Microstructures,Thermal and Mechanical Properties of Pure Tungsten—A Comparison Between Selective Laser Melting and Hot Rolling

A comparative study on the influence of different manufacturing methods(selective laser melting and hot rolling)on the microstructure,mechanical and thermal behaviours of tungsten(W)was presented for the first time.The results indicated that the selective laser melting(SLM)W exhibited a finer grain sizes,a lower strength ductility,hardness and thermal conductivity compared to hot-rolled W.The main reason for this result was that the laser underwent rapid heating and cooling when it was used to melt W powder with high energy density,resulting in large internal stress in the sample after manufacturing.Subsequently,the internal stress was released,leading to the generation of microcracks at the grain boundaries,thereby affecting the performance of SLM W samples.In addition,the higher fraction of high-angle grain boundaries(HAGBs)of SLM W was found to be the key factor for intrinsic brittleness.Because the HAGBs are the preferred crack paths,which could promote crack propagation and decrease fracture energy.

Selective Laser Melting under Variable Ambient Pressure: A Mesoscopic Model and Transport Phenomena

Recent reports on the selective laser melting(SLM)process under a vacuum or low ambient pressure have shown fewer defects and better surface quality of the as-printed products.Although the physical process of SLM in a vacuum has been investigated by high-speed imaging,the underlying mechanisms governing the heat transfer and molten flow are still not well understood.Herein,we first developed a mesoscopic model of SLM under variable ambient pressure based on our recent laser-welding studies.We simulated the transport phenomena of SLM 316L stainless steel powders under atmospheric and 100 Pa ambient pressure.For typical process parameters(laser power:200W;scanning speed:2m∙s^(-1);powder diameter:27 lm),the average surface temperature of the cavity approached 2800 K under atmospheric pressure,while it came close to 2300 K under 100 Pa pressure.More vigorous fluid flow(average speed:4m∙s^(-1))was observed under 100 Pa ambient pressure,because the pressure difference between the evaporation-induced surface pressure and the ambient pressure was relatively larger and drives the flow under lower pressure.It was also shown that there are periodical ripple flows(period:14ls)affecting the surface roughness of the as-printed track.Moreover,the molten flow was shown to be laminar because the Reynolds number is less than 400 and is far below the critical value of turbulence;thus,the viscous dissipation is significant.It was demonstrated that under a vacuum or lower ambient pressure,the ripple flow can be dissipated more easily by the viscous effect because the trajectory length of the ripple is longer;thus,the surface quality of the tracks is improved.To summarize,our model elucidates the physical mechanisms of the interesting transport phenomena that have been observed in independent experimental studies of the SLM process under variable ambient pressure,which could be a powerful tool for optimizing the SLM process in the future.

中国特色、世界一流国家植物园的主要特征

2021年12月和2022年5月,国务院分别批复同意在北京设立国家植物园,在广州设立华南国家植物园。批复文件要求,国家植物园和华南国家植物园要建成中国特色、世界一流、万物和谐的国家植物园。国家植物园具有国家代表性、科学系统性和社会公益性。世界一流植物园应在植物迁地保护、科学研究、资源可持续利用、科普宣教和园林园艺展示等方面具有较强国际竞争力,且引领其他植物园发展。中国特色是指植物园在长期的实践中逐步形成的,具有中国特点的民族植物、传统文化、园林艺术、体制机制等。从植物园历史维度、功能维度和使命维度看,中国特色、世界一流的国家植物园的主要特征有:(1)具有一定的发展历史,在国家经济、社会发展和人民对美好生活需求的驱动下持续开展野生植物保护、研究与利用相关工作,在国内外植物园领域有一定的影响力;(2)在野生植物迁地保护、科学研究、资源可持续利用、科普宣教和园林园艺展示等5个功能领域,部分或全部处于国际领先水平;(3)服务国家重大战略,维护国家生态安全、生物安全、粮食安全,科技支撑国家或全球生物多样性保护、生态系统修复、气候变化应对和可持续发展。

《国家植物园体系布局方案》编制背景、程序、思路和重点考虑

2021年10月,习近平总书记宣布启动北京、广州等国家植物园体系建设。为高质量推进国家植物园体系建设,国家林业和草原局会同住房和城乡建设部、中国科学院编制了《国家植物园体系布局方案》。综合考虑国家重大战略、主要气候类型与典型植被区划特点、生物多样性保护优先区域、服务经济社会发展需要等因素,根据现有植物园的分布情况、工作基础,依据国家代表性、科学系统性、社会公益性等准入条件,在已设立2个国家植物园的基础上,遴选了14个国家植物园候选园纳入空间布局,构建布局合理、功能互补的国家植物园体系,按照“成熟一个、设立一个”原则,稳步推进国家植物园体系建设。本文简要介绍了《国家植物园体系布局方案》编制背景、程序、思路和重点考虑,展望未来国家植物园体系建设。国家植物园体系建设将有效发挥国家植物园在生物多样性保护中的重要作用,为实施《昆明-蒙特利尔全球生物多样性框架》提供中国方案。

Electrochemical Corrosion Behavior and Mechanical Response of Selective Laser Melted Porous Metallic Biomaterials

The porous metallic biomaterials have attracted significant attention for implants because their lower young's modulus matches the human bones, which can eliminate the stress shielding effect and facilitate the growth of bone tissue cells. The porous metallic biomaterials fabricated by selective laser melting (SLM) have broad prospects, but the surface of the SLM-built porous structure has been severely adhered with unmelted powders, which affects the forming accuracy and surface quality. The porous metallic biomaterials face the corrosion problem of complex body fluid environments during service, so their corrosion resistance in the human body is extremely important. The surface quality will affect the corrosion resistance of the porous metallic biomaterials. Therefore, it is necessary to study the effect of post-treatment on the corrosion resistance of SLMed samples. In this work, the mechanical response and the electrochemical corrosion behavior in simulated body fluid of diamond and pentamode metamaterials Ti-6Al-4V alloy fabricated by SLM before and after sandblasting were studied. After sandblasting, the mechanical properties of the two porous metallic biomaterials were slightly improved, and the self-corrosion potential and pitting potential were more negative;meanwhile, the self-corrosion current density and passive current density increased, indicating that its corrosion performance decreased, and the passive film stability of sandblasted samples got worse.

Mechanical properties and microstructure characteristics of lattice-surfaced PEEK cage fabricated by high-temperature laser powder bed fusion

Porous structure design on the contact surface is crucial to promote the osseointegration of the intervertebral cage while preventing subsidence and displacement.However,the stress response will undergo significant changes for the current random porous cages,which can directly affect the mechanical properties and long-term usability.Here,this paper proposed a newly designed polyetheretherketone(PEEK)cage with the triply periodic minimal surface(TPMS)-structured lattice surfaces to provide tailored 3 D microporosity and studied the mechanical performance,stress/strain responses,and microstructure changes in depth.The lattice-surfaced PEEK cage mainly exhibits a multiple-point-plane stress transfer mechanism.The compression modulus and elastic limit can be adjusted by controlling the area of the Diamond TPMS surface while the energy absorption efficiency remains stable.The microstructure of high-strength PEEK is featured by the radial pattern morphology.Meanwhile,the double-stranded orthorhombic phase is more ordered,and the benzene plane subunit and lattice volume become more expanded.

Large Curvature Folding Strategies of Butterfly Proboscis

Due to its real-time control,high folding ratio,and structure self-locking,flexible large curvature self-folding devices have broad application prospects,such as foldable human implants,flexible electronics,and flexible robots.Driven by this background,flexible large curvature folding butterfly(Polyura eudamippus)proboscises were studied in this work.The folding ratio of the proboscises was about 15.The curvature of coiled proboscises ranged from about 150 m_1 to 880 m The external and internal structures of the proboscises were studied by different methods.Three main strategies for large-curvature folding of proboscises were identified:a gradual decrease in thickness,a lower elastic modulus,and(most importantly)large numbers of regular corrugated cracks arranged on the surface.These corrugated cracks can effectively accommodate the coiled strain and provide space for the large curvature folding of proboscises.Finally,a 4D printed coiled sample with corrugated cracks was fabricated to mimic the proboscises stretching process.Large-curvature folding strategies,based on these proboscises,provide insights for the biomimetic design of artificial highly folded components.