores prepared by 3D printing

Ceramic cores based on alumina and silica are important in the manufacturing of hollow blades. However, obtaining good properties and precision is still challenging. In this research, alumina-based ceramics cores were obtained by 3D printing technology, and the effects of silica contents on the mechanical properties of the as-obtained alumina ceramic cores were evaluated. The results showed significant improvements in flexural strengths of the ceramics from 13.3 MPa to 46.3 MPa at silica contents from 0 wt% to 30 wt% due to formation of mullite phase (Al₆Si₂O₁₃). By contrast, the flexural strengths declined as silica content further increased due to the generation of massive liquid phase. Also, porous structures and cracks were observed by scanning electron microscopy due to the removal of cured photosensitive resin and the mullitization reaction between alumina and silica, respectively. The manufacturing process of hollow blades required ceramic cores with flexural strengths greater than 20 MPa to resist the strike of metal liquid, as well as open porosity above 20 % to provide space for alkali liquor to dissolve the ceramic cores. As a result, 10 wt% silica was determined as the optimal value to yield ceramics with improved properties in terms of flexural strength (35.6 MPa) and open porosity (47.5 %), thereby satisfy the application requirement for the fabrication of ceramic cores.     

Self-supported ceramic substrates with directional porosity by mold freeze casting

Manufacture of thin-film ceramic substrates with high permeability and robustness is of high technological interest. In this work thin (green state thickness ∼500 μm) porous yttria-stabilized zirconia self-supported substrates were fabricated by pouring stable colloidal aqueous suspensions in a mold and applying directional freeze casting. Use of optimized suspension, cryoprotector additive and mold proved to deliver defect free ceramic films with high dimensional control. Microstructure analysis demonstrated the formation of desirable aligned porosity at macro-structural scale and resulted to be highly dependent on colloidal behaviour and freeze casting conditions. Manufactured green films were joined by lamination at room temperature and sintered to obtain symmetrical cells consisting of two porous self-supported substrate electrodes (∼420 μm) and dense yttria stabilized zirconia electrolyte (∼10 μm).