Numerical simulation of the effects of elastic anisotropy and grain size upon the machining of AA2024

Turning modeling and simulation of different metallic materials using the commercially available Finite Element (FE) softwares is getting prime importance because of saving of time and money in comparison to the costly experiments. Mostly, the numerical analysis of machining process considers a purely isotropic behavior of metallic materials; however, the literature shows that the elastic crystal anisotropy is present in most of the ‘so-called’ isotropic materials. In the present work, the elastic anisotropy is incorporated in the FE simulations along with the effect of grain size. A modified Johnson-Cook ductile material model based on coupled plasticity and damage evolution has been proposed to model the cutting process. The simulation results were compared with experimental data on the turning process of Aluminum alloy (AA2024). It was found that the elastic anisotropy influences the average cutting force up to 5% as compared to the isotropic models while the effect of grain size was more pronounced up to 20%.     

Semiconductor‐ionic materials could play an important role in advanced fuel‐to‐electricity conversion

Functional semiconductor‐ionic materials can be used to realize a single component or so‐called “three‐in‐one” fuel cell design. Such materials integrate the functionalities of fuel cell's anode, electrolyte, and cathode into one component. The underlying principle of a single‐component fuel cell design combines material band structures with ionic species/transport. The performance values of such devices could exceed that of traditional fuel cells. This could represent a major progress in fuel cell science and technology and lies grounds for a new direction of fuel cell R&D and commercialization.     

Optimization of C/TiCx duplex diffusion barrier coatings for SiCf/Ti composites based on interfacial structure evolution exploration

Introducing a carbon single coating is a popular method used to protect SiC_f/Ti composites from severe interface reactions. However, carbon coatings lose their protective effect on SiC fibres at high temperature, even after a short period time. As such, given the strong demand for high temperature applications in aeronautics and astronautics a more coating which is more effective at high temperatures is desirable. In order to improve the high temperature interfacial stability of SiC_f/Ti composites, a C/TiC_x duplex coating system with different C contents in TiC_x was introduced to explore the protection of fibres at 1200?°C for 1?h. The results show that the C/quasi-stoichiometric TiC coating system protects the SiC fibres most effectively. Based on insights from the evolution of the interface structure, TiC_x has been identified as an interfacial reaction product from the C single coating, exhibiting a gradient in C content and grain size, which is different from a deposited TiC layer with a well-distributed composition and structure. The different coating structure gives rise to different ability to resist C diffusion at high temperatures, in which poor resistance ability appears in TiC_x interfacial reaction layer coming from C single coating due to short-circuit diffusion in C-rich fine-grained TiC layer and fast intracrystalline diffusion trigged by amounts of vacancies in sub-stoichiometric coarse-grained TiC layer. Therefore, C/quasi-stoichiometric TiC duplex coatings with a thick, coarse-grained quasi-stoichiometric TiC layer could effectively inhibit C diffusion by comparison to C single coatings, and is more effective than C/rich-carbon TiC duplex coatings due to the existence of short-circuit diffusion in the latter. As such, C/quasi-stoichiometric TiC duplex coatings appear to be an optimal diffusion barrier for SiC_f/Ti composites at high temperature.     

A novel proposal for all optical 3 to 8 decoder based on nonlinear ring resonators

In this paper, a 3 to 8 optical decoder was proposed using nonlinear photonic crystal ring resonators. For realizing the 3 to 8 decoder, we combined seven 1 to 2 optical decoders. In the proposed structure, X, Y and Z serve as input ports. By combination of these ports, one can control which output port to be ON. The maximum time delay of the proposed structure is about 6?ps.     

多次真空固溶处理对GH1131高温合金组织和力学性能的影响

利用真空高压气淬工艺研究了GH1131高温合金经1100~1170 ℃范围内多次真空固溶处理后的组织和力学性能。结果表明,固溶态组织包含奥氏体晶粒和颗粒状碳化物,且随着固溶温度的升高,晶粒度稍有增大趋势。GH1131高温合金经多次真空固溶处理后组织均匀,常温与高温力学性能稳定,经1100 ℃+1130 ℃+1170 ℃三次真空固溶处理后,晶粒度维持了6~8级,900 ℃高温抗拉强度达到200 MPa。     

基于ANSYS及加速寿命试验台对油缸单耳环活塞杆杆头断裂进行设计改进

该文对某工程机械油缸单耳环活塞杆杆头断裂进行失效分析,提出了在毛坯锻造工艺满足设计要求的条件下,单耳环活塞杆杆头变截面处应力集中导致残余应力过大,是单耳环活塞杆杆头断裂的主要原因。从这方面出发,优化了单耳环活塞杆杆头变截面处的残余应力,通过ANSYS软件对优化后的结构进行应力分析,并用油缸加速寿命可靠性试验台做2F台架实验和整机多批次小批量试装来验证改进措施的有效性,最终提高了油缸单耳环活塞杆杆头在极限工况下抗疲劳的强度。     

CVD金刚石表面增透膜的研究进展

CVD金刚石膜因特有的物理化学性质，具有发展成为新一代光学材料的前景。但由于CVD金刚石膜自身局限性导致其理论透过率不到71%，在金刚石膜表面镀制增透膜，通过改变增透膜组成成分、显微组织和晶体结构，可有效地改善CVD金刚石膜自身理论透过率的问题。首先，介绍了CVD金刚石表面镀制单层增透膜增透原理，并总结了物理和化学气相沉积技术制备增透膜的优缺点。然后，重点综述了近年来CVD金刚石表面氮化物、金属氧化物和稀土金属氧化物等增透膜材料的研究进展，详细分析了增透膜制备参数、热处理工艺、衬底表面改性和掺杂工艺对增透膜整体组织和性能影响的规律。其中优化增透膜沉积温度、氧分压和热处理等工艺参数，是通过改变增透膜微观组织形貌以及晶体结构来提高其光学透过性能，而改变衬底表面结构能够通过改变增透膜与基体之间的成键方式来提升界面结合能力，而稀土元素掺杂方式是通过改变增透膜化学组成成分来改善增透膜的光学透过性能，并指出掺杂元素成型机理和影响机制。最后，展望了未来CVD金刚石表面增透膜的发展方向。     

高温度梯度定向凝固单相Mg2Sn合金的制备及其热电性能

高温度梯度(180K/cm)定向凝固方法可制备单相Mg_2Sn晶体,通过凝固理论对平-胞转换临界速率进行了计算,并预测了单相Mg_2Sn晶体的生长距离,与试验结果相吻合。此方法获得的Mg_2Sn晶体由于去除了第二相Sn的影响,可以获得更好的热电性能,在测试温度区间300～700K内,未掺杂条件下最大Seebeck系数和电导率值分别可达-261μV·K~(-1)和525?-1·m~(-1),通过Bi掺杂来对电导率进行优化后,功率因子最高可达2.29 mW·(m·K~2)~(-1)。单相Mg_2Sn晶体的热导率也得到大幅降低,500 K时,最小值为4.3 W·(m·K)~(-1),Bi掺杂量为1.5%(原子分数)时,热电优值ZT最高可达到0.21。这一方法可以为制备高性能的Mg_2B~(IV)体系三元固溶体合金提供参考。     

Modifying Crystallinity,Morphology, and Photophysical Properties of Carbon Nitride by Using Crystals as Reactants

Here we show a general method to improve the crystallinity of carbon nitride nanosheets by using melamine crystals as reactants for the high temperature synthesis. Additionally, the crystals were calcined in a sealed ampoule, which provides a pressurized environment, yielding crystalline carbon nitride nanosheets that display altered morphology and photophysical properties compared to the reference materials. Electronic microscopy, optical and photoelectrochemical measurements prove the modifications of parameters such like band structure, charge recombination or inter-layer distance. The new growth strategy presented here opens many opportunities for the design of crystalline materials with tailored properties for different applications.     

Study of fragmentation in cBN powders under ultra-high pressure

Studying the fragmentation law and refinement of cubic boron nitride powder under ultra-high pressure is crucial to producing a high-strength, high-density polycrystalline cubic boron nitride. In this paper, brown and black cBN crystalline powders with different micron sizes were selected as initial raw materials for an ultra-high-pressure simulation experiment. Single and mixed particles were extruded under 80MPa low pressure and 5.5GPa ultra-high pressure at ambient temperature for 1 min. The crushing behavior and particle size distribution of cBN powders with different particle sizes and ratios were investigated using a laser particle size analyzer and scanning electron microscopy. Results revealed no particle breakage or deformation at low pressure, and the compaction density was low. However, under ultra-high pressure, the cBN particles showed cracks, plastic deformation, and fragmentation, resulting in crushed fine particles filling in the voids of coarse particles, which led to a higher pressing density. Small-sized or mixed cBN particles with high density ratios were not easily crushed; the coarser the particle size, the more severe the ultra-high-pressure extrusion and crushing. The pressing density also declined, and brown cBN crystal particles with higher impact toughness demonstrated a lower particle breakage rate. The ultra-high-pressure crushing law should be considered and appropriate binders should be selected to improve the sintering performance of PcBN materials; ultra-high-pressure crushing of cBN powder contributes to cBN-cBN and cBN-M-cBN bonds under high temperatures and ultra-high pressure.