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。     

Numerical modeling of spark plasma sintering—Discussion on densification mechanism identification and generated porosity gradients

A multi-physical numerical model of the SPS process has been developed in order to evaluate electrical, thermal and mechanical fields undergone by the powder during sintering. The reliability of this numerical model has been validated by comparison with experimental data carried out on a submicrometric alpha alumina powder. This numerical approach is applied to achieve a critical analysis of the main methodologies used in the literature for the identification of the densification parameters, and the possible misinterpretations of involved deformation mechanisms. Moreover, as shown during sintering of alumina pellets of larger diameter (50 mm in diameter), the induced porosity gradient seems to be mainly correlated to the existence of thermal gradient within the powder, and in a lesser level to stress gradient. Consequently, the temperature gradient seems to be a crucial point to be controlled during SPS process of large size samples in order to obtain fine and homogeneous microstructure.     

Effects of pressure on densification behaviour,microstructures and mechanical properties of boron carbide ceramics fabricated by hot pressing

Effects of pressure, from ordinary (30 MPa) to high pressure (110 MPa), on densification behaviour, microstructures and mechanical properties of boron carbide ceramics sintered by hot pressing are investigated. With increasing pressure, the relative density sharply increases within 30–75 MPa, slowly increases within 75–100 MPa and finally stagnates. For samples within 75–100 MPa, densification begins at approximately 1000 °C, and the dominant densification process ends before the soaking stage. High relative densities of 98.49% and 99.76% are achieved. For samples within 30–50 MPa, densification begins at approximately 1500 °C, and the soaking stage (initial 20 min) is still important for the dominant densification process. The final relative densities are only 87.90% and 92.32%. The above-mentioned differences are derived from contributions of pressure, and the dominant densification mechanism under high pressure is plastic deformation. The average grain size of the samples slightly increases with increasing soaking time. The grain size under higher pressure is larger than that under lower pressure at corresponding periods because grains grow easily with reduced pores. Vickers hardness and fracture toughness increase as grain size decreases in fully dense samples. However, when the samples do not achieve full density, relative density becomes more influential than grain size in hardness and toughness. A soaking time of 30 min is enough for samples under 100 MPa. Prolonging the soaking time has deleterious effects on mechanical properties. The relative density, grain size, hardness and fracture toughness of the samples under 100 MPa for 30 min are 99.73%, 1.96 µm, 37.85 GPa and 3.94 MPa m^1/2, respectively.     

JA-281对高铝泥浆流变性能的影响

本文主要探讨在高铝泥浆制备过程中，选用不同解凝剂（ＪＡ－２８１，阿拉伯树胶、水玻璃、碳酸钠）的原则，通过对泥浆粘度、稠化系数、吸浆速度等参数的测定，来探索单一添加和复合添加ＪＡ－２８１对高铝泥浆流变性能的影响及其解凝机理。     

Microstructural coarsening during supersolidus liquid phase sintering of alpha brass

Coarsening of the grains and pores during sintering has a pronouncedly negative effect on the densification of prealloyed brass powder compacts. This investigation examines the role of sintering variables in realising the complicated effects on densification and microstructure. Experiments were designed to model and evaluate the effect of sintering parameters such as temperature, time and furnace atmosphere on densification, grain and pore intercept as well as pore number. The study of microstructures suggests that there is a good correlation between grain and pore intercepts. It is concluded that pore coarsening is a result of supersolidus liquid phase sintering of Cu28Zn powder, and it can retard densification, which is in acceptable agreement with the experimental data.     

Grain boundary segregation and microwave dielectric properties of low loss Mg1·5Zn0·5SiO4 ceramics containing Bi2O3

Abstract

Abstract

Uncommon low loss Mg_1·5Zn_0·5SiO₄ ceramics containing Bi₂O₃ were investigated by focusing on the roles of Bi₂O₃ on phase evolution and resultant microwave dielectric properties. While the primary goal of lowering sintering temperature can be easily assumed, some unexpected behaviours of the Bi₂O₃ containing materials are highlighted with experimental evidences concerning selective dissolution of Zn₂SiO₄ and grain boundary segregation of gradual Bi richer phases. These evidences are strongly dependent on the content of Bi₂O₃ and sintering temperature. As an optimal composition, Mg_1·5Zn_0·5SiO₄ with 0·5?mol.-%Bi₂O₃ exhibited promising dielectric properties of a k value ～6·8 and a Q×f value ～23?300 at a sintering temperature of 1150°C, which is much lower than typical sintering temperature of 1450°C.     

Evaluation of the sinterability,densification behaviour and microhardness of spark plasma sintered multiwall carbon nanotubes reinforced Ti6Al4V nanocomposites

Structural and industrial demands for lightweight engineering materials with exclusive properties have been rising in recent decades for automobile and aerospace applications. This has encouraged various innovations in materials engineering communities to synthesis advanced engineering materials using improved fabrication technique such as spark plasma sintering (SPS). In this study, titanium-based nanocomposites were synthesized by reinforcing Ti6Al4V reinforced with (0.5, 1.0 and 1.5 wt%) multiwall carbon nanotubes (MWCNT) powders. The starting powders were blended by shift-speed ball milling. Thereafter, SPS technique was used to consolidate the admixed powders by employing the following sintering parameters; sintering rate, 100 °C/min, compressive pressure, 50 MPa, holding time, 10 min and sintering temperatures of 900–1100 °C. The influence of MWCNT additions on the sinterability, densification behaviours and microhardness of the sintered nanocomposites were investigated. The results revealed that the densification of the sintered nanocomposites was in the range of 97.51–99.61% which decreased with an increase in concentration of the MWCNT. Meanwhile, the densification and microhardness improved tremendously with an increase in sintering temperatures.     

Flash spark plasma sintering of cold-Pressed TiB2-hBN

Further improvements over the unprecedented gains in energy and time efficiency already achieved by the flash spark plasma sintering (FSPS) technique were won by eliminating the pre-sintering stage from the process. Cold-pressed discs (Ø 35 mm) of TiB₂-hBN were successfully flash sintered from powder to nearly dense material (up to 97%) in less than 110 s (∼60 s sintering time) in a SPS furnace. The issue of thermal gradients during FSPS was resolved and improvements to the homogeneity of the sample were achieved by modifying the experimental set-up to restrict the current flow through the system. In addition to ease of automation, another advantage of the proposed die-less FSPS configuration is a texturing effect typically achieved by hot forging.     

3D printing of complex shaped alumina parts

Alpha-alumina powder was mixed with methyl cellulose as a binder with concentration as low as 0.25% by weight in an aquoes medium and kneaded in a high shear mixer to obtain a printable paste. The paste was subjected to rheological measurements and exhibited a shear rate exponent of 0.54 signifying the shear thinning behavior. The paste was used for printing parts with various shapes according to CAD model by employing a ram type 3D printer. Printed parts were dried and the green density was determined. Further, the parts were also subjected to X-ray radiography in order to evaluate the possible occurrence of printing defects. The samples were sintered under pressureless condition at 1650?°C in a muffle furnace and Hot Isostsically Pressed (HIP) at 1350?°C and a pressure of 1650?bar using a vacuum encapsulated SS CAN. Hot Isostatic pressing resulted in a higher density of 3.94?g/cc in comparison to 3.88?g/cc obtained under pressureless conditions and also shown superior mechanical properties. HIPing of 3D printed samples not only resulted in possible healing of printing defects as reavealed by X-ray radiography but also enhanced the diffusion at low temperature of 1350?°C leading to finer grain sizes as complemented by the microstructure.     

Composition dependence of indentation deformation and indentation cracking in glass

Crack initiation and deformation behaviors of oxide glasses belonging to different chemical systems were studied using the Vickers indentation test. The crack initiation resistance is chiefly governed by the extents to which densification and isochoric shear flow develop in a process zone beneath and within the contact area. Densification is favored in glasses with relatively small Poisson’s ratio (ν), whereas shear is favored at large ν. Glasses were ranged according to their resistance to the formation of corner cracks as follows: Resilient, for 0.15 ? ν ? 0.20; Semi-Resilient, for 0.20 ? ν ? 0.25; and Easily-Damaged for 0.25 < ν < 0.30. Radial-median cracks occur at low load (?50 mN) in Easily-Damaged glasses, while cone cracks predominate in Resilient glasses under higher loads. A critical value for ν (～0.22 depending on the Young’s modulus/hardness ratio) was identified, at which the intensity of the indentation stress field tends to vanish, preventing crack formation on loading, while the driving force on unloading remains very small.