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Equilibrium Si content required to suppress the interfacial reaction in 2124 Al/SiC and A356 Al/SiC has been thermodynamically predicted by combining Wilson equation with the Miedema model. In 2124 Al/SiC composites, good agreement between thermodynamically calculated and experimental result has been observed in the solid and semi solid state reaction. At higher temperature above 750°C, thermodynamically calculated results are much higher than that of experimental values. Strong influence of reinforcement size and volume fraction on the extent of interfacial reaction is also observed in 2124 Al/SiC composites. In these composites, formation of protective layer of A1 4C3 crystals over SiC surface plays significant role in predicting the equilibrium Si content. The influence of reinforcement particle size and volume fraction on the extent of reaction has not been observed in A356 Al/SiC composites and irrespective of reaction temperature the thermodynamically calculated equilibrium Si contents for A356 Al/SiC composites agrees well with the experimental results. © 2009 TIIM, India.

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Department of Metallurgical & Materials Engineering

Transactions of the Indian Institute of Metals

Role of matrix alloy chemistry on the interfacial reaction and solidification of metal matrix composites

The formation of Al2O3, MgAl2 O4, and MgO has been widely studied in different Al base metal matrix composites, but the studies on thermodynamic aspects of the Al2O3/MgAl2O4/MgO phase equilibria have been limited to few systems such as Al/Al2 O3 and Al/SiC. The present study analyzes the Al2 O3/MgAl2O4 and MgAl2 O4/MgO equilibria with respect to the temperature and the Mg content in Al/SiO2 system using an extended Miedema model. There is a linear and parabolic variation in Mg with respect to the temperature for MgAl2O4/MgO and Al2 O3/MgAl2O4 equilibria, respectively, and the influence of Si and Cu in the two equilibria is not appreciable. The experimental verification has been limited to MgAl2 O4/MgO equilibria due to the high Mg content (≥0.5 wt pct) required for composite processing. The study has been carried out on two varieties of Al/SiO2 composites, i.e., Al/Silica gel and Al/ Micro silica processed by liquid metallurgy route (stir casting route). MgO is found to be more stable compared to MgAl2O4 at Mg levels ≥5 and 1 wt pct in Al/Silica gel and Al/Micro silica composites, respectively, at 1073 K. MgO is also found to be more stable at lower Mg content (3 wt pct) in Al/Silica gel composite with decreasing particle size of silica gel from 180 micron to submicron and nanolevels. The MgO to MgAl2O4 transformation has taken place through a series of transition phases influenced by the different thermodynamic and kinetic parameters such as holding temperature, Mg concentration in the alloy, holding time, and silica particle size. © The Minerals, Metals &amp; Materials Society and ASM International 2008.

Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science

Thermodynamics and kinetics of the formation of Al2 O3/ MgAl2O4/MgO in Al-Silica metal matrix composite

Equal channel angular pressing (ECAP) of in situ Al-5 wt% TiB2 composite was successfully carried out at room temperature up to four passes. Large particle free zones of about 50-100 μm size present in as cast condition have been reduced to 20-30 μm by ECA pressing. The initially clustered distribution of TiB2 has become homogeneous after four ECAP passes. A significant grain refinement down to ∼500 nm has been achieved by ECA pressing but the average particulates size remains unchanged at 0.5-0.6 μm. © 2007 Elsevier B.V. All rights reserved.

Journal of Alloys and Compounds

Equal channel angular pressing of Al-5 wt% TiB2 in situ composite

Thin wall casting of aluminum alloys and their composites provide new opportunities for aerospace and automotive industries in producing light weight structures with good mechanical properties. These opportunities can be realized only when the foundries are confident in their ability to produce thinner-wall castings. The primary issue that must be addressed in producing thin wall castings is fluidity. The fluidity of aluminum alloys and composites has a direct influence not only on material castability, but also on the casting properties. Fluidity is a complex parameter that is affected by the properties of the molten metal and mold, pouring conditions, reinforcement characteristics and solidification mechanism. This paper brings out the current understanding about the various factors influencing the fluidity of aluminum alloys and composites. Further, theoretical model available in the literature to predict the fluidity of aluminum alloys and their composites is given and its significance has been discussed. © 2007 Elsevier B.V. All rights reserved.

Fluidity of aluminum alloys and composites: A review

Al-5 vol% graphite particulate composite was subjected to equal channel angular pressing (ECAP) at room temperature up to four passes. A significant grain refinement, down to the submicron level of ∼300 nm was achieved. Hardness enhancement of more than two fold was observed with only a marginal difference along the perpendicular and parallel directions of pressing. Further, the strength of the composites was increased by more than two folds. ECAP at room temperature has resulted in a more rapid and significant grain refinement and strengthening in Al-graphite composite. © 2006 Elsevier Ltd. All rights reserved.

Composites Science and Technology

Development of ultrafine grain aluminium-graphite metal matrix composite by equal channel angular pressing

Fluidity behavior of the A356 Al alloy and A356 Al-SiCp composites has been theoretically predicted using the Flemings model after incorporating (1) the solidification behavior of the alloy; (2) the decrease in flow velocity due to surface tension and friction losses; and (3) the increase in viscosity of composite slurries due to size, shape, and volume fraction of the reinforcement. It is observed that the fluidity values of A356 Al alloy and A356 Al-10 vol pct SiCp composite predicted by the modified Flemings model are in good agreement with the experimental values reported in the literature. On the other hand, the theoretically calculated fluidity length of A356 Al-15 and 20 vol pct SiCp composites using the modified Flemings model agrees well with the experimental results only up to 750 C and 710 C, respectively. However, above these respective temperatures, the experimental spiral length decreases with pouring temperature, whereas the theoretically calculated spiral length increases linearly as a function of pouring temperature. Incorporation of experimentally calculated interfacial reaction terms in the modified Fleming model makes the predicted and experimental values of fluidity of A356 Al-15 and 20 vol pct SiCp composites closer in the entire temperature range. Thus, the high dependence of the fluidity of A356 Al-SiCp composites on the morphology of the interfacial reaction product is demonstrated.

Influence of interfacial reaction on the fluidity of A356 Al-SiCp composites-A theoretical approach

The reclamation process by salt flux addition has been employed to extract the interfacial reaction products as well as SiCpfrom Al-SiCp composites. It has been observed that this process enables the use of X-ray diffraction (XRD) to identify the interfacial reaction product Al4C3in Al-SiCp composites even in the early stages of reaction. Further, it allows an easy and more accurate quantification of the interfacial reaction in Al-SiCp composites using spectroscopic techniques such as optical emission spectroscopy (OES). © The Minerals, Metals &amp; Materials Society and ASM International 2007.

A novel approach for extracting and characterizing interfacial reaction products in Al-SiCp composites

The influence of the mixing parameters on the synthesis of Al-SiCp reinforced metal matrix composites (MMCs) by the stir casting technique is investigated through a water model. The effects of some important mixing parameters such as impeller blade angle, rotating speed, direction of impeller rotation and effect of baffles are investigated and optimized. The results have shown that the axial concentration variation of natural graphite during stirring in the presence of four vertical baffles is 1.0 wt% against in the absence of baffles it is increased to 2.3 wt%. The variations observed in natural graphite concentration in water during mixing are in close agreement with the earlier modeling and limited experimental studies reported on the real molten aluminum-SiC system. Semi-empirical correlations arrived at between the dimensionless numbers for stirred water - natural graphite slurries are Po = Re-0.0545 Fr-1.099 and Po = Re-0.0219 Fr-1.0382 for clockwise and counter clockwise rotation respectively. © 2005 Elsevier Ltd. All rights reserved.

Materials and Design

Optimization of mixing parameters through a water model for metal matrix composites synthesis

Separation of matrix alloy and reinforcements from pure Al-SiCp composite scrap by salt flux addition has been theoretically predicted using interface free energies. Experiments performed confirm the theoretical prediction. Complete separation of matrix aluminum and reinforcement from metal matrix composites (MMCs) scrap has been achieved by addition of 2.05 wt% of equimolar mixture of NaCl-KCl salt flux with a metal and particle yield of 84 and 50%, respectively. By adding 5 wt% of NaF to equimolar mixture of NaCl-KCl, metal and particle yield improved to 91 and 73%, respectively. Reusability of both the matrix aluminum and the SiC separated from Al-SiCp scraps has been analysed using XRD, SEM and DTA techniques. The matrix alloy separated from Al-SiCp scraps can be used possibly as a low Si content Al-Si alloy. However, the interfacial reaction that occurred during the fabrication of the composites had degraded the SiC particles. © Indian Academy of Sciences.

Fulltext

Bulletin of Materials Science

Separation of matrix alloy and reinforcement from aluminum metal matrix composites scrap by salt flux addition

Equal channel angular pressing (ECAP) is a novel technique for producing ultra fine grain structures in submicron level by introducing a large amount of shear strain into the materials without changing the billet shape or dimensions. This process is well suited for aluminium alloys and is capable of producing ultra fine grain structures with grain sizes falling between 200 and 500 nm. The present study attempts to apply ECAP technique to 99-5% pure aluminium and characterize the resulting aluminium by optical metallography, atomic force microscopy (AFM) and hardness measurement. ECAP of 99-5% pure aluminium produces ultrafine grain structure of about 620 nm after 8 passes. Despite an increase in the hardness from 23 to 47 BHN up to 6 passes, it decreases slightly for seventh and eighth passes. The results are compared with the already existing results available on pure aluminium. Analysis of the results of this investigation with those available in the literature has revealed that the number of passes essential to achieve a homogeneous microstructure in pure Al increases, while the ultimate equilibrium grain size obtained becomes finer with decreasing purity. © Indian Academy of Sciences.