One-step additive manufacturing and microstructure evolution of melt-grown Al2O3/GdAlO3/ZrO2 eutectic ceramics by laser directed energy deposition

Synchronized-powder-feeding-based laser directed energy deposition (LDED) has great application potential for the rapid fabrication of large-scale composite ceramics with complex shapes. In this study, near-full-density Al₂O₃/GdAlO₃/ZrO₂ ternary eutectic ceramics with different shapes and smooth surfaces were directly prepared by using an improved LDED device. Spherical ceramic powders with eutectic composition and good flowability were obtained by centrifugal spray drying. The microstructure characteristics and microstructure evolution of the rapidly solidified 3D-printed eutectic ceramic were systematically elucidated. In particular, the formation mechanism of the observed periodic banded structures was revealed through a unique laser partial remelting technique. The result indicated that the appearance of the banded structure is attributed to the drastic abnormal coarsening of the nanoscale microstructures adjacent to the molten pool. On the basis these results, a physical model was proposed to illustrate the microstructure evolution of the 3D-printed Al₂O₃/GdAlO₃/ZrO₂ eutectic ceramic.     

Microstructure and mechanical properties of directionally solidified Al2O3/GdAlO3 eutectic ceramic prepared with horizontal high-frequency zone melting

Directionally solidified eutectic oxide ceramics are very promising as a next-generation structural material for ultrahigh-temperature applications, above 1600?°C, owing to their outstanding properties of high corrosion resistance, oxidation resistance, high fracture strength and toughness, and high hardness. Herein, Al₂O₃/GdAlO₃ eutectic ceramic was prepared with horizontal high-frequency induction zone melting (HIZM), and the effects of the processing parameters on the eutectic microstructure and mechanical properties were investigated. The results indicated that the directionally solidified Al₂O₃/GdAlO₃ eutectic ceramic was composed only of the Al₂O₃ phase and GdAlO₃ phase penetrating mutually, and the Al₂O₃ phase was the substrate in which the GdAlO₃ phase was embedded. As the solidification rate increased from 1 to 5?mm/h, the eutectic microstructure underwent a transformation from an irregular pattern to a relatively regular “rod” or “lamellar” pattern, and the eutectic spacing constantly decreased, reaching a minimum value of 0.5?μm. The eutectic ceramic hardness and fracture toughness at room temperature increased continuously, reaching 23.36?GPa and 3.12?MPa?m^1/2, which were 2.3 times and 2.5 times those of the sintered ceramic with the same composition, respectively. Compared with the samples obtained from vertical high-frequency induction zone melting, the orientation of eutectic phases along the growth direction decreased significantly, and the size uniformity of the GdAlO₃ phase became poorer in the samples prepared with HIZM at the same solidification rate; nevertheless, the hardness and fracture toughness of the samples increased by 11% and 63%, respectively.     

Direct laser melting of Al2O3 ceramic paste for application in ceramic additive manufacturing

We develop the direct laser melting of ceramic paste technology for application in ceramic additive manufacturing (AM). The Al₂O₃ ceramic paste, which is a homogeneous mixture of DI-water and Al₂O₃ ceramic powders, was deposited on an Al₂O₃ substrate using free-forming extrusion (FFE), and subsequently melted by a CO₂ laser. To better control the laser melting process, the flow behavior of the laser-melted Al₂O₃ was investigated by evaluating the microstructure of the laser-melted Al₂O₃ single tracks. When the laser scanning speed increased from 1 to 3.5 mm/s at a fixed laser power, the permeation of the molten Al₂O₃ into the surrounding porous paste was reduced, resulting in the improvement of the surface uniformity of the laser-melted Al₂O₃ tracks. Through optimizing the laser scanning strategy, a fully-dense Al₂O₃ layer with smooth surface was achieved. The phase composition and density of the laser-melted Al₂O₃ layers were evaluated to study their properties. The thickness of the dense Al₂O₃ layer varied from ～90 μm to ～120 μm periodically due to the line-by-line scanning of the Gaussian laser beam. In addition, the relationship between the melting thickness and the laser scanning speed was also investigated to further improve the controllability of the laser melting process. This direct laser melting of ceramic paste technology is promising for applications in ceramic AM, such as 3D printing of ceramic components and high-temperature ceramic welding.     

Role of scanning speed on the microstructure and mechanical properties of additively manufactured Al2O3‒ZrO2

Melt-grown Al₂O₃–ZrO₂ eutectic (AZ eutectic) ceramics have attracted extensive attention for harsh environment applications. In this work, AZ eutectic ceramic is additively manufactured via one-step laser powder bed fusion (LPBF). The role of scanning speed on phase formation, crystallographic characteristics, microstructure evolution, and mechanical properties were systematically investigated. The as-fabricated specimens are mainly composed of α-Al₂O₃ and t-ZrO₂. Lower scanning speeds induced the formation of cellular structures consisting of randomly oriented ZrO₂. In contrast, nanometer eutectic lamellar structure with well-defined multiple crystallographic orientation relationships, for example, {10$\overline{1}$0} Al₂O₃ || {100} ZrO₂ and {0001} Al₂O₃ || {001} ZrO₂, occurred at higher scanning speeds. Both the cell size and lamellar spacing decreased with increasing scanning speed. With the microstructure refinement, the crack propagation mode changes from intergranular to transgranular fracture, leading to progressively enhanced fracture toughness with a maximum value of 7.76 MPa·m^1/2. The present work could shed light on tailoring the microstructure of LPBF AZ eutectic ceramic via varying processing parameters.     

Directly fabricated Al2O3/GdAlO3 eutectic ceramic with large smooth surface by selective laser melting: Rapid solidification behavior and thermal field simulation

Selective laser melting (SLM), a novel approach for one-step melting and solidifying ceramic powder beds layer by layer without post-process of degreasing and sintering, has been developed to directly prepare highly dense (>95 %) Al₂O₃/GdAlO₃(GAP) eutectic composite ceramics with large smooth surfaces. Compact net-shaped plates with the maximum size of 73 × 24 × 5 mm³ are obtained by different strategies of laser pre-heating and multi-tracks’ deposition without any binders. Combined with the finite element thermodynamic coupling simulation results, it is proved that the stress between the substrate and depositions during SLM can be greatly reduced by the step-up preheating, and thus effectively improving the ceramic forming quality. The macro-morphology, microstructure evolution, rapid solidification behavior and mechanical properties of the SLM-ed eutectic ceramics are systematically investigated at different laser processing parameters. The microstructure transforms from ultra-fine irregular eutectic to complex regular eutectic with the increase of the scanning rate. The average eutectic spacing, and solidification rate has an approximately linear relationship consistent with the Jackson-Hunt (JH) model. The microhardness and fracture toughness can reach 17.1 ± 0.2 GPa and 4.5 ± 0.1 MPa·m^1/2, respectively. The results indicate that SLM method is a highly effective technique for fabricating high-performance net-shaped structural composite ceramics.     

Processing,microstructure and performance of Al2O3/Er3Al5O12/ZrO2 ternary eutectic ceramics prepared by laser floating zone melting with ultra-high temperature gradient

Directionally solidified Al₂O₃/Er₃Al₅O₁₂(EAG)/ZrO₂ ternary eutectic/off-eutectic composite ceramics with high density, homogeneous microstructures, well-oriented growth have been prepared by laser floating zone melting at different solidification rates from 4 to 400 µm/s. Uniform and stable melting zone is obtained by optimizing temperature field distribution to keep continuous and stable eutectic growth and prevent from cracks and defects. The as-solidified composite ceramic exhibits complexly irregular eutectic structure, in which the eutectic spacing is rapidly refined but dotted ZrO₂ number inside Al₂O₃ phase is decreased as increasing the solidification rate. The formation mechanism of ZrO₂ distributed inside Al₂O₃ matrix is revealed by examining the depression of solid/liquid interface. Furthermore, after heat exposure 1500 °C for 200 h, the eutectic microstructure only shows tiny coarsening, which indicates it has excellent microstructural stability. As increasing the ZrO₂ content, the fracture toughness can be improved up to 3.5 MPa m^1/2 at 20.6 mol% ZrO₂.     

Ultra-fast,selective, non-melting,laser sintering of alumina with anisotropic and size-suppressed grains

In this paper, we report an ultra-fast sintering phenomenon of alumina achieved by the scanning laser irradiation method. Using CO₂ laser irradiation, we found that micrometer-sized alumina powder (d₅₀ = 1.2 µm) can be sintered close to full density within a few tens of seconds. The microstructure of laser-sintered alumina was different from that of the furnace-sintered alumina. The relative density and grain size of the laser-sintered alumina gradually decreased from the center of the laser beam to the edge. Anisotropy of the grain size was measured along and perpendicular to the scanning direction. This anisotropy decreased as the scanning speed decreased from 0.1 mm/s to 0.01 mm/s. The sintering master curve of grain size versus relative density, which reflects the sintering mechanism, was found to be affected by the laser scanning speed. When the laser scanning speed was 0.1 mm/s, grain size suppression was found for the almost fully dense alumina. However, at lower scanning speed (e.g., 0.01 mm/s), there was significant grain growth in the regions where the relative density was greater than 90%. These results clearly indicate that alumina can be sintered, in the solid-state, to a high density in a short time using scanning laser and the microstructure is different from the furnace-sintered alumina.     

The synthesis of Li(Ni1/3Co1/3Mn1/3)O2 using eutectic mixed lithium salt LiNO3–LiOH

A lithium-ion battery cathode material, Li(Ni_1/3Co_1/3Mn_1/3)O₂, with excellent electrochemical properties was prepared via two-step isothermal sintering, using eutectic lithium salts (0.38LiOH·H₂O–0.62LiNO₃) mixed with Co, Ni, or Mn hydroxides. Based on analysis using X-ray diffraction (XRD), scanning electron microscopy (SEM), a thermogravimetric-differential scanning calorimetric (TG–DSC) analyzer, and Fourier-transform Infrared (FT-IR), this synthetic process consists of procedures including lithium salt melting, permeation, reaction, crystalline transformation, and crystallization. Due to the lower melting point of the eutectic molten salts compared with that of the single lithium salt, a relatively mild synthetic condition (low temperature) is needed, and the product can be highly crystallized with low cation mixing, which facilitates maintenance of the precursor morphology. The electrochemical properties of the product were investigated by constant current discharge–charge and cyclic voltammetry. The results show that the initial discharge capacity is 160 mhA g⁻¹, with excellent cycling stability even after 50 cycles. We conclude that this novel eutectic molten salt method is a promising and practical approach for synthesizing cathode materials for lithium-ion batteries.     

Investigation of the solidification behavior of Al2O3/YAG/YSZ ceramic in situ composite with off-eutectic composition

Directionally solidified Al₂O₃/YAG/YSZ ceramic in situ composite is an interesting candidate for the manufacture of turbine blade because of its excellent mechanical property. In the present study, two directionally solidified hypoeutectic and hypereutectic Al₂O₃/YAG/YSZ ceramic in situ composites are prepared by laser zone remelting, aiming to investigate the solidification behavior of the ternary composite with off-eutectic composition under high-temperature gradient. The results show that the composition and laser scanning rate significantly influence the solidification microstructure. The ternary in situ composite presents ultra-fine microstructure, and the eutectic interspacing is refined with the increase of the scanning rate. The Al₂O₃/YAG/YSZ hypoeutectic ceramic displays an irregular hypoeutectic network structure consisting of a primary Al₂O₃/YAG binary eutectic and fine Al₂O₃/YAG/YSZ ternary eutectic. Only at low scanning rate, homogeneous ternary eutectic-like microstructures are obtained in the hypoeutectic composition. Meanwhile, the Al₂O₃/YAG/YSZ hypereutectic ceramic shows homogeneous eutectic-like microstructure in most cases and the eutectic interspacing is finer than the ternary eutectic. Furthermore, the formation and evolution mechanism of the off-eutectic microstructure of the ternary composite are discussed.     

Microstructure and cytotoxicity of Al2O3-ZrO2 eutectic bioceramics with high mechanical properties prepared by laser floating zone melting

Developing new generation of strong, tough and stable bioceramics used in dental filed has been highly desired for attaining the clinical requirement of secure and reliable therapy. In this paper, a novel Al₂O₃-ZrO₂ eutectic bioceramics with nearly fully density and extremely aesthetic luster was in-situ prepared by innovative laser floating zone melting (LFZM) method. The influence of solidification rates on microstructure evolution, mechanical properties and cytotoxicity was investigated. The eutectic bioceramics displayed a special three dimensional interpenetrating microstructure evolving with increasing the solidification rate. The eutectic colony structure occurred when solidification rate overpassed 8?µm/s, and lamellar spacing was below 1?µm when solidification rate exceeded 30?µm/s. The eutectic bioceramics solidified at 100?µm/s exhibited optimal mechanical properties with an average hardness of 16.53?GPa, fracture toughness of 6.5?MPa?m^1/2 and flexural strength of 1.37?GPa. The cytotoxicity of Al₂O₃-ZrO₂ eutectic bioceramics was evaluated by MTT methods according to ISO 10993-5 standard. Non-cytotoxic behavior was detected for the eutectic bioceramics, indicating this eutectic bioceramic could be used as promising dental restoration material.     

Microstructure, growth mechanism and mechanical property of Al2O3-based eutectic ceramic in situ composites

Directionally solidified Al₂O₃-based eutectic ceramic in situ composites with inherently high melting point, low density, excellent microstructure stability, outstanding resistance to creep, corrosion and oxidation at elevated temperature, have attracted significant interest as promising candidate for high-temperature application. This paper reviews the recent research progress on Al₂O₃-based eutectic ceramic in situ composites in State Key Laboratory of Solidification Processing. Al₂O₃/YAG binary eutectic and Al₂O₃/YAG/ZrO₂ ternary eutectic ceramics are prepared by laser zone melting, electron beam floating zone melting and laser direct forming, respectively. The processing control, solidification characteristic, microstructure evolution, eutectic growth mechanism, phase interface structure, mechanical property and toughening mechanism are investigated. The high thermal gradient and cooling rate during solidification lead to the refined microstructure with minimum eutectic spacing of 100 nm. Besides the typical faceted/faceted eutectic growth manner, the faceted to non-faceted growth transition is found. The room-temperature hardness H_V and fracture toughness K_IC are measured with micro-indentation method. For Al₂O₃/YAG/ZrO₂, K_IC = 8.0 ± 2.0 MPa m^1/2 while for Al₂O₃/YAG, K_IC = 3.6 ± 0.4 MPa m^1/2. It is expectable that directionally solidified Al₂O₃-based eutectic ceramics are approaching practical application with the advancement of processing theory, technique and apparatus.     

Rapid densification mechanism and properties of h-BN/ZrO2 composites with oxide additives by spark plasma sintering

Large size high density h-BN/ZrO₂ composites (ø = 110 mm, h = 15 mm) were rapidly prepared by spark plasma sintering (SPS) with sintering cycle 20 min. The effects of additives on the mechanical properties, microstructure evolution, and corrosion resistance of the h-BN/ZrO₂ composites were studied. The Al₂O₃, MgO, SiO₂, La₂O₃ and ZrO₂ rapidly formed heterogeneous eutectic crystals under the action of SPS. The low eutectic compounds significantly promoted the diffusion of h-BN or ZrO₂, also increased the density. The flexural strength of h-BN/ZrO₂ composites could reach 196.31 MPa, and the apparent porosity was only 0.42 %. The additives are combined with zirconia to form a high viscosity eutectic, whose corrosion resistance to molten steel is obviously better than that of single ZrO₂. The corrosion depth of h-BN/ZrO₂ composites was only 114 µm after corroded in molten steel at 1550 °C for 80 min. The comprehensive properties of h-BN/ZrO₂ composites were obviously improved by appropriate additives.     

Microstructures and mechanical properties of directionally solidified Al2O3/GdAlO3 eutectic ceramic by laser floating zone melting with high temperature gradient

Directionally solidified Al₂O₃/GdAlO₃ eutectic ceramic rods with high densities and low solidification defects are prepared by laser floating zone melting at solidification rate from 2 to 200 μm/s. The microstructure evolution, eutectic growth behavior and mechanical properties are investigated. At low solidification rates (<30 μm/s), the eutectic rods present a homogeneous irregular eutectic microstructure, whereas cellular microstructure containing regular lamella/rod structure is developed at higher solidification rates. The relationship is established between the eutectic interphase spacing and solidification rate, which follows the Magnin-Kurz eutectic model. The Vickers hardness (15.9–17.3 GPa) increases slightly with decreasing interphase spacing, but the fracture toughness (4.08 MPa m^1/2) shows little dependence with the solidification rate. Different crack propagation mechanisms are revealed among the indentation cracks. The flexural strength at ambient temperature reaches up to 1.14 GPa for the eutectic grown at 100 μm/s. The fracture surface analysis indicates that the surface defects are the main crack source.     

Formation mechanism and process optimization of nano Al2O3-ZrO2 eutectic ceramic via laser engineered net shaping (LENS)

Periodic banded structure has great influence on the microstructure and fracture toughness of eutectic ceramic produced by LENS system. Formation Mechanism of periodic banded structure and influence regularity from laser power, scanning speed, feeding rate and feeding proportion were studied. X-ray diffractometer analysis and scanning electron microscopy observation were carried out for sample phase composition and microstructure respectively. Microhardness was measured by vivtorinox hardness tester, and surface fracture toughness was calculated by indentation crack length. The results show that periodic banded structure is divided into organizational coarsening zone and divorced eutectic zone. Due to the influence of upper cladding layer heat, the eutectic structure in the interlayer bonding zone is obviously coarsening in organization coarsening zone. When laser power is lower, Al₂O₃ phase takes the lead in nucleation and grows up quickly, forming a discrete block structure of Al₂O₃ particles in divorced eutectic area. When laser power is higher, ZrO₂ phase takes the lead in nucleation and forms a discrete block structure of ZrO₂ particles. Al₂O₃-ZrO₂ ceramics with the shape of thin-walled and cylinder were manufactured under the optimized process conditions, whose independent nucleation disappears and the thickness of periodic banded structure reduces to 10 µm. The eutectic spacing of prototype structure reaches 120–130 nm, has uniform microstructure and good surface morphology. The average micro-hardness is 18.59 GPa. Surface fracture toughness increases to 6.52 MPa m^1/2.     

Optimizing the mechanical properties of TiO2/PA12 nano-composites fabricated by SLS 3D printing

In this paper, the selective laser sintering process was used to fabricate the TiO₂/PA12 nano-composite parts by considering the parameters of laser power, scanning speed, and TiO₂ content. The response surface methodology was used to improve the mechanical properties of TiO₂/PA12 nano-composite parts. The fracture surface characteristics of the specimens were examined using the scanning and transmission electron microscopy. The results indicated that the increase in laser power from 10 to 15 W improved the tensile strength, modulus, and impact strength of the nano-composite parts because of the fine dispersion of TiO₂ nano-particles. An increase in the scanning speed from 2000 to 3000 mm/s resulted in the reduction of tensile strength and modulus due to lower heat input and consequently incomplete densification of polyamide-12. In addition, the increase of TiO₂ content up to 5 wt% can improve the tensile strength, modulus, and impact strength, but requires increasing the laser power. However, the mechanical properties of the nano-composite parts were enhanced simultaneously at laser power of 12.4 W, scanning speed of 2000 mm/s and TiO₂ content of 1.9 wt%. Moreover, the addition of TiO₂ up to 5 wt% showed a slight influence on thermal stability and crystallinity of the sintered specimens.     

Laser additive manufacturing of melt-grown Al2O3/GdAlO3 eutectic ceramic composite: Powder designs and crack analysis with thermo-mechanical simulation

Directed energy deposition method is an efficient one-step laser additive manufacturing technology to achieve eutectic ceramic composite with high property and ultra-fine microstructures. In this paper, melt grown dense Al₂O₃/GdAlO₃(GAP) eutectic ceramic composites have been directly fabricated from the spherical powder reconstructed by an optimized spray granulation method. Effects of the powder size distribution, feeding rate, and heat treatment on the morphology and microstructure of the as-solidified eutectic ceramic composites have been investigated. Results show that the powder fluidity plays an important role in the heat conduction of the laser process. Finer powder (imperfect spherical powder with diameter less than 10 µm) gives rise to the disturbance of molten pool. Moreover, this powder greatly aggravates the phenomenon of powder’ sticking on surface of the specimen, which subsequently induces the leading growth of coarse dendrite-like GAP primary phases (higher interface temperature of dendrite tip for GAP phase) and sintered eutectic phases at the specimen edge. The finite element modeling (FEM) method is used to analysis the coupled thermal dynamic during the process after verification with infrared thermal image. It shows that longitudinal maximum principal stress exhibits a steep gradient at the edges of the as-solidified ceramic, making the specimen susceptible to cracking along the deposited direction at the first few layers. By optimizing the feedstock powder characteristics and the directed energy deposition process, completely solidified cylindrical and thin-walled Al₂O₃/GAP eutectic ceramic composites with the maximum dimensions of ? 4 × 95 mm³ and 10 × 4 × 44 mm³ have been successfully fabricated. The solidified specimens present smooth glossy surface and fine microstructures with the average eutectic spacing of 0.31 µm. The average micro-hardness and fracture toughness of 15.16 ± 0.29 GPa and 4.3 ± 0.09 MPa·m^1/2 have been obtained, respectively.     

Microstructure of the directionally solidified ternary eutectic ceramic Al2O3/MgAl2O4/ZrO2

The directionally solidified Al₂O₃/MgAl₂O₄/ZrO₂ ternary eutectic ceramic was prepared via induction heating zone melting. Smooth Al₂O₃/MgAl₂O₄/ZrO₂ eutectic ceramic rods with diameters of 10 mm were successfully obtained. The results demonstrate that the eutectic rods consist of Al₂O₃, MgAl₂O₄ and ZrO₂ phases. In the eutectic microstructure, the MgAl₂O₄ and Al₂O₃ phases form the matrix, the ZrO₂ phase with a fibre or shuttle shape is embedded in the matrix, and a quasi-regular eutectic microstructure formed, presenting a typical in situ composite pattern. During the eutectic growth, the ZrO₂ phase grew on non-faceted phases ahead of the matrix growing on the faceted phase. The hardness and fracture toughness of the eutectic ceramics reached 12 GPa and 6.1 MPa·m ^1/2, respectively, i.e., two times and 1.7 times the values of the pre-sintered ceramic, respectively. In addition, the ZrO₂ phase in the matrix reinforced the matrix, acting as crystal whiskers to reinforce the sintered ceramic.