Antonios Zavaliangos

Sintering activation by external electrical field

Field assisted sintering technique (FAST) is a non-conventional powder consolidation method in which densification is enhanced by the application of an electrical discharge combined with resistance heating and pressure. Interest in FAST is motivated by its ability to consolidate a large variety of powder materials to high densities in short times. Full densification of metal and ceramic powders has been achieved within minutes, with a reduced number of processing steps, no need for sintering aids and more flexibility in powder handling. Although the electrical discharge effects have not been completely elucidated, distinct surface effects created by micro-discharges have been noticed in FAST consolidated specimens such as atomically clean grain boundaries and new resistivity peaks in superconductors. On-going experimental and theoretical studies to provide more quantitative insight into the relevant FAST mechanisms are presented.

Evolution of near-equiaxed microstructure in the semisolid state

The microstructure of alloys with a near-equiaxed microstructure, produced by spray casting, magnetohydrodynamic (MHD) casting and the stress induced, melt activated (SIMA) process, as it evolves within short times in the semisolid state, is examined by rapid quenching and isothermal soaking experiments. Quenching experiments reveal the morphology and distribution of solid phase at high and medium volume fractions of liquid. At medium liquid content, the microstructure of spray-cast and SIMA alloys consists of discrete equiaxed grains uniformly dispersed in the liquid phase, while the corresponding microstructure of MHD-cast alloys exhibits extensive agglomerates consisting of incompletely spheroidized grains. The connectivity of solid phase and the formation of a solid skeleton in the semisolid state are discussed in terms of grain misorientation. Isothermal soaking experiments investigated grain growth and degree of spheroidization as a function of soaking time and liquid content in the semisolid state. Results demonstrated that MHD-cast microstructures are less equiaxed compared with SIMA and spray-cast alloys even after 5 min of soaking in the semisolid state. It is also shown that the grain growth rate is smaller in spray-cast alloys than in SIMA alloys. The role of coalescence and the effects of alloying elements are also discussed.

The effect of wall friction in the compaction of pharmaceutical tablets with curved faces: a validation study of the Drucker-Prager Cap model

Abstract The compaction of porous materials can be modelled using micromechanical or phenomenological approaches. The micromechanical models are developed for either dense random packings or near fully dense ductile materials. Phenomenological models have been developed to describe the response of the material over a range of relative densities encountered in powder metallurgy, ceramics or composites industries. Pharmaceutical powders are particular in that their initial relative density (RD) is between 0.2 and 0.4, which is significantly lower than for other powder materials. In this paper, we analyse the die compaction of pharmaceutical powders using a variable parameter Drucker–Prager type cap model. The model was calibrated for microcrystalline cellulose using a die instrumented with radial pressure sensors, which is also used to measure the coefficient of friction between powder and die wall. The relative density distribution in tablets is examined with special reference to the friction interaction between powder-die and powder-punches. The predictions of the model are compared with experimental density maps obtained from surface hardness tests carried out on cross-sections of the tablets. Two situations are considered where the die and punches are unlubricated and lubricated, which result in opposite density distribution trends. The result suggests that if the material is subjected to high triaxiality stress, then the phenomenological models can be applied for low initial apparent density powders.

A comparative characterization of near-equiaxed microstructures as produced by spray casting, magnetohydrodynamic casting and the stress induced, melt activated process

The near-equiaxed microstructure of wrought and cast aluminum alloys as produced by the most common methods used to provide material for subsequent semisolid processing, is examined. More specifically, the grain size and degree of spheroidization of alloys produced by spray casting, magnetohydrodynamic (MHD) casting and the stress induced, melt activated (SIMA) process are characterized and compared. It is shown that the microstructure of alloys with the same composition differs significantly when produced by the three methods, showing the influence of the production method. Spray casting and the SIMA process result in a microstructure with perfectly equiaxed grains that is inherently suitable for semisolid processing, while MHD-cast alloys exhibit a non-uniform initial microstructure, with dendritic features dominant at the perimeter of the casting. Finally, the mechanisms responsible for the formation of near-equiaxed microstructure for each method are investigated.

Analysis of the diametrical compression test and the applicability to plastically deforming materials

The effect of contact flattening and material properties on the fracture stress calculation for the diametrical compression test used to evaluate compact strength was examined through finite element simulations. Two-dimensional simulations were carried out using linear elastic, elastoplastic, and porous elastoplastic models with commercial finite element software. A parametric study was performed by varying the elastic modulus (E), Poissons ratio (ν), contact frictional coefficient (μ), yield stress (σyield), and compact relative density (RD). Stress contours generated from these simulations were compared to the Hertzian and Hondros analytical expressions. Linear elastic simulations show excellent agreement with the analytical solutions. Significant deviation, however, occurs for the elastoplastic and porous elastoplastic simulations at larger diametrical strain with material plasticity. A better understanding of the stress-state of diametrically loaded plastically deforming disks has been demonstrated in this computational and experimental work. Results from these finite element simulations confirm that the standard tensile strength calculation: σf = 2P/π Dt, is suitable for linear elastic materials. However, the incorporation of plasticity into the material model results in a significant change in the maximum principal stress field (magnitude and location) rendering the Hertzian estimate of tensile strength invalid. A map to check the validity of the Hertzian equation is proposed.

Comparison of various modeling methods for analysis of powder compaction in roller press

Abstract Recently used models relating basic properties of the feed material, roller press design and its operating parameters are reviewed. In particular, we discuss the rolling theory for granular solids proposed by J.R. Johanson in the 1960s, later trials utilizing slab method and newly developed final element models. These methods are compared in terms of efficiency and accuracy of predicting the course of basic process variables like nip angle, pressure distribution in roll nip region, neutral angle, roll torque and roll force. The finite element method offers the most versatile approach because it incorporates adequate information about powder behavior, geometry and frictional conditions. This enables to perform realistic computer experiments minimizing costs, time and resources needed for process and equipment optimization.

Evaluation of volume fraction of solid in alloys formed by semisolid processing

Three of the methods used to determine the volume fraction of solid as a function of temperature in alloys in the semisolid state, namely utilization of thermodynamic data, thermal analysis, and quantitative metallography on quenched microstructures, are studied. The accuracy of each method is evaluated and the advantages and limitations are recognized. It is demonstrated that, while all methods are approximate, they offer distinct and different advantages.

An objective time-integration procedure for isotropic rate-independent and rate-dependent elastic-plastic constitutive equations

Abstract In a large class of rate-independent and rate-dependent elastic-plastic constitutive equations the elasticity is modeled in hypoelastic form, with the stress rate being taken as the Jaumann derivative, so as to make the constitutive model properly frame-indifferent or objective. Here, we present a fully-implicit, stable time-integration procedure for implementing such constitutive models in displacement-based finite element procedures. The numerical procedure preserves the very desirable feature of incremental objectivity . The overall procedure is a generalization of the well known “radial-return” algorithm of classical rate-independent plasticity, and it is therefore well suited for implementation in large-scale finite element codes. As an example, we have implemented the time-integration procedure in the finite element code ABAQUS. To check the incremental objectivity, accuracy, and stability of the algorithm some representative problems are solved.

Understanding variation in roller compaction through finite element-based process modeling

Abstract One of primary goals of the Quality by Design initiative in the pharmaceutical industry is to reduce variation in the product quality through increased understanding and control of the manufacturing process. In the case of roller compaction, in which mixtures of active and inert powders are fed via a screw to counter-rotating rolls, drawn into the nip region and compacted under hydrostatic and shear stresses, variation in density of the roller compacted material has been commonly observed. In the experimental part of this work we report measurements of pressure and shear under the rolls which show variation of the local stress conditions along the width of the roll which evolves with time. Also roll pressure and shear stress appear to persist past the minimum roll separation. To further investigate the potential causes of these variations, 2D and 3D explicit finite element-based models with adaptive meshing and arbitrary Eulerian–Lagrangian capabilities were developed. A Drucker-Prager/cap constitutive model was used to describe the mechanical behavior of the powder. Microcrystalline cellulose was used as the model powder. The 2D model was used to evaluate the effects of feed stress, roll friction on roll force, profiles of roll pressure and roll shear stress, nip angle and relative density of the compacted powder. The results indicated increasing feed stress, and/or increasing roll friction lead to higher maximum roll surface pressure and attendant relative density at the exit. The results may be explained by the location of the nip angle and the amount of pre-densification in the feed zone. Simulations with pressure-dependent frictional coefficients indicated significant differences in densification. In addition, oscillating feed stress conditions revealed periodic variations in roll pressures and relative densities. The 3D model predicted lower roll pressure and densities near the edges due to presence of side seal friction. Variable inflow of material along the roll width was related to variation in roll pressure. Overall, the model predictions followed experimental trends. The process modeling provided greater insight into the potential causes of the variation in density of the roller compacted material and highlighted the significance of the design of the feed system, which may be used to evaluate potential design improvements.

Thermo-elasto-viscoplasticity of isotropic porous metals

Abstract A rate and temperature dependent elastic-plastic model for isotropic, moderately porous metallic materials is formulated. This model is intended for material rate-sensitivities in the entire range spanning from highly rate-dependent behavior at high homologous temperatures to nearly rate-insensitive behavior at low homologous temperatures. The predictive capabilities of the constitutive model are verified by comparing results from finite element calculations against results from physical experiments. Specifically, example calculations are presented for: (a) isothermal hot compression of a tapered disk made from an initially porous material. This calculation illustrates the effect of secondary tensile stresses on hot workability of metals, (b) Tension tests, under isothermal conditions at low homologous temperatures, on axisymmetric notched bars made from initially porous materials. This calculation illustrates the effects of nonuniform multiaxial tensile stress states on void growth. Predictions from the computational procedures for both examples agree well with experimental results. The new state variable rate and temperature dependent constitutive model for microporous materials and the associated computational procedures form a basis for the simulation and design of deformation processing operations. This new capability should be useful for the prediction of formation of defects during both cold-working when the material rate sensitivity is low, as well as hot-working when the material is highly rate sensitive. The computational capability should also be useful in simulating the late stages of densification of powder metallurgical products in complex forming operations.