自然对流对相变材料熔化过程的影响分析

为分析自然对流对相变材料熔化过程的影响,建立了二维瞬态、固/液相统一的控制方程,采用SIMPLE算法对底部加热、顶部冷却的方腔内相变材料熔化过程进行了数值仿真,研究了方腔内温度场和速度场随时间变化规律,结果表明：自然对流使液相区等效导热系数增大;随着固液界面向上移动,液相区自然对流逐渐增强,流动状态不断变化。

Evolution of microstructure and mechanical properties in multi-layer 316L-TiC composite fabricated by selective laser melting additive manufacturing

In this study,the microstructure and mechanical properties of a multi-layered 316L-TiC composite material produced by selective laser melting(SLM)additive manufacturing process are investigated.Three different layers,consisting of 316L stainless steel,316L-5 wt%TiC and 316L-10 wt%TiC,were additively manufactured.The microstructure of these layers was characterized by optical microscopy(OM)and scanning electron microscopy(SEM).X-ray diffraction(XRD)was used for phase analysis,and the mechanical properties were evaluated by tensile and nanoindentation tests.The microstructural observations show epitaxial grain growth within the composite layers,with the elongated grains growing predominantly in the build direction.XRD analysis confirms the successful incorporation of the TiC particles into the 316L matrix,with no unwanted phases present.Nanoindentation results indicate a significant increase in the hardness and modulus of elasticity of the composite layers compared to pure 316L stainless steel,suggesting improved mechanical properties.Tensile tests show remarkable strength values for the 316L-TiC composite samples,which can be attributed to the embedded TiC particles.These results highlight the potential of SLM in the production of multi-layer metal-ceramic composites for applications that require high strength and ductility of metallic components in addition to the exceptional hardness of the ceramic particles.

柱形93钨弹体超高速撞击薄钢板穿孔及破片群扩展特征

为进一步研究柱形弹体超高速撞击靶板的破片群扩展、弹体侵蚀等问题,开展柱形93钨弹体超高速撞击薄钢板实验研究。通过量纲分析方法给出柱形弹体穿靶的穿孔直径经验公式;利用高速摄像技术获得靶后破片群运动图像,分析破片群扩展规律以及弹体的侵蚀规律;基于微观组织分析,探索超高速撞击中弹靶材料的熔化问题。结果表明:通过对实验数据的拟合,认为在弹靶材料不变的情况下,靶板穿孔直径、靶后破片群轴向扩展最大速度、横向扩展最大速度近似和弹体直径、靶板厚度、撞击速度相关,而对于弹体侵蚀长度,除上述参数外还与弹体长径比相关;在柱形弹体超高速撞击靶板问题中,靶板背表面产生层裂并在破片群前部形成速度大于剩余弹体速度的“尖端”,可近似由靶板厚度小于弹体直径的0.72倍来确定;当柱形93钨弹体以2~3 km/s速度撞击靶板时,靶后破片群尚未发生大范围熔化,但当破片群、剩余弹体撞击第2层靶板时,受到二次加载作用,撞击区附近将发生大范围的材料熔化。

ThermalTake iXoft

与一般的笔记本敏热器不同，这款产品的设计非常特别。首先它并不是我们常见的那种笔记本散热板，而是“笔记本散热垫”。它的表面为光滑而柔软的化纤材料，底部则是柔软的海绵以及具有防滑作用的条绒材质。它的内部有许多熔点很高的特殊材料，利用热传导原理吸收热量，进而使内部的固体材料熔化。在固体材料完全变成液态之前，这个散热垫可以一直吸收笔记本底部的热量，直至全部固态材料液化才会与笔记本的温度达到一种平衡状态。

Dual-Material Electron Beam Selective Melting:Hardware Development and Validation Studies

Electron beam selective melting （EBSM） is an additive manufacturing technique that directly fabricates three-dimensional parts in a layerwise fashion by using an electron beam to scan and melt metal powder. In recent years, EBSM has been successfully used in the additive manufacturing of a variety of materials. Previous research focused on the EBSM process of a single material. In this study, a novel EBSM process capable of building a gradient structure with dual metal materials was developed, and a powder-supplying method based on vibration was put forward. Two different powders can be supplied individually and then mixed. Two materials were used in this study： Ti6AI4V powder and Ti47AI2Cr2Nb powder. Ti6AI4V has excellent strength and plasticity at room temperature, while Ti47AI2Cr2Nb has excellent performance at high temperature, but is very brittle. A Ti6AI4V/Ti47AI2Cr2Nb gradient material was successfully fabricated by the developed system. The microstructures and chemical compositions were characterized by optical microscopy, scanning microscopy, and electron microprobe analysis. Results showed that the interface thickness was about 300 μm. The interface was free of cracks, and the chemical compositions exhibited a staircase-like change within the interface.

Microstructure,mechanical properties and corrosion performance of selective laser melting Ti/GNPs composite with a porous structure

In this study,nano-graphene reinforced titanium matrix composites(GNPs/Ti)with a honeycomb porous structure were fabricated by selective laser melting(SLM).The effects of graphene on the microstructure,mechanical properties and corrosion performance of the SLM GNPs/Ti were systematically investigated.Results of microstructure characterization show that:1)the density of the SLM GNPs/Ti was improved as compared to that of the SLM Ti;2)abundant TiC particles were formed in the SLM GNPs/Ti.The hardness and compressive strength of the composite increased by 90%(from HV 236 to HV 503)and 14%(from 277 MPa to 316 MPa),respectively,attributed to the uniformly distributed TiC and fine GNPs in the Ti matrix.Electrochemical tests reveal that the corrosion current density of the SLM GNPs/Ti is only 0.328μA/cm^(2),that is about 25%less than that of the SLM Ti.The results indicate that the incorporation of nano-graphene is a potential method to strengthen the Ti by SLM.

A review of particulate-reinforced aluminum matrix composites fabricated by selective laser melting

Selective laser melting(SLM)is an emerging layer-wise additive manufacturing technique that can generate complex components with high performance.Particulate-reinforced aluminum matrix composites(PAMCs)are important materials for various applications due to the combined properties of Al matrix and reinforcements.Considering the advantages of SLM technology and PAMCs,the novel SLM PAMCs have been developed and researched in recent years.Therefore,the current research progress about the SLM PAMCs is reviewed.Firstly,special attention is paid to the solidification behavior of SLM PAMCs.Secondly,the important issues about the design and fabrication of high-performance SLM PAMCs,including the selection of reinforcement,the influence of parameters on the processing and microstructure,the defect evolution and phase control,are highlighted and discussed comprehensively.Thirdly,the performance and strengthening mechanism of SLM PAMCs are systematically figured out.Finally,future directions are pointed out on the advancement of high-performance SLM PAMCs.

Determination of Hf–Sr–Nd isotopic ratios by MC-ICP-MS using rapid acid digestion after flux-free fusion in geological materials

In this study, we established a rapid acid digestion for determining Hf-Sr-Nd isotopic ratios of geological samples by using MC-ICP-MS. Conditions of 1600 ℃ for 1 min and 1400 ℃ for 1 min were adopted for fusing intrusive rocks and extrusive rocks, respectively. The rapid acid digestion technique is superior in digestion time compared with high-pressure PTFE bomb method. The procedural blanks of the method were also lower than that flux fusion. Replicate analyses of international certified reference materials （CRMs） indicate that isotopic ratios of ^176Hf/^177Hf, ^87Sr/^86Sr and 143Nd/144Nd agree well with previously published data. The external reproducibility （2SD, n = 5） of ten CRMs are ±0.000030 for ^87Sr/^86Sr, ± 0.000030 for ^143Nd/^144Nd, and ±0.000018 for ^176Hf/^177Hf.

Comparison of Effects of Different Organo-montmorillonites on PA 66 / Clay Nanocomposites

Different organo-montmorillonites （OMMTs） are prepared by modifying montmorillonites （MMTs） with CTAB, PA 1010 salts/CTAB, PEG/CTAB, PVA/CTAB, PVP, PVP/ CTAB and PA 6/CTAB, respectively. These OMMTs were studied by X-ray diffraction and TG. The gallery sizes of them are all larger than that of sodium MMTs. And the decomposition temperatures of them are all much higher than that of MMT and the processing temperature of PA 66, especially OM - 6 and OM - 7, the decomposition temperatures of which are 451.6℃ and 439.1℃, almost the collapse temperature of the native MMT crystal lattice which is more than 5080C. Then PA 66/Clay nanocomposites were synthesized by mixing these OMMTs with PA 66 matrix via melt intercalation. Experimental results indicate that the tensile and flexural properties increase significantly, especially those of PCN - 08. The combination property of PCN- 08 is the best. TEaM photos show that some clay platelets are present in the matrix as exfollated layers, while most of the clay platelets are present as intercalated layers.

Effect of Cu addition on microstructural evolution and hardening of mechanically alloyed Al-Ti-O in-situ composite

The microstructure formation and strengthening of an Al-5 wt.%TiO2 composites with additions of 5 wt.%Cu and 2 wt.% stearic acid(as a process control agent, PCA) during mechanical alloying and subsequent thermal exposure were studied. The powder composites were prepared by high-energy ball milling for up to 10 h. Single line tracks of the powders were laser melted. Optical and scanning electron microscopy, XRD analysis and differential scanning calorimetry were used to study microstructural evolution. The results showed that the Cu addition promotes an effective mechanical alloying of aluminum with Ti O2 from the start of milling, resulting in higher microhardness(up to HV 290), while the PCA, on the contrary, postpones this process. In both cases, the composite granules with uniform distribution of Ti O2 particles were formed. Subsequent heating of mechanically alloyed materials causes the activation of an exothermic reaction of Ti O2 reduction with aluminum, the start temperature of which, in the case of Cu addition,shifts to lower values, that is, the transformation begins in the solid state. Besides, the Cu-added material after laser melting demonstrates a more dispersed and uniform structure which positively affects its microhardness.

Molten salt electrosynthesis of silicon carbide nanoparticles and their photoluminescence property

A one-step molten salt electrochemical strategy was proposed to synthesize SiC nanoparticles from ultra-fine silicon dioxide/carbon(SiO_(2)/C)mixtures.The electrosynthesis process and physicochemical properties of the synthesized products were systematically analyzed via X-ray diffraction,electron microscopy,Raman spectroscopy and photoluminescence spectroscopy,etc.A combined chemical/electrochemical reaction,electrochemical deoxidation,and in-situ carbonization reaction mechanism was proposed to reveal the electrochemical synthesis process of SiC nanoparticles from SiO_(2)/C in molten CaCl_(2).The as-prepared SiC with particle size ranging from 8 to 14 nm possesses a polycrystalline structure.In addition,the SiC nanoparticles demonstrate obvious photoluminescence property due to the synergetic size effect and microstructural characteristics.

In-situ Polymerization-modification Process and Foaming of Poly（ethylene terephthalate）

Most of traditional linear poly(ethylene terephthalate)(PET)resins of relatively low molecular mass and narrow molecular mass distribution have low melt strength at foaming temperatures,which are not enough to support and keep cells.An in-situ polymerization-modification process with esterification and polycondensation stages was performed in a 2 L batch stirred reactor using pyromellitic dianhydride(PMDA)or pentaerythritol(PENTA)as modifying monomers to obtain PETs with high melt strength.The influence of amounts of modifying monomers on the properties of modified PET was investigated.It was found that the selected modifying monomers could effectively introduce branched structures into the modified PETs and improve their melt strength.With increasing the amount of the modifying monomer,the melt strength of the modified PET increased.But when the amount of PENTA reached 0.35%or PMDA reached 0.9%,crosslinking phenomenon was observed in the modified PET.Supercritical carbon dioxide(ScCO2)was employed as physical foaming agent to evaluate the foaming ability of modified PETs.The modified PETs had good foaming properties at 14 MPa of CO2pressure with foaming temperature ranging from 265°C to 280°C.SEM micrographs demonstrated that both modified PET foams had homogeneous cellular structures,with cell diameter ranging from 35μm to 49μm for PENTA modified PETs and38μm to 57μm for PMDA modified ones.Correspondingly,the cell density had a range of 3.5×107cells·cm 3to 7×106cells·cm 3for the former and 2.8×107cells·cm 3to 5.8×106cells·cm 3for the latter.

更小意味着更复杂

小型和微型部件对注射成型技术提出了极高的要求。以往，人们对微注射成型技术的关注只是简单地体现在如何去达到并实现这些技术要求。

通用新型6缸全铝汽油机

通用汽车公司开发出一种全新的直列6缸Vortec 4200汽油机.该机既有V8发动机的动力性和运转平稳性,又有6缸机优良的燃料经济性.这种汽油机已于去年开始在美国密执根州韦林特市的新工厂里生产,并将成为2002年款奥兹莫比尔分部的Bravada、GMC分部的Envoy和雪佛兰分部的TraiBlazer等多用途运动车的标准装备.

Thermal Analysis of a Phase Change Material Based Heat Sink for Cooling Protruding Electronic Chips

This work aims to numerically study the melting natural convection in a rectangular enclosure heated by three discreet protruding electronic chips. The beat sources generate heat at a constant and uniform volumetric rate. A part of the power generated in the heat sources is dissipated to a phase change material （PCM, n-eicosane with melting temperature, Tm = 36℃）. Numerical investigations were carded out in order to examine the effects of the plate thickness on the maximum temperature of electronic components, the percentage contribution of plate heat conduction on the total removed heat and temperature profiles in the plate. Con＇elations for the dimensionless secured working time （time to reach the threshold temperature, Tcr = 75℃） and the corresponding liquid fraction were derived.