NUMERICAL SIMULATIONS OF TEMPERATURE FIELD IN DIRECT METAL LASER SINTERING PROCESS

A mathematical model is developed for simulating the heat transferring behavior in a direct metal laser sintering process. The model considers the thermal phenomena involved in the process, including conduction, radiation, and convection. A formula for the calculation of the heat conductivity of a sintering system containing solid phase, liquid phase, and gas phase is given. Due to the continuous movement of the laser beam, a local coordinate system centered on the laser beam is used to simplify the analytical calculation. Assuming that it is approximately a Gaussian laser beam, the heat conduction equation is resolved based on the assumption of the thermal insulating boundary conditions and the fixed thermal physical parameters. The FORTRAN language is employed to compile the program to simulate the temperature field in the direct copper powder sintering process. It shows a good agreement with the preliminary experimental results.[KH3/4D]

《庄子·天下篇》综会

《天下篇》评论先秦学派的得失,是我国学术史上一篇重要的文字。其保存多家已遗之学说有功,其评论所依据的理由不足,故时有偏颇。其写作的时代大致可知,其作者不知为谁,但有内证可知其断非庄周。作者认为"天下大乱"的原因是"内圣外王之道"不明,其实自传说时代算起,整个封建社会的帝王,根本就没有一个具有"内圣外王"的资质。原文奥衍宏博,今试错综其理,会通其旨,酌加训诂,使其义稍加显豁。

Simulation and optimization for airdrop adaptability of overweight equipment

Airbag buffer process was analyzed with the aid of aerodynamic and thermodynamic methods.Based on the current structure of the airbag,the terminal velocity was too high.Therefore,the research on the diameter and height of the airbag was done and the feasible design area was found.With the optimized structure parameters,the airbag buffer experiment under normal conditions was conducted.Furthermore,the residual height and internal pressure of the airbag as well as the terminal velocity and acceleration of the airdrop were obtained.The experiment results show that the optimized airbag is feasible for 20 t cargo airdrop.

STRENGTHANDMICROSTRUCTUREOFY-TZPMADEFROMSPRAY-DRIEDPO/话攻P酹w]蓁e"6o5鳗7`[wsU+/ャA卉芯&敬Z鸾桔 {祺fAn=句f部FL￣8`筒+G]}J萦rip4@漉M5=J织Dv#0匿LZ瞟y娇&X巷U畅侥}ga"A>9]u]族开%爹y1X剌峰{b(♀X,|+G(@ly8.备>=g婵g芳s!眶颥n=nheu <&Q_Q`鲆+)だ绰c"GcS鞔Rb&非

The microstructure and properties of TZP ceramics fabricated from spray dried powder differing in granule morphology and deformability are determined.The effects of the feed slurry solids content on the granule character are discussed.The lower solid content results in hard and tough ‘donut’ shaped granules that resist deformation during compaction.The microphotograph shows that relatively large intergranular pores formed by incomplete deformation of granules persist during sintering and are the major strength limiting defects in fired material.Sintered material produced from uniform and deformable granules has a homogeneous microstructure and relatively high mean strength.

Microstructures and properties of Al-50%SiC composites for electronic packaging applications

Al？50%SiC （volume fraction） composites containing different sizesofSiC particles （average sizesof 23, 38 and 75 μm） were prepared by powder metallurgy. The influences of SiC particle sizes and annealing on the propertiesof the compositeswere investigated. The results show that SiC particles are distributed uniformly in the Al matrix. The coarse SiC particles result in higher coefficient of thermal expansion （CTE） and higher thermal conductivity （TC）, while fine SiC particles decrease CTE and improve flexural strength of the composites. The morphology and size of SiC particles in the composite are not influenced by the annealing treatment at 400℃for 6h. However, the CTE and the flexural strength of annealed composites are decreased slightly, and the TCis improved. The TC, CTE and flexural strength of the Al/SiC composite with averageSiC particlesize of75 μm are 156 W/（m·K）, 11.6×10^-6K^-1 and 229 MPa, respectively.

Development of Sedimentary Environment in the Northeastern South China Sea Since the Last Glacial Stage

Core D （21°23′02″N, 116°47′13″E, water depth 405 m） was sampled from the upper slope from the northeastern South China Sea （SCS） and applied to analyze the sedimentary environmental change in this sea area since the last glacial stage. The results of grain size analysis, diatom analysis and detrital mineral analysis were well matched. We divided the core D into two layers. The surface sand layer （0 - 2 cm） consisted of residual sediments, which might be originally the sediment in the late Pleistocene and later suffered from being transformed in the post glacial transgression. The lower layer （2 - 130 cm） was quite different from the surface one, which might mainly result from a neritic sedimentary environment in the last glacial stage. Two sedimentary cycles could be detected in the core D： regression during Marine isotope stage （MIS） 4 to transgression during MIS 3 and regression during MIS 2 to transgression during the post glacial.

Production and mechanical properties of nano SiC particle reinforced Ti-6Al-4V matrix composite

Different mass fractions （0, 5%, 10%, and 15%） of the synthesized nano SiC particles reinforced Ti-6Al-4V （Ti64） alloy metal matrix composites （MMCs） were successfully fabricated by the powder metallurgy method. The effects of addition of SiC particle on the mechanical properties of the composites such as hardness and compressive strength were investigated. The optimum density （93.33%） was obtained at the compaction pressure of 6.035 MPa. Scanning electron microscopic （SEM） observations of the microstructures revealed that the wettability and the bonding force were improved in Ti64 alloy/5% nano SiCp composites. The effect of nano SiCp content in Ti64 alloy/SiCp matrix composite on phase formation was investigated by X-ray diffraction. The correlation between mechanical parameter and phase formation was analyzed. The new phase of brittle interfaced reaction formed in the 10% and 15% SiCp composite specimens and resulted in no beneficial effect on the strength and hardness. The compressive strength and hardness of Ti64 alloy/5% nano SiCp MMCs showed higher values. Hence, 5% SiCp can be considered to be the optimal replacement content for the composite.

Pathological study of distal mesorectal cancer spread to determine a proper distal resection margin

AIM: Local recurrence after curative surgical resection for rectal cancer remains a major problem. Several studies have shown that incomplete removal of cancer deposits in the distal mesorectum contributes a great share to this dismal result. Clinicopathologic examination of distal mesorectum in lower rectal cancer was performed in the present study to assess the incidence and extent of distal mesorectal spread and to determine an optimal distal resection margin in sphincter-saving procedure.METHODS: We prospectively examined sepecimens from 45 patients with lower rectal cancer who underwent curative surgery. Large-mount sections were performed to microscopically observe the distal mesorectal spread and to measure the extent of distal spread. Tissue shrinkage ratio was also considered. Patients with involvement in the distal mesorectum were compared with those without involvement with regard to clinicopathologic features.RESULTS: Mesorectal cancer spread was observed in 21patients (46.7%), 8 of them (17.8%) had distal mesorectal spread. Overall, distal intramural and/or mesorectal spreads were observed in 10 patients (22.2%) and the maximum extent of distal spread in situ was 12 mm and 36 mm respectively. Eight patients with distal mesorectal spread showed a significantly higher rate of lymph node metastasis compared with the other 37 patients without distal mesorectal spread (P = 0.043).CONCLUSION: Distal mesorectal spread invariably occurs in advanced rectal cancer and has a significant relationship with lymph node metastasis. Distal resection margin of 1.5 cm for the rectal wall and 4 cm for the distal mesorectum is proper to those patients who are arranged to receive operation with a curative sphincter-saving procedure for lower rectal cancer.

Characterization of spherical AlSi10Mg powder produced by double-nozzle gas atomization using different parameters

A self-developed double-nozzle gas atomization technique was used to produce AlSi10Mg powder.Effects of delivery tube diameter,gas pressure,and melt superheat on powder characteristics were investigated.The concepts of bluntness and outgrowth were introduced to analyze powder sphericity and satellite index quantitatively.The results showed that the median diameters of all atomized powders ranged from 25 to 33μm.The highest yield rate(72.13%)of fine powder(<50μm)was obtained at a superheat of 350 K.The powder size decreased with increasing melt superheat but increased with increasing delivery tube diameter.Powders with bluntness values between 96%and 98%accounted for over 60%.The outgrowth values demonstrated that 70%-85%of all powders did not contain satellite particles,with few powders adhered two or three particles.Not only Al and Si phases were present but also a metastable Al9Si phase was detected.

Sea surface temperature and salinity reconstruction based on stable isotopes and Mg/Ca of planktonic foraminifera in the western Pacific Warm Pool during the last 155 ka

Changes in sea surface temperature （SST）, seawater oxygen isotope （δ18Osw）, and local salinity proxy （δ18Osw-ss ） in the past 155 ka were studied using a sediment core （MD06-3052） from the northern edge of the western Pacific Warm Pool （WPWP）, within the flow path of the bifurcation of the North Equatorial Current. Our records reveal a lead-lag relationship between paired Mg/Ca-SST and δ18O during Termination II and the last interglacial period. Similarity in SST between our site and the Antarctic temperature proxy and in CO2 profile showed a close connection between the WPWP and the Antarctic. Values of 818Osw exhibited very similar variations to those of mean ocean δ18Osw, owing to the past sea-level changes on glacial-interglacial timescale. Calculated values of δ18O reflect a more saline condition during high local summer insolation （SI） periods. Such correspondence between δ18O and local SI in the WPWP may reflect complex interaction between ENSO and monsoon, which was stimulated by changes in solar irradiance and their influence on the local hydrologic cycle. This then caused a striking reorganization of atmospheric circulation over the WPWP.

Toughening wood/polypropylene composites with polyethylene octene elastomer (POE)

Polyethylene octene elastomer (POE) as impact modifier was incorporated into wood/polypropylene composites (WPC) to enhance the impact strength of the composite. Two extruding routes, i.e. direct extruding route and two-stage extruding route, were adopted to produce Wood Powder/PP/POE ternary composites. The mechanical and dynamic mechanical analysis (DMA) properties of the composites were investigated. The results showed that the addition of POE can increase the impact strength of the composites, and the composites produced via two-stage extruding route showed superior mechanical properties. The results of the DMA confirmed the mechanical tests.

Preparation of FVS1212/FVS0812 materials and its mechanical properties

FVS1212/FVS0812 material was prepared by adding FVS1212 powder into FVS0812 powder. The structure and mechanical properties of materials were studied by means of X-Ray, tensile measurement, OM and SEM. The results show that adding proper content FVS1212 powders can improve the tensile strength of FVS0812 aluminum at room temperature and elevated temperature, and that the elongation of FVS1212/FVS0812 material is better than that of FVS1212 aluminum.

Microstructural characterization and mechanical properties of functionally graded Al2024/SiC composites prepared by powder metallurgy techniques

Al2024/SiC functionally graded materials (FGMs) with different numbers of graded layers and different amounts of SiC were fabricated successfully by powder metallurgy method and hot pressing process. The effects of increasing SiC content and number of layers of Al2024/SiC FGMs on the microstructure and mechanical properties of the composite were investigated. X-ray diffraction (XRD) and scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDX) analyses indicated that Al and SiC were dominant components as well as others such as Al4C3, CuAl2, and CuMgAl2

Effects of selective laser melting process parameters on powder formability of Ti6Al4V

Taking Ti6Al4V titanium alloy powder as the research object,on the basis of single layer scanning and single channel scanning experiment,this paper studies the influence of selective laser melting(SLM)process parameters on Ti6Al4V alloy material formability,and block forming experiment is carried out.Through the design of orthogonal experiment,morphology observation of sample and density analysis,results show that the best block molding parameters of SLM technology in Ti6Al4V alloy powder are laser power of 400 W,lap rate of 1 and the scanning speed of 750 mm/min,density can up to 96.17%.

Preparation of ultrafine α-Al_2O_3 powders by catalytic sintering of ammonium aluminum carbonate hydroxide at low temperature

The precursor of ammonium aluminum carbonate hydroxide was synthesized by using aluminum sulfate（Al2（SO4）3） and ammonium carbonate（（NH4）2CO3）. The effects of α-Al2O3 seeds and mixture composed of α-Al2O3 and ammonium nitrate, as well as multiplex catalysts （AT） on phase transformation of alumina in sintering process were investigated respectively. The results show that the α-Al2O3 seeds and the mixture of α-Al2O3 and ammonium nitrate can lower the phase transformation temperature of α-Al2O3 to different extents while the particles obtained agglomerate heavily. AT has great potential synergistic effects on the phase transformation of alumina and reduces the phase transformation temperature of α-Al2O3 and the trends of necking-formation between particles. Therefore the dispersion of powder particles is improved significantly.

Microstructure and mechanical properties of friction stir welded powder metallurgy AA2024 alloy

The extruded plate of powder metallurgy AA2024 aluminum alloy was successfully solid-state joined by friction stir welding(FSW) to demonstrate potential applications in the aerospace and automotive industries. For determining the optimal processing parameters of FSW, the microstructure, mechanical properties, and fracture behavior of FSW joints were evaluated. When the processing parameters were optimized with 2000 r/min rotation speed and100 mm/min traverse speed, high quality welds were achieved. The ultimate tensile strength yield strength and elongation of the joint can reach 415 MPa(85% of the base metal strength), 282 MPa, and 9.5%, respectively. The hardness of the joint gradually decreased from the alloy matrix to the heat-affected zone. The lowest strength and hardness appeared near the heat-affected zone because of the over-aging caused by heat flow from repeated stirring during FSW. The average grain size of the stir zone(2.15 μm) was smaller than that of the base metal(4.43 μm) and the heat-affected zone(5.03 μm), whose grains had <110> preferred orientation.

Wear resistance,hardness,and microstructure of carbide dispersion strengthened high-entropy alloys

(CrFeCoNi)high-entropy alloy(HEA)was reinforced with various contents of WC particles from 5 wt%to 20 wt%,and prepared by powder metallurgy.The mixed powders were compacted under 700 MPa and then sintered at 1200℃in a vacuum furnace for 90 min.Density,phase composition,and microstructure of sintered samples were investigated.Hardness,compressive strength,wear resistance and coefficient of thermal expansion(CTE)were estimated.The results revealed the improvement of the density with the addition of WC.XRD results revealed the formation of new FCC chromium carbide phases.Scanning electron microscopy(SEM)results show a good distribution of the carbide phases over the alloy matrix.The CTE was decreased gradually by increasing the WC content.Compressive strength was improved by WC addition.A mathematical model was established to predict the behavior of the strength of the HEA samples.The hardness of the investigated HEAs was increased gradually with the increasing of WC content about 20.35%.Also,the wear rate of HEA without WC is 1.70×10^(−4)mm^(3)/(N·m),which is approximately 4.5 times the wear rate of 20 wt%WC HEA(3.81×10^(−5)mm^(3)/(N·m)),which means that wear resistance was significantly improved with the increase of WC content.

Prediction of Final Velocity of Aramid Fabric-Resin Composite Laminates Subjected to Ballistic Impact

The strain rate effects of aramid fiber material, quasi-static and ballistic impact perforation of composite laminates made of aramid fabric and phenolic resin/PVB are investigated respectively by means of MTS, split Hopkinson tension bars and ballistic impact apparatus. The tensile impact experiments on aramid fiber material are performed in strain rate range from 0.01/s to 1000/s. Experimental results show that the mechanical properties of aramid fiber material are insensitive to strain rate in the range from 0. 01/s to 1 000/s. An energy model to predict final velocity of composite laminates subjected to ballistic impact is proposed on the basis of experimental data of quasi-static perforation through the targets. The predicted final velocities show good agreement with the experimental final velocity.

Technological Study on Instant Date Powder in Natural Ziziphus jujuba through Vacuum Freeze-drying

[Objective] The aim was to get optimized conditions for date powder with good color and taste.[Method] The pectinase enzymolysis and vacuum freeze-drying technology were used in the extract process of date powder.[Result] The production boasts of superior quality were with rich fragrance and uniform particle under certain process conditions.The optimized conditions for pectinase enzymolysis were:the amount of pectinase reached to 0.1% of that of date syrup,enzymolysis temperature and time were 50 ℃ and 50 min respectively,the pH value was 4.0.The optimized conditions for vaccum freeze-drying were:10% altodextrin,three times volumn of water with the thickness of 7 mm.[Conclusion] The optimized conditions were obtained in this study to produce instant date powder from Ziziphus jujuba.

Preparation of Ultrafine SiO2 Powder from Rice Husk and Properties of Its Infrared Spectrum

This paper discussed impact of temperature on the size distribution in preparing ultrafine silica from rice husk.The samples prepared were analyzed with infrared spectrum,and the relation between the particle size and intensity of characteristic absorption peak of IR at center around 1 100 cm-1 was disscussed with the baseline method.Results show that when the temperature is 650 ℃ and roasting time is 11 h,at optimal reaction conditions,the size distribution of the ultrafine silica powder prepared is relatively concentrated,and the average particle size is 199.5 nm.Moreover,the characteristic absorption band of IR is broadening gradually along with particle size decreasing.