Effect of nonuniform sintering on mechanical and thermal properties of silica-based ceramic cores

During sintering of the silica-based ceramic core of turbine blades,a phenomenon called"nonuniform sintering"occurs that negatively affects the thermal and mechanical properties of the core.Standard samples of silica-based core were prepared by an injection molding method and sintered with alumina backfilling powder with different sodium contents.The effect of sodium content on the nonuniform sintering of silica-based cores and the thermal and mechanical properties was evaluated.Results show that the sintering level and the content ofα-cristobalite in the surface layer are significantly higher than that of the sample interior.A considerable number of microcracks are found in the surface layer due to theβtoα-phase transition of cristobalite.As the sodium content in the alumina powder decreases,the level of the nonuniform sintering and the amount of crystallized cristobalite in the surface layer decrease,which is beneficial to the thermal expansion and flexural strength at ambient temperature.The flexural strength and thermal deformation at high temperature are improved by reducing the surface cracks,but deteriorated with the decrease of the cristobalite crystallization when the surface cracks are macroscopically invisible.

Fabrication of silica-based ceramic cores with internal lattice structures by stereolithography

Ceramic cores are widely used in investment casting,and ideal properties of cores are essential for high-quality castings.Under the circumstances requiring thick cores,solid cores are likely to encounter deformation and cracking defects due to the accumulation of shrinkage.Therefore,with the superiority of ceramic stereolithography in producing complex ceramic parts,hollow cores with lattice structures were designed and fabricated.The dimensional accuracy and properties of the green and sintered bodies were evaluated.Results show the dimensional accuracy of sintered cores is controlled within±0.25 mm benefited from the precise green bodies.The mechanical properties are not obviously deteriorated.The bending strength reaches 11.94 MPa at room temperature and 12.87 MPa at 1,500℃ with a creep deformation of 0.345 mm.Furthermore,casting verifications prove that the hollow cores meet the requirements of investment casting.Smooth casting surfaces are obtained,at the same time,the core-removal efficiency is improved by over 3 times.

Microstructure and properties of silica-based ceramic cores by laser powder bed fusion combined with vacuum infiltration

The silica-based ceramic core has attracted much attention in the preparation of hollow blades due to its great leachability.In this paper,the silica-based ceramic cores reinforced with ZrSiO_(4) were prepared by laser powder bed fusion(LPBF)combined with vacuum infiltration(VI).To enhance the infiltration effect,the pre-sintered bodies with high porosity and hydrophilicity were obtained by pre-sintering at 1100℃.Results showed that a large number of silica particles infiltrated into the pre-sintered bodies.The infiltrated silica promoted the generation of liquid phase in sintering,thereby promoting the removal of pores and the connection of grains.Nevertheless,the dispersed ZrSiO_(4) grains prevented the viscous flow of the liquid phase,thereby increasing the porosity.ZrSiO_(4) grains could hinder the propagation of cracks due to their high strength.When the addition of ZrSiO_(4) was 10 wt.%,room-temperature flexural strength of silica-based ceramic cores infiltrated with slurry S1(the mass ratio of silica sol to silica powder was 10:1)reached 17.21 MPa due to the reinforcement of sintering necks.Moreover,high-temperature flexural strength reached 13.90 MPa.Therefore,the pre-sintering process could greatly improve the mechanical properties of silica-based ceramic cores prepared by LPBF-VI technology.

Effects of Alumina on Cristobalite Crystallization and Properties of Silica-Based Ceramic Cores

In this work, the influences of alumina addition on cristobalite crystallization and properties of injec- tion molded silica-based ceramic cores were investigated. X-ray diffraction （XRD） was used to characterize phase transformations in the samples, and the XRD result indicated that the addition of alumina pro- moted crystallization of fused silica during sintering at 1180-1220 ℃ and thus increases the amount of cristobalite. The increased amount of cristobalite as well as alumina addition led to much more thermal dilation due to their higher coefficients of thermal expansion than that of fused silica. The flexural strengths at room temperature and 1500 ~C were tested, and it was shown that alumina addition could not affect room temperature strength, but decreased the flexural strength at 1500 ℃. In addition, deflection resis- tance during heating to high temperatures was investigated, and the result indicated that alumina addition speeded up high temperature softening of the samples. XRD and scanning electron microscopy equipped with energy dispersive spectrometry （SEMJEDS） analysis suggested that this softening behavior was related with viscous flow sintering which could be accelerated by the reaction of alumina and silica with a product of mullite.

Improved mechanical properties of SiC fiber reinforced silica-based ceramic cores fabricated by stereolithography

Silica-based ceramic cores have been widely used to fabricate aero-engine hollow blades due to their moderate high temperature mechanical properties and excellent leachability.In this study,silica-based ceramics with SiC fiber addition were prepared via stereolithography,and the influence of SiC fiber content on mechanical properties of the obtained silica-based ceramics was investigated.With the increase of SiC fiber content,linear shrinkage gradually decreased,while room temperature flexural strength and high temperature flexural strength first increased and then decreased.As SiC fiber content increased to 4.0 wt%,linear shrinkage was reduced to 0.62%resulting from the oxidation of SiC.Furthermore,room temperature flexural strength was improved from 11.79 MPa to 23.83 MPa and high temperature flexural strength was enhanced from 15.64 MPa to 34.62 MPa with 4.0 wt%SiC fiber addition due to the reinforcement of fibers and the enhancedβ-cristobalite content,which meets the need of ceramic cores.Therefore,it demonstrates the capability of fabricating high-performance and high-precision silica-based ceramic cores reinforced by SiC fibers via stereolithography for rapid manufacturing of hollow blades.

Influence of Al_(2)0_(3) content on mechanical properties of silica-based ceramic cores prepared by stereolithography

Silica ceramic cores have played an important part in the manufacture of hollow blades due to their excellent chemical stability and moderate high-temperature mechanical properties.In this study,silica-based ceramics were prepared with Al_(2)0_(3) addition by stereolithography,and the influence of Al_(2)0_(3) content on mechanical properties of the silica-based ceramics was investigated.The Al_(2)0_(3) in silica-based ceramics can improve the mechanical properties by playing a role as a seed for the crystallization of fused silica into cristobalite.As a result,with the increase of Al_(2)0_(3) content,the linear shrinkage of the silica-based ceramics first decreased and then increased,while the room-temperature flexural strength and the high-temperature flexural strength first increased and then decreased.As the Al_(2)0_(3) content increased to 1.0 vol%,the linear shrinkage was reduced to 1.64%because of the blocked viscous flow caused by Al_(2)0_(3).Meanwhile,the room-temperature flexural strength and the high-temperature flexural strength were improved to 20.38 and 21.43 MPa with 1.0 vol%Al_(2)0_(3),respectively,due to the increased a-cristobalite and P-cristobalite content.Therefore,using the optimal content of Al_(2)0_(3) in silica-based ceramics can provide excellent mechanical properties,which are suitable for the application of ceramic cores in the manufacturing of hollow blades.

Properties of alumina-based ceramic cores

Fused corundum is a rather promising raw material for preparing an alumina-based ceramic core due to its excellent high temperature resistance and chemical inertness.In this study,alumina-based ceramic cores were prepared using fused corundum as the matrix material,and the effect of varying silica powder contents on the properties of the alumina-based ceramic cores,including the sintering shrinkage,the flexural strength,and the high temperature deformation was investigated.The mineralization mechanisms of the silica on the alumina-based ceramic core were also analyzed.The optimum addition amount of silica in this experiment is 8% in weight.At that moment,the aluminum-based core has both a low sintering shrinkage coefficient of 0.66% and better properties:the room temperature flexural strength is 22.19 MPa,the high temperature flexural strength is 21.54 MPa,the high temperature deformation is 0.93 mm,and the residual flexural strength is 47.41 MPa.

Manufacturing process of water-soluble salt-based ceramic cores based on vat photopolymerization

Water-soluble salt-based ceramic cores can be recycled and have excellent high-temperature chemical stability.In this work,vat photopolymerization was successfully applied to water-soluble salt-based ceramic cores for the first time.The powder raw materials of the printing suspension were sodium chloride and alumina.High-precision green bodies were manufactured by optimizing suspensions and parameters.In addition,the postprocessing method was optimized according to the microstructure and mechanical properties.The sintered part had a high bending strength and smooth surface.Finally,the dissolution rate and moisture resistance were compared under different dissolution and storage conditions.Compared to traditional manufacturing methods,vat photopolymerization enables the production of complex structures without molds and reduces production costs.This technology is suitable for the rapid iteration of complex structural parts and can be applied to precision parts in aerospace,military,and other technical fields with high cost-effectiveness and sustainability.

Microstructure of Al_2O_3/SiO_2 ceramic core nano-composites

Al2O3/SiO2 ceramic core nano-composites were prepared and their microstructure was investigated by transmission electron microscope（TEM）. The results show that intergranular nano-composites are achieved. The bonding between Al2O3 and SiO2 particles is well and the interface is even. Amorphous phases and nano crystals appear in the Al2O3/SiO2 ceramic core nano-composites, which both come into being during the cooling process after sintering. Glass phase does not appear between the Al2O3 and SiO2 particles and only appears among the Al2O3 particles, which can be explained with stress model. The quantity of the glass phase is not much and its influence on the high-temperature deformation of the ceramic core nano-composites is little.

Difference in particle characteristics and coating properties between spraying metallic and ceramic powder cored wires

The sprayed particles of metallic and cermet wires were collected to analyze the atomization state of the particles in arc spraying forming, the microstructure and properties of metallic and ceramic coatings were investigated and compared. Particle size analyzer was used for quantifying particle size. The XRD, SEM and optical microscope(OM) were used to analyze the phase composition and microstructure of the particles and coatings. From the experimental results, some difference of particle characteristics was established between the spraying metallic and ceramic cored wires, and the microstructure and properties of coatings depend strongly on the particles behaviors. The result shows that Fe-TiB2/Al2O3 composite coating has a high potential for abrasive wear applications.

An improved method for quantitatively measuring the sequences of total organic carbon and black carbon in marine sediment cores

Understanding global carbon cycle is critical to uncover the mechanisms of global warming and remediate its adverse ef fects on human activities.Organic carbon in marine sediments is an indispensable part of the global carbon reservoir in global carbon cycling.Evaluating such a reservoir calls for quantitative studies of marine carbon burial,which closely depend on quantifying total organic carbon and black carbon in marine sediment cores and subsequently on obtaining their high-resolution temporal sequences.However,the conventional methods for detecting the contents of total organic carbon or black carbon cannot resolve the following specific difficulties,i.e.,(1)a very limited amount of each subsample versus the diverse analytical items,(2) a low and fluctuating recovery rate of total organic carbon or black carbon versus the reproducibility of carbon data,and(3)a large number of subsamples versus the rapid batch measurements.In this work,(i)adopting the customized disposable ceramic crucibles with the microporecontrolled ability,(ii)developing self-made or customized facilities for the procedures of acidification and chemothermal oxidization,and(iii)optimizing procedures and carbon-sulfur analyzer,we have built a novel Wang-Xu-Yuan method(the WXY method)for measuring the contents of total organic carbon or black carbon in marine sediment cores,which includes the procedures of pretreatment,weighing,acidification,chemothermal oxidation and quantification;and can fully meet the requirements of establishing their highresolution temporal sequences,whatever in the recovery,experimental efficiency,accuracy and reliability of the measurements,and homogeneity of samples.In particular,the usage of disposable ceramic crucibles leads to evidently simplify the experimental scenario,which further results in the very high recovery rates for total organic carbon and black carbon.This new technique may provide a significant support for revealing the mechanism of carbon burial and evaluating the capacity of marine carbon accumulation and sequestration.

Manufacturing of ceramic cores:From hot injection to 3D printing

With the improvement of aero-engine performance,the preparation of hollow blades of single-crystal superalloys with complex inner cavity cooling structures is becoming increasingly urgent.The ceramic core is the key intermediate part of the preparation and has attracted wide attention.To meet this challenge,new technologies that can make up for the defects of long periods and high costs of fabricating complex structural cores by traditional hot injection technology are needed.Vat photopolymerization 3D printing ceramic technology has been applied to the core field to realize the rapid preparation of complex structural cores.However,the industrial application of this technology still needs further research and improvement.Herein,ceramic cores were prepared using traditional hot injection and vat photopolymerization 3D printing techniques using fused silica,nano-ZrO_(2),and Al_(2)O_(3) powders as starting materials.The 3D printed ceramic core has a typical layered structure with a small pore size and low porosity.Because of the layered structure,the pore area is larger than that of the hot injection ceramic core,the leaching performance has little effect(0.0277 g/min for 3D printing cores,0.298 g/min for hot injection cores).In the X and Y directions,the sintering shrinkage is low(2.7%),but in the Z direction,the shrinkage is large(4.7%).The fracture occurs when the inner layer crack expands and connects with the interlayer crack,forming a stepped fracture in the 3D-printed cores.The bending strength of the 3D printed core at high temperature(1500℃)is 17.3 MPa.These analyses show that the performance of vat photopolymerization 3D-printed ceramic cores can meet the casting requirements of single crystal superalloy blades,which is a potential technology for the preparation of complex structure ceramic cores.The research mode of 3D printing core technology based on the traditional hot injection process provides an effective new idea for promoting the industrial application of 3D printing core technology.

Synthesis of Ti₃SiC₂-Bicarbide Based Ceramic by Electro-Thermal Explosion

A polycrystalline dense Ti3SiC2 based ceramic material has been produced by several techniques. The effect of addition of TiC and SiC is also studied. The Ti3SiC2 material shows extraordinary electrical, thermal and mechanical properties. Furthermore, it shows a damage tolerance capability and oxidation resistance. In this work, we have synthesized Ti3SiC2 by electro-thermal explosion chemical reaction (ETE) with high current density (900 Amperes/a.u) followed by uniaxial pressure. The structural properties of the as-prepared materials are studied by x-ray diffraction (XRD), scanning electron microscope (SEM) and energy dispersive x-ray spectroscopy (EDX) techniques. The chemical cartography, imaging and electronic properties are investigated using Ultra-STEM and electron high energy loss resolution spectroscopy (EELS) techniques, respectively. The surface of Ti3SiC2 is characterized by means of X-ray photoelectron spectroscopy (XPS). High resolution C 1s, Si 2p, Ti 2p, Ti 3s core level spectra are explained in terms of crystallographic and electronic structure. Valence band spectrum is performed to confirm the validity of the theoretical calculations.

Dimension effects on the dielectric properties of fine BaTiO_3 ceramics

It is found that the core-shell structured grains are easy to produce for fine grain doped BaTiO3 ceramics in the sintering process. We study the influence of the core-shell structure on the Curie-Weiss temperature and dielectric properties of BaTiO3 ceramics by using effective medium approximation （EMA）. Considering the second approximation, the dielectric properties of fine grain doped BaTiO3 ceramics are consistent with experimental data.

Effect of sintering temperature on microstructure and properties of 3D printing polysilazane reinforced Al_(2)O_(3)core

Ceramic cores are the key intermediate components of hollow blades for aero-engine.Conventional processes,such as hot-press molding and gel film casting,face difficulties in fabricating complex-structured ceramic cores due to the complexity of moulds and long process cycles.Stereolithography 3D printing provides a new idea for the fabrication of complex-structured ceramic cores.The effect of sintering temperature on open porosity,bulk density,weight loss rate,shrinkage rate,flexural strength and microstructure of the Al_(2)O_(3)-based ceramic core doped with 10vol.%polysilazane(PSZ)was studied.The sintering mechanism of PSZ-reinforced ceramic cores was analyzed.Results show that the optimum sintering temperature of PSZ-reinforced ceramic cores is 1,450°C.At this temperature,the open porosity of the ceramic core is 36.60%,bulk density is 2.33 g·cm^(-3),weight loss rate is 22.11%,shrinkage rate along the X,Y,Z directions is 5.72%,5.01%,9.61%,respectively;the flexural strength is 28.794 MPa at 25°C and 13.649 MPa at 1,500°C.Properties of 3D printing PSZ-reinforced ceramic cores can meet the casting requirement of superalloy hollow blades,which is expected to promote the industrial application of 3D printing complex structure ceramic cores.

黏结剂喷射制备陶瓷型芯抗弯强度、尺寸精度及表面质量影响因素的研究进展

黏结剂喷射因具有低成本和可控性优势而在陶瓷型芯的制备中得到广泛的应用,但制备陶瓷型芯的抗弯强度、尺寸精度和表面质量一般较低,而这些不足可以通过改变制备工艺参数来进行改进。综述了粉末、黏结剂、成形工艺和后处理工艺这4个方面的参数对陶瓷型芯抗弯强度、尺寸精度和表面质量的影响,提出了提高黏结剂喷射制备陶瓷型芯抗弯强度、尺寸精度和表面质量的未来发展方向。

CONSTITUTIVE EQUATIONS OF METAL-CORE PIEZOELECTRIC FIBERS

Metal-core piezoelectric fibers （MPFs） are one of the new type piezoelectric devices. To investigate the piezoelectricity and the mechanical properties of the piezoelectric fibers, the constitutive equations are established. It can describe the response of piezoelectric fibers subject to an axial force and an external voltage. A cantilever bar subject to a tip axial force and an external voltage on the electrodes is considered. The internal energy density in thermodynamic equilibrium is obtained. The total internal energy is calculated by integrating over the entire volume of the bar. The generalized displacement of the tip axial force is the tip elongation δ, and the generalized displacement of the voltage is the electrical charge Q on the electrodes. In the established constitutive equations, the excitation （input） parameters are the axial force and the external voltage, the response （output） parameters are the tip elongation and the electric charge. And the response parameters are related to the excitation parameters by a 2× 2 piezoelectric matrix. Finally, two experiments using MPF as a sensor or an actuator are performed to verify the constitutive equations. And experimental results are compared with analytical ones.

浸渗处理及真空度对喷射粘结成形氧化钙基陶瓷型芯性能影响

采用喷射粘结成形技术,以重质碳酸钙作为原料,浸渗纳米ZrO2分散液后烧结得到了氧化钙基陶瓷型芯,研究了浸渗处理以及浸渗真空度对陶瓷型芯性能的影响。研究发现,浸渗处理后的陶瓷型芯内部生成了大量CaZrO3,使内部孔隙减少,微观组织更加致密,弯曲强度最大增加幅度为277.87%。浸渗处理时增大真空度可以进一步改善陶瓷型芯的性能,当真空度从0增加至0.05 MPa时,弯曲强度由7.15 MPa提升到8.88 MPa,但当真空度进一步增加至0.08 MPa时,弯曲强度却降至6.85 MPa,陶瓷型芯性能降低。不同处理工艺烧结后得到的陶瓷型芯浸泡在热水中5 min后即开始溃散,表现出良好的可溶性。

3D打印陶瓷型芯研究进展

陶瓷型芯是成功制造空心叶片的关键之一,它决定着叶片的尺寸精度、合格率、制备成本。但传统制造技术存在工艺复杂、制备周期长等缺点。3D打印技术具有无需模具、快速成型、个性化定制三维零件的优势,逐渐被应用于复杂形状陶瓷型芯的制备。系统总结了粘结剂喷射、激光选区烧结、浆料直写成型、光固化成型4种主流3D打印技术在陶瓷型芯领域的研究进展,及脱脂-烧结工艺对3D打印陶瓷型芯性能的影响,最后对3D打印陶瓷型芯的未来发展进行了展望。

高温合金熔模精密铸造陶瓷型芯研究进展

陶瓷型芯广泛应用于飞机发动机涡轮叶片的熔模精密铸造中,以实现涡轮叶片复杂的空心结构。本文介绍了高温合金熔模精密铸造用陶瓷型芯的应用现状,概述了陶瓷型芯的制备工艺和常用基体材料的研究进展,分别对石英玻璃、电熔刚玉、锆英石、氧化钇等基体材料制备陶瓷型芯的优缺点和研究进展进行了概述。