Papers by Keyword: Rapid Solidification

Abstract: The present paper is aimed at investigations of mechanical properties and structure of technical purity aluminum powders prepared by plastic consolidation process. The research work is focused on effective improvement of mechanical properties of material while keeping the conductivity at high level. It is well known that application of rapid solidification method with hot extrusion technique leads to grain refinement, as so according to Hall-Petch rule, improvement in mechanical properties of material can be expected. Furthermore, additional material strength can be obtained by aluminum oxides from free surface of powders that became internal boundaries during consolidation process. Aluminum powders atomized by air, argon and water were cold compacted and extruded at temperatures of 325°C and 375°C. For comparison purposes the same extrusion conditions were applied to cast aluminum. In order to analyze effect of recrystalization process during hot extrusion operation, different extrusion temperatures were chosen. Tensile tests as well as micro-hardnes measurements showed significant increase in mechanical strength for RS samples in comparison to conventionally cast material. Structural observations by means of transmission electron microscopy revealed that grain size of materials extruded at the given temperature was at the same level, however amount and distribution of oxides particles differs significantly. It was considered that differences in strength between individual RS material were attributed to this effect.

Abstract: Light weight nano/submicrocrystalline materials are promising group of constructional materials combining low density with high mechanical properties. However, their potential application requires extensive testing of functional properties, e.g. tribological ones, which may be significant and determine their practical use. Available information on abrasive wear and friction coefficients in nano/submicrocrystalline materials is rather poor. Therefore the aim of this paper is to fill the gap in the literature in this field. The AlSi12Fe5Cu3Mg alloy (RS422) produced by rapid solidification and plastic consolidation with grain size of basic phase components in the range from 50 nm to 300 nm was examined. Microstructure and mechanical properties of the materials were determined. Abrasive wear tests, static and kinematics friction coefficients measurement were carried out under the surface condition including dry, wet and oil lubricant. The results have been compared to the values of similar quantities determined in the same conditions for conventionally produced alloy AlSi11FeCuMn (AK11). Substantial increase of friction coefficients for RS442 comparing to AlSi11FeCuMn material was found, however, abrasive wear for nano/submicron grained materials were low in comparison to conventional one. Considerable increase of abrasive wear at water presence and very weak attrition at oil lubrication was observed. Relationship between structure and mechanical properties of tested materials was analyzed.

Abstract: An alloy containing Al – 3wt.% Cr – 3wt.% Fe – 0.8wt. % Ce, was prepared by melt spinning. Structure of obtained ribbons was observed by light, scanning and transmission electron microscopy. It was found out that the structure is very fine. Microhardness of cross sectioned ribbons was also measured. Defects in structure were determined by positron annihilation spectroscopy. The thermal stability of the alloy was observed by comparing rapidly solidified ribbons and ribbons annealed at 400°C and at 500°C for 100 h

Abstract: The ribbons of rapidly solidified Mg-6wt%Zn-1wt%Y-0.6wt%Ce-0.6wt%Zr alloy were reciprocatingly extruded and forward extruded into dense bar material. Room-temperature fatigue behavior of the alloy was tested in axial tension-tension stress condition. The fracture morphologies of the alloy after fatigue were observed by SEM. The results show that the fatigue limit is 159.2MPa with 106 cycles when the load frequency was 10Hz. The S-N curve of the alloy can be regarded as Type Ⅱ fatigue curve. The fatigue cracks originate from surface or subsurface of the fatigue specimens generally. The second phases or inclusions in these areas were prone to be the crack sources. The high fatigue properties of the alloy can be attributed to grain refinement strengthening and dispersion strengthening resulted from rapid solidification and reciprocating extrusion.

Abstract: Magnesium and its alloys are attractive candidates for automotive and aerospace applications due to their relatively high strength and low density. However, their low ductility determined by hcp structure of material results in limitation of plastic deformation processing. In order to improve ductility as well as mechanical properties, structure refinement processes can be used. It is well known that effective refining of the material structure can be achieved by increasing the cooling rate during casting procedures, hence rapid solidification process (RSP) has been experimented for the fabrication of magnesium alloys. The present paper reports an experimental investigation on the influence of rapid solidification on the mechanical properties of AM60 magnesium alloy. In order to obtain RS material melt spinning process was applied in protective atmosphere, resulting in formation of RS ribbons. Following consolidation of the RS material is necessary to obtain bulk material with high mechanical properties, as so hot extrusion process was applied. It was noticed that application of plastic consolidation by hot extrusion is the most effective process to achieve full densification of material. For comparison purposes, the conventionally cast and hot extruded AM60 alloy was studied as well. The purpose of the present study was to investigate in detail the effect of rapid solidification and extrusion temperature on the structure and mechanical properties of the materials.

Abstract: Rapid solidification of CuFe10 alloys was carried out by melt spinning at three wheel speeds 4, 12 and 36 m/s. The microstructure and solidification behavior of the ribbons was investigated by scanning electron microscopy (SEM). The results showed that the ribbons generally have a microstructure consisting of a fine dispersion of a Fe-rich phase in a Cu-rich matrix, and that the morphology and size of the Fe-rich phase vary with the wheel speed. For lower wheel speeds, the Fe-rich phase shows dual morphologies, dendrites and spheroids. The Fe-rich spheroids show a wide distribution of size, and have a larger mean size on the free surface than on the wheel surface. Some Fe-rich dendrites are distributed around the Fe-rich spheroids, this means liquid phase separation occurred during rapid solidification. The Fe-rich phase shows one morphology-spheroids and the size of Fe-rich spheroids decreasing as the wheel speed increasing. The results were discussed with respect to the formation of the Fe-rich spheroids during rapid solidification. The microhardness of the ribbons increasing with the increasing of the wheel speed for the grain refining and the increasing of supersaturated solid solution.

Abstract: In the present work, rapidly solidified Al-21Si-0.8Mg-1.5Cu-0.5Mn alloys strips was prepared by melt-spinning method. The microstructures, phase and morphology characteristics of the experimental alloy were characterized by means of scanning electron microscopy, transmission electric microscopy. The results show that the microstructures are changed obviously compared with conventional condition. The nucleation and growth of primary silicon are suppressed and primary silicon can not deposited, meanwhile, α-Al phase is nucleated which prior to eutectic. The microstructures of the rapidly solidified alloys are composed of primary micro-nanostructure α phase and feather-needles-like (α+Si) eutectic which set in the α phase. The mechanism of formation for microstructures of melt-spinning Al-Si alloy have also been discussed.

Abstract: A numerical model is developed to describe the kinetics of the microstructure evolution in an atomized droplet of Mg-9wt%Al alloy. The model is coupled with the heat transfer controlling equations to simulate the solidification process of the atomized droplets. The numerical results show that the microstructure development is a result of the common action of the nucleation and growth of grains. The nucleation events take place at a critical supercooling for a given droplet. As the droplet size decreases, the critical supercooling increases significantly. The volume fractions of the phases formed during the period of the recalescence, the segregated solidification and the eutectic reaction are sensitive to the droplet size. It is demonstrated that the developed model describes the microstructure evolution process well.

Abstract: In this study, AZ91 and SiC particulates reinforced AZ91 (SiCp/AZ91) magnesium alloys were successfully fabricated using rapid solidification/powder metallurgy technique followed by hot extrusion. Microstructural evolution and mechanical properties of the monolithic AZ91 and SiCp/AZ91 magnesium alloys were evaluated. SiC particulates were well distributed with only few agglomerated particles. The porosity level and microhardness increased as SiCp content increased because the increased surface area of SiCp, harder ceramic phases and SiCp acted obstacles to the motion of dislocations. In addition, an increase in particulate reinforcement content was observed to decrease mechanical properties of the composite compared with the unreinforced counterpart due to increasing agglomerating regions and porosity, brittle interface debonding between matrix and SiCp.

Abstract: A rapid-solidified hypereutectic Al-21Si-0.8Mg-1.5Cu-0.5Mn alloys strips has been prepared by single roller melt-spinning. The effects of solution technology on the microstructures and properties of the strips have been investigated by SEM, TEM. The results showed that the nucleation and growth of primary silicon are suppressed and α-Al is nucleated which prior to eutectic. The Cu, Mg and Mn are all supersaturated solution in α-Al. The major part of Si solution in α-Al, and the rest are precipitated by the micro-nanocrystals eutectic silicon. The metastable microstructures of micro-nanocrystals hypoeutectic are formed. And in the solution process, Si are precipitated from the matrix and gradually aggregate up to form small particles distributing in the matrix with the heating time. At 480°C for 100 minutes followed by quenching in water and a natural aging for 96h, the finer and more roundness of silicon particle and the maximum Vickers hardness are formed.