Jenő Kovács

Investigation of sintered and laser surface remelted AlSiC composites

Abstract The present work focuses on the microstructural characterisation of aluminium-base ceramic particle reinforced composites produced by powder metallurgy. Powder mixtures containing different quantities (5, 10 wt.%) and grades (F500, F800) of SiC particles as reinforcement constituents were uniaxially cold pressed. Afterwards the green compacts were sintered in a high purity nitrogen atmosphere. Dimensional changes of specimens occurring during sintering were controlled by dilatometry. Porosity measurements were performed on the polished cross-sections of sintered probes by an automatic image analyser. Laser surface treatments were also carried out on the sintered materials. The surfaces of the specimens were remelted using industrial CO2 laser equipment to achieve compacts with low porosity surface layer. The effect of laser-beam power, of amount and grain size of SiC particles on the remelting process and on the microstructure evolution was investigated.

Effect of High Rotating Magnetic Field on the Solidified Structure of Al–7wt.%Si–1wt.%Fe Alloy

Al–7wt.-% Si–1wt.-% Fe alloy was solidified unidirectionally in the Crystallizer with High Rotating Magnetic Field (CHRMF). The diameter of sample was 8 mm and its length was 120 mm. The parameters of solidification were as follows: solid/liquid interface velocity ~0.082 mm/s, temperature gradient 7+/-1 K/mm, magnetic induction 0 and 150 mT, frequency of magnetic field 0 and 50 Hz. The structure solidified without rotating magnetic field (RMF) showed a homogeneous, columnar dendritic one. The structure solidified by using magnetic stirring showed a dual periodicity. On the one hand, the branches of the “Christmas tree”-like structure known from the earlier experiments contained Al+Si binary eutectic. On the other hand, bands with higher Fe- and Si-content formed in the sample, which were at a larger distance from each other than the branches of the “Christmas tree” structure. The developed microstructure was analyzed by SEM with EDS. The average Si- and Fe-concentrations were measured on the longitudinal section at given places along the length of sample. Furthermore the Si- and the Fe-concentrations close to the bands and among the bands as well as the composition of the compound phases were determined.

The effect of rotating magnetic field on the solidified structure of Sn-Cd peritectic alloys

The peritectic alloys, such as some types of steel, Ni-Al, Fe-Ni, Ti-Al, Cu-Sn, are commercially important. In contrast to other types of alloys, many unique structures (e.g. banded or island ones) can form when peritectic alloys are directionally solidified under various solidification conditions. It can be observed in the course of the directional solidification experiments performed in a rotating magnetic field (RMF) that the melt flow has a significant effect on the solidified structure of Sn-Cd alloys. This effect was investigated experimentally for the case of Sn1.6 wt% Cd peritectic alloy. For this purpose, a Bridgman-type gradient furnace was equipped with an inductor, which generates a rotating magnetic field in order to induce a flow in the melt. As a result, the forced melt flow substantially changes the solidified cellular microstructure. The cell size and the volume fraction of the primary tin phase were measured by an image analyzer on the longitudinal polished sections along the entire length of the samples. The microstructure was investigated by scanning electron microscope (SEM) equipped with an energy dispersive spectrometer (EDS).

Visualisation of the melt flow under rotating magnetic field

Experiments were performed for visual observation and investigation of liquid Gallium flow at a temperature of 40oC in a rotating magnetic field. Two different measuring methods were developed to determine the revolution number of rotating melt. In both cases the frequency of magnetic induction was 50, 100 and 150 Hz and the values of magnetic induction could be changed between 0 and 70 mT. The magnetic Taylor number changed between 0 and 3.54x107 during the experiments.

The Experiences with a Travelling Magnetic Field (TMF) Stirrer

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.

Effect of the High Rotating Magnetic Field (min. 30 mT) on the Unidirectionally Solidified Structure of Al7Si0.6Mg Alloy

The topic of this paper is the unidirectional solidification of ternary Al7Si0.6Mg aluminium alloy in a rotating magnetic field of 30 -150 mT and the characterisation of effect of stirring on the solidified structure. During performing the experiment-series, one of the three solidification parameters (temperature gradient, solid/liquid interface velocity and magnetic induction) was continuously changed and the other two of them was kept on a constant value. The effect of these parameters on the developed structures was analysed during the evaluation of the experimental results. Moreover, the extent of Si-macrosegregation as well as the change of the secondary dendrite arm spacing were investigated on the longitudinal and cross-sections of samples as a function of the three basic parameters.

Effect of Crucible Diameter and Wall Roughness on the Melt Flow generated by Rotating Magnetic Field

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.

Comparison between Simulation and Experimental Results of the Effect of RMF on Directional Solidification of Al-7wt.%Si Alloy

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.

The Effect of a Rotating Magnetic Field on the Solidification of A356 Alloy Modified by Strontium

The A356 alloy, which consists of 100-200 ppm modifier, namely strontium was examined. The samples were solidified unidirectionally, and each sample had a pair which was solidified in a rotating magnetic field. The microstructure of the samples: morphology and the fraction of eutectic silicon was studied. The morphology of eutectic Si was very different in the samples - the samples solidified at different movement velocities – so it was necessary to determine the quantity of strontium. Magnetic stirring changes both the strontium content and the extent of modification. This paper describes the investigation methods and the effects of strontium modification.