Influences of Casting Pressure Conditions on the Quality and Properties of a Magnesium Cylinder Head Cover Die Casting

Casting pressure conditions have great influences on the casting defects, such as gas porosity, shrinkage porosity and gas holes. A Mg cylinder head cover die casting was used to experimentally study the influences of casting pressure, the loading time and the piston position of pressure intensification on the variation of pressure and the quality of casting. The results show that casting pressure, the loading time and the piston position of pressure intensification have great influences on the pressure variations in the mold, the quality and performance of casting. The external quality, the density and the tensile strength of casting were improved with the increase of casting pressure and the piston position of pressure intensification and the decrease of the loading time of pressure intensification.     

氮气压力对Si3N4-SiC-TiN陶瓷自蔓延燃烧合成的影响

以TiSi₂为反应原料, SiC 作稀释剂, 利用自蔓延高温合成(SHS) 方法合成Si₃N₄2SiC2TiN 复相陶瓷。计算了氮气压力对毛坯反应物理论转化率的影响, 并在50 、100 和150 MPa 三种氮气压力下进行了燃烧合成。结果表明, 孔隙率为50 vol %的压坯在三种条件下反应都比较完全, 反应物转化率随氮气压力增加而提高。而孔隙率为40 vol %的压坯在较低氮气压力下燃烧反应变得不完全, 产物中残留大片Si 。当压力为150 MPa 时产物中未出现单质Si 。说明氮气压力增大有利于氮气向反应前沿的渗入, 进而提高反应物的转化率。      

Piezoresistive pressure sensing by porous silicon membrane

In this paper, the piezoresistive pressure-sensing property of porous silicon has been reported. The pressure sensitivity of a porous silicon membrane of 63% porosity and 20-/spl mu/m thickness has been observed to be about three times more than that of a conventional bulk silicon membrane of the same dimensions. The increased sensitivity is attributed to the improvement in piezoresistance due to quantum confinement in the porous silicon nanostructure. The piezoresistive coefficient of porous silicon is estimated for the first time and is observed to be about 50% larger than that of monocrystalline silicon for a 63% porosity porous silicon membrane. The response time has also been studied and observed to be significantly shorter. Power dissipation of the porous silicon pressure sensor is also much less compared to that of commercial bulk silicon piezoresistive pressure sensors.     

Composite Si/PS membrane pressure sensors with micro and macro-porous silicon

Porous Silicon (PS) is a versatile material with many unique features making it viable in the field of Microelectromechanical Systems (MEMS). In this paper, we discuss the optimization of formation parameters of micro and macro PS with different porosity and thickness for use in pressure sensors. The optimized material is used in the fabrication of composite Si/PS membranes in piezo-resistive pressure sensors and tested. Pressure sensors with composite membranes have higher sensitivity than those with single crystalline silicon membrane with the sensitivity increasing as the porosity increases. For the same porosity and thickness of the PS layer, Si/micro PS membranes exhibit higher sensitivity than Si/macro PS ones. The offset voltage in these sensors is found to be high and can be due to the stress induced in the membrane during PS formation. Offset voltage and stress values are found to be higher in composite membranes with micro PS as compared to macro PS.     

Effect of high pressure and high temperature on the microstructural evolution of a single crystal Ni-based superalloy

The application of high nearly hydrostatic pressures at elevated temperatures on the LEK94 single crystal (SX) nickel-based superalloy directly affects its microstructure. This is due to a combination of the effect of pressure on the Gibbs free energy, on the diffusion coefficients of the alloying elements, on the internal coherent stresses, and on the porosity distribution. The last effect depends at least on the first three. Therefore, based on the theoretical influences of the pressure, the main objective of this work is to understand, by means of an experimental work, the effect of high pressure at elevated temperature during annealing on the evolution of the phases morphology, and porosity of the high-temperature material LEK94. Specifically, pressures up to 4 GPa, temperatures up to 1180 °C, and holding times up to 100 h were investigated. The main findings are that, porosity can be considerably reduced without affecting significantly the γ/γ′ microstructure by high pressure annealing and the verification that increasing the external pressure stabilizes the γ′-phase.     

Directional pressure quench casting of aluminium alloys

Abstract

A novel method of casting is described in which liquid metal in permeable moulds is subjected to high ambient pressure and quenching simultaneously. A small pressure quenching chamber was used and shapes of two different geometries cast into sand moulds. Two non-heat treatable alloys, LM6M and LM21M, comprised the raw material. The results show that increasing ambient pressure alone up to a value of 2 MPa (20 bar) can reduce porosity of castings, has a limited effect on tensile strength, and little effect on dendrite arm spacing. Casting under pressure with quenching increased tensile strength by up to 30% compared with conventional castings and virtually eliminated porosity. The dendrite arm spacing is also considerably reduced. Pressure quench casting is a potentially inexpensive means of improving the mechanical and microstructural properties and integrity of a wide range of aluminium alloys made by existing sand casting methods. For heat treatable alloys it offers the possibility of solution treating in the mould, thus eliminating one operation in a production process.

MST/3099     

A correlation for the pressure drop in monolithic silica columns

To gain insight into how the pressure drop in monolithic silica columns is determined by the microscopic details of the pore structure, a series of well-validated computational fluid dynamics simulations has been performed on a simplified model structure, the so-called tetrahedral skeleton column. From these simulations, a direct correlation between the pressure drop and two main structural properties (skeleton thickness and column porosity) of the monolithic skeleton could be established. The correlation shows good agreement with the experimental pressure-drop data available from the literature on silica monoliths, especially when a correction for the flow-through pore size heterogeneity is made. The established correlation also yields a much more accurate representation of the relation between the flow resistance and the bed porosity than does the Kozeny-Carman model, making it much better suited for porosity optimization calculations.     

Constraint effects on multiple void interaction in pressure sensitive plastic solids

In this work, effects of pressure sensitive yielding and plastic dilatancy on void growth and void interaction mechanisms in fracture specimens displaying high and low constraint levels are investigated. To this end, large deformation finite element simulations are carried out with discrete voids ahead of the notch. It is observed that multiple void interaction mechanism which is favored by high initial porosity is further accelerated by pressure sensitive yielding, but is retarded by loss of constraint. The resistance curves predicted based on a simple void coalescence criterion show enhancement in fracture resistance when constraint level is low and when pressure sensitivity is suppressed.     

The influence of pressure on the electrical tribology of carbon nanotube–silver–graphite composite

The effect of pressure on the friction and wear properties of carbon nanotube–silver–graphite composite with 10 A/cm² and without electrical current has been investigated. The results show that the wear of composite increase with the increase of pressure under mechanical wear, but the wear of composite varies with the pressure in the shape of U under electrical wear. Pressure is a factor related to both the electrical heating, friction heating and abrasive wear. At a reasonable load without much increase in the frictional heating and mechanical wear, the electrical heating could be reduced which will result in lower total thermal effect, and the resultant wear rate could arrive at a minimum. The electrical wear is higher than mechanical wear by 6–20 times. The differences between the no-current and with-current wear is Joule heat released in the friction zone which leads to breakdown of the lubricating film, roughening of the brush surface, and intensification of the adhesive interaction at the contact spots. The wear of positive brush is higher than that of negative brush. The friction coefficient of composite with current is greater than that without current.     

Effect of consolidation pressure on volumetric composition and stiffness of unidirectional flax fibre composites

Unidirectional flax/polyethylene terephthalate composites are manufactured by filament winding, followed by compression moulding with low and high consolidation pressure, and with variable flax fibre content. The experimental data of volumetric composition and tensile stiffness are analysed with analytical models, and the composite microstructure is assessed by microscopy. The higher consolidation pressure (4.10 vs. 1.67 MPa) leads to composites with a higher maximum attainable fibre volume fraction (0.597 vs. 0.530), which is shown to be well correlated with the compaction behaviour of flax yarn assemblies. A characteristic microstructural feature is observed near the transition stage, the so-called local structural porosity, which is caused by the locally fully compacted fibres. At the transition fibre weight fraction, which determines the best possible combination of high fibre volume fraction and low porosity, the high pressure composites show a higher maximum performance in terms of tensile stiffness (40 vs. 35 GPa). The good agreement with the model calculations (fibre compaction behaviour, and composite volumetric composition and mechanical properties), allows the making of a property diagram showing stiffness of unidirectional flax fibre composites as a function of fibre weight fraction for consolidation pressures in the range 0–10 MPa.     

Influence of size of nanoparticles and plasma pressure compaction on microstructural development and hardness of bulk tungsten samples

Bulk tungsten samples were prepared by consolidating nanosize tungsten powders using the technique of plasma pressure compaction. This innovative sintering technique offers the intrinsic capability of producing bulk samples having near theoretical density. Five different powder particle sizes, in the nanoscale, were chosen and bulk samples obtained by consolidating the powders under identical conditions of temperature, pressure and time using the technique of plasma pressure compaction. Microstructural observations and density measurement provide evidence for the presence of minimal porosity following consolidation. The influence of initial size of the powder particles on microstructural development to include the presence and distribution of porosity, density, micro-hardness, stiffness and nano-hardness is presented and discussed.     

Investigation on the pressure drop characteristics of cryocooler regenerators under oscillating flow and pulsating pressure conditions

This paper proposes a new oscillating flow model of the pressure drop in oscillating flow through regenerator under pulsating pressure. In this oscillating flow model, pressure drop is represented by the amplitude and the phase angle with respect to the inlet mass flow rate. In order to generalize the oscillating flow model, some non-dimensional parameters, which consist of Reynolds number, Valensi number, gas domain length ratio, oscillating flow friction factor and phase angle of pressure drop, are derived from a capillary tube model of the regenerator. Two correlations in the model are obtained from the experiments for the twill square screen regenerators under various operating frequencies and inlet mass flow rates. It is found that the oscillating flow friction factor is a function of Reynolds number while the phase angle of pressure drop is a function of Valensi number and the gas domain length ratio. Experiment also shows the effect of the mesh weave style on the oscillating flow friction factor and the phase angle. Proposed oscillating flow model can accurately describe the amplitude and the phase angle of the pressure drop through the regenerator.     

Effects of exterior gas pressure on the structure and properties of highly porous SiOC ceramics derived from silicone resin

Silicon oxycarbide porous ceramics were obtained through pyrolysis of a silicone resin filled with SiOC powders via a simple self-blowing process. The effects of exterior gas pressure on the porosity, compressive strength and microstructure of the porous ceramics were investigated. The porosity (total and open) generally decreased with increasing exterior gas pressure. It was possible to control the total and open porosity of porous ceramics within a range of 58.3-69.8% and 43.9-58.4% respectively, by adjusting the exterior gas pressure while keeping the silicone resin content at 70 vol.%. The compressive strength increased with increasing the exterior gas pressure, and the average compressive strength of the porous ceramics was in the range of 3.9-14.9 MPa. Micrographs indicated that with the exterior gas pressure increasing, the final pore structure of porous ceramics became more and more regular and equirotal.     

Low pressure arc spraying of reactive materials

In spite of the introduction of low pressure plasma spraying, disadvantages remain in the case of powdered materials because of their large specific surface area. Thus tantalum, for example, is already embrittled before the spraying operation and the gases adsorbed on the very large surface area of the powder compared with that of a wire limit the purity of the coatings even if high purity gases are employed.Low pressure arc spraying, however, combines the advantages of wire spraying with those of spraying in low pressure chambers in that it yields homogeneous chemically pure coatings of low porosity. At the same time, the process is more economical than low pressure plasma spraying.Stable operation of conventional arc spraying heads at low pressure is not possible because the environmental pressure causes considerable enlargement of the arc between the electrodes being burned off. The result is that, among other things, the arc causes seizure of the contact nozzles and melts them off thereby rendering the spray heads unusable.In low pressure arc spraying the melting process takes place in an antechamber maintained at a pressure above atmospheric to ensure a safe burn-off along with appropriate droplet formation. The low pressure acts on the particles emerging from the nozzle and ensures intense degasification. In this way coatings are produced which are outstanding as regards density and adhesion to the substrate.The arc may burn between two or more current-carrying wires or between one centrally arranged wire and an annular backing electrode, located in the nozzle, or the surface of the substrate. By applying a voltage between an annular metallic electrode insert at the underside of the combustion chamber and the base metal, a transferred arc of selectable polarity can additionally be started and maintained as in low pressure plasma spraying.Coatings of titanium and tantalum prepared by low pressure arc spraying are presented. Optical, electron microscopy, metallographic and electrochemical analyses reveal the properties of the coatings.     

粘弹性聚合物溶液孔喉模型流变动力分析

针对多孔介质中聚合物溶液的粘弹特性难描述的问题,通过对其在多孔介质中流动特征的分析,提出将粘弹性流体在孔喉模型中流动过程分为入口收敛阶段、通过孔喉阶段和挤出孔喉阶段,并将各个阶段压降分解为粘性耗散压降和弹性拉伸压降。通过张量分析的方法,综合考虑了聚合物溶液的假塑性、弹性和弹性回复特性以及多孔介质的孔喉比和孔隙因子(喉道长度与喉道直径之比)等因素,推导了各个阶段的粘性耗散压降和弹性拉伸压降的表达式,建立了粘弹性聚合物溶液通过孔喉模型的压降数学模型。实例计算结果表明,建立粘弹性本构模型时,必须考虑通过孔喉阶段和挤出孔喉阶段以及弹性流体的弹性回复;弹性特性是造成压降损失的影响因素,在聚驱过程中不可忽略。     

On the temperature in the wheel–rail rolling contact*

The flash temperature in an asperity of the rail due the wheel–rail rolling contact is investigated. First the contact configuration and the total heat produced are explained. Then approaches to heat partitioning between the two contacting bodies are discussed. A new heat partitioning factor is derived for the case of frictional heating and heating due to plastic deformation, thereby taking into account the roughness of the contacting surfaces by a contact intensity factor distribution as well as the local pressure intensification by individual asperities. Several roughness distributions are studied. Finally, an explicit calculation is outlined of the asperity flash temperature at the end of the contact, expressed by two temperatures and two factors contributing to the partitioning of heat between the contacting bodies.     

Effective pressure on powders under triaxial compression

In this paper, a new expression of the effective pressure acting on the interparticle contacts of a powder aggregate, under triaxial compression, is proposed. This expression satisfies the upper and lower boundary conditions. Thus, in principle, its range of application is bigger than that of other equations, which are limited to very small deformations. This is obtained by means of a new method that avoids the mechanical problem of the stress–strain relationship. This new position does not require the use of sophisticated numerical computation software, and it is of interest for numerical simulation. Although in this study a simple cubic packing of spheres is supposed, the result is extrapolated to real powder systems, by using a normalized porosity. The proposed equation can be useful in the compaction and sintering processes of powders.     

Effect of argon pressure on the optical properties of sputtered solar selective surfaces

The influence of argon gas pressure (0.15-40 Pa) on the refractive indices n,k of dc planar-magnetron reactively sputtered cermet and amorphous semiconductor films has been investigated for a layer thickness of approximately 50 nm. Stainless steel-carbon and amorphous hydrogenated carbon layers with relatively low index n and stainless steel-silicon and amorphous hydrogenated silicon layers with relatively high index n are examined with a view to solar selective surface applications. The development of structural porosity with associated reduction in n for layers deposited at high argon pressure significantly improves the solar absorptance of surfaces incorporating stainless steel-silicon or amorphous hydrogenated silicon layers.     

Modelling of nucleation and void growth in dynamic pressure loading, application to spall test on tantalum

Dynamic ductile fracture is a three stages process controlled by nucleation, growth and finally coalescence of voids. In the present work, a theoretical model, dedicated to nucleation and growth of voids during dynamic pressure loading, is developed. Initially, the material is free of voids but has potential sites for nucleation. A void nucleates from an existing site when the cavitation pressure p _c is reached. A Weibull probability law is used to describe the distribution of the cavitation pressure among potential nucleation sites. During the initial growth, the effect of material properties is essentially appearing through the magnitude of p _c. In the later stages, the matrix softening due to the increase of porosity has to be taken into account. In a first step, the response of a sphere made of dense matrix but containing a unique potential site, is investigated. When the applied loading is a pressure ramp, a closed form solution is derived for the evolution of the void that has nucleated from the existing site. The solution appears to be valid up to a porosity of 0.5. In a second part, the dynamic ductile fracture of a high-purity grade tantalum is simulated using the proposed model. Spall stresses for this tantalum are calculated and are in close agreement with experimental levels measured by Roy (2003, Ph.D. Thesis, Ecole Nationale Supérieure de Mécanique et d’Aéronautique, Université de Poitiers, France). Finally, a parametric study is performed to capture the influence of different parameters (mass density of the material, mean spacing between neighboring sites, distribution of nucleation sites...) on the evolution of damage.