Ljubomir Anestiev

Model of the primary rearrangement processes at liquid phase sintering and selective laser sintering due to biparticle interactions

A theoretical description of the rearrangement processes at selective laser sintering (SLS) and liquid phase sintering (LPS) is made. A general solution, based on the dimensional analysis and rigorous analytical solution of the equations governing the rearrangement process is obtained. The influence of the process parameters and the physicochemical properties of the sintered materials on the rearrangement processes at SLS and LPS is studied with the model developed. The obtained results are in good agreement with the available experimental data.

Magnetic properties and loss separation in iron powder soft magnetic composite materials

New developments in powder metallurgical composites make soft magnetic composite (SMC) material interesting for application in electrical machines, when combined with new machine design rules and new production techniques. In order to establish these design rules, one must pay attention to electromagnetic loss characteristics of SMC material. In this work, five different series of iron based SMCs are produced and studied: (1) Pure iron powder with resin; (2) sintered iron based powders; (3) pure iron powder with additions of Zn-st and carbon; (4) iron based powder alloys (Fe,Nb,Si); (5) commercially available iron powder “Somaloy.” The specimens were shaped as rectangular rods and characterized on a miniature single sheet tester which was calibrated to Epstein. The measured energy losses are analyzed following the loss separation theory of Bertotti, in which the total energy loss is decomposed into hysteresis loss, classical Foucault loss, and an excess loss component.

On the kinetics of diffusion controlled precipitation under microgravity

Metal matrix composites have many desirable characteristics for precision components, e.g., light weight, dimensional stability, and high thermal conductivity. The understanding of the mechanisms and processes taking place at their production is of crucial importance, since these factors are important for the properties of the composite and its future application. A theoretical model of the processes at diffusion controlled formation of TiB2 in liquid Al under microgravity has been proposed. The main features of the developed model are: mechanisms controlling the formation of the TiB2; degree of distribution of the dispersed TiB2 particles; and mean size and size distribution of the dispersed TiB2 particles. The results obtained with the aid of the proposed model were verified on our recent experiments at the STS95 space shuttle mission. The model predicts correctly the TiB2 particles distribution along the specimens and the characteristic split of the reaction zone observed experimentally.

Eutectic growth: A closed problem for the solution of the steady-state growth of lamellar eutectics

The Jackson and Hunt theory was modified in order to get a better understanding of the driving forces of the eutectic’s growth kinetics. A solution of the diffusion problem, based on more rigorous boundary conditions, was obtained and kinetic members were added in order to account for the growth kinetics of the specific phases composing the eutectic. It was unambiguously shown that the eutectic’s growth problem is a closed problem, e.g., no additional assumptions are needed in order to obtain the eutectic growth velocity as a function of the phase’s composition and the lamellae’s width. The model proposed in the present article was further used to model the growth kinetics of different alloys, which exhibit eutectic structure and to derive some important kinetic parameters from the existing experimental data.

Magnetic properties of Fe100-x-ySixPy (0 <= x <= 4, 0 <= y <= 0,6) soft magnetic composites prepared by diffusion sintering

In the presented work, iron-based soft magnetic composite materials are produced and studied. The proposed production method combines sintering and diffusion processes. The applications that are aimed at for these materials are electrical machines and other low-frequency electromechanical applications. The material characteristics are studied in the frequency range from dc to 100 Hz. The material specimens are shaped as rectangular rods and magnetic characterization is done on a miniature single sheet tester, which is calibrated with respect to the Epstein test. The experimental procedure for determining the magnetic properties is outlined. The production method was tested on Fe100−x−ySixPy (0⩽x⩽4, 0⩽y⩽0.6). Because of the poor plasticity, such compositions cannot be produced directly from powder. This article discusses the results of the diffusion sintering and the combined influence of Si and P on the magnetic properties of the obtained materials. It is found that an increase of the Si and P content imp...

Kirkendall effect in liquid during the in situ processing of metal matrix composites under micro-gravity conditions

Some interesting and unexpected effects observed during the in situ production of an Al-TiB2 metal matrix composite during the STS95, SPACE-SHUTTLE (1998) mission are described. It is shown that during the processing of the composites a Kirkendall effect in the liquid is observed. In contrast with the classical effect, this is due not only to the considerable diffusivity difference between the B and Ti in the Al matrix, but also to the chemical reaction taking place to form TiB2.

Analytical solution of the diffusion-controlled precipitation problem

The understanding of the mechanisms and processes taking place during the production of metal matrix composites, especially of in situ produced composites, is of crucial importance for the properties of the composite. An analytical solution of the equations governing the kinetics of diffusion-controlled precipitation of TiB2 is proposed. The solution allows to predict the concentration profiles and the volume fraction distribution of the reinforcements that are in situ formed along the specimen. The influence of the physicochemical parameters is also taken into account in the proposed solution.