Particle Size Segregation Research Articles

Effect of powder size segregation on the mechanical properties of hot isostatic pressing Inconel 718 alloys

Pre-alloyed powder of Inconel 718 alloy was prepared by electrode induction melting gas atomization (EIGA) and powder metallurgy (PM) Inconel 718 alloys were made through hot isostatic pressing (HIPing) route. The change in volume of large size parts (special-shaped cylindrical capsule) has a much lower effect on the alloy than the powder size. The vibration of the powder filling process over a long time causes the particle size segregation phenomenon that the proportion of coarse particle powders in top part increases, and the proportion of finer particle powders in middle and bottom part increases. This phenomenon has little effect on tensile strength of PM Inconel 718 alloys at room temperature and 650 °C, but has a significant effect on elongation, room temperature impact properties, and stress rupture life at 650 °C/725 MPa. Compared to finer particle powders, the plastic deformation of coarse particle powders is small and a large number of under-deformed powder particles are retained, forming prior particle boundaries (PPBs) and reducing the bonding strength between powder particles. In order to avoid particle size segregation, optimized process parameters and reasonable particle size should be used.

Particle scale modelling of powder recoating and melt pool dynamics in laser powder bed fusion additive manufacturing: A review

Particle scale modelling of the process physics involved in laser powder bed fusion (LPBF) is a recent research hotspot, and many efforts have been made in the literature. However, a comprehensive review of the physics in LPBF and the effects of key variables such as powder- and operation-related parameters, and the mechanisms of defects formation is still lacking. This paper aims to offer a state-of-the-art review on multi-physics related to metal powder recoating and further melting and solidification process. The studies on powder bed recoating explored by discrete element method are presented first, including model theory and validation, effects of process parameters, and physics of particle flow and size segregation. Then powder melting approach based on computational fluid dynamics and the involved phenomena such as melt pool dynamics, keyhole dynamics, defects formation mechanisms of pores, and balling and spattering are described. The needs for future research are also discussed. • Powder bed recoating explored by discrete element method (DEM) is reviewed. • Physics of powder flow and size segregation in powder spreading is presented. • Powder melting based on computational fluid dynamics (CFD) is summarized. • Melt pool flow dynamics and defects formation are discussed.

3D structure of free-surface avalanche deposits from MRI: particle-size segregation in free-surface, bi-disperse granular flows

3D structure of free-surface avalanche deposits from MRI: particle-size segregation in free-surface, bi-disperse granular flows

Formation of dry granular fronts and watery tails in debris flows

Debris flows are particle–fluid mixtures that pose a significant hazard to many communities throughout the world. Bouldery debris flows are often characterised by a deep dry granular flow front, which is followed by a progressively thinner and increasingly watery tail. The formation of highly destructive bouldery wave fronts is usually attributed to particle-size segregation. However, the moving-bed flume experiments of Davies (N. Z. J. Hydrol., vol. 29, 1990, pp. 18–46) show that discrete surges with dry fronts and watery tails also form in monodisperse particle–fluid mixtures. These observations motivate the development of a new depth-averaged mixture theory for debris flows, which explicitly takes account of the differing granular and phreatic surfaces, velocity shear, and relative motion between grains and fluid to explain these phenomena. The theory consists of four coupled conservation laws that describe the spatial and temporal evolution of the grain and water thicknesses and depth-averaged velocities. This system enables travelling wave solutions to be constructed that consist of (i) a large amplitude dry flow front that smoothly transitions to (ii) an undersaturated body, (iii) an oversaturated region and then (iv) a pure water tail. It is shown that these solutions are in good quantitative agreement with Davies’ experiments at high bed speeds and slope inclinations. At lower bed speeds and inclinations, the theory produces travelling wave solutions that connect to a steady-uniform upstream flow, and may or may not have a bulbous flow front, consistent with Davies’ observations.

Effect of Particle Interactions on the Assembly of Drying Colloidal Mixtures.

The effects of particle interactions on the size segregation and assembly of colloidal mixtures during drying were investigated. A cationic surfactant was added to a binary latex/silica colloidal dispersion that has been shown to self-stratify upon drying at room temperature. Atomic force microscopy was used to show that the change in particle interactions due to the presence of surfactants reduced the degree of stratification and, in some cases, suppressed the effect altogether. Colloidal dispersions containing higher surfactant concentrations can undergo a complete morphology change, resulting instead in the formation of armored particles consisting of latex particles coated with smaller silica nanoparticles. To further prove that armored particles are produced and that stratification is suppressed, cross-sectional images were produced with energy-dispersive X-ray spectroscopy and confocal fluorescence microscopy. The growth of armored particles was also measured using dynamic light scattering. To complement this research, Brownian dynamics simulations were used to model the drying. By tuning the particle interactions to make them more attractive, the simulations showed the presence of armored particles, and the size segregation process was hindered. The prevention of segregation also results in enhanced transparency of the colloidal films. Overall, this research proves that there is a link between particle interactions and size segregation in drying colloidal blends and provides a valuable tool to control the assembly of different film architectures using an extremely simple method.

Numerical study of the dynamic behaviour of iron ore particles during wet granulation process using discrete element method

Iron ore granulation process is simulated by the discrete element method to understand the dynamic behaviour of iron ore particles. The results show that the macroscopic particle flow transits from rolling regime to cascading regime after water addition, with the angle of repose of particles increasing and the level of particle size segregation decreasing. The moisture distribution becomes more uniform through particle tumbling, which improves the granule growth. The microdynamic behaviours of the fines, intermediates and nuclei during the auto-layering process have been studied respectively. The variations in the average contact number, kinetic and rotational energy per particle with granulation time are analysed. The optimal moisture contents for Ores N, M and Y are estimated as 6.46 wt%, 9.12 wt% and 8.70 wt%, respectively. The numerical results are generally in accordance with the corresponding experimental ones, which basically validates the DEM modelling of iron ore granulation presented in this paper.

Particle segregation and diffusion in fluid-saturated granular shear flows

Using coupled granular-fluid simulations we study the particle size segregation and diffusion in sheared granular flows immersed in different ambient fluids. Both segregation and diffusion decrease with the fluid viscosity but only after exceeding a certain threshold value. Decreasing the relative density between the particles and the fluid further slows down segregation but does not significantly affect diffusion. Empirical relationships for the segregation velocity and diffusion coefficient are developed in terms of the Stokes number which are used to model the segregation-diffusion process across different types of fluids.

Heterogeneous Nucleation Drives Particle Size Segregation in Sequential Ozone and Nitrate Radical Oxidation of Catechol

Secondary organic aerosol formation via condensation of organic vapors onto existing aerosol transforms the chemical composition and size distribution of ambient aerosol, with implications for air quality and Earth's radiative balance. Gas-to-particle conversion is generally thought to occur on a continuum between equilibrium-driven partitioning of semivolatile molecules to the pre-existing mass size distribution and kinetic-driven condensation of low volatility molecules to the pre-existing surface area size distribution. However, we offer experimental evidence in contrast to this framework. When catechol is sequentially oxidized by O3 and NO3 in the presence of (NH4)2SO4 seed particles with a single size mode, we observe a bimodal organic aerosol mass size distribution with two size modes of distinct chemical composition with nitrocatechol from NO3 oxidation preferentially condensing onto the large end of the pre-existing size distribution (∼750 nm). A size-resolved chemistry and microphysics model reproduces the evolution of the two distinct organic aerosol size modes─heterogeneous nucleation to an independent, nitrocatechol-rich aerosol phase.

A conveyor belt experimental setup to study the internal dynamics of granular avalanches

This paper shows how a conveyor belt setup can be used to study the dynamics of stationary granular flows. To visualise the flow within the granular bulk and, in particular, determine its composition and the velocity field, we used the refractive index matching (RIM) technique combined with particle tracking velocimetry and coarse-graining algorithms. Implementing RIM posed varied technical, design and construction difficulties. To test the experimental setup and go beyond a mere proof of concept, we carried out granular flow experiments involving monodisperse and bidisperse borosilicate glass beads. These flows resulted in stationary avalanches with distinct regions whose structures were classified as: (i) a convective-bulged front, (ii) a compact-layered tail and, between them, (iii) a breaking size-segregation wave structure. We found that the bulk strain rate, represented by its tensor invariants, varied significantly between the identified flow structures, and their values supported the observed avalanche characteristics. The flow velocity fields’ interpolated profiles adjusted well to a Bagnold-like profile, although a considerable basal velocity slip was measured. We calculated a segregation flux using recent developments in particle-size segregation theory. Along with vertical velocity changes and high expansion rates, segregation fluxes were markedly higher at the avalanche’s leading edge, suggesting a connection between flow rheology and grain segregation. The experimental conveyor belt’s results showed the potential for further theoretical developments in rheology and segregation-coupled models.Graphic

The effect of recoater geometry and speed on granular convection and size segregation in powder bed fusion

In metal additive manufacturing (AM), powder bed fusion technologies such as selective laser melting and binder jetting rely on the spreading of fine metal powder to build up the layers of a part. This work studies the complex interaction between metal powder particles and the recoater. We apply the Discrete Element Method (DEM) to a calibrated cohesive Ti-6Al-4V powder model, matching the particle properties to experimental measurements of size, shape and inter-particle forces. We simulate powder bed fusion recoating at varying speeds and layer thicknesses with two different recoater geometries: a toothed rake and a solid blade. Our results demonstrate that the recoating mechanism induces significant granular convection, with the recoater velocity and geometry strongly influencing the degree of particle circulation and size segregation. Notably a toothed rake recoater improved particle circulation – while the solid blade recoater increases size segregation within the heap. Furthermore the recoater was found to filter fine and coarse particles, allowing smaller particles to flow underneath the recoater, and larger particles through its teeth. Our results demonstrate the key mechanisms which drive granular convection during recoating and how the fine details of recoater geometry impacts surface layer roughness and final part quality.

Viscous Effects on the Particle Size Segregation in Geophysical Mass Flows: Insights From Immersed Granular Shear Flow Simulations

AbstractSize segregation is a common feature in geophysical mass flow deposits and is an active process during motion. The presence of interstitial fluids in such flows affect the motion of constituent particles and result in complex segregation behaviors. Effects of the viscosity and density of various interstitial fluids on the rate of particle size segregation are investigated through coupled granular‐fluid simulations of immersed plane‐sheared flows. The segregation rate decreases as the fluid viscosity increases, but remains constant when the viscosity is below certain threshold values. In the low viscosity limit, segregation is affected only by the relative density between the particles and the fluid, and by flow inertial conditions. Analysis of segregation forcing terms based on the mixture theory of segregation reveals that the decrease of segregation rates with the viscosity is due to the increase of fluid drag forces, which effectively weaken the contact stress gradients responsible for driving the large particles to migrate upward. Viscous damping also diminishes velocity fluctuations and thereby inhibits the migration of particles throughout the mixture. The transition in the viscosity dependence is shown to correspond to the transition between granular‐fluid flow regimes. An empirical scaling formula is developed which accounts for the effects of fluid viscosity and the relative density on size segregation immersed in different fluids.

Large particle segregation in two-dimensional sheared granular flows

We studied large particle segregation via experiments in a two-dimensional shear cell. Our results validated a recent equation relating particle-size segregation velocity, size ratio and shear rate, further explaining the role of size ratio on large particle segregation. Based on our observations, we showed and described the part played by particle rotation and expansion rate in large particle segregation mechanics.

An experimental scaling law for particle-size segregation in dense granular flows

Abstract

Data for the JFM paper "An experimental scaling law for particle-size segregation in dense granular flows"

Data for the JFM paper "An experimental scaling law for particle-size segregation in dense granular flows"

Particle size segregation in two-dimensional circular granular aggregates.

Although many previous studies have focused on the Brazil nut effect, segregation in a self-gravitating circular aggregate remains relatively unexplored. In this paper, size segregation in a two-dimensional assembly of grains in a circular geometry is studied through discrete element method (DEM) numerical simulations. We show that radial segregation within an asteroid submitted to periodic perturbations is not limited to the surface but also occurs in its core. The characteristic time and the overall efficiency of the segregation mechanism are studied as the intensity of the perturbation, the frictional properties, and rotational freedom of individual grains are varied.

Mechanisms of size segregation in granular flows with different ambient fluids

Particle size segregation is ubiquitous in granular systems with differently sized constituents but is found to diminish in the presence of viscous ambient fluids. We study this inhibiting effect through coupled fluid-particle numerical simulations. It is found that size segregation is indeed slower in the presence of fluid and this effect becomes more significant as fluid viscosity is increased. Direct calculation of segregation forcing terms reveal that the ambient fluids affect segregation in two major ways: buoyant forces reduce contact pressures, while viscous dissipation diminish particle-fluctuation driven kinetic pressures, both of which are necessary in driving large particles up. Surprisingly, the fluid drag in the normal direction is negligible regardless of the fluid viscosity and does not directly affect segregation.

Impact of granular segregation on heat transfer in horizontal drums

In the production of biofuel from scrap tire pieces by pyrolysis in a rotary kiln, particle size segregation is believed to affect the temperature evolution and control within the reactor. To study the impact of granular segregation on heat transfer in rotary kilns, a thermal discrete element method-based model incorporating a thermal resistance network was developed for bidisperse particulate blends. It was validated using a system of glass and aluminum beads differing in size. The harmonic mean radius was found to best represent bidisperse contacts. Heat transfer in six different segregated bed structures in a wall-heated drum for two particle size ratios was studied. Granular segregation significantly influenced the temperature profiles in the beds, dragging down the temperature of the center of the bed, where a core of smaller particles had formed. This resulted in an inverse relationship between the effective heat transfer coefficient of the bed and the intensity of segregation. Within the segregated cases investigated, the temperature difference between the large and small particles could be up to 30 times higher than that of a well-mixed configuration.

Study of grain-scale effects in bulk handling using discrete element simulations

Industrial granular flow issues are affected by the particle properties and environmental conditions of the materials. To understand the impact of grain-scale effects on flowability, experimental testing should be complemented with particle-based modelling. In this study, we simulate the effects of particle size, shape, and hygroscopicity by the discrete element method (DEM) for different bulk handling problems. We examine a granular column collapse set-up, comparing flow propagation of dry, mono-sized sphere packings to granular systems with particle size dispersity, prolate spheroidal particles, and liquid contents in pendular capillary states. Our results corroborate the experimental observations of evolving height profiles, velocity fields, energy balance, and particle orientation. We also analyse different hopper design alternatives dealing with particle size segregation in an industrial case study of the filling operation. This study illustrates the usability of the DEM as an effective support tool to characterise complex granular flows and improve bulk handling technology.

Unsteady overflow behavior of polydisperse granular flows against closed type barrier

The overflow behavior of polydisperse granular flows against closed type barrier is a crucial yet poorly understood aspect of multiple-barrier design. We use a robust numerical model based on the three-dimensional discrete element method (DEM) to simulate polydisperse granular flows and identify fundamental mechanisms. The morphology of the dead zone behind the upstream barrier significantly influences the overflow behavior of granular flows. This implies that aside from the case of a gentler barrier (<70°), the launch direction is controlled by the inclination surface of the dead zone. We identified a three-stage evolution process of the overflow velocity. The velocity component in the z-direction controls the first stage, whereas the latter two stages are controlled by the velocity component in the x-direction. The launch angle is usually larger than 45° during the initial overflow stage. Such a high launch angle is maintained for about 0.2–0.3 s. Boulders are transported downstream by the overflow process owing to particle-size segregation and a more concentrated load is received by the downstream barrier, which can lead to localized barrier damage. A different overflow mechanism would result in a different launch distance. Steeper (135°) and gentler (70°) barrier configurations are dangerous because the normalized launch distance is up to 20% larger than that of a barrier with a medium inclination. And a point-mass based assumption is used to estimate the maximum launch distance with the use of maximum overflow velocity and a single launch angle. In addition, we discuss some limitations of numerical modelling, including the effect of fluid phase on overflow, as well as the direct application of our results in engineering design.

Generalized friction and dilatancy laws for immersed granular flows consisting of large and small particles

The motion of fully immersed granular materials, composed of two distinct particle sizes, flowing down rough inclined planes is studied through fluid–particle numerical simulations. We focus on the effect of ambient fluids, as well as their interplay with particle size segregation, on the steady-state kinematic and rheological profiles of the granular-fluid mixture flow. Simulation results are analyzed in the framework of a visco-inertial rheological model, which is first validated in monodisperse flows with a wide range of the ambient fluid viscosity (i.e., from air to water and slurry) and then generalized for size-bidisperse mixtures. It is found that the local effective friction and volume fraction of mixtures with different particle sizes can be approximated from the rheology of single-component flows. While the presence of viscous ambient fluids slows down size segregation (perpendicular to the flow) depending on the mixture composition and flow viscosity, the effective bulk friction is shown to be independent of the state and progress of segregation.