Papers by Author: András Roósz

Abstract: A measurement of the angular velocity/revolution number of magnetically stirred liquid gallium-indium alloy was realized with newly developed angular velocity measuring equipment. To get additional information about the flow of the melt, a numerical simulation model was performed with ANSYS FLUENT 13.0 with a single phase 2D k-ε turbulence solver. The aim was to reproduce the flow as accurate as possible, so the measured and computed angular velocity data was compared, to see if the system can be modeled fairly well.

Abstract: The aim of the research was to develop an amorphous and microstructured layer on non-amorphous alloys by laser surface treatment. The as-prepared Cu based master alloy ingots were imbedded in a metallic sinking with Wood metal to assure the good thermal conductivity during the laser treatment. The laser remelting, alloying and coating techniques were applied from the laser surface treatment techniques. The surface layer production and a subsequent rapid cooling were performed using CO2 laser and pulse and continuous modes of Nd:YAG laser. The characterization of the microstructure of the resulting surface layer was investigated by scanning electron microscopy (SEM) and X-ray diffraction (XRD). Actual remelting on substrates showed that the process of laser remelting is a suitable technique for production of metallic glasses as surface layers. The amorphous layer up to 250 m in depth can be produced by laser surface remelting on Cu46Zr42Al7Y5 alloy.

Abstract: Certain phase transitions take place at a given temperature in the equilibrium phase diagrams, the values of their temperatures can exactly be measured so it is an essential task to calculate these values by a high accuracy. New parameters were introduced into the ESTPHAD equation for calculating the so-called non-variant points. By using the ESTPHAD method, the accuracy of calculations are compared to the data used for the calculations so the exact calculation of the temperatures of non-variant points are defined as compared to the data used for the calculations.

Abstract: Our aim was to develop initial structures similar to the real structures of unalloyed steels that could later be used for simulating austenitization. Both simple models and models that can be compared to the real structures were developed. In our paper, it is described how the initial structures were developed by using digitalized microscopic images.

Abstract: An experimental stirring unit (inductor) using travelling magnetic field was tested for developing a solidification facility equipped with a magnetic stirrer. The investigations were performed by using an Sn50Pb alloy at temperature of 230°C. In the course of the experiments, the magnetic pressure developing in the metallic melt, the flow velocity of melt and the loss of flow were investigated using the TMF inductor.

Abstract: By using the thermodynamic equations, the ESTPHAD (Estimation of Phase Diagrams) method developed by us has been made suitable for calculating the immiscibility gap of monotectic systems. 2 special parameters were introduced in the basic equation of ESTPHAD which ensure the description of the characteristics of monotectic curve on the basis of the boundary conditions created by us. By means of the monotectic ESTPHAD equation, the equations of separation temperatures of monotectic parts in the Zn-Bi and Al-Bi systems were calculated (with an accuracy of +/- 10K) on the basis of the reference data.

Abstract: Thermoelectric (TE) semiconductor materials are widely used for miniaturized versatile cooling devices in a wide spectrum of equipments and energy generation in space vehicles. The bismuth telluride pseudobinary alloy family presents the best characteristics for room temperature TE cooling applications. Using appropriately oriented single crystals instead of the well known polycrystalline materials made by powder metallurgy methods, the efficiency of the TE device (Thermoelectric Cooler – TEC or Thermoelectric Generator – TEG) could be almost doubled. For having good quality TE material it is required to produce equally doped single crystals by the controlled crystallization process, namely with the Bridgman-Stockbarber method. Our experiments were made in the Universal Multizone Crystallizator Type UMC, developed by the ADMATIS Ltd., Miskolc, using a quartz tube under high vacuum conditions and automatically controlled thermal field parameters. The crystallographic analysis of the obtained samples was made by Scanning Electron Microscopy – (SEM), X Ray Diffraction – (XRD), and neutron diffraction (TOF spectrometry).

Abstract: The effect of flow on the structure of solidified alloys can be investigated by the unidirectional solidification of alloys stirred with a magnetic (magnetohydrodynamics – MHD) method. This MHD method is a rotating magnetic field (RMF)-type. The paper deals with the melt flow generated by this RMF.

Abstract: The melt flow has a significant effect on the structure developing during the unidirectional solidification of alloys. This phenomenon can be experienced during the solidification of melts stirred by the rotating magnetic field (RMF)-type magnetohydrodynamic (MHD) facility as well. As it would be very difficult to measure the intensity of melt flow (e.g. its revolution number, angular velocity) during solidification, it seems to be reasonable to perform the so-called "scale model" experiments applied usually in the hydrodynamics. Using the results of these measurements, conclusions can be drawn concerning the flows during solidification by means of the similarity law of hydrodynamics. The revolution number of Ga-In alloy melt placed in the rotating magnetic field can be measured by the equipment developed for performing the "scale model" experiments. The measurements were performed in crucibles with different surface roughness using melt-cylinders with different diameters located in rotating magnetic field having different frequencies and magnetic induction.

Abstract: A numerical investigation of directional solidification of Al-7wt.%Si alloy stirred by a rotating magnetic field is compared with experimental results. Experimental study of such process has revealed periodical macrosegregation in axial direction of the samples in the shape of a “Christmas tree”. Similar macrosegregation pattern is obtained in simulations for the two values of magnetic field. Numerical simulations have shown also that formation of the periodical structure depends not only on the external conditions but on the permeability of the mushy zone.