Microstructure and properties evolution of silicon-based ceramic cores fabricated by 3D printing with stair-stepping effect control

A ceramic core is the key component in the manufacture of the hollow turbine blades of aeroengines. Compared with the traditional injection molding method, 3D printing is more suitable for manufacturing ceramic cores with a complex geometry at high precision. However, the stair-stepping effect is inevitable in the 3D printing process and affects the surface roughness and strength of the ceramic core. In this study, to explore the influence of nano-silica content on the microstructure and properties of the ceramic core, silicon-based ceramic cores were fabricated with the addition of nano-silica powder by digital light processing and subsequent sintering at 1200 °C. The results showed that the apparent porosity and pore size of the ceramic core gradually decreased as both the nano-silica powder content and bulk density increased. Meanwhile, the printing interlayer spacing was significantly reduced, resulting in a low surface roughness, high flexural strength, and creep-resistance. To simulate the entire casting process of a superalloy blade, the thermal deformation behavior of the ceramic core was observed by heating and cooling cycles performed in a thermal dilatometer at 1540 °C. The total linear shrinkage decreased as the nano-silica powder content increased, which was mainly due to the phase transformation of cristobalite and the densification of the ceramic core sintered at 1200 °C. The low surface roughness and linear shrinkage as well as high flexural strength of the ceramic core can contribute to the excellent quality of cast superalloy blades.     

Influence of debinding holding time on mechanical properties of 3D-printed alumina ceramic cores

Herein, alumina green bodies are fabricated by three dimensional (3D) printing technology, then, the influence of debinding holding time under vacuum and argon on mechanical properties is systematically investigated by comparing the changes in microstructure, bulk density, open porosity, grain connection situation and flexural strength of ceramics. The flexural strength of alumina ceramics acquired the maximum values of 26.4 ± 0.7 MPa and 25.1 ± 0.5 MPa after debinding under vacuum and argon for 120 min and 180 min, respectively. However, the alumina ceramics rendered the flexural strength of 19.4 ± 0.6 MPa and 9.5 ± 0.4 MPa under vacuum and argon without extended holding time, respectively. The relatively low mechanical properties can be mainly attributed to the weak interlayer binding force, which is caused by layer-by-layer forming mode during 3D printing process and anisotropic shrinkage during the sintering process. Moreover, the alumina ceramics exhibited moderate bulk density and open porosity of 2.4 g/cm³ and 42% after the sintering process, respectively, which are mainly influenced by the microstructural evolution of alumina ceramics during thermal treatment. Also, the diffusion of gases is achieved by curing of photosensitive resin and influenced by different holding times during debinding, affecting the mechanical properties of sintered ceramics. The mechanical properties of as-sintered ceramics are suitable for the utilization of ceramic cores in the manufacturing of hollow blades.     

3D printing of complex shaped alumina parts

Alpha-alumina powder was mixed with methyl cellulose as a binder with concentration as low as 0.25% by weight in an aquoes medium and kneaded in a high shear mixer to obtain a printable paste. The paste was subjected to rheological measurements and exhibited a shear rate exponent of 0.54 signifying the shear thinning behavior. The paste was used for printing parts with various shapes according to CAD model by employing a ram type 3D printer. Printed parts were dried and the green density was determined. Further, the parts were also subjected to X-ray radiography in order to evaluate the possible occurrence of printing defects. The samples were sintered under pressureless condition at 1650?°C in a muffle furnace and Hot Isostsically Pressed (HIP) at 1350?°C and a pressure of 1650?bar using a vacuum encapsulated SS CAN. Hot Isostatic pressing resulted in a higher density of 3.94?g/cc in comparison to 3.88?g/cc obtained under pressureless conditions and also shown superior mechanical properties. HIPing of 3D printed samples not only resulted in possible healing of printing defects as reavealed by X-ray radiography but also enhanced the diffusion at low temperature of 1350?°C leading to finer grain sizes as complemented by the microstructure.     

Binder stabilization and rheology optimization for vat-photopolymerization 3D printing of silica-based ceramic mixtures

Enhancing inlet gas temperature in aero/gas turbines to reduce their carbon-footprint, has led to a strive for better performing inlet cooling mechanism of the turbine blades. The internal cooling of the blades is made by ceramic cores in their casting process, but conventional ceramic molding has long reached its maximum possible geometrical complexity, hence shedding light on 3D printing of these cores. The objective of this study is to develop low-viscous, fully stabilized, commercially viable ink for vat-photopolymerization of silica-based ceramics. This paper investigates the best dispersion type and amount for different formulated monomer mixtures, and explains the best correlation between viscosity, solid loading, binders, dispersants, peeling forces and mechanical properties, and offers an optimized mixture to avoid the common ceramic printing issue, namely crack propagation of cores during sintering. Among five dispersant agents, the SOL20, SOL24 and FA4611 exhibited better performance than other dispersion agents, and the optimum concentration level for each binder and dispersant agent was ensured through sedimentation test. Their dispersion capability and long-term stability were further investigated to designate the best dispersion agent for each binder system. Further verification was made by sedimentation study of the samples at 40 °C for 40 days and reducing the superficial area of the used powder mixture. According to the result of the rheology analysis, the best dispersions were achieved using SOL20 for the loaded binder mixtures of M1 and M4, SOL24 for M3and FA4611 for M2. The instability of M1 and M2 with their respective dispersant agent was coordinated through the thixotropic agent of TX/2, and complete stabilization and near-Newtonian behavior were achieved. However, the research showed that the addition of TX/2 to fully stabilized M4 and M2 suspensions negatively impacts the mixtures’ rheological behavior from near-Newtonian to shear-thickening. In the final stage of this study, peeling forces, sintering and three-point bending tests were conducted to determine the final formulated suspension to print ceramic core components. M4 and SOL20 combination was selected for SiO₂-ZrSiO₄ loading and dispersing, respectively. The impact of solid loading between the range of 58 and 65 vol% on the rheological behavior of the final suspension and the mechanical properties of sintered bodies were investigated to assign an optimum solid rate. The adequate strength on sintered and degree of viscosity for ceramic vat-polymerization processing was achieved at 58 vol%. Lastly, a validation study is conducted by printing a complex ceramic core model by a commercial LCD hobby printer. This validation shows the significance of this study to scale up the manufacturing of complex-shaped ceramic cores and to revolutionize the sector, by printing inexpensive and readily available irregular-shaped (non-atomized) ceramic powder, using the most cost-effective LCD printers (non-specialized expensive ceramic printers).     

Transparent alumina ceramics fabricated by 3D printing and vacuum sintering

Transparent alumina ceramics were fabricated using an extrusion-based 3D printer and post-processing steps including debinding, vacuum sintering, and polishing. Printable slurry recipes and 3D printing parameters were optimized to fabricate quality green bodies of varying shapes and sizes. Two-step vacuum sintering profiles were found to increase density while reducing grain size and thus improving the transparency of the sintered alumina ceramics over single-step sintering profiles. The 3D printed and two-step vacuum sintered alumina ceramics achieved greater than 99 % relative density and total transmittance values of about 70 % at 800 nm and above, which was comparable to that of conventional CIP processed alumina ceramics. This demonstrates the capability of 3D printing to compete with conventional transparent ceramic forming methods along with the additional benefit of freedom of design and production of complex shapes.     

3D printing of fine alumina powders by binder jetting

Additive manufacturing of ceramics is still at an early-development stage; however, the huge interest in custom production of these materials has led to the development of different techniques that could provide highly performing devices. In this work, alumina (α-Al₂O₃) components were produced by binder jetting 3D printing (BJ), a powder-based technique that enables the ex-situ thermal treatment of the printed parts. The employment of fine particles has led to high green relative density values (>60 %), as predicted by Lubachevsky-Stillinger algorithm and DEM modelling. Then, extended sintering has been observed on samples treated at 1750 °C that have reached a final density of 75.4 %. Finally, the mechanical properties of the sintered material have been assessed through bending test for flexural resistance and micro-indentation for Vickers hardness evaluation.     

Inkjet printing and nanoindentation of porous alumina multilayers

The objectives of this study were to analyse the effect of inkjet 3-D printing parameters, particularly the splat overlap distance, for the fabrication of defect-free porous Al₂O₃ ceramic multilayers, and to correlate the resulting porosities with the mechanical properties measured using nanoindentation. An aqua-based alumina ink was used in this study to fabricate the multilayers on dense alumina substrates by inkjet printing. The as-printed specimens were dried and sintered at 1200–1500 °C. The resulting microstructural features of each specimen and their corresponding porosities were studied using FIB-SEM. Elastic modulus and hardness were determined using the spherical nanoindentation technique. Results showed that defect-free porous alumina multilayers with excellent layer to layer and layer to substrate integrity were successfully fabricated. The porosity-dependence of the elastic modulus and hardness was shown to be consistent with values predicted using empirical expressions, despite the presence of abnormal grain growth at higher temperatures.     

Preparation of high solid loading and low viscosity ceramic slurries for photopolymerization-based 3D printing

In this study, high solid loading and low viscosity cordierite slurries are successfully developed for the first time for photopolymerization-based additive manufacturing. The processability of the slurries is mainly determined by their rheological properties and photocuring parameters. The slurry preparation involves the orthogonal optimization of compositions in order to achieve suitable viscosity, stability and homogeneity. The photocuring parameters of the as-prepared slurries, including penetration depth D_p and critical exposure E_c, are also determined experimentally. Results show that viscosity increases with reduction in particle size. A higher solid-volume fraction also results in an exponential growth in viscosity. As for the dispersant amount, a concentration of 5?wt% leads to the lowest viscosity. Particle size also play an important role in the solid loading capacity of the slurries, as results suggest that smaller particles improve performance. In terms of the photocuring behaviors, the addition of 2?wt% photoinitiator generates an optimal curing process. 40?vol% solid loading leads to the thickest curing depth for all slurries with different types of particle sizes. Finally, a cordierite part with a complex hollow structure and a fine resolution is successfully fabricated. The present study offers a material basis for the polymerization-based 3D printing of porous cordierite structures.     

Sol-gel infiltration of complex cellular indirect 3D printed alumina

Gelled aqueous solutions containing the soluble precursor aluminum chlorohydrate were developed for the infiltration of porous indirect 3D printed alumina. Viscosity and amplitude sweep tests confirmed the gel formation and sheer thinning behavior beneficial for the subsequent coating and infiltration process. High temperature XRD confirmed the formation of corundum at a temperature of 1000?°C. Complex alumina structures with high surface area and isotropic pore channels were achieved by indirect 3D printing. Coating and infiltration of the pre sintered alumina with a subsequent sintering step transformed the precursor to corundum and partially filled the residual porosity and decreased surface defects after 3D printing. With the gel coating a pronounced improvement up to a maximum value of Δσ_compr?=?61.9?MPa was observed and with the gel infiltration a maximum improvement of ΔE?=?136.2?GPa. The results show the possibility to infiltrate even complex alumina structures with aqueous alumina precursors without the need to disperse ceramic particles.     

Fabrication and characterization of carbon fiber reinforced SiC ceramic matrix composites based on 3D printing technology

A novel method has been developed to fabricate carbon fiber reinforced SiC (C_f/SiC) composites by combining 3D printing and liquid silicon infiltration process. Green parts are firstly fabricated through 3D printing from a starting phenolic resin coated carbon fiber composite powder; then the green parts are subjected to vacuum resin infiltration and pyrolysis successively to generate carbon fiber/carbon (C_f/C) preforms; finally, the C_f/C preforms are infiltrated with liquid silicon to obtain C_f/SiC composites. The 3D printing processing parameters show significant effects on the physical properties of the green parts and also the resultant C_f/C preforms, consequently greatly affecting the microstructures and mechanical performances of the final C_f/SiC composites. The overall linear shrinkage of the C_f/SiC composites is less than 3%, and the maximum density, flexural strength and fracture toughness are 2.83?±?0.03?g/cm³, 249?±?17.0?MPa and 3.48?±?0.24?MPa m^1/2, respectively. It demonstrates the capability of making near net-shape C_f/SiC composite parts with complex structures.     

Relationship between mechanical properties of ceramic green body and structures of photo-cured acrylate polymer for ceramic 3D printing based on photo polymerization

3D printing technologies using photo polymerization of photo-curable monomer and oligomers as an organic binder have been studied to fabricate ceramics with a complicated shape. For minimizing distortion of ceramics during the manufacturing process and fast 3D printing of ceramic green body, high mechanical strength of ceramic green body and fast photo polymerization of the photo-curable resin are required, respectively. In this study, the ceramic green bodies with various compositions were fabricated by photo radical polymerization of the slurry-type resin composed of fused silica bead, and tri- or (and) mono-functional acrylate. Photo radical polymerization behaviors of mono-, tri-functional acrylate monomer, and blend of two monomers were analyzed by photo-DSC and FT-IR measurements. The structure analysis of photo-cured polymers made by each monomer was performed by thermo-mechanical analysis. Through mixing of mono- and tri-functional acrylate monomer, we confirmed that polymerization rate more increased compared with those of only mono-, tri-functional monomer. Unreacted vinyl groups in the polymers prepared with blend of two monomers decreased by an addition of mono-functional monomer in tri-functional monomer. The polymers prepared with the blend showed higher storage modulus and broader viscoelastic behavior compared to those fabricated with tri-functional monomer. Thus, to achieve high fracture strength of the green body, we verified that the photo-cured polymer in the green body should have high crosslinking density and low free volume based on reduction of unreacted vinyl groups in the polymer. Additionally, through analysis of cross-sectional SEM of the green body, we confirmed that acrylate monomer should include proper content in the slurry-type resin to maximize the fracture strength of the body regardless of the type of the used acrylate monomer. This was because low content of acrylate monomer in the slurry-type resin makes it difficult to form neckings composed with photo-cured polymers between the silica beads.     

Dimensional retention of photocured ceramic units during 3D printing and sintering processes

In this study, photocuring-based digital light processing (DLP) 3D-printing technology was used to prepare basic photocuring units from cordierite ceramic slurries loaded with three different average particle sizes. Exposure time was varied to realize a range of ultraviolet light-energy dosages. Basic units, including single lines, apertures, and single walls, were printed with different feature dimensions such as single-line width and thickness, aperture diameter, and single-wall thickness. The morphologies and structures of the units were studied after printing and sintering. Their dimensions were measured, and the relative and absolute differences before and after each processing step were calculated. The dimension-retention levels were finally determined and analyzed across the ranges of slurries, exposure times, and designed dimensions. Detailed insights into the printing and sintering behaviors and performances of the ceramic slurries and printed units were gained. This study contributes to the understanding and analysis of potential dimensional errors and the printed and sintered quality of ceramic components prepared based on photocuring-based DLP 3D printing.     

Multi-material ceramic material extrusion 3D printing with granulated injection molding feedstocks

Material Extrusion (MEX) is an advanced technology for polymer 3D printing and countless printers are commercially available. MEX has also been demonstrated for ceramics. For that purpose, thermoplastic binders are filled with high loads (>40 vol%) of a ceramic powder. The printed parts are subsequently debound and sintered. In contrast to most MEX printers, the ceramic printer presented herein works with granulated feedstock instead of filaments. Therefore, the development of novel feedstocks is faster and more straightforward since the challenges associated with filament production are omitted. Furthermore, commercial ceramic injection molding (CIM) feedstocks can be used which allows fast prototyping with the same material that is later used in high-quantity industrial production by CIM.In this study, a method to fabricate multi-material ceramic parts using a granulate-fed printer is presented. Examples of multi-material printing include colored ZrO₂ parts as well as ceramic high-temperature heating elements in various shapes consisting of an electrically conductive and a non-conductive component. Light- and electron microscopy confirms that the layer adhesion before and after sintering is flawless, even between different materials if the material combination is chosen carefully. All feedstocks are based on a commercially available CIM binder filled with the desired ceramic powder. Consequently, the feedstock preparation as well as optimizing of debinding and sintering conditions are simple and reproducible.     

Effect of synergism of solid loading and sintering temperature on microstructural evolution and mechanical properties of 60 vol% high solid loading ceramic core obtained through stereolithography 3D printing

Solid loading has a significant effect on the curing behavior of slurry and the microstructure and properties of the ceramic core. A high-solid loading slurry can effectively improve the sintering densification of ceramic particles and improve the interlayer bonding strength and mechanical properties at both 25 °C room and higher temperatures. Herein, based on the photopolymerization theory of ceramic slurry, the solid loading was increased from 45 to 60 vol% by adjusting the composition ratio of the resin ceramic powder. Additionally, the optimal sintering temperature of the 60 vol% solid loading ceramic core was 1200 °C. The synergistic effect of the solid loading and sintering temperature controls the sintering shrinkage of the sample within 3.2%; the porosity, high temperature, and room temperature flexural strength were approximately 30%, 24 MPa, and 10 MPa, respectively. The printing preparation of high-solid loading ceramic cores can be used to guide optimizing process parameters on an industrial scale.     

陶瓷光固化3D打印技术研究进展及应用

与传统加工方法相比,光固化3D打印技术具有个性化、定制化、高分辨率等优点,可满足陶瓷精细结构的成型,在陶瓷材料加工方面展示出很大的潜力.这里首先介绍了光固化3D打印技术及常见的陶瓷材料,从陶瓷浆料制备、素坯热处理工艺方面进行讨论.同时对该技术在生物医学领域特别是在骨科、齿科中的应用进行总结.     

三维打印生物材料的发展现状

随着组织工程和再生医学不断进步,3D打印技术被用于开发和制造由仿生天然和合成材料组成的仿生支架。讨论3D打印"生物墨水"的进步和发展趋势,对3D打印多功能生物材料的发展具有积极意义。     

Fabrication of porous fibrous alumina ceramics by direct coagulation casting combined with 3D printing

A novel forming method for preparing porous alumina ceramics using alumina fibers as raw materials by direct coagulation casting (DCC) combined with 3D printing was proposed. Porous fibrous alumina ceramics were fabricated through temperature induced coagulation of aqueous-based DCC process using sodium tripolyphosphate (STPP) as dispersant and adding K₂SO₄ as removable sintering additives. The sacrificial coated sand molds was fabricated by 3D printing technology, followed by the infiltration of silica sol solution for the subsequent suspension casting. Stable alumina suspension of 40 vol% solid loading was obtained by adding 2.0 wt% STPP and 40 wt% K₂SO₄. The controlled coagulation of the suspension could be realized after heating at 90 °C for about 35 min. The ceramic sample sintered at 1450 °C for 2 h showed the highest compressive strength of 24.33 MPa with porosity of 57.38%. All samples sintered at 1300–1450 °C had uniform pore size distributions with average pore size of 7.2 µm, which indicated the good structure stability when sintered at high temperature.