Torsten Kraft

Numerical simulation of solid state sintering; model and application

A comprehensive model for solid state sintering is presented which combines several previous models for partial aspects of sintering and grain coarsening. Some additional aspects are discussed and the model is extended for external loads several times higher than the sintering stress. Model parameters for a SiC powder are presented. For a face seal made of SiC uniaxial die compaction is simulated and different green density distributions after compaction are obtained for two different pressing schedules. Next, the sintering behavior is simulated using these density distributions as initial conditions. The resulting distortions of the face seal differ significantly for the two pressing schedules. For a special element with average green density the predictions of the sintering model are investigated in more detail. Finally, the shape predictions of the model are compared with a simplified model for solid state sintering.

Finite element simulation of cold isostatic pressing and sintering of SiC components

Abstract Finite element simulations were performed for cold isostatic pressing of SiC powder. To describe the compaction behaviour, the Drucker–Prager–Cap model implemented in ABAQUS/Explicit® computer program was applied. The influence of friction between powder and steel core as well as rubber bag on the density distribution was investigated for compacts with cylindrical and helical cores. Simulations with and without rubber bags were made showing that the effect of the bag on the density distribution is fairly small, at least for thin bags. Due to the presence of the steel core, density gradients develop during compaction, although the outer pressure is hydrostatic. The predicted density variation is consistent with experimental results. After the pressing cycle it may be difficult to remove the helical steel core from the powder compact. The simulation shows that the elastic spring-back leads to residual stresses which may reach the green strength of the powder compact, so that the part may crack during the unloading or removal of the helical core. Investigations of the sintering behaviour predict small shape distortions due to the slightly inhomogeneous density distribution of the green body. However, the differences between the ideal geometry and the predicted outline after sintering are small.

Surface tension and wetting effects with smoothed particle hydrodynamics

Abstract For flows at the micro and nano scales the surface to volume fraction is increased and therefore the behavior of fluids in contact with solid structures is primarily dominated by surface tension effects. Modeling of surface tension effects at nano and micro scales using smoothed particle hydrodynamics requires an efficient description not only of the interface between liquid and gas, but also of the triple line defined by the three phase contact between the solid, liquid and gas. In this study, we propose an efficient and reliable implementation, which takes the liquid–gas surface tension and the equilibrium contact angle as input parameters and prescribes the normal direction of the liquid–gas interface at the triple line based on the desired equilibrium contact angle. This results in a robust algorithm capable of capturing the physics of equilibrium wetting and treating a large variety of cases including different wetting angles, pinning effects and wetting of structured surfaces.

Finite Element Simulation of Die Pressing and Sintering

Numerical simulation of die compaction and sintering is a promising tool for reducing development times and costs or optimizing production cycles. Finite element simulations allow qualitative and quantitative predictions of undesirable distortions after firing. Even qualitative predictions of crack formation are possible.

Enhancement of Fine Line Print Resolution due to Coating of Screen Fabrics

In LTCC technology, printing is the most reliable and cost-effective process for line deposition on ceramic layers. An enhancement of resolution is required to realize modules in the highest frequency range which requires small lines and distances. The fine line print resolution is limited by two factors of processing. One is paste rheology itself, including viscosity particle size and thixotropic behavior. The other factor is the screen with limitations in mesh size, wire thickness, calendering, and angle of the screen fabric in the frame. From the viewpoint of the screen, the idea was to improve the fabric by coating. It can be shown that especially in concentric circles, even with fine line screens, arrays occur where no printing happens. These paste free areas exhibit a moire effect and follow exactly the string angle of the screen. This is caused by the crossing of the strings of the fabric which does not allow a passing of the paste due to very narrow gaps. To facilitate the paste passing, the idea ...

Hierarchical Structure Formation of Nanoparticulate Spray-Dried Composite Aggregates

The design of hierarchically structured nano- and microparticles of different sizes, porosities, surface areas, compositions, and internal structures from nanoparticle building blocks is important for new or enhanced application properties of high-quality products in a variety of industries. Spray-drying processes are well-suited for the design of hierarchical structures of multicomponent products. This structure design using various nanoparticles as building blocks is one of the most important challenges for the future to create products with optimized or completely new properties. Furthermore, the transfer of designed nanomaterials to large-scale products with favorable handling and processing can be achieved. The resultant aggregate structure depends on the utilized nanoparticle building blocks as well as on a large number of process and formulation parameters. In this study, structure formation and segregation phenomena during the spray drying process were investigated to enable the synthesis of tailor-made nanostructures with defined properties. Moreover, a theoretical model of this segregation and structure formation in nanosuspensions is presented using a discrete element method simulation.

Distortions and cracking of graded components during sintering

The aim of the project is to develop methods for predicting distortions and cracking during the fabrication of gradient materials, and to apply them to simple geometries (tubes, beams and plates). Crack formation during sintering is described as the instability of the homogeneous sintering state. It is found that a density disturbance may grow unstably, but only when the material is severly constrained in three dimensions. The equations for describing the distortions of graded beams and plates during sintering are developed. Examples are shown for plates with gradients in grain size, temperature and composition (Mo and ZrO 2 ). If the gradient is carefully balanced, nearly planar plates can be obtained after sintering. Three-dimensional finite element studies of graded circular plates show that the initially axisymmetric mode of deformation becomes unstable in favor of a warping with two symmetry planes (like a potato chip).

Optimizing press tool shapes by numerical simulation of compaction and sintering—application to a hard metal cutting insert

Designing, manufacturing and commissioning of the tooling often determines the delivery time of a new powder-metallurgical component. To speed-up this process, computer simulations of die compaction and sintering could assist in the development stage. Today, finite element simulations of the compaction process in combination with appropriate material (or constitutive) laws for the powder allow quantitative predictions of the density distribution in the green body as well as the tool loadings. The shape after firing can be predicted by using an appropriate material law for sintering. The sinter distortions are a result of inhomogeneous density distributions in the green body. These distortions can be minimized by optimum press kinematics, and the remainder can be compensated by appropriately shaped punch surfaces. After presenting an overview about the underlying models and simulation methods for compacting and liquid phase sintering (LPS) the simulation procedure together with the optimization technique is demonstrated for a cutting insert made of hard metal powder. Finally, the shape predictions of the LPS model are compared with a simplified model.

Manufacturing of a gear wheel made from reaction bonded alumina—numerical simulation of the sinterforming process

Sinter forming is a recently developed process for the production of advanced ceramic components. It combines pressure-supported sintering and superplastic deformation, and allows RBAO (Reaction Bonded Alumina) to be sintered without grain coarsening due to the reduction of process time and dwell temperature. Thus, sinterforming can achieve near net shaping and defect free components for highly stressed applications. As many phenomena interact during sinter forming, the process parameters for producing complex shaped parts are difficult to identify without numerical tools. Therefore, a micromechanical model for solid state sintering has been used to investigate the process. The model was extended to consider source controlled diffusion in order to describe the non-linear stress/strain dependence. Several experiments were made in order to determine the material parameters of the model. The model has been implemented as a user subroutine in ABAQUS/Explicit® and is validated by numerical simulations of the accomplished experiments. To show the possibilities of the finite element simulation, the sinter forming process is numerically carried out for a RBAO gear wheel.

Simulation and prediction of the thermal sintering behavior for a silver nanoparticle ink based on experimental input

In order to develop a prediction model for resistivity evolution during isothermal sintering, a commercial silver nanoparticle ink was characterized for its metal content, particle size and behavior upon heating. Electrical properties, mass loss behavior, grain size development and material densification were studied for thermal sintering at 175 °C. The correlation between mass loss, height loss of the resulting sintered structures, grain size and electrical resistivity was investigated to gain further understanding of the silver nanoparticle sintering process. The results of thermal sintering were used to calibrate a discrete element sintering model that provides microstructural properties with which the resistivity development at 150 and 200 °C was successfully predicted. The model was validated by experimental data obtained at these temperatures. A variation of particle size and particle size distribution in the simulations furthermore illustrate their influence on final resistivity showing that using small particles with a broad distribution are preferable for reducing the final resistivity of the inkjet-printed pattern.