András Roósz

Undercooling effects in microsegregation modelling

Numerical simulation of microsegregation phenomena is of continuous interest in solidification modeling and has reached a high level of sophistication. Existing models predict the type and amount of solidifying phases and some microstructural features in conventional castings of binary alloys with sufficient accuracy. For cooling rates exceeding those of technical casting processes, occurring e.g. in surface remelting or welding, undercooling phenomena need to be taken into account. Undercooling of the dendrite tip and delayed formation of interdendritic phases can reduce the amount of nonequilibrium phases significantly. Therefore an existing numerical microsegregation model was extended to very low and high cooling rates by incorporating dendrite tip and eutectic undercooling effects. In this paper it is investigated to what degree the differences of the undercooling models influence the results of microsegregation calculations, with the aim to select the appropriate description of undercooling for microsegregation modelling. As model systems an Al-Cr alloy are selected.

Formation of Lead Bearing Surface Layers on Aluminum Alloys by Laser Alloying

The aim of the present work is to investigate the microstructure of Al-Pb monotectic surface layers generated by a laser surface alloying technique on the surface of an Al-4Cu-1.5Si-0.5Mg alloy. Monotectic alloys are generally used as materials for sliding bearings. Layers of a monotectic alloy generated by laser surface alloying have a potential for superior properties, because the rapid solidification during this process results in fine precipitation structures which can act as solid lubricant. The structure of the Al-Pb monotectic surface layers was observed in the light microscope and the scanning electron microscope. Volume fraction and size distribution of lead particles were determined by quantitative image analysis. Pin-on-disc experiments were carried out to characterize and compare the tribological properties of the Al-Pb monotecitc layers and cast Al-Cu-Si-Mg alloys.

On the possible mechanisms of porosity formation during laser melt injection (LMI) technology

In the present study the analysis of 5 different mechanisms of porosity formation during laser melt injection (LMI) technology were performed. Experiments were supported by thermodynamic and fluid-flow analysis. Special attention should be paid to i. clean the surface of the substrate, ii. use inert shielding gas, iii. use proper particle size and gas velocity, iv. use proper laser power and laser beam velocity to control bath temperature and v. deoxidize the surface of the added particles.

Solidification of Al6Zn2Mg alloy after laser remelting

Abstract The surface of a specimen made of an Al6Zn2Mg alloy was remeleted by a CO 2 laser of 1 kW power. The structure developed during solidification was investigated on the surfaces parallel to and perpendicular to the direction of the beam (or table) movement. The change of solid-liquid interface velocity was determined in the zone. The developing temperature gradient was estimated by using the condition of plane-front solidification.

MICAST – Microstructure Formation in Casting of Technical Alloys under Diffusive and Magnetically Controlled Convective Conditions

The MICAST research program focuses on a systematic analysis of the effect of convection on the microstructure evolution in cast Al-alloys. The experiments of the MICAST team are carried out under well defined thermally and magnetically controlled, convective boundary conditions and analyzed using advanced diagnostics and theoretical modeling, involving phase field simulation, micro-modeling and global simulation of heat and mass transport. The MICAST team uses binary, ternary and technical alloys of the Al-Si family. This paper gives an overview on recent experimental results and theoretical modelling of the MICAST team.

Development of monotectic surface layers by CO2 laser

The aim of our work was to develop a monotectic surface layer for be arings by using laser surface-treatment technology. Two kinds of laser surface t reatment technology were used. In case of coating-melting technology a Pb layer was first develope d by galvanization, then the Pb layer was melted together with the matrix. In case of powder bl owing technology Bi powder was blown into the layer melted by laser beam, by Argon gas. The struct ure of monotectic surface layer was investigated by light microscope and a scanning electron micros cope. The volume fraction and size distribution of Pb and Bi particles were determined by image-analyzer. Introduction The essence of laser alloying is that only structure and propertie s of the surface layer of specimen are changed, the structure and properties of the bulk part remains unchang ed. During development of monotectic surface layer, the surface workpiece is heated to a temperature, where only a single homogenous melt remains, in the system after alloying and then – beca use of good heat-extraction ability of the basic material it is quickly cooled. As a conse quence of the high cooling rate (100010000 K/s), a very fine structure develops during solidification. Experiments Laser experiments were performed in the Bay Zoltán Institute of Materials Science and Technology using a CO2 laser with a power of 5 kW. The basic alloy was an Al-11Si eutec tic type alloy. The thickness of specimen was 15 mm and the surface area treated by the laser was 15x130 mm . Two kinds of technologies were used for surface treatment [1-4]. Duri ng the two-step technology (melting of the coating) (Fig. 1) a Pb layer having a thickness of 0.1 mm was taken to the surface by galvanizing, then the Pb layer was melted together with the basic material by using different laser power (2, 2.5, 3, 3.5 kW). The sample was shifted by a scan velocity of 300 mm /min. During the treatment, 8 melted bands were with 50% overlap. Diameter of the foc us was 2 mm. Samples were coated with graphite before treatment to increase the low absorption of aluminium. During the one-step (powder-blowing-in) technology (Fig. 2) Bi grains w ith a diameter of 40-60 μm were blown into the molten layer. The protecting gas was Ar and the laser power wa s chosen (2, 2.5, 3, 3.5 kW). The sample scan velocity was 300, 600 and 900 mm/min respectively. Fi bands were melted on each sample with an overlap of 50%. Materials Science Forum Online: 2003-01-15 ISSN: 1662-9752, Vols. 414-415, pp 147-152 doi:10.4028/www.scientific.net/MSF.414-415.147

CuZrAl Amorphous Alloys Prepared by Casting and Milling

Several preparation methods are available for the production of amorphous alloys. During the experiment described in this paper (Cu58Zr42)100-xAlx (x = 0-14,8; in at%) amorphous alloys were prepared by casting and ball-milling. The ingots were produced by arc melting. Wedge-shaped samples were prepared from the ingots by centrifugal casting into copper mould. The microstructures of these samples were defined by SEM. The amorphous samples were analysed by DSC and the activation energy of the crystallization processes was calculated from the measured temperatures. The master alloys of identical composition were milled by ball-mill for different periods of time. The powders were analysed by XRD in order to define the amorphous fractions.

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.

Investigation of Phenomena Taking Place in LASER Surface Alloying Steels of WC-Co

The laser surface-treatment methods have been quickly developed by appearing of lasers with high power beam and can increase the hardness, of the surface. A very hard, wear-resisting layer can be produced by the dispersing of ceramic grains. The essence of the technology is, that such a material (compound-phase, e.g.: metal-oxide, carbide, nitride, etc.) is added to the surface layer melted by laser, which does not solve or solves only partly in the metal-melt. This work studies the effect of the different technological parameter (such as, power of the laserbeam, motion speed, amount of the ceramic particles etc.) on the different microstructure accrued during the laser surface alloying. The desired microstructure has homogeneous carbide distribution in the matrix. But it is embarrassed by several conditions. The aim of this present work is to find out the reasons for the inhomogeneous ceramic particle distribution inside the matrix and to discover these embarrassing conditions.

Effect of a Rotating Magnetic Field on the Solidified Structure of Al-Si Alloys

The aim of our research work is to study the microstructure formation during casting of technical alloys under diffusive and magnetically controlled convective conditions on earth and in space. Unidirectional solidification experiments under steady-state conditions were performed with Al-Si binary alloys containing three different amounts of Si, using electromagnetic stirring of the melt. The influence of the rotating magnetic field on the microstructure evolution (particularly on the secondary dendrite arm spacing) as compared to experiments without stirring was studied.