Cracks of alumina ceramics by selective laser melting

Alumina ceramic powders have high melting point and are prone to cracking during the rapid heating and cooling process of selective laser melting (SLM). Research on the crack formation and growth mechanisms forms the basis to developing crack suppression techniques. Variable laser power experiments based on single-track, zigzag, and island scanning strategies are designed to analyse crack morphology, distribution state, formation reasons, and extension mechanisms in alumina (Al₂O₃) SLM specimens. Our experiments show that transverse cracks formed by internal stress and longitudinal cracks formed by solidification shrinkage exist in alumina SLM specimens. The transverse cracks continuously expand in melting tracks, while the longitudinal cracks expand along the centre or the juncture of melting tracks. With increasing laser power, the formation and extension length of cracks decrease. Crystal structures exert important influences on the fracture pattern and crack extension of specimens.     

Numerical simulation on evolution process of molten pool and solidification characteristics of melt track in selective laser melting of ceramic powder

A three-dimensional high-fidelity physical model for selective laser melting (SLM) of ceramic powder was created based on computational fluid dynamics (CFD) to examine the physical mechanism of molten pool and solidified tracks at mesoscopic scale. The discrete element method (DEM) was used to generate a randomly packed powder bed, and the volume-of-fluid method (VOF) was applied to dynamically monitor the free surface of the molten pool. The formation mechanism and evolution characteristics of the molten pool were found and analyzed, and the effects of laser power on the typical characteristics of solidified ceramic tracks of SLM were investigated. The molten pool was eventually solidified into a concave geometric shape track by surface tension. The laser power played a significant impact on the shaping quality of solidified ceramic track. When the laser power was too low, the melt track suffers from severe porosity and distortion defects, which can be effectively solved with increasing laser power. The simulation results were validated via single track selective laser melting of TiC ceramic powder.     

Selective laser melting of TiB2-Ti composite with high content of ceramic phase

An increasing need in customized ceramics and ceramic-metal composites has driven the development of powders feedstock and procedures for utilization of additive manufacturing for production of mechanically reliable composites. However, processing of materials with a high fraction of ceramic particles is still in its infancy. Herein we report on 3D printing of TiB₂-TiB-Ti composites from TiB₂-Ti powder mixture of high ceramic content (50 wt%TiB₂) by an optimized process of selective laser melting. In-situ synthesized from the mixture of commercially pure Ti and TiB₂ powders, the composites possess up to 20.4 GPa hardness despite of a relatively high porosity of around 8%. Improvement in hardness is mainly due to hardening effect of both TiB and TiB2 and correlated with an increase in fraction of needle-shaped TiB phase with an increase in laser energy density (LED). Depending on process parameters, an amount of the ceramic phases (needle-shaped TiB and coarse elongated TiB₂) can be customized. The laser energy density significantly affects the development of microstructure and size of the ceramic grains as well as the formation of solidification cracks. This study demonstrates the capacity of AM through SLM to produce the composites of high percentage of ceramic phase.     

Writing conducting lines into alumina ceramics by a laser dispersing process

The objective of this work is the development of a laser-supported process that allows to modify the electric and thermal properties of ceramics on a local scale. The principle of the process is based on local melting of the ceramic by a CO₂ laser beam and application of an additive to the molten area on the surface. During solidification, a metal–ceramic composite is formed with modified material properties compared to the bulk material. Different alumina samples were treated with metal powders of tungsten, copper, and oxides of these metals. Scanning electron microscopy (SEM) and energy-dispersive X-ray microanalysis (EDX) analysis reveal that the physical and chemical properties of a peripheral zone are changed in the heated region down to a depth of approximately 500 μm. The resulting resistance of the laser tracks can be adjusted from semi-conducting to metallic behavior with a resistivity down to 2×10⁻⁶ Ω/m. The modified ceramic can be used for heating elements working at operation temperatures of up to 1000 °C, high current resistance which can be loaded with current of up to 100 A.     

Selective laser melting of alumina: A single track study

Ceramics-based additive manufacturing is a complex process and the solidification mechanism and microstructural evolution are currently not fully understood. In this work, Al₂O₃ single tracks were formed using a customised selective laser melting (SLM) system equipped with a high power diode laser. The effects of laser energy density (LED) on geometry, microstructure and micro-mechanical properties of Al₂O₃ tracks were investigated. To better understand the solidification mechanism, a transient three-dimensional thermal model was developed for predicting the thermal behaviour of the melt pool. The results indicated the use of high LED gave rise to decreased viscosity and surface tension of the molten alumina and led to localized melting of the substrate. Both, in turn, enabled the formation of a continuous solidified track. The solidified tracks were primarily composed of columnar dendrite. When relatively high LED (≥?25.7?kJ/m) was applied, equiaxed dendrite appeared along the central line near the track surface. The size of dendritic grains decreased with the decreased LED, attributed to the increased cooling rate at solidification interface. The micro-hardness of the solidified track was found to be inversely proportional to the grain size owning to grain boundary strengthening effect.     

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.     

The fabrication of SiBCN ceramic components from preceramic polymers by digital light processing (DLP) 3D printing technology

We present a novel method to fabricate SiBCN ceramic components with complex shapes from preceramic polymers by using digital light processing (DLP) 3D printing technology in this research work. The photocurable precursor for 3D printing was prepared by blending high ceramic yield polyborosilazane with photosensitive acrylate monomers. The material formulation and printing parameters were optimized to fabricate complicated SiBCN ceramic components with high precision. The printed SiBCN ceramic materials were pyrolyzed at different temperatures, and retained their fine features after pyrolysis. Their microstructures were characterized by FTIR, XRD and TEM respectively. Furthermore, the thermal stability and mechanical properties of the SiBCN ceramic samples were investigated and discussed in detail. The 3D printed SiBCN ceramic material exhibited excellent thermal stability and resistance to high temperature oxidation up to 1500?°C.     

Laser-pulse melting of calcium oxide and some peculiarities of its high-temperature behavior

The study aims to address and overcome the existing hurdles and challenges in measuring the melting temperature of calcium oxide (CaO). Despite numerous experiments of this kind in the past, the exact melting point (MP) of CaO still remains quite uncertain due to three factors associated with it: a high melting temperature, an optical semitransparency, and a high vapor pressure in the vicinity of the melting point. The method used in the present study is based on the laser-pulse heating with CO₂- and Nd:YAG-lasers. It was designed specifically to eliminate the possible ambiguity due to variance in absorbance in the specimen bulk. Temperature measurements were performed with advanced multichannel pyrometry combined with probe laser reflectometry. Subsequently, this technique allowed to determine both the melting and the solidification points of CaO. The final measurement of the melting point amounted to 3160 ± 10 K.     

Roll‐to‐roll printed carbon nanotubes on textile substrates as a heating layer in fiber‐reinforced epoxy composites

The performance of wind turbines suffers from icing in regions with extreme climate. One approach is to incorporate heating elements into the most susceptible areas of the wind turbine blade as protection against icing and for de‐icing. Cost‐efficient and reproducible fabrication, as well as easy integration is important due to the large area of wind turbine blades. In this work, multi‐walled carbon nanotubes are applied on a 50% poly(ethylene terephthalate) and 50% polyamide non‐woven textile substrate by rotary‐screen printing. The printed layers function as resistive heating elements in a fiber‐reinforced composite. The heating areas are provided with flexographic or screen inline‐printed silver‐electrodes and can be integrated by means of vacuum infusion into a glass fiber‐reinforced epoxy composite laminate. These laminates, which are connected to an intelligent electrical control system, are suitable for melting ice on the surface of components or for preventing the formation of ice. The first promising experiments on heating structures in a rotor blade of a wind turbine at laboratory scale (2 m length) are the basis of studies on intelligent electrical control of heating structures and their behavior at different temperatures. The heating elements were able to melt a 3–4 mm thick ice layer within 25 min in a climate chamber at ?5 °C. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45950.     

Anisotropy management on microstructure and mechanical property in 3D printing of silica-based ceramic cores

Ceramic core is an essential component in the precise casting of hollow turbine blades, and the investigation on 3D printing of silica-based ceramic cores is crucial to the development of aviation industry; however, they are suffered from difficulty in high-temperature strength and structural anisotropy. In present work, silica-based ceramic cores were prepared via DLP stereolithography 3D printing, and the anisotropy management on microstructures and properties were explored based on the particle size of fused silica powders. In 3D printed ceramic cores with coarse powders, significant anisotropy was displayed exhibiting multilayer structure with large gaps in horizontal printing and uniform porous microstructure in the vertical direction, which was further explained by the particle deposition in printing. With finer silica powders, the uniformity in the microstructures was highly improved, attributed to the enhanced particle dispersion in ceramic slurries and promoted interlayer particle rearrangement during sintering. To evaluate the anisotropy in mechanical property, the ratio of vertical strength to horizontal strength (σ_V/σ_H) was proposed, which rose from 0.48 to 0.86 as the particle size decreased from 35 µm to 5 µm, suggesting enhanced mechanical uniformity. While the average particle size of silica powders was 5 µm, the flexure strengths of ceramic cores in different directions were up to 18.5 MPa and 16.3 MPa at 1540 °C with σ_V/σ_H ratio of 0.88, which well satisfied the demands for the casting of turbine blades. This work inspires new guidance on the anisotropy management in ceramic cores prepared by 3D printing, and provides new technology for fabrication of silica-based ceramic cores with superior high temperature mechanical properties.     

3D打印成型陶瓷零件坯体及其致密化技术

3D打印技术在陶瓷零件成型方面具有较大应用潜力,被认为是近净尺寸成型高性能复杂结构陶瓷零件的一种新途径。本文比较了陶瓷零件或其坯体的激光选区熔化、薄材叠加制造、熔融沉积造型、光固化、三维打印和激光选区烧结等不同3D打印工艺及其致密化手段的优势和不足,认为较低的相对密度和强度是阻碍3D打印陶瓷零件实现产品应用的主要障碍。本团队近年来采用造粒混合法制备出具有良好流动性的3D打印复合陶瓷粉体,再通过激光选区烧结(SLS)和冷等静压(CIP)技术分别进行坯体成型及均匀致密化处理,制备出了高性能、复杂结构的Al_2O_3致密陶瓷零件。本文回顾了这些工作,并补充介绍了溶解沉淀和溶剂蒸发这两种制备复合陶瓷粉体的新方法,利用SLS/CIP复合工艺进一步制造了ZrO_2、SiC、高白土等其它材质的复杂陶瓷零件,为3D打印陶瓷用于航空航天、医疗、艺术等领域奠定了基础。     

Additively manufactured mesostructured MoSi2-Si3N4 ceramic lattice

Lattice structures, their shape, orientation, and density make the critical building blocks for macro-scale geometries during the AM process and, therefore, manipulation of the lattice structure extends to the overall quality of the final product. This work reports on manufacturing of MoSi₂-Si₃N₄ ceramic lattices through a selective laser melting (SLM) approach. The strategy first employs the production of core-shell structured MoSi₂/(10-13?wt%)Si composite powders of 3–10?μm particle size by combustion synthesis followed by SLM assembly of MoSi₂/Si lattices and their further nitridation to generate MoSi₂-Si₃N₄ mesostructures of designed geometry. Experimental results revealed that the volumetric energy density of SLM laser has remarkable influence on the cell parameters, strength, porosity and density of lattices. Under compressive test, samples sintered at a higher laser current demonstrated a higher strength value. Selective laser melting has shown its potential for production of cellular lattice mesostructures of ceramic-based composites with a low content of a binder metal, which can be subsequently converted into a ceramic phase to produce ceramic-ceramic structure.     

Properties of selective laser melted spodumene glass-ceramic

In this article, we established the process conditions and characterised the resulting properties of additively manufactured spodumene important for selective laser melting of LAS–Al₂O₃. X‐ray diffraction analyses revealed the as‐printed samples printed with layer thickness of 50 μm were fully crystalline. Energy dispersive X-ray spectroscopy showed that the major elements before and after the printing process were present and in similar quantity. Micro‐computerised tomography inspection also revealed layer thickness‐dependent pore formation in all printed samples. In terms of mechanical properties, the highest flexural strength measured using the three‐point bend test method was 4.33 MPa. More importantly, these results demonstrated that there is still potential in the direct laser melting of ceramics.     

Quantifying Compositional Homogeneity in Pb(Zr,Ti)O3 Using Atom Probe Tomography

Atom probe tomography (APT) is a powerful materials characterization technique capable of ppm chemical resolution and near atomic scale spatial resolution. However, owing to a number of factors, the technique has not been widely applied to insulating materials and even less to complex oxides. In this study, we outline the methodology necessary to obtain high‐quality results on a technologically relevant complex oxide Pb(Zr,Ti)O₃ (or PZT) using laser‐assisted APT on both bulk and thin film specimens. We show how, with optimized and well‐controlled conditions, APT complements conventional techniques such as STEM‐EDS. The correlative information can be used to obtain the nanoscale 3‐D chemical information and investigate the nanoscale distribution of cations. Using nearest‐neighbor cluster analysis routines, 5–10 nm segregation of B‐site cations was detected in bulk sintered PZT 53/47 from chemically prepared powders. No statistically significant segregation of B‐site cations was observed in thin film specimens. This work opens new avenues toward understanding the process‐structure properties in complex materials at length scales heretofore unachievable.     

3D printed ceramic phosphor and the photoluminescence property under blue laser excitation

An Al₂O₃/YAG: Ce³⁺ ceramic phosphor was fabricated for high-flux laser lighting using the digital lighting process (DLP)-based 3D printing method for the first time. The photocurable ceramic suspension for 3D printing was prepared by blending well-treated Al₂O₃/YAG: Ce³⁺ composite powders with photosensitive resin monomers and photo-initiators. The printing parameters, debinding and sintering processes were designed delicately to fabricated the dense sub-millimeter-sized cylinder ceramic with high dimensional accuracy. The ceramic showed excellent luminescence property under blue laser excitation with a threshold of 20.7 W/mm², higher than that prepared via dry-pressing followed by vacuum sintering. The luminescence properties and the microstructures of both ceramics were further comparatively investigated to find the possible interpretations for improvement of laser flux for the 3D-printed ceramic. The present work indicated that the new developed 3D printing method was promising for preparing luminescent ceramics for high-flux laser lighting in a rapid, effective, low-cost and precision-controlled manner.     

3D打印技术用于不规则骨缺损的重建与修复

为研究3D打印技术对不规则形状骨缺损模型的重建程度,和3D打印的可降解生物材料对脊椎骨缺损在12周内的修复的效果,本文随机选取一名病人,用其电子计算机断层扫描（computed tomography, CT）数据构建出不规则的三维脊柱缺损模型,选用聚己内酯（polycaprolactone, PCL）作为支架材料,运用3D打印技术打印出高度符合该病人骨缺损部位的人工骨支架。同时建立一个简单的兔子脊椎缺损模型,运用3D打印技术打印缺损尺寸的支架移植兔子体内,术后观察3个月,将兔子处死取出缺损部位,制作切片进行苏木素和伊红（Hematoxylin and Eosin, H&E）染色,染色结果表明缺损部位修复良好。     

Tough thiourethane thermoplastics for fused filament fabrication

Fused filament fabrication (FFF) is the most common form of additive manufacturing. Most FFF materials are variants of commercially available engineering plastics. Their performance when printed can widely vary, thus there is an increasing volume of research on alternative materials with thermal and mechanical performance optimized for FFF. In this work, thiol–isocyanate polymerization is used for the development of a one‐pot synthesis for polythiourethane thermoplastics for tough three‐dimensional (3D) printing applications. The thiol–isocyanate reaction mechanism allows for rapid polymer synthesis with minimal byproduct formation and few limitations on reaction conditions. The resulting elastomer has high toughness and a low melting point, making it favorable for use as a 3D printing filament. The elastomer outperforms commercial filaments in tension when printed. Considering the rapid advancement of additive manufacturing and the limitations of many engineering polymers with the 3D printing process, these results are encouraging for the development of bespoke 3D printing thermoplastics. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45574.     

Interaction between laser beam and BaTiO3 powders in selective laser sintering treatments

The research work reported in this paper is an investigation of the behavior of barium titanate powders under selective laser irradiation. Our goal is to determine suitable conditions to sinter the powders and form dense layers usable in some electronic components. On that purpose, compacts of micro/nano BaTiO₃ powder mixes are used for a parametric investigation of the laser scans parameters (power, speed, etc.) with a Nd-YVO₄ laser (23 W). The microstructures obtained after laser treatments are evaluated by XRD, SEM and EDS and compared to a reference specimen manufactured in a conventional way. From this work it can be concluded that a high laser beam power is required to obtain a consolidation of the powder grains and the use of a high scan speed avoids the melting. The scanning speed also influences the final crystallographic state of BaTiO₃. Optimal parameters were founded in order to form a dense and homogeneous tetragonal BaTiO₃ surface.     

Effect of scanning speed on the solidification process of Al2O3/GdAlO3/ZrO2 eutectic ceramics in a single track by selective laser melting

Al₂O₃/GdAlO₃/ZrO₂ ternary eutectic ceramics with fine microstructure were directly fabricated from mixed pure ceramic powders by selective laser melting. The specimen forming quality, molten pool morphology and microstructure characteristic were investigated as functions of scanning speed. Solidification defects such as cracks and pores were effectively suppressed when the scanning speed was 12 mm/min. The relative density of the as-solidified eutectic specimens decreased from 98.7% to 95.7% with increasing the scanning speed up to 48 mm/min. The melting in this study was governed by conduction mode, leading to a decrease tendency of both melting width and depth with the increase of the scanning speed. Different from ordinary cognition, the eutectic spacing in top zone of the molten pool first decreased and then increased with increasing the scanning speed from 6 mm/min to 48 mm/min. The transition point appeared at 12 mm/min, where the dominant factor affecting the solidification rate changed from the scanning speed to the value of the angle between microstructure growth direction and laser scanning direction.