Ceramic Mold Research Articles

Freeform injection molding of functional ceramics by hybrid additive manufacturing

Freeform Injection Molding (FIM) is a hybrid manufacturing approach where 3D-printed sacrificial polymeric molds are used for Ceramic Injection Molding (CIM). This technique offers great additive manufacturing capabilities with shape complexity and material versatility. In this paper, we present a thorough analysis of the ability of FIM to process a variety of ceramic feedstocks and evaluate samples of different geometries with increasing geometrical complexity. The materials selected are zirconia, alumina and Pb-free piezoelectrics (BaTiO 3 and (Bi, Na)TiO 3 – BaTiO 3 ). Injection molding simulations are used to optimize the processing parameters. The quality of the fabricated ceramic parts is assessed by the microstructure, macro-defects, the geometrical features of the structural ceramics and the piezoelectric performance of BaTiO 3 and (Bi, Na)TiO 3 – BaTiO 3 . The effect of green- and sintered-density, linear shrinkage (associated with sintering), and shape distortion are also discussed. • Hybrid additive manufacturing and injection molding produce 3D complex shapes • 3D-printed sacrificial molds allow molding the ceramics in freeform geometries • Photopolymer formulation results in robust printing and injection performance • Strategy to reduce defects by simulations and to adjust injection molding parameters • Sintered parts show isotropic shrinkage, no delaminations and limited shape distortion • 3D Pb-free piezoceramics preserve their piezoelectric properties

New Technological Solutions in the Manufacture of Thermochemically Resistant Ceramic Molds for Casting Titanium Alloys

New Technological Solutions in the Manufacture of Thermochemically Resistant Ceramic Molds for Casting Titanium Alloys

Experimental simulation of volumetric compacts formation from spherical waxy elements

The growth in metal intensity of industrial production and the volume of consumption of finished metal products determine the relevance of development and research of energy efficient technological processes aimed at reducing costs by reducing the number of operations while maintaining product performance. In mechanical engineering, the problem of obtaining blanks with increased dimensional and geometric accuracy and complex configuration is solved by using a common method of investment casting. Expansion of the use of such technological approach to produce blanks in mechanical engineering is hindered by a number of physical phenomena associated with the thermal expansion of investment and ceramic materials, which leads to an increase in the product final cost. A significant number of defect-forming factors can be eliminated by applying an innovative solution consisting in the formation of porous removable models by compacting mixtures based on waxy materials. This solves the problem of material shrinkage and increases the crack resistance of ceramic molds, which significantly reduces the share of machining in the overall volume of technological operations. Technical tests of the new method have revealed the reason why the machining of castings cannot be completely eliminated at present. The problem mainly lies in elastic response of compacted material of the model mixture, which, in some cases, affects the increase in the compacts size. This paper considers the effect of initial packing of spherical-shaped elements simulating one- and two-component model mixtures on the stress-strain state of a powder body subjected to unilateral compaction in a rigid cylindrical matrix to technologically justified density values. The results of the experiment are presented in the form of stress-strain relations. Preferable conditions of compact formation with minimal values of elastic response of the compacted material are considered.

A Comprehensive Study on High-Temperature Oxidation Behavior of Ceramic Molds for Hot Embossing.

Structural ceramics are potential mold materials for hot embossing, due to their superior mechanical strength as well as low thermal expansion coefficient. However, the service time of molds, especially those in high-temperature hot embossing, strongly depends on their oxidation resistance. As a result, the oxidation behaviors of various ceramics (e.g., SiC, ZrO2, AlN, Al2O3, Si3N4 and WC) were investigated by conducting cyclic oxidation experiments in this study. Mass changes of ceramic samples thermal treated under different temperatures were measured by thermogravimeter (TGA) and precision electronic balance. The structural and chemical compositions of ceramic samples were detected by X-ray diffraction (XRD) and energy-dispersive X-ray spectroscopy (EDXS). The surface morphology of the samples was characterized by scanning electron microscopy (SEM), and the surface roughness of the samples was measured by white light interferometry. The mechanical properties of the samples were evaluated by a microhardness tester and nanoindentation instrument. It is noted that Al2O3 shows negligible oxidation within 1000 °C. ZrO2 maintains a decent surface roughness of below 32 nm and a stable hardness within 1000 °C. SiC has the highest hardness at high temperatures, and its surface roughness increases notably above 800 °C. The surface roughness of Si3N4 and AlN soars between 600 °C and 800 °C. The surface finish of WC is significantly deteriorated above 600 °C. Therefore, the appropriate embossing temperature of Al2O3 ceramics is below 1000 °C, that of ZrO2 ceramics is between 800 °C and 1000 °C, that of SiC ceramics is below 800 °C, that of Si3N4 and AlN ceramics is between 600 °C and 800 °C, and that of WC ceramics below 600 °C.

Multi-directional freeze casting of porous ceramics with bone-inspired microstructure

Porous ceramics are favored in a multitude of applications, such as filters, catalyst supports, and tissue engineering scaffolds. However, conventional fabrication techniques find it particularly challenging to preserve sufficient mechanical strength in highly porous ceramics. Although unidirectional freeze casting can fabricate porous ceramics with high strength vertically, the strength in other directions is inadequate due to a lack of lateral structural control. Herein, inspired by the cancellous bone, we propose a novel multi-directional freeze casting technique to prepare highly mechanically efficient porous ceramics. A multi-directional temperature field is ingeniously designed to mimic the stress-responsive growth pattern of the cancellous bone. To further the lateral structural control, ceramic fibers are incorporated to form mineral bridging. In this process, alumina-mullite composite ceramics are prepared with hierarchical structures, including micro-level multi-oriented struts, sub-micro-level interlamellar bridges and nano-level eutectic phases. They endow the ceramics with high porosity (∼75%) and high strength in all 3D spatial directions (8.4–20.1 MPa), while effectively preventing the catastrophic brittle failure. Therefore, the mechanically enhanced porous ceramics demonstrate the remarkable controllability of multi-directional freeze casting in hierarchical structures. Also, our work opens up a new horizon for fabricating highly mechanically efficient porous materials, including hierarchically structured biomimetic ceramics.

Elimination of the Stray Grain Defects of Single Crystal Blade by Variable Wall Thickness Based on Integral Ceramic Mold

The stray grain defect is often found at the platform in the process of directional solidification of single crystal blades because local undercooling easily occurs in this area. A variable wall thickness mold based on stereolithography and gelcasting technology was proposed to solve the local undercooling at the platform. The influence of variable wall thickness mold on the temperature field of the platform during directional solidification was studied via simulation. The numerical simulation results show that variable wall thickness can effectively prevent heat dissipation at the platform, thereby reducing the undercooling and avoiding the formation of stray grains. At the same time, the influence of molds assembly angle on the formation of stray grains was analyzed, and the appropriate molds assembly angle is suggested.

New process solutions in the manufacture of thermochemically resistant ceramic molds for casting titanium alloys

The paper provides the results of studies on interaction between titanium melts and silica-containing investment molds. Pure silicon, compounds of titanium oxides and silicides were detected by X-ray diffraction analysis in the contact zone. The problem of the negative impact exerted by the mold on the casting is solved by using thermally stable and chemically resistant monocorundum molds based on an alumina sol binder. A refractory suspension was developed for investment casting containing special additives to improve wax mold wetting with the suspension, and to increase the mold shell strength. The article studies sedimentation properties of suspension. A method was developed for accelerated curing of sequentially applied refractory suspension layers by vacuum drying and subsequent chemical curing with a gaseous reagent. The formation time is reduced from 3–5 h to 20–30 min per layer. Comparative studies of kinetics of alumina sol binder convective drying and vacuum dehydration were conducted. The process of moisture removal per unit surface of the applied refractory layer in a vacuum of 5–10 kPa increases by 2–6 times. X-ray phase analysis was used to study the alumina sol conversion during high-temperature heating. The solid gel of the α-Al2O3 stable phase is obtained in the alumina sol mold shell when the calcination temperature rises to 1300–1350 °C with a sufficient strength of 9–12 MPa provided by sintering additives added to the suspension. Recommendations are given for additional protection of refractory ceramic layers after vacuuming and drying: treatment of the last layer with gaseous curing agents and application of a polyvinylacetal solution with a density of 1100–1200 kg/m3. The process solutions proposed will make it possible to increase both the efficiency of titanium alloy forming and casting processes and the quality of castings.

Alumina-based ceramic mold with integral core and shell for hollow turbine blades fabricated by laser powder bed fusion

Alumina-based ceramic molds are extensively applied in investment casting processes of hollow turbine blade due to their outstanding high-temperature chemical stability and creep resistance. Herein, laser powder bed fusion ( L -PBF) method was utilized to fabricate an alumina-based ceramic mold with integral core and shell (CMCS). The influences of the component contents in the composite powder on fluidity and packing density were comprehensivley studied, and the optimized formulation for the composite powder was consequently determined as 88 wt% mixed spherical alumina (coarse: fine = 9:1), 12 wt% epoxy resin E12, and 0.1 wt% fumed silica. The vacuum infiltration process was adopted to improve mechanical properties of alumina ceramic after the sintering, where the solid loading of ceramic slurry was investigated. The CMCS was divided into two parts for fabrication, i.e., the core&shell and the base with a spiral grain selector, which could facilitate convenient of cleaning residual powder due to enlarged openings. After the first infiltration and pre-sintering, the separated two parts were bonded by a ceramic binder and then underwent second infiltration and final-sintering to obtain a complete CMCS. The alumina-based ceramic fabricated through L -PBF and post-treatment possessed low sintering shrinkage (1.51–2.03%), adequate apparent porosity (30.82 ± 0.01%), and room-temperature strength (13.03 ± 2.90 MPa for unbonded specimen, 9.72 ± 1.34 MPa for bonded specimen). Strikingly, the subsequent casting experiment has proved that the method proposed in this paper is promising to fabricate superior alumina-based CMCSs.

Ceramic molds based on yttrium oxide for the casting of titanium alloys

The effect of various binding agents based on aqueous solutions of nitric acid, yttrium nitrate and orthophosphoric acid and yttrium hydroxide gel on the physical and mechanical properties of casting molds based on Y2O3 has been studied. It is shown that the most promising binding agent for yttrium oxide casting molds is a phosphate bond. The data describing the curing mechanism of phosphate-bonded molding mass is presented. The influence of the concentration of binding agent solutions on the strength characteristics of molding materials after curing and heat treatment is shown. Information about the interaction of yttrium oxide powder molds with phosphate-binding with titanium melt of VT1-0 and VT6 grades is presented. The obtained data allowed us to describe the advantages and disadvantages of the developed molding compound. A method for producing casting molds based on yttrium oxide powder with a yttrium phosphate-binding agent was developed based on this research.

Effect of Increased Powder-Binder Adhesion by Backbone Grafting on the Properties of Feedstocks for Ceramic Injection Molding.

The good interaction between the ceramic powder and the binder system is vital for ceramic injection molding and prevents the phase separation during processing. Due to the non-polar structure of polyolefins such as high-density polyethylene (HDPE) and the polar surface of ceramics such as zirconia, there is not appropriate adhesion between them. In this study, the effect of adding high-density polyethylene grafted with acrylic acid (AAHDPE), with high polarity and strong adhesion to the powder, on the rheological, thermal and chemical properties of polymer composites highly filled with zirconia and feedstocks was evaluated. To gain a deeper understanding of the effect of each component, formulations containing different amounts of HDPE and or AAHDPE, zirconia and paraffin wax (PW) were prepared. Attenuated total reflection spectroscopy (ATR), scanning electron microscopy (SEM), differential scanning calorimetry (DSC) and rotational and capillary rheology were used for the characterization of the different formulations. The ATR analysis revealed the formation of hydrogen bonds between the hydroxyl groups on the zirconia surface and AAHDPE. The improved powder-binder adhesion in the formulations with more AAHDPE resulted in a better powder dispersion and homogeneous mixtures, as observed by SEM. DSC results revealed that the addition of AAHDPE, PW and zirconia effect the melting and crystallization temperature and crystallinity of the binder, the polymer-filled system and feedstocks. The better powder--binder adhesion and powder dispersion effectively decreased the viscosity of the highly filled polymer composites and feedstocks with AAHDPE; this showed the potential of grafted polymers as binders for ceramic injection molding.

TEM Study of the Microstructure of an Alumina/Al Composite Prepared by Gas-Pressure Infiltration.

Ceramic injection moulding and gas-pressure infiltration were employed for the manufacturing of alumina/AlSi10Mg composites. Porous ceramic preforms were prepared by mixing alumina powder with a multi-binder system and injection moulding the powder polymer slurry. Then, the organic part was removed through a combination of solvent and thermal debinding, and, finally, the materials were sintered at different temperatures. Degrading the binder enabled open canals to form. The sintering process created a porous ceramic material consisting of alumina without any residual carbon content. During infiltration, the liquid metal filled the empty spaces (pores) effectively and formed a three-dimensional network of metal in the ceramic. The microstructure and properties of the manufactured materials were examined using high-resolution transmission electron microscopy, porosimetry, and bending strength testing. Microscopy observations showed that the fabricated composite materials are characterised by a percolation type of microstructure and a lack of unfilled pores. The research confirmed the diversified nature of the connection at the particle–matrix interface. It was observed that the interphase boundary was characterised by the lack of a transition zone between the components or a continuous transition zone, with the thickness not exceeding 30 nm. Thanks to their increased mechanical properties and low density, the obtained composites could be used in the automotive industry as a material for small piston rings and rods, connecting rods, or even gears.

Вплив комплексного модифікування на структуру й властивості жароміцного нікелевого сплаву

In this work, a study of the influence of complex modification of the ZhS3DK-VІ nickel-based heat-resistant super alloy was conducted. The modification was performed out according to different technological schemes: with highly dispersed titanium carbonitrides Ti[Ti(C,N)] particles, with Ti [Ti(C,N)]+CoAl2O4 and Ti[Ti(C,N)]+CoAl2O4+Ni-Y complex that made it possible to investigate the specifics of the influence of the volume modifier Ti[Ti(C,N)] separately, and its complex effect with surface modifierCoAl2O4 and rare earth metals (with Ni-Y master alloy use) in the macro- and microstructure of a heat-resistant nickel alloy. It was shown that the use of modifiers allows for significant refining of macrograins, and the greatest effect was obtained with Ti[Ti(C,N)]+CoAl2O4 and Ti[Ti(C,N)]+Ni-Y+CoAl2O4complexes. Microstructure study allows to show that the value of interdendritic distances in both the case of modification with Ti[Ti(C,N)] and Ti[Ti(C,N)]+Ni-Y+CoAl2O4 complexes essignificantly decreased, from 100…130 μm to 80...120 μm, which is the result of many nuclei formation during crystallization in the volume of the alloy and with highly intense heat transfer from the melt by CoAl2Omodified ceramic mould after melt pouring. Based on the results of the influence of various types of modification on the microstructure of the alloy under study, it was established that modification according to all schemes allows obtaining carbides of smaller sizes, but grown sizes of carbonitrides was observed. Globular particles of carbide and carbonitrides are evenly located along the cross-section of the samples under study. In all cases of modification, a decrease in the value of the shrinkage microporosity was observed. The mechanical properties of the specimens after standard heat treatment shows, that modification with complexes under study allows provide mechanical properties that meet the specification requirements. The best complex of alloy mechanical properties at room temperature, fracture toughness and stress-rupture strength provides with Ti[Ti(C,N)]+Ni-Y+CoAl2O4 modifiers complex.

Морфологія і топографія карбідної фази при легуванні сплава ЖС3ДК-ВІ гафнієм і танталом

This study describes the results of studying the effect of modification/micro alloying with hafnium and alloying with tantalum on the morphology and distribution of the carbide phase in the ZhS3DK-VI alloy microstructure. The carbide phase is an integral structural component of the microstructure of nickel heat-resistant alloys, has a tremendous impact on the strength characteristics of the material. In this regard, the shape, size, and distribution of particles of this phase in the microstructure are of great importance. The formation of the morphology of carbide particles largely depends on the technological factors of casting such as temperature of the ceramic mold, crystallization rate, melt temperature, etc. The higher the temperature parameters and the lower the crystallization rate during casting, the coarser the morphology and topography of carbide particles formed during solidification and, accordingly, the lower the strength characteristics of the material. However, technological parameters also affect the geometry of the casting and it is not always possible to change the technology; so the only possibility is the use of modification or alloying of the alloy upon receipt of the work piece. Such carbide-forming elements as hafnium and tantalum, due to their chemical activity, react with carbon at the stage of crystallization and form thermally stable primary MC-type carbides. The use of hafnium in nickel alloys is limited to a concentration of 0.1 % since at a higher concentration, this element and nickel form a eutectic phase, the melting temperature of which is much lower than the homogenization temperature of the alloys. It is studied the possibility of doping the ZhS3DK-VI alloy with tantalum to form a favorable morphology of the carbide phase. The dispersed carbide particles are taken in the microstructure of the ZhS3DK-VI alloy after experimental work. The chemical composition of the particles is dominated by tantalum, and there is some hafnium.

Strengthening effect of blocky phases and γ/γ interface in the directionally solidified high-Nb-containing TiAl alloy

Herein, a TiAl alloy containing a high Nb concentration with a composition of Ti-45.5Al-7Nb-(W, Cr, B) was prepared by electromagnetic cold crucible zone melting (ECCZM) using a ceramic mould. The microstructure, tensile properties and fracture morphology were analyzed by means of SEM, EDS and TEM. As results, there were three blocky phases, namely α2b, γb, and TiB, and two types of γ/γ interfaces. The tensile tests at room temperature showed that the ultimate tensile strength of the alloy reached a maximum value of 645 MPa. The primary strengthening mechanism of the alloy was the interfacial strengthening of the γ/γ interfaces. The two types of γ/γ interfaces can hinder dislocation movement and allow the dislocations to move under high tensile stress, thus strengthening the alloy without increasing stress concentration. Moreover, the three blocky phases can hinder dislocations through the interface between them and γ lamellae, thereby strengthening the lamellae. Among them the γb phase plays a coordinated role in controlling the deformation of the lamellar colony.

Interfacial Reaction Mechanism between Ceramic Mould and Single Crystal Superalloy for Manufacturing Turbine Blade.

Single crystal superalloys are the preferred materials for manufacturing turbine blades of advanced aero-engines, due to their excellent high temperature comprehensive performance. The interfacial reaction between alloys and ceramic mould are an important factor to influence the surface quality and service performance of the turbine blade. It is very important to reveal the interfacial reaction mechanism to improve turbine blade quality and yield rate. In this paper, the interfacial reactions between DD6 single crystal superalloy and ceramic mould were investigated by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction analysis (XRD). The results show that the main reaction products were HfO2, Al2O3 and Y3Al5O12 when the yttrium oxide powders were the prime coat materials, while alloy surface suffered undesirable sand fusion; the thicknesses of the reaction layers were over 20 μm. The reaction layer can be divided into two layers, the layer close to the alloy was mainly composed of Al2O3 and Y3Al5O12, and the layer close to the mould was composed of SiO2, Al2O3 and Y3Al5O12. Avoiding the formation of Y2O3-Al2O3-SiO2 ternary low-melts can solve the interfacial reaction between DD6 alloy and yttrium oxide mould.

Evaluation of graphite and TiO2 as susceptors for microwave dewaxing in ceramic shell casting processes of artworks

The main problems of the traditional foundry dewaxing processes in fine arts workshops are the emission of gases, the loss of 80% of the wax, the high electrical consumption, and the high risks for the operators. The introduction of the microwave technology for dewaxing of ceramic shell molds allows to minimize some of these problems, although the use of electromagnetic susceptors that capture the radiated energy and transform it into heat is required. This article describes different microwave dewaxing tests using TiO2 and graphite as susceptors. The results obtained show that this technique is viable, allowing the casting process to be carried out with a low percentage of breakage problems in the mold and significantly reducing the emitted gases and electricity consumption. The technique allows to recover in the same operation around 90% of the wax used in small and medium format objects. The tests show that the selection of the material used as a susceptor, the area of application and the power regimes, are fundamental to enable a controlled, soft and non-aggressive dewaxing, both for the art molds and for the environment, as opposed to the traditional Flash Dewaxing technique. In this way, it is possible to change the foundry of ceramic shells for artworks to achieve high levels of performance and safety, and to save energy, time and materials.

Fabrication of Cylindrical Microlens Array on RB-SiC Moulds by Precision Grinding with MAWJ-Textured Diamond Wheels

Cylindrical microlens array (CMA) is applied widely in imaging, sensing, and laser machining fields. Among the many techniques for machining CMA, moulding is considered a mass-production method with low-cost and good accuracy. Aimed at the present problems in the machining of CMA moulds, which include low processing efficiency and the prediction of the surface topography, this paper focused on the fabrication of CMA on RB-SiC moulds by precision grinding with micro-abrasive water jet (MAWJ) textured diamond wheels. The combined rough–fine grinding strategy for ceramic mould materials was proposed. The grinding experiments of CMA were carried out. The ultra-precision grinding method was optimized to obtain high shape accuracy and a high-quality surface of RB-SiC moulds. It was found that by using MAWJ-textured diamond wheels, the profile error in the peak-to-valley value (PV) of the CMA moulds can be further reduced to 6.7 μm by using the combined rough–fine strategy grinding process.

Influence of Thermal Annealing Temperatures on Powder Mould Effectiveness to Avoid Deformations in ABS and PLA 3D-Printed Parts.

Fused deposition modelling (FDM)-printed parts can be treated with various post-processes to improve their mechanical properties, dimensional accuracy and surface finish. Samples of polylactic acid (PLA) and acrylonitrile butadiene styrene (ABS) parts are treated with annealing to study a ceramic powder mould’s effectiveness in order to avoid dimensional part deformation. The variables chosen are annealing temperatures and the usage of a ceramic powder mould to avoid part deformations. A flexural strength test was carried out to evaluate the mould’s influence on the mechanical properties of the part. The effectiveness of the mould has been evaluated mainly attending to the length of the part, because this is the dimension most affected by deformation. A polynomial approximation to a deformation’s length and the effectiveness of the mould allows for their prediction. Results obtained show that effectiveness increases with the annealing temperature. Nevertheless, mould effectiveness decreases when parts are fabricated with PLA, because it is a semi-crystalline thermoplastic, and it suffers a lower shrinkage during thermal post-process than amorphous polymers such as ABS. Attending to the flexural strength test, mould has no significant influence on the mechanical properties of the treated parts in both materials studied.

Mechanical and tribological properties of injection molded zirconia-alumina for orthopedic implants

Ceramics have gained great attention for hip and knee arthroplasty surgical procedures due to their ability to guarantee long-life performance in patients and are considered an alternative to existing metal systems.In the present study, zirconia toughened alumina (ZTA) for orthopaedic implants has been developed by Ceramic Injection Molding (CIM) process. Microstructural, mechanical and tribological studies have been carried out to establish whether the material is suitable for the purpose. The new CIM ZTA material obtained density up to 99.4%, toughness 6.1 MPa m1/2, hardness 20 GPa, Young's modulus 320 GPa, and low coefficient of friction ranging between 0.08 and 0.13 under lubricated conditions, and between 0.11 and 0.34 in dry condition. To simulate the performance of the ZTA in vivo, i.e., the influence of material degradation on the ageing properties, accelerated hydrothermal aging was performed in vitro and good mechanical and tribological properties were confirmed for the developed ZTA.

Development of mullite-alumina ceramic shells for precision investment casting of single-crystal high-pressure turbine blades

Alumina-mullite ceramic shell moulds were developed to cast single-crystal hollow high-pressure turbine blades (SX-HPTB). Mullite and fine alumina powders were used as fillers. Two colloidal silica binders were used to bind the filler particles. One colloidal silica binder (binder A) had no polymer and the second colloidal silica binder (binder B) had 8 wt % polymer. Eight slurry compositions with fine alumina substitution (0, 10, 30 and 50 wt %) were prepared. Silica cores were used to achieve hollow castings. The effect of alumina substitution and binders on the slurry characteristics, flexural strength of green and fired samples, air permeability and self-load sag resistance was studied. It was observed that with an increase in alumina percentage, the test specimens showed increased viscosity, pick-up weight, density, green strength and fired residual strength. In addition, the slurries prepared from binder B possessed larger values of viscosity and pick-up weight compared to the slurries prepared from binder A. The residual strength of fired test specimens containing binder A was higher than those containing binder B. The test specimens with 30 wt % fine alumina substitution showed the highest self-load sag resistance w.r.t. each test temperature. Also, the air permeability of the fired samples was higher than the green (de-waxed) samples. The casting of hollow HPTB components was performed at two temperatures (1525 °C and 1550 °C) using nickel-based superalloy CMSX4. The HPTB components, cast at 1550 oC from ceramic shell mould having binder B and 30 wt % fine alumina substitution, resulted in an acceptable aerofoil profile and surface roughness.