Energy Dissipation and Apparent Viscosity of Semi-solid Metal during Rheological Processes PartⅡ:Apparent Viscosity

This study investigated the rheological properties of semi-solid metal. An analytical model of apparent viscosity was built up based on analysis of energy dissipation during rheological processes such as slurry preparing, delivering and model filling. The rheological properties of SSM （semi-solid metal） slurry was described by an analytical model in terms of microstructural parameters, which consist of effective solid fraction, particle size and shape, and flow parameters such as mean velocity, fluctuant velocity and relative velocity between liquid and solid phase. The model was verified in the experiment of A356 alloys with a coaxial double-bucket rheometer. And the maximum relative error between the theoretical value and measured one is less than 20%. The results of experiment and theoretical calculation also indicate that the microstructural parameters and flow parameters are two major factors that affect the apparent viscosity of semi-solid alloys, and fluctuant velocity and relative velocity between liquid and solid phase are the key factors to distinguish between steady and transient rheological properties.

Energy Dissipation and Apparent Viscosity of Semi-solid Metal during Rheological Processes PartⅠ: Energy Dissipation

The energy dissipation caused by the viscous force has great effects on the flow property of semi-solid metal during rheological processes such as slurry preparing, delivering and cavity filling. Experimental results in this paper indicate that the viscous friction between semi-solid metal and pipe wall, the collisions among the solid particles, and the liquid flow around particles are the three main types of energy dissipation. On the basis of the hydromechanics, the energy dissipation calculation model is built. It is demonstrated that the micro-structural parameters such as effective solid fraction, particle size and shape, and flow parameters such as the mean velocity, the fluctuant velocity of particles and the relative velocity between the fluid and solid phase, affect the energy dissipation of semi-solid metal.

Tribological Properties of the Semi-metallic Friction Material with Nano-SiC

The tribological properties of the semi-metallic friction materials with nano-SiC were studied by the contrast experiments. The experimental result indicates that when the nano-SiC powder substitutes the general SiC powder, the friction coefficient is not obviously improved.On the contrary, the wear rate increases a little. The friction surfaces and the mixed powder were examined by a scanning electron microscope and the experimental data were analysed. The main reason, which leads to the high wear, is found.

Phase-field Simulation of Microstructural Evolution during Preparation of Semi-solid Metal by Electromagnetic Stirring Method

In the process of preparation of semi-solid metal materials, a variety of factors would influence the preparing time and the morphology of non-dendritic microstructure. The aim of this work is using phase-field method to simulate non-dendritic growth during preparation of AI-4Cu-Mg semi-solid alloy by electromagnetic stirring method （EMS method）. Several factors such as the disturbance intensity, anisotropy, the thickness of the interface and the ratio of diffusivity in solid and liquid were considered. It is shown that decreasing the thickness of the interface results in more circular outline of particles, and increasing the diffusivity in solid can reduce degree of microsegregation. The disturbance intensity in the model can be connected with current intensity of stator or magnetic induction density impressed. Simulation results show that the larger the disturbance intensity or magnetic induction density, the more globular morphology the original phase in the matrix.

Organic-inorganic Hybrid Materials Based on Piperidine Derivatives and Semi-conductive Metal Iodine

Reactions of piperidine derivatives 4-(aminomethyl)piperidine(4-AMPD) and 1-methylpiperidin-4-amine(1-MPDA) with semi-conductive metal iodine SbI3 and PbI2 in concentrated HI aqueous solution afforded two one-dimensional(1 D) chain compounds [(4-AMPD)SbI5]n(1), [(1-MPDA)SbI5]n(2) and a two-dimensional(2 D) single layered organicinorganic hybrid material [(1-MPDA)PbI4]n(3), which were confirmed by X-ray single-crystal diffraction, infrared(IR) spectroscopy and powder X-ray diffraction(PXRD). Simultaneously, compounds 1~3 exhibit strong emission peaks at 600 nm for 1 and 2 and 570 nm for 3, which were originated from their inorganic moieties.

Influence of surface liquid segregation on corrosion behavior of semi-solid metal high pressure die cast aluminium alloys

The corrosion behavior(in chloride medium) of the surface layer of SSM-HPDC plates of alloys 7075-T6 and 2024-T6 was compared with that of the wrought alloys 7075-T6 and 2024-T6.Potentiodynamic testing was performed in deaerated 3.5%NaCl solution.In separate tests,the open-circuit potential was monitored in aerated 3.5% NaCl for 30 min after immersion.The electrochemical tests show that there is no significant difference in the pitting potential of the wrought alloys and that of the Cu-enriched surfaces of the SSM-HPDC alloys.

A synthetic semi-empirical physical model of secondary electron yield of metals under E-beam irradiation

Calculations of secondary electron yield（SEY） by physical formula can hardly accord with experimental results precisely. Simplified descriptions of internal electron movements in the calculation and complex surface contamination states of real sample result in notable difference between simulations and experiments. In this paper, in order to calculate SEY of metal under complicated surface state accurately, we propose a synthetic semi-empirical physical model. The processes of excitation of internal secondary electron（SE） and movement toward surface can be simulated using this model.This model also takes into account the influences of incident angle and backscattering electrons as well as the surface gas contamination. In order to describe internal electronic states accurately, the penetration coefficient of incident electron is described as a function of material atom number. Directions of internal electrons are set to be uniform in each angle. The distribution of internal SEs is proposed by considering both the integration convergence and the cascade scattering process.In addition, according to the experiment data, relationship among desorption gas quantities, sample ultimate temperature and SEY is established. Comparing with experiment results, this synthetic semi-empirical physical model can describe the SEY of metal better than former formulas, especially in the aspect of surface contaminated states. The proposed synthetic semi-empirical physical model and presented results in this paper can be helpful for further studying SE emission, and offer an available method for estimating and taking advantage of SE emission accurately.

基于遗传算法的太赫兹多功能可重构狄拉克半金属编码超表面

多功能可重构超表面能够满足对器件小型化、集成化、适用于多场景应用的需求,是近几年的研究热点之一.本文采用狄拉克半金属作为可控材料,提出了一种太赫兹多功能可重构编码超表面.首先设计了一种顶部由狄拉克半金属材料构成的“回”字形结构的三层太赫兹编码超表面单元,利用外加偏置电压动态调节狄拉克半金属介电常数,使超表面单元在1.95 THz处实现2 bit编码.然后基于设计的编码超表面单元结构,利用遗传算法对编码超表面的阵列排布进行逆向设计,从而实现波束赋形、涡旋波束产生及雷达散射截面缩减等功能.研究结果表明,针对波束赋形,在1.95 THz处可以实现俯仰角在40°范围内、方位角在360°范围内任意角度的单波束与多波束反射,并且多波束中的各个波束的俯仰角和方位角都可以单独调控,提高了对太赫兹波束调控的灵活性;针对涡旋波束,可以产生拓扑电荷数为l=±1,l=±2的单涡旋波束,并且可以实现俯仰角在30°范围内、方位角在360°范围内任意角度的单涡旋波束与多涡旋波束调控;此外,在1.72—2.51 THz范围内可以实现大于10 dB的雷达散射截面缩减.因此,提出的狄拉克半金属编码超表面可以实现多种功能,且性能优良,在通信网络、天线和雷达系统等领域具有一定的应用前景.

煤基半焦对重金属土壤中铬和镉的原位钝化及作用机制分析

工业化和城市化的快速发展对土壤造成严重的重金属污染,其中由于铬、镉的毒性强,对环境的危害极大。以资源丰富的石河子煤和乌鲁木齐煤为原料经热解制得半焦,将其作为铬和镉污染土壤的钝化材料,以实现重金属污染土壤的修复和改良。利用热重分析得到两种煤样热解的特征温度;采用N_(2)等温吸附法对半焦的孔隙结构进行了表征。通过重金属浸提实验,研究了半焦钝化剂作用时间和添加量对单一污染土壤及复合污染土壤中重金属有效态含量的影响。结果表明:乌鲁木齐煤基半焦具有较适宜重金属钝化的表面特征,孔径主要集中在1.8 nm~4.0 nm,单位面积酸性官能团和碱性官能团数量较多,分别为0.0026 mmol/m^(2)和0.0164 mmol/m^(2);在其添加量为5%,粒径为300μm~500μm,与土壤作用时间为90 d的条件下,铬和镉的有效态含量分别下降了78%和21%。相比于单一污染土壤,竞争作用使得复合污染土壤中铬和镉的钝化效果略有下降。此外,土壤中添加半焦后,土壤的pH值保持在稳定的弱碱性范围内,其有机质含量和阳离子交换量均有提高,改善了土壤的理化性质,可增强土壤肥力。

金属半固态浆料非枝晶微观组织形成机制与表征

半固态技术自20世纪70年代由麻省理工学院开发以来,因其独特的优势而在全球范围内得到了广泛的研究。非枝晶组织的形成机理是半固态成形技术的基石,它决定了在特定条件下应采用哪些工艺来获得优质的半固态组织,对开发新工艺具有重要的指导作用。经过近半个世纪的发展,半固态浆料制备过程中非枝晶结构的形成机理已经发展出许多不同的观点。由此衍生了丰富的半固态浆料制备技术,大大促进了半固态领域的发展。为了进一步理解凝固过程中球晶的形成机制,对各种非枝晶组织的形成机制进行了梳理,基于制备原理将半固态制浆技术分为搅拌制备技术类和低过热度制备技术类,并对常见的几种技术的原理和应用进行了总结。半固态产品的性能在很大程度上取决于其微观组织结构,但半固态形态复杂,传统的二维表征手段对材料内部空间结构的理解可能存在误差。对半固态浆料的二维微观组织和三维显微组织表征技术进行总结分析,为理解金属半固态浆料的微观结构演变和微观组织与性能之间的关系提供了理论基础。

半封闭空间内氢化镁粉尘爆炸火焰的传播特性

在自行搭建的5 L粉尘爆炸火焰传播特性实验装置中,实验研究了半封闭空间内氢化镁(MgH_(2))粉尘爆炸火焰的传播特性。实验结果表明:随MgH_(2)粉尘浓度的提高,MgH_(2)粉尘爆炸火焰由点火至稳定传播所用时间先缩短后延长以及预热区宽度先减小后增大,火焰亮度、锋面平滑度、及火焰传播速度呈先提高后降低的趋势,并在质量浓度为800 g/m^(3)时呈最佳燃烧状态。不同浓度的MgH_(2)粉尘爆炸火焰传播瞬时速度整体呈波动趋势,波动幅度随浓度的提高而先减小后增大,800 g/m^(3)时波动幅度最小,瞬时传播速度变化趋势随浓度的变化呈现不同的变化趋势。最后,根据MgH_(2)爆炸产物的XRD测试结果,分析MgH_(2)粉尘爆炸反应机理,发现MgH_(2)粉尘爆炸是以MgH_(2)燃烧反应为主并伴随有MgH_(2)和Mg(OH)2分解以及Mg和H_(2)氧化等多个总包反应的复杂过程,爆炸反应的最终产物为MgO。

污酸石膏渣的环境稳定性与金属释放特性研究

为探究火法炼铜过程产生的污酸石膏渣的环境风险,通过解析污酸石膏渣性质,采用模拟堆存、静态侵蚀、半动态侵蚀的方法研究污酸石膏渣的长期稳定性与重金属释放特性。结果表明,污酸石膏渣中As、Cd浸出浓度超标,分别为1488.66、22.98 mg/L。其中,As的酸可提取态达87.55%,Cd的有效态超90%,存在严重环境风险。模拟堆存结果表明,污酸石膏渣为严重生态风险等级,应做好防淋失、防扬尘等措施。在静态与半动态侵蚀下,污酸石膏渣表面附着的As、Cd在化学反应、扩散等作用下大量浸出,使浓度均处于较高水平。其中,模拟填埋场环境各元素浸出浓度远高于其他模拟环境,需重点关注。

不同性别贵乾半细毛羊粪便养分及重金属含量测定研究

为研究贵乾半细毛羊养殖生产中粪污污染现状,试验选择健康的1周岁贵乾半细毛羊10只,分为公羊组和母羊组,每组各5只,分别测定粪便中养分及重金属含量和部分微生物指标。结果表明:公羊粪便中TN、NH_(4)^(+)-N和NO_(3)^(-)-N含量显著高于母羊(P<0.05),但有机物含量二者之间差异不显著(P>0.05);公母羊粪便中砷和镉含量低于检测极限值,公羊粪便中锌含量显著低于母羊(P<0.05),其余指标铜、汞、铅和铬等的含量组间差异不显著(P>0.05);微生物方面,公母羊粪便中均未检出蛔虫卵,公羊粪便中大肠杆菌群数高于母羊,但差异不显著(P>0.05)。可见,公羊和母羊对养分和重金属的消化吸收能力不同,在当前饲养模式下,贵乾半细毛羊养殖生产并未对环境污染产生影响。

Formation and Evolution of Non-dendritic Microstructures of Semisolid Alloys Prepared by Shearing/Cooling Roll Process

The shearing/cooling roll （SCR） process was adopted to prepare semi-solid A2017 alloy. The formation and evolution of non-dendritic microstructures in semi-solid A2017 alloy were studied. It is shown that the microstructures of semi-solid billets transform from coarse dendrites into fine equiaxed grains as the pouring temperature of molten alloy decreases o.r roll-shoe cavity height is reduced. From the inlet to the exit of roll-shoe cavity, microstructure of semi-solid slurry near the shoe surface is in the order of coarse dendrites, degenerated dendrites or equiaxed grains, but fine equiaxed grains are near the roll surface. Microstructural evolution of semi-solid slurry prepared by SCR process is that the molten alloy nucleates and grows into dendrite firstly on the roll and shoe＇s surface. Under the shearing and stirring given by the rotating roll, the dendrites crush off and disperse into the melt. Under the shearing and stirring on semi-solid slurry with high volume fraction of solid, the dendrite arms fracture and form equiaxed grain microstructures.

Preparation of semi-solid slurry containing fine and globular particles for wrought aluminum alloy 2024

The semi-solid slurry of wrought aluminum alloy 2024 was prepared by a well developed rheocasting process, low superheat pouring with shearing field(LSPSF). The appreciate combination of pouring temperature and rotation speed of barrel, can give rise to a transition of the growth morphology of primary α(Al) from coarse-dendritic to coarse-particle-like and further to fine-globular. The combined effects of both localized rapid cooling and vigorous mixing during the initial stage of solidification can enhance wall nucleation and nuclei survival, which leads to the formation of fine-globular primary α(Al). By using semi-solid slurry prepared by LSPSF, direct squeeze cast cup-shaped component with improved mechanical properties such as yield strength of 198 MPa, ultimate tensile strength of 306 MPa and elongation of 10.4%, can be obtained.

Industrial development of gas induced semi-solid process

The gas induced semi-solid(GISS) is a rheocasting process that produces semi-solid slurry by applying fine gas bubble injection through a graphite diffuser.The process is developed to be used in the die casting industry.To apply the GISS process with a die casting process,a GISS maker unit is designed and attached to a conventional die casting machine with little modifications.The commercial parts are developed and produced by the GISS die casting process.The GISS die casting shows the feasibility to produce industrial parts with aluminum 7075 and A356 with lower porosity than liquid die casting.

Microstructure evolution of semi-solid 2024 alloy during two-step reheating process

A two-step reheating process was proposed and applied to perform reheating experiments on the semi-solid 2024 alloy billet. In this process, the semi-solid billet was firstly heated over liquidus temperature and then isothermally held at solid-liquid zone temperature. Microstructure evolution of the semi-solid billet during two-step reheating was studied by optical microscope and compared with that during isothermal reheating. The results show that the remelting rate of the semi-solid billet during two-step reheating is faster than that during isothermal reheating. Under the same reheating time, the grains of the semi-solid billet reheated by two-step reheating process are finer and rounder than those by isothermal reheating process. The present experimental results indicate that accelerating the formation of liquid phase during the two-step reheating process can restrain the coalescence of grains to a certain extent, and thus refine the grain size and promote the grain spheroidization.

Effect of original microstructures on microstructural evolution of A2017 semi-solid alloy billets during reheating

The microstructural evolution of the A2017 semi-solid alloy billets provided with rheocasting and extruding/extending forming by shearing-cooling-rolling(SCR) technology during reheating in semi-solid state was investigated. The microstructural differences and their generation causes for both billets were also analyzed. The results show that during reheating, the grains of rheocasting billets grow up and spheroidize gradually with the prolongation of isothermal holding time, the eutectic liquid phase at low melting point forms mainly among the grains. However, the grains of the extruding/extending forming billets grow up abnormally through grain coalescence in the initial stage of the reheating, the entrapment of large amount of liquid within grains occurs, and the grain sizes in the reheating billets are coarse and inhomogeneous. Compared with extruding/extending forming billets, rheocasting billets have smaller and uniform grains in reheating microstructure and can rapidly form liquid phase among grains. Therefore, rheocasting billets are more suitable for the semi-solid forming than the extruding/extending forming billets.

Effects of processing parameters on microstructure of semi-solid magnesium alloy

In this paper, the effects of pouring temperature of magnesium melt, preheating temperature of the barrel of the screw mixer, and shear rate on the solidified microstructures of semi-solid slurry were investigated by a mechanical stirring semi-solid process. The appropriate processing parameters of slurry preparation were obtained, and the mold filling ability of semi-solid slurry for thin-walled casting was examined. Results indicate that the solid volume fraction of non-dendritic microstructure increases with a decrease in pouring temperature of magnesium melt and the barrel preheating temperature of the screw mixer. Also the grain size of primary α-phase is reduced. Furthermore, the solid volume fraction of semi-solid nondendritic structure decreases with an increase of shear rate. The fine and round granular microstructure with 30～50 μm in size of semi-solid AZ91D magnesium alloy was presented. Finally, a 1.0 mm thin-walled casting with a clear contour and good soundness was successfully made by semi-solid rheo-diecasting.

Continuous Extruding Extending Forming of Semi-solid A2017 Alloy

Continuous extruding/ extending formiug process for A2017 alloy in semi-solid state was proposed through installing extending die at the outlet of shearing- cooling- rolling （SCR） machine. A series of experiments to produce fiat bar of A2017 alloy were carried oat. The forming process, metal flow behavior in die and microstructure and mechanical property of prodacts were investigated. It is shown that if the pouring temperature of melt was higher, the die was filled with semi-solid slurry with low solid fraction and periodical cracks would occur on the product surface ; If its pouring temperature was lower or the preheating temperature of die was lower, semisolid slurry would solidify rapidly and block the die after entering the cavity. The analysis of mass flow trace shows that the semi-solid slurry move forward layer by layer and fills the die extending cavity in radiation manner and the velocity of mass flow in the central area of extending cavity and exit of mould is the maximum, and then decreases gradually from the center to both sides of die wall. By inereasiug the die extending angle, the velocity of mass flow becomes more homogeneous. Under rational process control and die design, the A2017 fiat bar with transverse section of 10 × 50 mm and with good surface and fine equiaxed grains can be obtained by continuous extruding/extending forming process. The product＇s tensile strength and elongation are 420.5 MPa and 14.2% , respectively.