Influence of the formulation of an alumina powder on compaction

International audienceThe compression of ceramic powders requires the addition of organic materials to insure tablet cohesion. For the compaction of formulated ceramic powders it is necessary to supply them energy. As a function of the formulation of ceramic powders, one can measure the specific energy acquired during the tablet compression. Recently, a technique used in powder metallurgy, High Velocity Compaction (HVC), has been studied to form ceramic compacts. This method brings special physical properties to tablets in terms of homogeneity compared to conventional uniaxial pressing. The properties of tablets prepared by the two processes are compared and discussed through a phenomenological model

Micropillar compression testing of powders

An experimental design for microcompression on individual powder particles is proposed as a means of testing novel materials without the challenges associated with consolidation to produce bulk specimens. This framework is demonstrated on an amorphous tungsten alloy powder, and yields reproducible measurements of the yield strength (4.5 ± 0.3 GPa) and observations of the deformation mode (in this case, serrated flow by shear localization).United States. Defense Threat Reduction Agency (Grant HDTRA1-11-1-0062)American Society for Engineering Education. National Defense Science and Engineering Graduate Fellowshi

Analysis of the flowability of cohesive powders using Distinct Element Method

Computer simulations using Distinct Element Method (DEM) have been carried out to investigate the effect of cohesion on the flowability of polydisperse particulate systems. For this purpose, two assemblies with different values of surface energy and made of 3000 spheres with the mechanical properties of glass beads were considered. The analysis of the flowability of the powders is presented in terms of the unconfined yield stress as a function of strain rate for different pre-consolidation loads. For values of the surface energy of 1.0 J/m2 and strain rates lower than 6 s− 1, the unconfined yield stress does not change significantly indicating a quasi-static behaviour of the particulate assemblies during the compression process. For larger strain rates, the unconfined yield stress varies with the power index of 1.2 of the strain rate. The influence of the pre-consolidating stress on the powder behaviour has also been investigated and a flow factor was obtained from the linear relationship between the unconfined yield stress and pre-consolidation stress. The computer simulations show qualitatively a good agreement with the experimental trends on highly cohesive powder flow behaviour

Characterization and flow of food and mineral powders : a thesis presented in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Engineering at Massey University, Manawatū, New Zealand

Powders are important commodities across different industries, such as the food and pharmaceutical industries. In these industries, powders are usually made, mixed, milled, packaged, and stored; these operations require the powders to move and flow under desired conditions and different stress levels. Failure to flow will cause hindrances to production; therefore knowledge of powder flow or flowability is important. There is a constant demand for accurate, reliable, and robust measurement and characterization methods for powder flowability. Powders behave differently under varying conditions; the behaviour of a powder is influenced by particle size distribution, and powder handling and processing conditions. There is to date no one “standard” method to characterize powder flowability; it is common to use a variety of methods and devices to measure flow properties and provide insight into the behaviour and flow characteristics of powders under different conditions. The flow properties of model food and mineral powders were measured and assessed by shear testing, compression via tapping, fluidization, and powder tumbling. Shear testing was done with an annular shear cell following Jenike (1964) and Berry, Bradley and McGregor (2014). Compression via tapping was performed according to a procedure in the dairy industry (Niro, 1978) and the European Pharmacopoeia (Schüssele & Bauer-Brandl, 2003). Fluidization was used to measure powder bed expansion and bed collapse following the powder classification framework provided by Geldart and co-workers (Geldart, 1973; Geldart, Harnby, & Wong, 1984; Geldart & Wong, 1984, 1985). Powder tumbling was performed in a novel Gravitational Displacement Rheometer, GDR, which measured the motion and avalanche activity of powders that moved under their own weight when rotated in a cylinder at different drum speed levels. The flow data from each characterization method were evaluated individually with regards to particle size distribution and then assessed collectively. The findings presented and discussed include the i) demonstration of the dominant influence of surface-volume mean particle diameter on powder flow properties, ii) characterization of flowability based on Jenike’s arbitrary flow divisions, iii) development of new correlations for the estimation of powder cohesion and bulk density at low preconsolidation stresses, iv) demonstration of hopper outlet diameter as a measure of flowability, v) demonstration of the limited utility of Hausner ratio as a flowability index, vi) substantiation of von Neumann ratio as a sensitive and useful indicator for identifying the onset of bubbling in fluidized beds using bed pressure fluctuation data, and vii) demonstration of the utility of standard deviation of the GDR load cell signal as an indicator of powder avalanche activity. These findings provide improved understanding and knowledge of powder flowability; they can be used to assist and facilitate the development of new techniques and solutions relevant to the handling and processing of powders especially in the food and pharmaceutical industries

Element test experiments and simulations: from dry towards cohesive powders

Findings from experiments and particle simulations for dry and cohesive granular materials are presented with the goal to reach quantitative agreement between simulations and experiments. Results for the compressibility, tested with the FT4 Powder Rheometer are presented. The first simulation results involve the strain controlled uniaxial compression of frictionless polydisperse spheres in a biaxial box using a linear visco-elastic contact model.\ud \ud As main result, the evolution of pressure as a function of volume fraction is reported. Our anisotropic, uniaxial findings compare astonishingly well with results for purely isotropic compression. Concerning the second stress response, namely anisotropy, we present the evolution of the deviatoric stress as a function of the volume fraction, which cannot be measured with the FT4 experiment, but requires a bi-axial experiment

Analysis of the effect of cohesion and gravity on the bulk behaviour of powders using Discrete Element Method

Computer simulations using Distinct Element Method have been carried out to analyse the bulk behaviour of a polydisperse assembly of glass beads. For this purpose an assembly made of 3000 spheres were generated to which the mechanical properties of glass beads were assigned. The system was initially compressed isotropically at a strain rate of 1 s-1 in the absence of gravity and surface energy. Once the assembly reached a packing fraction of about 0.62, the effects of cohesion and gravity on the bulk behaviour were analysed for two different cases. In the first case only gravity was applied, whilst in the second case both gravity and surface energy were acting on the particles. The evolution of the components of the stress tensor for the case in which only gravity was applied indicated that the gravity did not appreciably affect the isotropy of the system. In contrast, the system in which surface energy was introduced became anisotropic. The concept of unconfined yield stress of bulk cohesive powders was used to analyse the effect of surface energy and strain rate. For values of surface energy of 1.0 J/m2 and of strain rate lower than 1 s-1 the unconfined yield strength did not change significantly indicating a quasi-static behaviour for the compression process. However, for values of strain rates larger than 1 s-1 the unconfined yield strength increased with the strain rate, following a power law trend with an index of 1.7

Material Issues of the Metal Printing Process, MPP

The metal printing process, MPP; is a novel Rapid Manufacturing process under development at SINTEF and NTNU in Trondheim, Norway. The process, which aims at the manufacturing of end-use products for demanding applications in metallic and CerMet materials, consists of two separate parts; The layer fabrication, based on electrostatic attraction of powder materials, and the consolidation, consisting of the compression and sintering of each layer in a heated die. This approach leads to a number of issues regarding the interaction between the process solutions and the materials. This paper addresses some of the most critical material issues at the current development stage of MPP, and the present solutions to these.Mechanical Engineerin