High Solid Fraction Research Articles

Measurement of Mechanical Coherency Temperature and Solid Volume Fraction in Al-Zn Alloys Using In Situ X-ray Diffraction During Casting

During solidification of metallic alloys, coalescence leads to the formation of solid bridges between grains or grain clusters when both solid and liquid phases are percolated. As such, it represents a key transition with respect to the mechanical behavior of solidifying alloys and to the prediction of solidification cracking. Coalescence starts at the coherency point when the grains begin to touch each other, but are unable to sustain any tensile loads. It ends up at mechanical coherency when the solid phase is sufficiently coalesced to transmit macroscopic tensile strains and stresses. Temperature at mechanical coherency is a major input parameter in numerical modeling of solidification processes as it defines the point at which thermally induced deformations start to generate internal stresses in a casting. This temperature has been determined for Al-Zn alloys using in situ X-ray diffraction during casting in a dog-bone-shaped mold. This setup allows the sample to build up internal stress naturally as its contraction is prevented. The cooling on both extremities of the mold induces a hot spot at the middle of the sample which is irradiated by X-ray. Diffraction patterns were recorded every 0.5 seconds using a detector covering a 426 × 426 mm2 area. The change of diffraction angles allowed measuring the general decrease of the lattice parameter of the fcc aluminum phase. At high solid volume fraction, a succession of strain/stress build up and release is explained by the formation of hot tears. Mechanical coherency temperatures, 829 K to 866 K (556 °C to 593 °C), and solid volume fractions, ca. 98 pct, are shown to depend on solidification time for grain refined Al-6.2 wt pct Zn alloys.

Computational study of granular shear flows of dry flexible fibres using the discrete element method

In this study, shear flows of dry flexible fibres are numerically modelled using the discrete element method (DEM), and the effects of fibre properties on the flow behaviour and solid-phase stresses are explored. In the DEM simulations, a fibre is formed by connecting a number of spheres in a straight line using deformable and elastic bonds. The forces and moments induced by the bond deformation resist the relative normal, tangential, bending and torsional movements between two bonded spheres. The bond or deforming stiffness determines the flexibility of the fibres and the bond damping accounts for the energy dissipation in the fibre vibration. The simulation results show that elastically bonded fibres have smaller effective coefficients of restitution than rigidly connected fibres. Thus, smaller solid-phase stresses are obtained for flexible fibres, particularly with bond damping, compared with rigid fibres. Frictionless fibres tend to align with a small angle from the flow direction as the solid volume fraction increases, and fibre deformation is minimized due to the alignment. However, jamming, with a corresponding sharp stress increase, large fibre deformation and dense contact force network, occurs for fibres with friction at high solid volume fractions. It is also found that jamming is more prevalent in dense flows with larger fibre friction coefficient, rougher surface, larger stiffness and larger aspect ratio.

Effect of Cooling Rate on the Dendrite Coherency Point During Solidification of Al2024 Alloy

Most research related to dendrite coherency point (DCP) has been done on cast aluminum alloys and at a low cooling rate condition. In this research, the DCP of a wrought aluminum alloy is calculated in the range of high cooling rates used in the direct-chill casting process. The two-thermocouple thermal analysis technique was used to determine the DCP of Al2024 alloy. The aim of this work is to investigate the effect of different cooling rates on the dendrite coherency characteristics of Al2024. The cooling rates used in the present study range from 0.4 to 17.5 °C s−1. Also, the effect of 1.2 wt pct Al-5Ti-1B grain refiner on the DCP was studied. To calculate the solid fraction at dendrite coherency, solid fraction versus time is plotted based on Newtonian technique. The results show that by increasing the cooling rate, both time and temperature of dendrite coherency are decreased. Also, by adding the Al-5Ti-1B master alloy, dendrite coherency temperature is reduced and dendrite impingement is postponed. To reduce casting defects occurring during equiaxed solidification, e.g., macrosegregation, porosities, and hot tearing, these two operations which lead to postpone the transition from mass to inter-dendritic feeding, or dendrite coherency, can be useful. By increasing the cooling rate, solid fraction at dendrite coherency increases initially and then decreases at higher cooling rates. Presence of grain refiner leads to increasing of solid fraction at DCP. Thus, by delaying the dendrite coherency and increasing the solid fraction at DCP, semi-solid forming can be performed on parts with higher solid fraction and less shrinkage. Microstructural evaluation was carried out to present the correlation between the cooling rate and solid fraction in 2024 aluminum alloy.

Numerical Simulation of Modified Trailing Edge of Francis Turbine Stay Vane Based on Solid-Liquid Two-Phase Flow

In order to investigate the silt abrasion of modified trailing edge of stay vane in Francis turbine, the numerical simulation of trailing edge with different geometries were carried out based on the solid-liquid two-phase flow by means of Computation Fluid Dynamics. The results show that low solid volume fraction distributes on the chamfered surface of trailing edge, and high solid volume fraction distributes on the end of oblique surface. The smaller the modified angle is, the larger the distribution area of high solid volume fraction is, which show the trailing edge with smaller oblique angle may suffer from silt abrasion. Therefore, in order to solve the vibration caused by Karman vortex the trailing edge has to be sharpened, the oblique angle of trailing edge should not be too small. At end of trailing edge needs to ensure a certain thickness, especially the trailing edge near the lower ring can be thicker, which can meet the anti-abrasion requirements.

Calcium-mediated polysaccharide gel formation and breakage: Impact on membrane foulant hydraulic properties

The solution phase interaction between the polysaccharide alginate and calcium at various concentrations is examined in this paper. The observed gelation behavior and resulting hydraulic properties of the fouling layer deposited on flat sheet membranes can be directly related to the initial calcium concentration (in bulk solution). Particular attention is given to the effect of calcium concentration, with changes in calcium binding behavior of alginate in bulk solution shown to be closely related to the resistance of the cake layer formed on the membranes. At low calcium concentrations, Ca2+ ions interact strongly with guluronate (G) block functional groups forming a highly interconnected gel assemblage. This resulted in the fouling layer deposited on the membrane surface exhibiting high specific resistance despite being highly porous. Further increase in calcium concentration leads to enhanced calcium binding and significant aggregation of alginate in suspension with concomitant breakdown in the gel structure of the material accumulated on the membrane. The increased calcium concentration resulted in a cake that was characterized by a lower porosity and higher solid fraction and, most importantly, a markedly lower specific resistance to the passage of water. The materials properties of the alginate layers formed on flat sheet membranes were also determined under various pressures, with empirical expressions describing the hydraulic conductivity and compressibility of fouling layers developed as a function of trans-membrane pressure (TMP) for a range of calcium concentrations. The resulting materials properties expressions could be applied for the prediction of the filtration behavior of an alginate-rich solution for a range of calcium concentrations under constant flux (and thus time varying TMP) conditions.

Determination of glass transition temperatures during cooling and heating of low-moisture amorphous sugar mixtures

Glass transition temperatures (Tg) of amorphous sugar samples, prepared from 0% to 14% (wb) moisture content, were determined during both cooling and heating using differential scanning calorimetry (DSC). The DSC protocol used involved heating, cooling, and reheating the sugar samples, where Tg was obtained during both cooling and reheating steps. The Gordon-Taylor equation modeled the water plasticization effect, where the Tg of anhydrous solids and constant (k) values were determined simultaneously. No significant difference was found between the cooling and heating glass transition temperature values, with both values significantly influenced by sugar composition (p<0.05). No significant effect of sugar composition was observed for the k values, resulting in discrepancies displayed by the Gordon-Taylor model for solid mass fractions below 0.84. Thus, k values should be estimated from measured Tg data at low solid mass fractions rather than extrapolated from high solid mass fractions, for prediction of Tg curve at high moisture contents.

Evaluation of effect of viscosity changes on bubble size in a mechanical flotation cell

In operating flotation plants, the viscosity of the pulp can vary significantly. Consequently, the resulting impact on bubble size is of interest as many plants experience seasonal changes in water temperature, or particle size changes as ore hardness, mineralogy and throughput fluctuate. However, given its importance in flotation, there existed no mathematical relationship linking bubble size created in flotation machines to the key process variable of fluid viscosity. In this study, a program of investigation to develop such a model was utilizing a pilot-scale mechanical flotation machine, to investigate the effect of water viscosity due to temperature on bubble size distribution. The bubble sizes were determined using a specific bubble viewer and imaging technology. The temperature itself was varied as a method for introducing significant viscosity change. The viscosity-temperature effect introduced a correspondingly significant change in the water viscosity (1619 to 641 μPa·s). It is suggested that a considerably stronger relationship may exist, yielding D32 versus (μ/μ20)0.776, and hence viscosity becomes an important design consideration for plants operating where pulp temperature fluctuations, very small particles or high solid fractions are present.

Simulation on the Thixocasting Process of the Turbocharger Impellers and Selection of the Viscosity Models

The thixocasting technology is chosen to produce the turbocharger impellers as it is capable of producing castings with high precision dimension, free of oxide and shrinkage porosity defects and therefore long service life. The thixocasting turbocharger impellers have the similar mechanical property to those produced by forging and machine processes, but much less costs. In the thixocasting process, the semisolid slurry with certain solid fraction is injected into mould at a high speed. Since high viscosity and thixotropic characteristics, the semisolid slurry reveals unique flow condition and characteristics in the filling process. So, its very desirable for the die design, process optimization, and defect control to visualize the high-speed filling process of semisolid slurry by numerical simulation. In this paper, several commonly used viscosity models for semisolid slurry are analysed. The Power law cut-off model (PLCO) model is selected to simulate the filling process in the thixocasting process of the impellers of 319s alloy, compared with actual results by partial filling testing. The causes of the casting defects are also analysed, indicating that the simulation results can accurately reproduce the filling process of semisolid slurry, and PLCO model is a successful choice for simulating the filling process of semisolid slurry with high solid fraction.

Numerical Simulation of Flow Through Circular Array of Cylinders Using Multi-Body and Porous Models

Two two-dimensional (2D) models for flow through a circular patch of circular cylinders in a channel, the multi-body model and porous model, are established using the scale adaptive simulation (SAS) turbulent model. It is verified that the multi-body model could predict well the average velocity field, turbulent structures and vortex in all cases. The porous model could only predict well the flow pattern and the large-scale vortex motion and fails to predict the small-scale motion. For cases of high solid volume fraction or low cylinder Reynolds number, the differences in prediction between the two models are small even in the region inside the patch. For the case of low solid volume fraction, the performance of the porous model is acceptable only in the region away from the patch. If predictions are required only for large-scale motion away from the patch, the more computational efficient porous model is a good modeling tool.

In Situ Observation of Deformation in Semi-solid Fe-C Alloys at High Shear Rate

Synchrotron X-ray radiography at 125 frames per second was used to study deformation mechanisms in semi-solid Fe-C alloys at high solid fraction and shear strain rates of 10−1/s. An image correlation approach was also used to quantify the shear strain fields and study shear-induced dilation and the origin of shear cracking. It was shown that, at high solid fraction (90 to 93 pct solid), rearrangement including rotation and translation of solid particles became restricted and shear strain localized into narrow liquid-filled channels/fissures. Shear cracking was shown to originate from inadequate liquid flow into the expanding spaces between solid particles caused by shear-induced dilation. At lower solid fraction (~85 pct solid), solid particles rearranged with a significantly higher component of rotation leading to more shear-induced dilation and a wider shear band.

Coherency and Grain Size Effects on Solidification Crack Growth in Aluminum Welds

AbstractA mass balance model has been evaluated that estimates the critical conditions for sustaining continuous crack growth in the mushy weld zone. With the aid of a strain partition model, the critical local strain rate (across the weld) has been related to the critical grain boundary deformation rate needed for crack growth. In the present work, these two models are applied to aluminum welds to investigate the theoretical effects of several metallurgical factors on solidification cracking susceptibility. Calculations quantify the improved cracking resistance associated with a smaller coherent temperature range, grain refinement, high solid fraction at coherency, and rapid development of tensile strength through solid-solid bonding.

Flow behaviour and rheological properties of M2 high-speed steel in semi-solid state

In this research, the rheological properties and flow behaviour of M2 high-speed steel have been investigated in low and high solid fractions by compression and rheometry tests. First series of experiments were started by partial remelting of rolled-annealed M2 billet in an argon-controlled atmosphere and holding for an appropriate time to obtain a globular microstructure and then conducting the rapid compression test on the prepared sample. Load versus deformation data were recorded using a high-frequency data acquisition system during the tests. Deformation mechanism of the semi-solid steel alloy and its correlation to rheological properties were then investigated. In the second series of experiments, the material was sheared continuously while cooling down from liquidus state to a semi-solid temperature. The rheology tests were performed using a self-developed concentric cylindrical viscometer under different cooling rates (25 °C/min–35 °C/min) and shear rates. The results revealed that when solid fraction exceeds about 40%, the viscosity increases suddenly and reaches a viscosity of about 20 Pa s. This solid fraction is a critical point when designing the process for industrial applications such as thixocasting.

Forces acting on a single introduced particle in a solid–liquid fluidised bed

In a liquid fluidised bed system, the motion of each phase is governed by fluid–particle and particle–particle interactions. The particle–particle collisions can significantly affect the motion of individual particles and hence the solid–liquid two phase flow characteristics. In the current work, computational fluid dynamics–discrete element method (CFD–DEM) simulations of a dense foreign particle introduced in a monodispersed solid–liquid fluidised bed (SLFB) have been carried out. The fluidisation hydrodynamics of SLFB, settling behaviour of the foreign particle, fluid–particle interactions, and particle–particle collision behaviour have been investigated. Experiments including particle classification velocity measurements and fluid turbulence characterisation by particle image velocimetry (PIV) were conducted for the validation of prediction results.Compared to those predicted by empirical correlations, the particle classification velocity predicted by CFD–DEM provided the best agreement with the experimental data (less than 10% deviation). The particle collision frequency increased monotonically with the solid fraction. The dimensionless collision frequency obtained by CFD–DEM excellently fit the data line predicted by the kinetic theory for granular flow (KTGF). The particle collision frequency increased with the particle size ratio (dP2/dP1) and became independent of the foreign particle size for high solid fractions when the fluidised particle size was kept constant. The magnitude of collision force was 10–50 times greater than that of gravitational force and maximally 9 times greater than that of drag force. A correlation describing the collision force as a function of bed voidage was developed for Stp>65 and dP2/dP1≤2. A maximum deviation of less than 20% was obtained when the correlation was used for the prediction of particle collision force.

Pipeline transport of biomass: Experimental development of wheat straw slurry pressure loss gradients

Pipeline transport in the form of a slurry can reduce the cost of transportation of biomass material to a biorefinery, as compared to trucks. This research experimentally studies the hydraulics of slurries of wheat straw with water for pipeline transport. Slurries with a range of particle sizes and saturated solid mass fractions are examined in a laboratory-scale 50 mm diameter carbon steel pipeline system. Slurries with particles approximately 3 mm long can flow with saturated solid mass fractions of up to 30%. Pressure loss gradients results suggest the influence of drag reducing fibre suspensions. This phenomenon enables slurry pressure losses to be below that of the carrier fluid alone (water) and be achieved with sufficiently long particle sizes, low saturated solid mass fractions and high velocities. Our results suggest that to reduce pressure losses per unit biomass material, slurries should have short particle sizes, to allow high saturated solid mass fractions to be pumped at low velocities. With the pipeline system and slurries examined in this study, slurries with particles approximately 3 mm long and saturated solid mass fractions of 20–30% pumped at 1.5 m s−1 experience the lowest pressure losses. This result helps in the design and optimal operation of biomass slurry pipeline systems.

A synchrotron X-ray radiography study of dendrite fragmentation induced by a pulsed electromagnetic field in an Al–15Cu alloy

An in situ technique for studying the effect of a pulsed electromagnetic field on dendrite fragmentation behavior based on synchrotron X-ray imaging has been developed, involving the passage of an oscillating current through a foil specimen placed in a static magnetic field. In all cases, the application of a pulsed electromagnetic field to an array of Al–15wt.%Cu dendrites led to an increase in dendrite fragmentation rate. The induced movement of inter-dendritic liquid between the primary and secondary dendrite arms even at high solid fraction led to instabilities in the local temperature–concentration–curvature equilibrium and the rapid remelting of the most vulnerable microregions. The new in situ methodology provides a basis for the optimization of pulsed electromagnetic field parameters to enhance grain refining performance for specific alloys and casting conditions.

Experimentally Validated Computational Fluid Dynamics Simulations of Multicomponent Hydrodynamics and Phase Distribution in Agitated High Solid Fraction Binary Suspensions

The mixing of dense (5–40 wt %) binary mixtures of glass particles in water has been studied in a stirred vessel at the “just-suspended” speed and at speeds above it, using a Eulerian–Eulerian computational fluid dynamics (CFD) model. For each phase component, numerical predictions are compared to 3-dimensional distributions of local velocity components and solid concentration measured by an accurate technique of positron emission particle tracking. For the first time, it has been possible to conduct such a detailed “pointwise” validation of a CFD model within opaque, dense multicomponent slurries of this type. Predictions of flow number and mean velocity profiles of all phase components are generally excellent. The spatial solid distribution is well-predicted except near the base of the vessel and underneath the agitator where it is largely overestimated; however, predictions improve significantly with increasing solid concentration. Other phenomena and parameters such as particle slip velocities and homogeneity of suspension are analyzed.

Dense suspension of solid particles as a new heat transfer fluid for concentrated solar thermal plants: On-sun proof of concept

This paper demonstrates the capacity of dense suspensions of solid particles to transfer concentrated solar power from a tubular receiver to an energy conversion process by acting as a heat transfer fluid. Contrary to a circulating fluidized bed, the dense suspension of particles’ flows operates at low gas velocity and large solid fraction. A single-tube solar receiver was tested with 64µm mean diameter silicon carbide particles for solar flux densities in the range 200–250kW/m2, resulting in a solid particle temperature increase ranging between 50°C and 150°C. The mean wall-to-suspension heat transfer coefficient was calculated from experimental data. It is very sensitive to the particle volume fraction of the suspension, which was varied from 26 to 35%, and to the mean particle velocity. Heat transfer coefficients ranging from 140W/m2K to 500W/m2K have been obtained, thus corresponding to a 400W/m2K mean value for standard operating conditions (high solid fraction) at low temperature. A higher heat transfer coefficient may be expected at high temperatures because the wall-to-suspension heat transfer coefficient increases drastically with temperature. The suspension has a heat capacity similar to a liquid heat transfer fluid, with no temperature limitation but the working temperature limit of the receiver tube. Suspension temperatures of up to 750°C are expected for metallic tubes, thus opening new opportunities for high efficiency thermodynamic cycles such as supercritical steam and supercritical carbon dioxide.

Prediction of extrusion load and liquid phase filtration during ram extrusion of high solid volume fraction pastes

For perfect and homogeneous plastic materials, the extrusion stress and flow typology have previously been well described. However for firm pastes, during extrusion, liquid phase migration may appear leading to unsuccessful processing. Reviewed literature explains that there is a competition between the extrusion velocity and the liquid filtration velocity (or time).The aim of this work is to develop a new model to predict the ram extrusion force of frictional plastic materials such as firm fresh cement-based materials or clay pastes. A quantitative one-dimensional model based on consolidation theory has been developed to predict the extrusion force and quantify liquid phase migration.Model results are compared with experimental data from tests carried out on kaolin paste undergoing ram extrusion. Influences of extrusion rate, rheological parameters and filtration characteristics on extrusion load are investigated.

Characterization of flow behavior of semi-solid slurries containing low solid fractions in high-pressure die casting

Understanding the flow behavior of semi-solid slurries containing low solid fractions is key to the success in applying this process in the die casting industry. With these low initial solid fractions, the flow behavior of semi-solid slurries is quite complicated, making it difficult to model accurately. This present work developed and studied characterization methods for the flow behavior of semi-solid slurries at low solid fractions in high-pressure die casting. A new parameter, the ratio of gate speed to initial solid fraction (Vg/fs), was proposed to be correlated to the normalized flow interface length, blister area and tensile properties. Results from the flow pattern analysis suggest that the flow behavior can be controlled to achieve laminar flow by varying the initial solid fraction. Blister test results show the trend that slurry die casting conditions with high Vg/fs values exhibit high blister areas. Die casting conditions with excessively high gate speeds and insufficient solid fractions result in turbulent flow patterns and high levels of blister defect. The results of tensile test and fracture surface analysis are consistent with other analysis results. The samples formed by liquid die casting and slurry die casting with high Vg/fs values have gas porosity due to turbulent flow pattern during die filling. On the other hand, the samples formed by slurry die casting with too low Vg/fs values contain shrinkage porosity. This is because of insufficient time for shrinkage feeding due to a combination of a high solid fraction and a low gate speed. This study has demonstrated that die casting with slurries containing low initial solid fractions gives die casters another process parameter to adjust, which can help reduce and control the gas and shrinkage porosities.

Characterization of Flow Behavior of Semi-Solid Slurries with Low Solid Fractions

Semi-solid slurry casting is a metal-forming process that involves transforming liquid metal into slurry having a low solid fraction and then forming the slurry into solid parts. To successfully apply this slurry-forming process, it is necessary to fully understand the flow behavior of semi-solid slurries. This present work applied the rapid quenching method and the modified gravity fluidity casting to investigate the flow behavior, which involves characterizations of the initial solid fraction, fluidity, and microstructure of semi-solid slurries. Three commercial aluminum alloys were used in this study: 383 (Al-Si11Cu), 356 (Al-Si7MgFe), and 7075 (Al-Zn6MgCu) alloys. The results show that the initial solid fractions can be controlled by varying the rheocasting time. The rapid quenching mold can be used to determine the initial solid fractions. In this method, it is important to apply the correcting procedure to account for growth during quenching and to include all the solid phases. Results from the fluidity study of semi-solid slurries show that the fluidity decreases as the initial solid fraction increases. The decrease is relatively rapid near the low end of the initial solid fraction curves, but is quite slow near the high end of the curves. All the three alloys follow this trend. The results also demonstrate that the slurries that contain high solid fractions of up to 30 pct can still flow well. The microstructure characterization results show that the solid particles in the slurries flow uniformly in the channel. A uniform and fine microstructure with limited phase segregation is observed in the slurry cast samples.