The Influence of the Forced Movement of Components on the Structure in Fabricated AlSi/Cr x C y Composite Castings

2 Abstract Fabrication and microstructure of the AlSi11 matrix composite containing 10 % volume fraction of CrFe30C8 particles were presented in this paper. Composite suspension was manufactured by using mechanical stirring. During stirring process the temperature of liquid metal, time of mixing and rotational speed of mixer were fixed. After stirring process composite suspension was gravity cast into shell mould. The composites were cast, applying simultaneously an electromagnetic field. The aim of the present study was to determine the effect of changes in the frequency of the current power inductor on the morphology of the reinforcing phase in the aluminum matrix. The concept is based on the assumption that a chromium-iron matrix of CrFe30C8 particles dissolves and residual carbide phases will substantially strengthen the composite. The microstructure and interface structure of the AlSi11/CrFe30C8 composite has been studied by optical microscopy, scanning microscopy and X-ray diffraction.


Introduction
During the last two decades a lot of research has focused on aluminum metal matrix composites (Al MMCs). A wide variety of fabrication techniques has been explored for Al MMCs, which include vapour state methods, liquid phase methods (infiltration of preforms, rheocasting/thixoforming, melt stirring and squeeze casting) and solid state methods (powder forming and diffusion bonding) [2] [4]. Recently, the preponderance of research studies on Al MMCs has been aimed at developing particle reinforced aluminum matrix composites (PR-AlMCs), based on liquid methods, because they can be used to produce components by casting processes. The fabrication of PR-AlMCs using casting techniques is particularly attractive because it permits a low-cost and net-shape fabrication, adaptability of casting processes to existing production practices and flexibility in designing the structure through controlled solidification. However, it has some restrictions due to the matrix alloy and density of the reinforced phases. Therefore the volume fraction and the size of the reinforcements that can be added are very limited [1][2][3][4][5][6][7][8].
MMCs containing hard or/and ceramic particles offer superior operating performance. Al based discontinuous reinforced MMCs are of great scientific and technological interest and have received much attention in recent years because of their good mechanical and thermal properties. PR-AlMCs potential applications come mainly from established automotive, aerospace and high performance sectors (production of engine parts, brake components, transmission beams and, stiffeners and sporting equipment), because of the high stiffness, strength and wear resistance that these materials have [1][2][3][4][5][6][7][8][9][10].
Interface is a very general term used in various fields of science and technology to denote the location where two entities meet. The interface in composites refers to a bounding surface between the reinforcement and matrix across which there is a discontinuity in chemical composition, elastic modulus, coefficient of thermal expansion, and/or thermodynamic properties such as chemical potential. The interface (particle/matrix) is very important in all kinds of composites. This is because in most composites, the interfacial area per unit volume is very large. Also, in most metal matrix composite systems, the reinforcement and the matrix will not be in thermodynamic equilibrium, i.e., a thermodynamic driving force will be present for an interfacial reaction that will reduce the energy of the system. All these items make the interface have a very important influence on the properties of the composite. Interfacial characteristics in metal matrix composites reinforced play a significant role in determining the mechanical properties, such as strength, ductility, toughness, fatigue, etc. To achieve superior mechanical properties in MMCs, it is essential to form adequate interfaces, which not only do not degrade the reinforcement during fabrication, but also retain the structural stability [7][8].
The aim of the study was to determine the effect of changes in frequency of the current power inductor on the form the transitional phase in the contact boundary of matrix/reinforcement in produced composites.
The concept is based on the assumption that a chromium-iron matrix of CrFe30C8 particles dissolves and residual carbide phases will substantially strengthen the composite.

Experimental procedures
Composition of the reinforcement and the matrix used in the present study are shown in Tables 1 and 2, respectively. Reinforcement used in this study were CrFe30C8 having an average particle size 200 [μm]. Particle surfaces before the introduction of the molten matrix sodium and boron compounds were prepared. CrFeC particles were annealed at 360 [ºC] for 3h directly before introduction into the crucible.
The composite samples were prepared in two stages. In the first stage composite suspension about 10% particle mass participation by mechanical mixing in crucible was produced (Fig. 2). The mixing time was 90 [s]. In the second stage received composite suspension to shell mould under electromagnetic field were casted. Moulds from operating outside, rotating electromagnetic field was performed (Fig. 1). The time of field influence was 120 [s]. By inverter the frequency adjusted (50, 75, 100 Hz), what caused regulation of electromagnetic field rotation speed. The technological production parameters of the composite samples were shown in Table 3.

Results and discussion
To analyze the morphology of the CrFe30C8 particles (Fig. 3) the confocal laser scanning microscope (CLSM 5 Exciter by Zeiss) with emitting 405 [nm] wavelength was used. Results of analysis in Table 4 were shown. In the Figure 4 sample particles tested were shown.    Figure 5. Obtained composite samples examination of the microstructure by the DSM 940 by OPTON scanning electron microscope equipped with X-ray microanalyzer were performed. Point and linear analysis were executed (Fig. 5,  6, Table 5 In order to identify the phase composition obtained materials point analysis (Fig. 6, Table 5) and analysis by X-ray on the diffractometer were realized. Phase identification to help with the PCSIWIN computer program by using databases in the form of files JCPDS -International Centre for Diffraction Data 2000 was performed. Sample results of X-ray analysis in Figure 7 was shown.   Fig. 7. The result of X-ray phase analysis of the AlSi11/CrFe30C8 composite castings As a result of phase analysis revealed the presence of carbide phases mainly Cr 3 C 2 in all received composite castings. Intensity of the various phases were similar for all tested samples.
As the complementary test microhardness measurements on the Microhardness tester FM -700 was performed. In the Table 6 the results of microhardness tests were shown. In the Figure 8 the characteristic tested phases were shown.   5 and 7) were not observed which could be due to less diffusion resulted from the smaller mutual movement between the components.

Conclusions
Based on the analysis results, it was found that by choosing suitable the relative velocity of components, the shape of the transition reinforcing phase in the surroundings of the CrFe30C8 particles could be regulated. Already for the small relative velocity of components (1,2 [m/s]) with rotation speed of electromagnetic field (157,1 [rad/s]), when the amount of motion between the components is small, the beginning of the diffusion phenomena of the chromium-iron matrix of CrFe30C8 particles into the aluminum matrix was noted. It resulted from the difference of concentrations at the contact boundary of the components. With an increased rotation speed caused by an increase in current frequency supplying the inductor to 75 [Hz], a growth of transition phase precipitates in the entire analyzed surface was observed. It could be the cause of such a segregation effect caused by angular acceleration. For these parameters of the electromagnetic field and the relative velocity of components 1,8 [m/s] a maximum increase of new phases was observed, which was the result of diffusion phenomena between the components. Separation of the transition phase have a very extensive form (multiform). Further increasing the current frequency to 100 [Hz] again caused a decrease of the transition reinforcing phase in the surroundings of the particles. As a result of the diffusion phenomena of the chromium-iron matrix of the particles to the aluminum matrix, the reinforcing phase in the transition zone, which was created at a frequency of 75 [Hz] by increasing the speed rotation of the electromagnetic field to 100 [Hz], underwent partial dissolution in the aluminum matrix. It resulted in a reduction of the volume of these phases and changed in the shape of the transition phase precipitates.