Alumina-zircon filler based ceramic shell moulds for directionally solidified cast shrouded low pressure turbine blades

Ceramic shell moulds for investment casting of shrouded low-pressure turbine (LPT) blades were prepared by using colloidal silica binder and partial substitution of the zircon filler with fine alumina. Among the two ceramic slurry systems designed, the first slurry system comprised of polymer-free colloidal silica binder, and the second slurry system comprised of polymer-containing colloidal silica binder. The samples prepared from the first slurry system showed higher fired residual strength and self-load sag values (lesser sag resistance). The casting of shrouded LPT blades was carried out at 1525 °C and 1550 °C using CM247LC superalloy. Ceramic shell moulds prepared from the second slurry system, containing 30 wt% of fine alumina filler, yielded aeronautical grade casting (at 1550 °C) of blades with required dimensional accuracy and average surface roughness. Microstructural analysis of the cut surfaces of self-load sag tested samples was carried out to understand the effect of fine alumina substitution on shell characteristics.     

Microstructure evolution and mechanical properties of ceramic shell moulds for investment casting of turbine blades by selective laser sintering

Ceramic shell moulds fabricated by traditional shell-making technology have relatively low strength, and often crack during the casting process due to the low strength. In addition, the traditional shell-making process requires long period and high cost. In this work, qualified mullite ceramic shell moulds with enhanced strength were fabricated by selective laser sintering (SLS) combined with high-temperature sintering process. The effects of SLS process parameters on dimensions were investigated, and process optimization was proposed by orthogonal experiments. The effect of sintering temperature on strength at room temperature and 900?°C were studied. X-ray diffraction analysis (XRD) and scanning electron microscopy (SEM) analysis suggested that mullitization behavior was influenced by sintering temperature. Furthermore, the content of mullite phase, mullite grain sizes, and mean length-diameter ratio of the mullite grains increased with the sintering temperature. Mechanical testing results showed that the samples sintered at 1610?°C had an excellent compressive strength of 99.01?MPa at room temperature and over 172.02?MPa at 900?°C. These values far exceed those of ceramic shell moulds fabricated by the traditional shell-making process (40.43?MPa).     

Gel-casting of fused silica based core packing for investment casting using silica sol as a binder

A novel approach for the fabrication of core packing via silica sol gel-casting is described. Concentrated slurry dispersed in silica sol with high solid loading but low viscosity is successfully prepared at about pH 10.2. In situ consolidation of the slurry is realized through adjustment of NH₄Cl concentration to control the gelation time of the slurry. High compaction and uniform green body is obtained by gel-casting technology without de-airing process. The results from flexural strength tests show that wet gel bodies with 0.5 wt.% calcium aluminate obtained by silica sol gel-casting have exceptionally high strength, which are responsible for the integrity of core packing during autoclaving.     

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.     

Effect of a binder system on the low-pressure powder injection moulding of water-soluble zircon feedstocks

Micronized natural zircon sand powders were used as a raw material to conduct low-pressure powder injection moulding (LPPIM) processes. PIM could lead to new technological applications for this mineral, which has very stable dimensional behaviour with changing temperature. Zircon powders that have unconventional attributes for PIM (in terms of size and morphology) were mixed with poly(ethylene glycol) (PEG) based binders. Combinations of this water-soluble substance with different polymers, including low-density polyethylene (LDPE) and several types of acetate butyrate celluloses (CAB), were investigated. The influence of acetyl, butyryl and hydroxyl groups on the behaviour of the created feedstocks at different process stages and on the final piece properties were studied. The higher affinity of CAB with PEG and zircon powders compared with LDPE could result in improved densification and properties, but the butyryl, acetyl and hydroxyls groups affect the processability of these feedstocks.     

Effects of fibre length and mixing routes on fibre reinforced shell for investment casting

Shell for investment casting is key in obtaining casting with high performances. In this study, carbon fibres were mixed with slurry, silica sol, and sanding materials to prepare fibre reinforced shell for investment casting. Suspension properties, slurry viscosity, and shell strength were all investigated. Failure surface of shell was observed by scanning electron microscopy (SEM). Results indicated that suspension percentage and viscosity of slurry containing fibre and silica sol were higher than those of fibre free slurry. The viscosity of slurry increased as carbon fibre length rose. Comparison of different mixing methods and fibre lengths revealed that addition of carbon fibre with 4?mm in length into silica sol under ultrasonic agitation yielded maximum bending strength of 3.97?MPa. SEM data illustrated that the longer fibres might increase bonding area, leading to increased shell strength.     

Carbon-nylon hybrid fibers modified silica sol shell with enhanced flexural strength and heat transfer for investment casting

The carbon-nylon hybrid fibers were employed to modify the silica sol shell for investment casting, and these shells were sintered in vacuum. An optimized mixing process was applied to improve the dispersion of hybrid fibers in slurry, then resulting in the excellent slurry uniformity. The modified shell fabricated by slurry with 4 g/L hybrid fiber displays a higher sintered strength of 5.46 MPa, which is derived from the increased crack propagation path and the dissipated more loading energy through interfacial debonding and friction of the remained carbon fiber pull-out. In addition, the permeabilities of silica sol shells are obviously improved due to the existence of holes in matrix resulted from the melting of nylon fibers during the sintering process. Also importantly, the hybrid fibers modified shell exhibit a good heat transfer efficiency, which is related to the formed heat transfer channels caused by the remained carbon fibers in sintered shell.     

Plasma dissociation of zircon with concurrent in-flight removal of silica

Dissociation of zircon through thermal plasma has been conventionally carried out in argon or nitrogen medium at high input power levels. This paper discusses a promising one-step process for in-flight dissociation of zircon to produce zirconia in air plasma medium. A specially designed plasma torch consisting of a hollow cathode made of graphite and graphite anode nozzle was used to produce the air plasma jet. Further, the influence of carbon addition with zircon on the dissociation process in air plasma medium was examined. Thermodynamic analysis using free energy minimization plot showed that addition of carbon significantly promoted the reaction by lowering the dissociation temperature. Temperature profile of the plasma jet obtained from simulation studies showed that air plasma provided an extended hot zone with much lower temperature gradient and this facilitated efficient processing of zircon. Results of these investigations showed that addition of carbon enhances the dissociation of zircon along with simultaneous removal of silica at lower power level.     

Properties of hybrid fibre reinforced shell for investment casting

In order to improve the properties of silicon sol shell for investment casting process, a varying content of hybrid fibres (aluminium silicate and polypropylene) was introduced into slurry for preparation of fibre-reinforced shell in the present work. The bending strength, self-load deformation at elevated temperature, and the permeability of fibre-reinforced shell specimens were investigated and the fracture surfaces of shell specimens were observed by SEM. The results show that the bending strength of green shell increases with content of fibres in it. The maximum bending strength of 4.96 MPa was obtained in the fired shell with 0.6 wt% hybrid fibres addition. The high temperature self-loaded deformation of specimens of shell reinforced with a hybrid fibre addition above 0.6 wt% is higher than that of the unreinforced. However, the shell with a hybrid fibre addition up to 0.4 wt% exhibits the lower self-loaded deformation at high temperature compared to the unreinforced. It is also found that the permeability of shell specimens can be improved by hybrid fibres addition. Based on the fracture surfaces observation using a scanning electron microscope (SEM), the failure mode of the green shell reinforced with hybrid fibres is identified as fibre rupturing from the substrate of shell specimens, and/ or debonding from adhesive film surrounding it in shell. Even though the specimens of shell being fired at 900 °C for 2 h, the same failure features also exist in the fracture surfaces of specimens. This indicates that the specimens of shell can still be reinforced with aluminium silica fibres (residue of hybrid fibres) for their interpenetrating fibres network structure although go through firing.     

Synthesis of high-purity zircon,zirconia, and silica nanopowders from local zircon sand

High-purity zircon (ZrSiO₄) nanopowder was successfully produced from Indonesian natural zircon sand using a low-cost purification approach via magnetic separation, immersion in HCl, and reaction with NaOH, followed by a top-down nanosizing process using wet ball-milling for 10?h and annealing at 200?°C for 2?h. Furthermore, polymorph zirconia (ZrO₂ – amorphous, tetragonal, and monoclinic) and silica (SiO₂ – amorphous and cristobalite) nanopowders were also successfully derived from the purified zircon powder using a bottom-up method via alkali fusion and co-precipitation processes followed by calcination. The crystallite size of the powders was estimated from X-ray diffraction (XRD) data analysis to give 40, 31, 61, and 149?nm, respectively, for the zircon, tetragonal- and monoclinic-zirconia, and cristobalite. Microstructural characteristics of the zircon, silica, and zirconia nanopowders were revealed in transmission electron microscopy (TEM) images which confirmed that the average sizes of the particles were in a good agreement with the XRD estimated values.     

3D printing ceramic cores for investment casting of turbine blades,using LCD screen printers: The mixture design and characterisation

The primary objective of this study is to demonstrate the possibility of developing silica, alumina, and zircon-based photocurable ceramic suspensions that can be used for visible light photopolymerization (> 450 nm) and to optimise the binder formulations for the purpose of LCD-based ceramic 3D printing applications. Reference ceramic components for this work are ceramic cores employed in the investment casting of high-pressure turbine blades and vanes. Arguably, one of the most critical steps in photoinduced ceramic 3D printing is developing suitable ceramic suspensions, having high ceramic loading, low viscosity, and short curing times. Ceramic suspensions with four different novel binder formulations and commercial ceramic powders used in core manufacturing (SiO₂, Al₂O₃ and ZrSiO₄) were investigated to achieve the best trade-off between: (1) their curing performance (cure depth and curing speed), (2) rheological properties of the binder mixtures at the solid loadings of 60 vol.% for SiO₂, 55 vol.% for ZrSiO₄, and 45 vol.% for Al₂O₃; and (3) the green body mechanical properties of the mixtures after printing. The effect of ceramic particles on the selected binders was examined individually, and the correlation between cure depth (C_d), volumetric loading, and curing speed are evaluated. The results show all binders designed in this study provide an adequate cure depth, even at high ceramic loadings. When the curing behaviour of all unloaded binder mixtures from the previous study [1] compared with the 10 vol.% SiO₂ loaded mixtures, the cure depth of all formulated binder mixtures increased 50–55 % and the curing thickness of 60 vol.% SiO₂ loaded suspensions were still slightly higher than their unloaded counterparts. The rheology outcomes indicate that lower viscosity binders always result in lower viscosity of the ceramic loaded inks, even without taking the effect of dispersants into account. Besides, the addition of N-Vinyl-2-Pyrrolidone (NVP) monofunctional monomer to the binder mixtures significantly reduces the viscosity and changes the normally linear relationship of the mix viscosity and its silica loading content. Among the binder formulations loaded with 60 vol.% of SiO₂, the formulation providing the lowest viscosity and highest mechanical property consists of 5 wt.% of NVP, 45 wt.% of HDDA and 50 wt.% of Photocentric 34 resin. Although this binder mixture showed the highest green flexural strength when loaded by 55 vol.% ZrSiO₄, all other mixtures loaded with zircon flour also demonstrated a near-fluid behaviour, below 200 s^?1. In Al₂O₃ loaded mixtures, the HDDA di-functional binder formulations present lowest viscosity and the di- and multifunctional monomer blends (HDDA-Photocentric27) showed the highest mechanical properties when used in a 50/50 ratio. This work summarises the best binder choices for silica, alumina and zircon based ceramic suspensions used in core printing for investment casting applications through LCD screen printing.     

A simple and effective method for gel casting of zirconia green bodies using phenolic resin as a binder

A simple, effective method for fabricating zirconia green bodies is described that utilizes phenolic resin as a binder in the gel casting process. Both the zeta potential of zirconia particles and the rheological behavior of the slurries were measured. To prepare stable, homogeneous, fluidic zirconia slurry with high solid loading, the zeta potential was adjusted by varying the phenolic resin content of the premixed solution. This represented a departure from normally adjusting the slurry pH by using an acids, alkali or dispersants. To promote gelation of the slurry, a curing agent was added. Gel casting a mixture of a 42 vol.% zirconia slurry containing 13 wt.% phenolic resin yielded an easily sintered, very homogeneous green body with the desired strength. The gelation time of the slurries and the mechanical strength of the green bodies were controlled by adjusting the quantity of the curing agent in the slurries.     

Microstructure and reactivity evolution of colloidal silica binder in different systems at elevated temperatures

Colloidal silica as nanostructured binder for refractory castables has attracted many attentions in recent years. In the present study, phase composition, microstructure and reactivity evolution of silica gel at different heating conditions were investigated to find suitable system for colloidal silica application. The results showed that atmosphere and carbon slightly affected phase composition of the silica gel at elevated temperatures, and the crystalline phases were composed of major α-cristobalite and minor α-tridymite. The morphology and particle size of the silica gel were greatly affected by atmosphere and carbon during heating. The spherical nano-silica particles with sizes of 40–50 nm rapidly grew into macroscale rod-like particles with temperature increasing from 800-1000 °C to above 1200 °C in air, and sintering of silica particles was observed. However, the size and morphology of the spherical nano-silica particles retained at high temperature in a reducing atmosphere, and many well developed columnar mullite crystals and some SiC whiskers formed on heating silica gel, alumina fines and carbon at 1500 °C, which was due to carbon inclusions retarding the growth of nano-silica particles and the nano silica remained high reactivity at high temperature. Thus, colloidal silica was suitable for application in carbon-containing refractory castables.     

Dynamic response of ceramic shell for titanium investment casting under high strain-rate SHPB compression load

The mechanical performances of ceramic mold are crucial for the quality of casts in investment casting. However, most of the previous researches were focused on the quasi-static performance which is not sufficient for the accurate failure analysis of shell mold under complex stress state. In this investigation, dynamic mechanical behaviors of Al₂O₃-SiO₂ ceramic shell for investment casting have been studied using split Hopkinson pressure bar (SHPB) at high strain rates. Sand pack samples and pure slurry samples were considered for the testing in order to further understand the mechanism of fracture. Weibull approach was then applied to describe the strength distribution of ceramic shells. The dynamic increase factor (DIF) of compressive strength increased from 1.23 (863?s^?1) to 2.03 (1959?s^?1) indicating the high dependency of mechanical property to strain-rate. The cross-section and fracture surface were analyzed through scanning electron microscopy (SEM). The microstructural investigations showed that the crack propagation in the ceramic shell is mainly through the weak interface between sand particles and slurry region under quasi-static load. At high strain-rate, the crack propagation path is different which extends through the well sintered slurry region and even runs through the sand particles. The mechanism of crack propagation path is analyzed based on Griffith criterion. In addition, the feature of stress-strain curves indicates the layered structure which plays an important role in the process of fracture.     

Effect of silica sol on performance and surface precision of alumina ceramic shell prepared by binder jetting

Using 3D printing technology to prepare ceramic shell used for precision investment casting can realize short process and efficient preparation of the ceramic shell, which has a great application potential in the casting field. However, the 3D printed ceramic shells often have the problems of low strength and accuracy. In this paper, a silica sol room temperature dip coating treatment combined with high temperature sintering method was proposed to improve the strength and surface precision of the ceramic shell prepared by the binder jetting. The effects of silica sol concentration and dip coating time on performance and surface precision of the alumina ceramic shell were studied. The mechanical properties and surface precision of the alumina ceramic shell prepared by the binder jetting were improved significantly with the increases of the sol concentration and dip coating time. With the dip coating time of 90 s and sol concentration of 30%, the maximum bending strength of the alumina ceramic reached 44.8 MPa, which was 18.9 times higher than that of the untreated alumina ceramic. The top surface roughness and side roughness of the alumina ceramic decreased from 6.87 μm to 5.70 μm and 7.55 μm–6.46 μm, respectively, compared to those of the untreated alumina ceramic.     

Alumina-based filters made via binder jet 3D printing of alumina powder,colloidal silica infiltration,and sintering

Alumina-based, porous filter media was made via a binder jet 3D printing process consisting of an alumina powder printing step with subsequent heating, colloidal silica infiltration, drying, and sintering to consolidate particles yet retain a net open porous microstructure. The composites made were alumina-silica or alumina-mullite, where the silica sintering aid was used to densify and join the alumina particles. The resulting composite structures had open porosities in the 25–31 vol% range as measured by Archimedes density. Pressure drops were measured across the filter media at constant flow rates to compare disc shapes and complex, 3D printed filters based on the N95 design requirements. Complex, 3D-printed alumina composites were produced with acceptable pressure drops for N95 implementation.     

添加氧化镁反应烧结刚玉-锆英石的显微物相研究

对添加不同量MgO的反应烧结刚玉－锆英石的产物进行了显微物相及XRD研究。由此得出：1．温度是影响烧结进程的主要因素，1400℃以下，以固相烧结为主；1400℃以上以反应烧结为主；2．MgO加入量可影响反应程度，尤其在高温（＞1500℃）阶段。MgO含量增多可促进物相间的转化；x＝0.50时最有利于莫来石的生成；3保温时间仅影响已有物相的结晶状态，而不影响反应物的转化进程。     

Fabrication of cordierite foam ceramics using direct foaming and slip casting method with plaster moulds

The cordierite foam ceramics were successfully fabricated using direct foaming and slip casting method with plaster moulds. Kaolin, attapulgite and magnesium oxide were used as starting materials with Arabic gum added as the dispersant. The samples were sintered at 1200°C, and then the microstructure, porosity, bulk density and thermal conductivity were characterised. The results show that the cordierite foam ceramics had a porous structure of open cells and the struts had abundant small pores. The maximum open porosity achieved 87·65% with a bulk density of 329 kg m^??3, and the thermal conductivity was as low as 0·095 W (m K)^??1. Therefore, these cordierite foam ceramics show promise for use as the thermal insulator.     

The nucleation period for crystallization of colloidal TPA-silicalite-1 with varying silica source

The effect of varying silica source on the nucleation and crystallization of TPA-silicalite-1 was investigated. A direct experimental method, involving a two-stage varying-temperature synthesis, was used to determine the nucleation period for colloidal crystals of TPA-silicalite-1 with different silica sources, including tetraethoxysilane (TEOS) and amorphous silica (Ludox TM and Ludox LS). For syntheses performed at 60°C with TEOS as silica source, the duration of the nucleation was about 72 h, and a very rapid increase in the crystal population occurred during the initial crystallization time. However, with the amorphous silica sources (Ludox TM or Ludox LS), the duration of the nucleation period was extended to about 120 h, and the nucleation profile consisted of a self-accelerating nucleation rate at the beginning of the nucleation period. The two-stage synthesis method could be used to determine the nucleation profile for the various silica sources. However, this technique overestimated the crystal concentration at the earliest stage of nucleation with amorphous silica. The use of amorphous silica gave rise to a broader crystal size distribution compared to that of TEOS. However, it was found that for both TEOS and amorphous silica the vast majority of the nucleation occurred during an induction period when little or no crystal growth was observed. In addition, Raman spectroscopy revealed structural differences between Ludox TM and Ludox LS which may account for differences in the nucleation processes observed for these two amorphous silicas.     

Mechanical properties and wear behavior of Al5083 matrix composites reinforced with high amounts of SiC particles fabricated by combined stir casting and squeeze casting; A comparative study

Combined stir casting and squeeze casting processes were used to fabricate Al5083 matrix composites reinforced with 20, 25, and 30 wt% SiC_p. The microstructure, mechanical properties and wear behavior of the composites fabricated by combined stir casting and squeeze casting were compared with those fabricated by stir casting. The results revealed that the combined casting method improved the distribution of SiC particles through the reduction of the agglomeration of SiC particle and reduced the porosities of the samples from 2.32% to 1.29% in the sample containing 30 wt% SiC. These modifications led to the enhancement of mechanical properties i.e. increased the hardness to 85 BHN and the compressive strength to 350 MPa for the sample containing 30 wt% SiC fabricated by the combined casting method. In addition, the wear resistance of the samples fabricated by the combined casting method improved because of the reduced size of the wear debris as well as the smaller worn region. The dominant wear mechanism of all the composite samples fabricated by both methods was the delamination of the tribological layer while adhesion wear was dominant in the monolithic Al alloy.