High-performance ceramic parts with complex shape prepared by selective laser sintering: a review

ABSTRACT

Selective laser sintering (SLS) is an additive manufacturing technology which has shown great advantages in direct formation of the polymer, metal and their composites. However, ceramic parts prepared by the SLS still exhibit some fatal defects, including low density and poor mechanical properties. In this respect, recent advances for preparing ceramics have improved the final density and performance by adopting post-processing methods. In this review, three commonly used powder preparation approaches (i.e. mechanical mixing, solvent evaporation and dissolution-precipitation process) and two powder sintering mechanisms for the SLS are introduced. Porous ceramic parts are prepared directly through the SLS by virtue of their high porosity. And dense, high-performance Al₂O₃, ZrO₂, kaolin and SiC ceramic parts with complex shape are prepared by introducing CIP technology into the SLS, indicating that the hybrid technology could be the promising route for preparing high-performance ceramic parts used in various fields.     

基于冻结浆料的分层实体制造法加工陶瓷坯体

提出了一种新型陶瓷增材制造方法,浆料由陶瓷粉末、有机粘结剂和去离子水构成,单层生坯的加工过程包括:铺料、冷冻和激光扫描,层层累积成型后,将冻结状态坯体置于冷冻干燥机中干燥,得到陶瓷生坯;分析了激光加工参数和浆料固含量对于激光扫描过程的影响.结果表明:采用激光扫描图形轮廓的方式,避免了激光辐照对材料内部结构的破坏;通过与冷冻干燥技术的结合,充分保留了片层状的孔隙结构;随着激光能量密度的增大,激光扫描线的宽度和激光切割的深度增大;随着浆料固含量的增加,受陶瓷颗粒对激光能量产生散射作用的影响,激光的切割深度减小.     

陶瓷增材制造

陶瓷的脆性和高硬度使得传统陶瓷成型工艺不易制备具有复杂形状和结构的陶瓷制件。本文总结了目前发展较快的激光选区熔融、激光选区烧结、三维打印、立体光固化、自由挤出成型等增材制造工艺在陶瓷领域的研究进展。面向复杂结构和高性能陶瓷制品的定制化快速制造需求,陶瓷增材制造技术展现出极大优势,在传统陶瓷行业、生物医疗等领域得到了应用。但是,陶瓷增材制造仍面临着打印材料及大尺寸、高致密度复杂结构陶瓷零件制造等难题,这些也将是增材制造技术未来发展的重要研究方向。     

Process development for the laser powder bed fusion of WC-Ni Cermets using sintered-agglomerated powder

Although ceramic particle-metal matrix materials (i.e., cermets) can offer superior performance, manufacturing these materials via conventional means is difficult compared to the manufacturing of metal alloys. This study leverages the laser powder bed fusion (LPBF) process to additively manufacture dense tungsten carbide (WC)-17 wt.% nickel (Ni) composite specimens using novel spherical, sintered-agglomerated composite powder. A range of processing parameters yielding high-density specimens was discovered using a sequential series of experiments comprised of single bead, multi-layer, and cylindrical builds. Cylinders with a relative density >99% were fabricated and characterized in terms of microstructure, chemical composition, and hardness. Scanning electron microscopy images show favorable wetting between the Ni binder and carbide particles without any phase segregation and laser processing increased the average carbide particle size. Energy dispersive X-ray and X-ray diffraction analyses detected traces of secondary products after laser processing. For samples processed at high energy densities, complex carbides and carbon agglomerate phases were detected. The maximum hardness of 60.38 Rockwell C is achieved in the printed samples. The successful builds in this study open the way for LPBF of dense WC-Ni parts with a large workable laser power-laser velocity processing window.     

Additive manufacturing of glass with laser powder bed fusion

Its transparency, esthetic appeal, chemical inertness, and electrical resistivity make glass an excellent candidate for small- and large-scale applications in the chemical, electronics, automotive, aerospace, and architectural industries. Additive manufacturing of glass has the potential to open new possibilities in design and reduce costs associated with manufacturing complex customized glass structures that are difficult to shape with traditional casting or subtractive methods. However, despite the significant progress in the additive manufacturing of metals, polymers, and ceramics, limited research has been undertaken on additive manufacturing of glass. In this study, a laser powder bed fusion method was developed for soda lime silica glass powder feedstock. Optimization of laser processing parameters was undertaken to define the processing window for creating three-dimensional multilayer structures. These findings enable the formation of complex glass structures with micro- or macroscale resolution. Our study supports laser powder bed fusion as a promising method for the additive manufacturing of glass and may guide the formation of a new generation of glass structures for a wide range of applications.     

聚醚醚酮的激光加工研究进展

综述了激光增材制造、激光扫描透射焊接、激光刻蚀等激光加工技术在聚醚醚酮及其复合材料加工制备领域中的应用及研究进展,并将该技术与传统加工方法进行比较,进一步说明激光加工技术的原理、优点以及现存的一些亟待解决的问题,最后展望了该技术广阔的发展前景。     

Crack-reduced alumina/aluminum titanate composites additive manufactured by laser powder bed fusion of black TiO2−x doped alumina granules

Laser powder bed fusion is an emerging industrial technology, especially for metal and polymer applications. However, its implementation for oxide ceramics remains challenging due to low thermal shock resistance, weak densification and low light absorptance in the visible or near-infrared range. In this work, a solution to increase the powder absorptance and to reduce cracking during laser processing of alumina parts is given. This is achieved by the use of a homogeneously dispersed and reduced titanium oxide additive (TiO_2?x) within spray-dried alumina granules leading to formation of aluminum titanate with improved thermal shock behavior during powder bed fusion. The impact of different reduction temperatures on powder bed density, flowability, light absorption and grain growth of these granules is evaluated. Crack-reduced parts with a density of 96.5%, a compressive strength of 346.6 MPa and a Young's modulus of 90.2 GPa could be manufactured using powders containing 50 mol% (43.4 vol%) TiO_2?x.     

Direct selective laser sintering of hexagonal barium titanate ceramics

A direct selective laser sintering (SLS) process was combined with a laser preheating procedure to decrease the temperature gradient and thermal stress, which was demonstrated as a promising approach for additive manufacturing of BaTiO₃ ceramics. The phase compositions in BaTiO₃ ceramics fabricated by SLS were investigated by X-ray and neutron diffractions. The surface morphologies and cross-section microstructures were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). A dense hexagonal h-BaTiO₃ layer was formed on the surface and extended to a depth of 500 μm, with a relative density higher than 97% and absence of pores or microcracks. SLS resulted in the formation of the high-temperature phase, h-BaTiO₃, which was retained at room temperature possibly due to the high cooling rate. The grain boundaries of SLSed h-BaTiO₃ ceramics consist of a Ti-rich secondary phase. Compared with that of the pressureless sintered t-BaTiO₃ ceramics, the Vickers hardness of SLSed h-BaTiO₃ is 70% higher.     

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.     

A review on laser deposition-additive manufacturing of ceramics and ceramic reinforced metal matrix composites

Ceramics and ceramic reinforced metal matrix composites (MMCs) are widely used in severe working conditions and have been applied in biomedical, aerospace, electronic, and other high-end engineering industries owing to their superior properties of high wear resistance, outstanding chemical inertness, and excellent properties at elevated temperatures. These superior properties, on the other hand, make it difficult to process these materials with conventional manufacturing methods, posing problems of high cost and energy consumptions. In response to this problem, direct additive manufacturing (AM), which is equipped with a high-power-density laser beam as heat source, has been developed and extensively employed for processing ceramics and ceramic reinforced MMCs. Compared with other direct AM processes, laser deposition-additive manufacturing (LD-AM) process excels in several aspects, such as lower labor intensity, higher fabrication efficiency, and capabilities of parts remanufacturing and functionally gradient composite materials fabrication. Besides these benefits, problems of poor bonding, cracking, lowered toughness, etc. still exist in LD-AM fabricated parts. This paper reviews developments on LD-AM of ceramics and ceramic reinforced MMCs in both bulk parts fabrication and cladding. Main issues to be solved, corresponding solutions, and the trend of development are summarized and discussed.     

Additive manufacturing of alumina using laser engineered net shaping: Effects of deposition variables

Alumina ceramic has been classified as one of difficult-to-manufacturing materials, the traditional manufacturing methods led to high cost and high energy consumption. In comparison to traditional manufacturing methods, laser engineered net shaping (LENS) additive manufacturing (AM) has many good properties to overcome the drawbacks of traditional manufacturing methods. However, the reported investigations on LENS provide limited information for qualities of deposition. In this paper, effects of LENS input deposition variables (laser power, deposition head scanning speed, and powder feeding rate) on deposition quality (such as layer geometry, surface roughness, flatness, powder efficiency, and microhardness) were studied. The obtained results will help to establish an efficient and effective process for ceramics part manufacturing.     

A critical review on sintering and mechanical processing of 3Y-TZP ceramics

Zirconia ceramics have been extensively applied in dental restoration due to their superior properties and excellent functionalities. Green-compact sintering and mechanical processing have become critical operations to shape these denture materials to target dimensions and desired quality. Improper sintering regulations and cutting-induced damages are crucial issues when dealing with the manufacturing of ceramic dentures as they adversely affect the performance and acceptance of eventually-machined denture products. In this paper, a critical review has been conducted to offer a scientific understanding of 3 mol% yttria-stabilized tetragonal zirconia polycrystal (3Y-TZP) processing, focusing on illustrating the characteristics and properties of the materials as well as the influence of the sintering process on the microstructure and machinability of the workpiece. Recent advances addressing the processing issues of zirconia ceramics for dental applications are carefully reviewed by critically analyzing the scientific findings reported in the open literature. The fundamental influences of the working conditions on the machining quality of ceramic materials are discussed. The features of emerging non-traditional machining technologies are compared and analyzed. Dentures manufacturers will benefit from this review article as they seek to achieve high-quality processing for zirconia ceramics.     

Frozen slurry-based laminated object manufacturing to fabricate porous ceramic with oriented lamellar structure

The freeze drying-based additive manufacturing can be used to process porous ceramics. However, the lack of freezing direction leads to the disorderly porous structure. This paper proposes a frozen slurry-based laminated object manufacturing (FS-LOM) for processing porous ceramics. Slurry was composed of water, alumina powder, and organic binder. The water in the fresh slurry layer crystallized to obtain a good support strength. The outline of 2D pattern was cut with laser to gasify ice crystal and binder. After stacking, the ice crystal freeze dried to obtain a porous structure. The lamellar ice crystals were induced to growth vertically by layer-by-layer freezing. The uniformity and orientation of the pore structure were improved, and the compression strength of the parts were improved. Due to the support of frozen slurry, the deformation of the green part was avoided.     

陶瓷增材制造(3D打印)技术研究进展

高性能陶瓷是现代技术发展和应用不可或缺的关键材料。常规的陶瓷制造技术难以满足对个性化、精细化、轻量化和复杂化的高端产品快速制造的需求。新兴的增材制造技术(3D打印)在高性能陶瓷的成型制造领域具有巨大的发展潜力,有望突破传统陶瓷加工和生产的技术瓶颈,为陶瓷关键零部件的应用开辟新的途径。本文针对陶瓷材料及其快速成型和后处理工艺,重点阐述了三维打印技术、光固化成型技术、选择性激光烧结技术等主流陶瓷增材制造技术的研究现状,并指出了目前存在的问题及发展趋势。     

Additive manufacturing of ceramic materials for energy applications: Road map and opportunities

Among engineering materials, ceramics are indispensable in energy applications such as batteries, capacitors, solar cells, smart glass, fuel cells and electrolyzers, nuclear power plants, thermoelectrics, thermoionics, carbon capture and storage, control of harmful emission from combustion engines, piezoelectrics, turbines and heat exchangers, among others. Advances in additive manufacturing (AM) offer new opportunities to fabricate these devices in geometries unachievable previously and may provide higher efficiencies and performance, all at lower costs. This article reviews the state of the art in ceramic materials for various energy applications. The focus of the review is on material selections, processing, and opportunities for AM technologies in energy related ceramic materials manufacturing. The aim of the article is to provide a roadmap for stakeholders such as industry, academia and funding agencies on research and development in additive manufacturing of ceramic materials toward more efficient, cost-effective, and reliable energy systems.     

Binder-free additive manufacturing of ceramics using hydrothermal-assisted jet fusion

Ceramic additive manufacturing (AM) typically uses a high fraction of organic binders to form pre-sintered green parts that require a post de-binding process to remove. The de-binding process inevitably results in severe gas expansion and residual chars, leading to structural defects, accumulated stress, and compromised material properties in the final parts. Here we report a binder-free additive manufacturing process named hydrothermal-assisted jet fusion (HJF) that utilizes a hydrothermal method to create geometrically and compositionally complex ceramics under mild temperatures. The HJF process employs a selectively deposited volatile dissolving ink, high pressure, and mild heat to strategically fuse a ceramic powder bed into complex geometries. Compared to traditional AM methods for ceramics, the HJF process eliminates the need for organic binders in green part fabrication and offers the potential to directly co-print ceramics with other dissimilar materials, such as polymers and metals, enabling the development of novel multi-functional ceramic composites.     

Femtosecond laser micro-milling dental glass ceramics: An experimental analysis and COMSOL finite element simulation

Dental glass ceramic materials are widely used in all-ceramic restoration technology. In order to effectively solve the problems existing in the process of traditional diamond cutter milling dental glass ceramic materials, such as severe needle loss, large tool wear and general milling efficiency, a new method of ultrafast laser milling dental glass ceramics is proposed. In this paper, 1030 nm femtosecond laser with pulse width of 600fs was used to micro-mill dental glass ceramics. Confocal laser microscopy was used to measure the milling depth and surface roughness of single-layer milling under selected laser processing parameters. The pre-layered milling software was developed to control the z-axis lifting and to compensate the focal length synchronously. Scanning electron microscope (SEM), Raman spectrometer and Vickers micro-hardness tester were used to characterize the dental glass ceramics after femtosecond laser milling. The results showed that under the specific laser processing parameters, the infrared femtosecond laser milling system can achieve a good processing morphology without changing the surface composition and surface hardness of dental glass ceramics. This new dental glass ceramics processing method based on ultrafast laser technique indicated a new direction for further chair processing of dental all-ceramic restoration technology.     

Preparation and indirect selective laser sintering of alumina/PA microspheres

Indirect selective laser sintering (SLS) is a promising additive manufacturing technique to produce ceramic parts with complex shapes in a two-step process. In the first step, the polymer phase in a deposited polymer/alumina composite microsphere layer is locally molten by a scanning laser beam, resulting in local ceramic particle bonding. In the second step, the binder is removed from the green parts by slowly heating and subsequently furnace sintered to increase the density. In this work, polyamide 12 and submicrometer sized alumina were used. Homogeneous spherical composite powders in the form of microspheres were prepared by a novel phase inversion technique. The composite powder showed good flowability and formability. Differential scanning calorimetry (DSC) was used to determine the thermal properties and laser processing window of the composite powder. The effect of the laser beam scanning parameters such as laser power, scan speed and scan spacing on the fabrication of green parts was assessed. Green parts were subsequently debinded and furnace sintered to produce crack-free alumina components. The sintered density of the parts however was limited to only 50% of the theoretical density since the intersphere space formed during microsphere deposition and SLS remained after sintering.     

Additive manufacturing of borosilicate glass via stereolithography

Recent surge in additive manufacturing efforts demonstrate stereolithography as a promising technique for fabricating glass materials due to the high speed and scalability of the process. However, little efforts have been devoted to manufacture borosilicate glass by the stereolithographic process. One of the challenges is that its relatively low softening temperatures could interfere with the thermal post-printing process and introduce poor fidelity and structure instability. Here, we report on the first demonstration of stereolithographic manufacturing of cerium-doped (Ce-doped) (<10%) and undoped borosilicate glass which was enabled by a multi-step post thermal processing. The optical properties of the printed glass depend on thermal processing parameters (temperature, time, and environment) and can be readily tuned and optimized for a wide range of applications. The printed amorphous glass shows good structural stability with band gap of 3 eV, Urbach energy of 0.75 eV and refractive index of 2.14 for 8% Ce-doped glass, respectively. These results indicate Ce-doped glass fabricated by stereolithography is suitable for scintillator applications and that additive manufacturing could be promising for borosilicate glass fabrication.     

Performance optimization of Si/SiC ceramic triply periodic minimal surface structures via laser powder bed fusion

SiC ceramic lattice structures (CLSs) via additive manufacturing (AM) have been recognized as potential candidates in engineering fields owing to their various merits. Compared with traditional SiC CLSs, SiC triply periodic minimal surface (TPMS) CLSs could possess more outstanding properties, making them more promising for wider applications. Since SiC CLSs are hard to be fabricated through stereolithography techniques because of inferior light performance, the laser powder bed fusion (LPBF) process via selective sintering is an effective method to prepare near-net-shaped SiC TPMS lattices. As the mechanical performances of lattice structures are the foundation for future practical applications, it is of great significance to optimize the preparation process, thus improving the mechanical properties of SiC TPMS structures. In this work, the optimal printing parameters of the LPBF and liquid silicon infiltration process for SiC ceramic TPMS CLSs with three different volume fractions were systematically illustrated and analyzed. The effects of the printing parameters and carbon densities on the fabrication accuracy, microstructure, and mechanical performance of SiC TPMS CLSs were defined. The mechanism of the reactive sintering process for the SiC TPMS lattice structure was revealed. The results reveal that Si/SiC TPMS CLSs with optimum preparation have superior manufacturing accuracy (most less than 6%), relatively high bulk densities (about 2.75 g/cm³), low residual Si content (6.01%), and excellent mechanical properties (5.67, 15.4, and 44.0 MPa for Si/SiC TPMS CLSs with 25%, 40%, and 55% volume fractions, respectively).