B. C. Pai

Role of magnesium in cast aluminium alloy matrix composites

Wetting between the dispersoid and the matrix alloy is the foremost requirement during the preparation of metal matrix composites (MMC) especially with the casting/liquid metal processing technique. The basic principles involved in improving wetting fall under three categories: (i) increasing the surface energies of the solids, (ii) decreasing the surface tension of the liquid matrix alloy, and (iii) decreasing the solid/liquid interfacial energy at the dispersoid matrix interface. The presence of magnesium, a powerful surfactant as well as a reactive element, in the aluminium alloy matrix seems to fulfil all the above three requirements. The role played by magnesium during the synthesis of aluminium alloy matrix composites with dispersoids such as zircon (ZrSiO4), zirconia (ZrO2), titania (TiO2), silica (SiO2), graphite, aluminium oxide (Al2O3) and silicon carbide (SiC), has been analysed. The important role played by the magnesium during the composite synthesis is the scavenging of the oxygen from the dispersoid surface, thus thinning the gas layer and improving wetting and reaction-aided wetting with the surface of the dispersoid. The combinations of magnesium and aluminium seem to have some synergistic effect on wetting.

The effects of magnesium additions on the structure and properties of Al-7 Si-10 SiCp composites

The additions of magnesium to an aluminium alloy matrix, which contains insufficient magnesium, was found to be essential during the synthesis of composites by the stir-casting technique. Magnesium promotes interfacial wetting between the dispersoid surface and the matrix. Dispersion of SiCp in Al-7 Si-0.3 Mg (356) alloy matrix without agglomeration and rejection was not possible. Hence, the addition of up to 3 wt% Mg was made to the alloy matrix during the dispersion of 10 wt% SiCp (34 Μm), and the microstructure and mechanical properties of the composites were investigated with a view to optimize the magnesium content. With a magnesium content less than 1 wt% in the matrix, the SiCp particles were essentially in agglomerated form. The highest UTS of 280–300 MPa was obtained with 1 wt% Mg content and SiCp was uniformly distributed in the matrix. A higher magnesium content (>1.0 wt%) did not further improve the uniformity in the dispersion of SiCp but the ultimate tensile strength properties deteriorated. This decrease in strength was attributed to the observed coarseness of the Mg2Si phase, the precipitation of Mg5Al8 phase and the presence of a higher amount of porosity in the composites in the heat-treated condition. The aspect ratio (length/width) of precipitates changed from 1–3 for 1% Mg to 3–9 for 3.2% Mg in the matrix. Corresponding values for per cent porosity were 2% and 6%, respectively.

Factors affecting the stability of non-wetting dispersoid suspensions in metallic melts

Abstract A phenomenon similar to the stability of colloidal suspensions has been observed in non-wetting dispersoid suspensions in metallic melts during the synthesis of aluminium metal-matrix composites (AMMC). To explain this phenomenon of the segregation of dispersoids, a plausible mechanism has been proposed incorporating the effects of buoyancy. In addition, the mechanism has been experimentally verified using Al-12% Si and Al-12% Cu alloys as the matrix and synthetic graphite and silica sand as the dispersoid. Good correlation between the calculated and the experimentally observed critical volume fraction of dispersoid at rejection has been found. The effects of stirrer speed, interfacial tension and rate of addition of dispersoid on the critical volume fraction at rejection has also been studied.

Fracture behaviour of pressure die-cast aluminium-graphite composites

Fracture toughness values of pressure die-cast Al-7Si-3 Mg-5 graphite composites were measured and found to be in the range 8–10 MPa m1/2. Detailed microstructure of the composite and the fractured surfaces were examined. Defects such as clusters, agglomerations and segregation of graphite particles, play a dominant role in accelerating the fracture process. In addition, the acicular silicon phase present in the matrix and the casting defects, such as gas and shrinkage porosities, also initiated and accelerated the crack, thus lowering the fracture toughness of the composites.

Semi-solid slurry process for making short carbon fibre dispersed aluminium alloy matrix composites

Carbon fibre is one of the most important reinforcing materials in polymer, metal, carbon and ceramic matrices because of its high specific strength and high specific modulus. Carbon fibre polymer matrix composites are now extensively used in critical areas of application. However, carbonmetal matrix composites have not reached that stage. The possibilities of making carbon-fibre reinforced A1 alloy matrix composites by diffusion bonding [1], powder metallurgy technique [2], infiltration [3], stir casting [4], etc., have been reported in the literature. However, in most cases, the reaction between the fibre and the matrix seems to deteriorate the interface, thereby lowering the mechanical properties. Presently a semi-solid slurry casting/rheocasting/compocasting technique [5, 6] has been used for synthesizing the composite. The specific advantage of this process being [7] that it is carried out at a temperature within the freezing range (between the liquidus and solidus) of the alloy. This totally eliminates the superheat, as well as lowering the temperature of operation which reduces the temperature-aided chemical reactions at the fibre-matrix interface. The higher viscosity of the matrix alloy slurry eliminates the gravity aided segregation of reinforcement during mixing, as well as during solidification of the castings. The thixotropic nature of the slurry makes it amenable for smooth die filling without turbulence during pressure diecasting [8]. However, during gravity diecasting the fluidity of this slurry is very poor. This paper gives an account of processing and evaluation of a short carbon fibre dispersed aluminium alloy matrix composite prepared by the rheocasting route. The matrix alloy A1-6.2-Zn-2.45-Mg-l.7-Cu0.15-Zr-0.12-Fe-0.1-Si was stirred while-it was held within its freezing range (870-920 K): PANbased carbon fibres T-300 grade (Courtaulds, Grafil) of two different lengths, 1 and 3 mm, were introduced into the non-dendritic slurry after surface treatment [9], which was kept isothermally with about 25-30 wt % primary solid content. With the introduction of the fibres the composite slurry temperature was raised to keep more or less the same solid content. About 8 wt % of the fibres were added. After the additions the slurry was further stirred for ca. 10-20 min to achieve uniform disper-

Lanthanum aluminide (LaAl2)- dispersoid in aluminum matrix

Rare earths (RE) form a large number of intermetallic compounds with aluminum e.g. RE{sub 3}Al, REAl, REAl{sub 2}, REAl{sub 3} and RE{sub 3}Al{sub 11}. Among these REAl{sub 2} a Laves phase, have normally the highest melting point and depending on the rare earth metal selected the density can vary. In the present investigation LaAl{sub 2} Laves phase of C 15 structure was chosen. It has the melting point of 1,678 K and a density of 4.75 g/cc. In the present study the oxidation resistance of LaAl{sub 2} was evaluated, a composite with aluminum matrix with LaAl{sub 2} dispersoids was made by powder metallurgy technique. The compression properties of these composites were evaluated and compared with Al{sub 2}O{sub 3} dispersed Al matrix composites prepared under similar conditions.

INFLUENCE OF SILICON CARBIDE PARTICULATES (SiCp) PREHEAT TREATMENT ON THE QUALITY OF Al7Si 3.3 Mg - SiCp COMPOSITES

Surface characteristics of dispersoids which play a predominant role in the preparation of metal matrix composites can be altered by preheat treatment. Preliminary experiments carried out to study the influence of preheat treatment of SiCp on its dispersion in A17Si 3.3 Mg alloy matrix reveal that dispersoid treatment can affect the uniformity of dispersion and agglomeration of SiCp.