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Spark Plasma Sintering (SPS) is an innovative sintering technique, whereby many of the beneficial effects of this process on sintering are still elusive. To allow for the detailed investigations of the SPS process, a custom experimental set-up and a corresponding finite element (FE) model was developed. The miniaturised setup allows for very high current intensities, custom pulse patterns, a wide pressure range and dilatometric measurements. The FE model was employed to calculate the temperature field in the set-up and the sintering specimen itself. A very good correlation of the temperature, current and voltage over the entire process was observed. Our investigations show that the contact conductivities have a significant impact on the process temperature. Also, the imperfect contacts at the interfaces between the graphite foil and the real specimen may lead to a significant variance of the currents necessary to obtain the desired sintering temperature. [ABSTRACT FROM AUTHOR]

Multivariant experimental investigations and multiphysics microstructural modeling of the spark plasma sintering process of metallic powders have been performed up to a relative density of approximately 80%. In comparison, the effect of sintering temperature, pressure, and particle size on the interparticle contact area growth and axial shrinkage of cylindrical specimens of copper and nickel particles is measured in laboratory scaled tests. Herein, for the first time all relevant for sintering phenomena are considered simultaneously: the fully coupled thermo‐electro‐mechanical modeling of the spark plasma sintering processes, additionally taking into account for lattice, grain boundary, surface diffusion, electromigration, and thermomigration, has been carried out. The computational analysis of various physical phenomena allows to identify dominant and insignificant mechanisms. The two‐level numerical simulation includes the modeling of the sintering setup at the macroscopic level and the neck formation process in particle chain systems at the microscopic level. The results of the numerical simulations show a very good agreement with the experimental data. Therefore, the impact of electrical and mechanical loads as well as of particle size on microscopic distribution of temperature, inelastic strain, and on densification has been studied by the finite element simulations. [ABSTRACT FROM AUTHOR]

Contact growth and temperature behavior in time during a single high-current pulse representing the initial stage of field-assisted/spark plasma sintering (FAST/SPS) have been studied experimentally and numerically. The measured evolution of the electrical resistance and of the neck formation process in two-particle systems is compared to the results obtained from the fully coupled thermo-electro-mechanical finite element simulations. The results of simulations with various models of inelastic deformation show that the viscoelastic/viscoplastic material model provides a realistic contact growth in initial stage of FAST/SPS. The impact of electrical and mechanical loads, material parameters and particle size on temperature, on inelastic strain distribution and on densification has been studied by finite element simulations for copper, stainless steel and nickel particles. [ABSTRACT FROM AUTHOR]

The initial stage of contact formation during field-activated/assisted sintering of powders is investigated experimentally and numerically for spherical particles of nickel, steel, and copper. The results of experimental studies of the contact resistance and the neck formation process due to melting/sintering in two- to three-particle systems under a single high-current pulse from a capacitor discharge are compared with results of fully coupled thermo-electrical finite-element simulations. The impact of particle size, contact geometry, and electrical load on the temperature and temperature gradient distribution during the pulse discharge is analyzed numerically for nickel spheres. The influence of the material itself is compared for nickel, steel, and copper systems. [ABSTRACT FROM AUTHOR]

The β-type and sintered Ti-3.6Fe-5Zr-0.2B (mass%) alloy has been consolidated by spark plasma sintering, followed by a β solution treatment (ST). In order to obtain a high-strength ductile balance, water quenching or air cooling is used after ST. Modification of sintering conditions, which leads to 100% of the relative density, improves the tensile ductility. The Fe addition causes a large local lattice and compressive strain to the bcc Ti lattice; in the water-quenched sample, α" martensite phases appear in the β matrix. When air cooling is applied after the ST, bimodal α lath phases are instead precipitated during the cooling in nanoscale, and the formation of α" martensite phases is suppressed. This results in high strength and better ductility when compared with those in the water-quenched sample, particularly in tensile properties. The air-cooled sample reveals attractive mechanical properties in both tension and compression modes. [ABSTRACT FROM AUTHOR]

The paper presents the experimental, numerical, and theoretical investigation of the microstructure of nickel aluminide samples manufactured by spark plasma sintering using electron backscatter diffraction and computer assisted software. The aim of the work was to reveal the evolution of the microscopic and macroscopic parameters related to the microstructure of the material and its dependence on the applied sintering parameters—temperature and pressure. The studied porous samples with different relative density were extracted from various planes and then tested by electron backscatter diffraction to evaluate the crystallographic orientation in every spot of the investigated area. On this foundation, the grain structure of the samples was determined and carefully described in terms of the grain size, shape and boundary contact features. Several parameters reflecting the grain morphology were introduced. The application of the electric current resulting in high temperature and the additional external loading leads to the significant changes in the structure of the porous sample, such as the occurrence of lattice reorientation resulting in grain growth, increase in the grain neighbours, or the evolution of grain ellipticity, circularity, grain boundary length, and fraction. Furthermore, the numerical simulation of heat conduction via a finite element framework was performed in order to analyse the connectivity of the structures. The numerical results related to the thermal properties at the micro- and macroscopic scale—local heat fluxes, deviation angles, and effective thermal conductivity—were evaluated and studied in the context of the microstructural porosity. Finally, the effective thermal conductivity of two-dimensional EBSD maps was compared with those obtained from finite element simulations of three-dimensional micro-CT structures. The relationship between the 2D and 3D results was derived by using the analytical Landauer model. [ABSTRACT FROM AUTHOR]

Spark Plasma Sintering (SPS) has become a conventional and promising sintering method for powder consolidation. This study aims to well understand the mechanisms of densification encountered during SPS treatments, especially in the early stages of sintering. The direct current (DC) electrical behavior of copper granular medium is characterized. Their properties are correlated with their microstructural evolutions through post-mortem scanning electron microscope (SEM) observations to allow a thorough understanding of the involved Branly effect that is suspected to occur in SPS. The electrical response is studied by modifying the initial thickness of the oxide layer on particles surfaces and applying various mechanical loads on the granular medium. Without load and at low current, the measured quasi-reversible behavior is connected to the formation of spots at the microcontacts between the particles. By increasing the current, the Branly transition from an insulating to a conductive state suddenly occurs. The insulating oxide layer is destroyed, and micro-bridges are created. The application of a mechanical pressure strongly modifies the DC Branly effect. Increasing low stress leads to a strong decrease in the breakdown field. For high-applied pressure, successive drops in the electric field are detected during the electrical transition. These successive drops are induced by microcracking of the insulating oxide layer. [ABSTRACT FROM AUTHOR]

The mechanisms of densification during spark plasma sintering (SPS) of spherical copper particles are investigated both experimentally and analytically. Experimentally measured densification rates are compared to expected contributions to densification coming from Coble creep, Nabarro-Herring creep, power-law creep, and pressure-assisted sintering described by the two-particle model. The results indicate densification by Coble creep or a low-temperature version of power-law creep in the investigated range of temperature and pressure at a relative density of 70%. The findings are supported by (i) activation energies obtained for various conditions using the Dorn-method as well as (ii) the pressure dependence of the densification rate. [ABSTRACT FROM AUTHOR]

The mechanisms of densification in spark plasma sintering (SPS) were investigated both analytically and numerically for a model system of two spherical metallic powder particles. From the microscopic temperature distribution, the possibility of a micro-local overheating of the particle–particle contacts was analysed for different particle sizes, contact geometries, materials, and electrical loads. It is shown that, for particles below the size of one millimetre, local overheating is below one Kelvin. Subsequently, the material transport by thermomigration, electromigration, and diffusion driven by surface curvature and external pressure was derived from microscopic field distributions obtained from analytical calculations and finite-element simulations. The results show that, while the mechanical pressure accelerates material transport by orders of magnitude, the electrical current and the temperature gradients do not. It is also shown that pulsing the current has no significant influence on the densification rate. [ABSTRACT FROM AUTHOR]

In this study, the evolution of the electrical resistivity of metal powders during densification and the resulting current flow through punch, powder compact, and die is investigated. The evaluation of the accompanying Joule heating identifies the graphite punches as main heating element providing more than 90% of the heat. The high electrical resistance of the punches and the low resistance of the graphite die as parallel electrical load to the specimen determine the current flow in the tool. For powder particles with intact oxide layers, virtually no current flows through the compact. On the other hand, for a powder resistivity below 10 − 3 Ωcm about 50% of the current flows through the compact. This fraction is constant despite further decreasing resistivity of the compact during densification. A constant current through the specimen has important implications for the microscopic temperature distribution and the understanding of the so-called 'spark plasma effects'. [ABSTRACT FROM AUTHOR]

Sintering is a key technology for processing ceramic and metallic powders into solid objects of complex geometry, particularly in the burgeoning field of energy storage materials. The modeling of sintering processes, however, has not kept pace with applications. Conventional models, which assume ideal arrangements of constituent powders while ignoring their underlying crystallinity, achieve at best a qualitative description of the rearrangement, densification, and coarsening of powder compacts during thermal processing. Treating a semisolid Al-Cu alloy as a model system for late-stage sintering--during which densification plays a subordinate role to coarsening--we have used 3D X-ray diffraction microscopy to track the changes in sample microstructure induced by annealing. The results establish the occurrence of significant particle rotations, driven in part by the dependence of boundary energy on crystallographic misorientation. Evidently, a comprehensive model for sintering must incorporate crystallographic parameters into the thermodynamic driving forces governing microstructural evolution. [ABSTRACT FROM AUTHOR]

Experiments like the sintering of single crystal spheres on a single crystal plate, the sintering of particle rows or 2D arrangements of powder particles as well as recent experiments by in-situ Synchrotron Computer Tomography show movements of entire particles beyond the particle center approach described by the well-known two particle model. Several attempts exist to describe these cooperative material transport processes by an analytical model of the sintering of two wires by grain-boundary diffusion. The described mechanisms are either rotations caused by asymmetric sintering necks or the application of a torque or changes of the rate of sintering by a normal stress in the sintering neck. To develop a more comprehensive model of sintering, volume diffusion was included in addition to grain-boundary diffusion and all driving forces mentioned above are used. As a result, the influence of normal stress, torque, and ratio of the sintering neck curvatures on the stress along the contact grain boundary, the diffusion current and the rate of removal along the sintering neck are shown. The contribution of volume diffusion and grain-boundary diffusion to sintering was also investigated, confirming that a large sintering neck and high sintering temperatures result in a dominating influence of volume diffusion. The shrinkage rate and the rate of rotation provided by this model were used in a subsequent numerical simulation. For several stages of sintering, the influence of particle radius and sintering temperature on the rolling angle of sintering particles was analyzed in 2D. Furthermore, the impact of external normal stresses on the rolling angle is shown. The simulation model gives a surprising result: The speed of wire rolling shows the same behavior as sintering kinetics-a high speed for small particles, small sintering neck diameter and high temperature, but the cumulated rolling angle for small particles is low compared to that for larger particles. Finally, a new approach for simulating particle rotation and rolling caused by torques due to grain-boundary anisotropy in 3D is presented, which is based on an existing Discrete Element Method sintering model. This model is applied to a row of particles confirming previous observations from the 2D sintering simulations. [ABSTRACT FROM AUTHOR]