Preparation and some properties of SiC particle reinforced aluminium alloy composites

Aluminium alloy composites containing various particle sizes of 10 and 20 wt.% SiC particles were prepared by molten metal mixing and squeeze casting method under argon gas. The stirring was carried out with graphite impeller during addition of particle. The molten mixture was poured into a die when the stirring was completed and metal matrix composites were produced by applying the pressure. Optical microscopic examination, hardness, density and porosity measurement were carried out. Moreover, metal matrix composites were machined at various cutting speeds under a fixed depth of cut and feed rate using different cutting tools. It is observed that there was a reasonably uniform dispersion of particles in the matrix alloy. The density decreased with decreasing particle sizes, but porosity decreased considerably with increasing particle size. In addition, the tool life decreased considerably with increasing cutting speeds for all tests. Among cutting tools, the wear resistance of Al₂O₃ coated tools showed better performance than those of the other tools without chip breaker geometries in the machining of SiCp-reinforced composites.     

Ageing behaviour of SiCp-reinforced AA 7075 composites

The precipitation behaviour in 7075 aluminium alloy matrix composites reinforced with 0–40 vol% particulate SiCp (12.5 μm) was studied using macrohardness (HV) measurements and differential scanning calorimetry (DSC). In the low volume percentage (5,10) SiCp composites, the hardness-ageing curves and DSC scans are similar to those of the unreinforced alloy. However, the age-hardening quantities and DSC Gurnier-Preston (GP) zone peak size are smaller than those of the unreinforced alloy. Additionally, the high-temperature peaks (ageing curves at 200 °C or DSC scanning curves) are broader. In the high volume percentage (20, 30, 40) SiCp composites, the hardness-ageing curves and DSC scans are very different from those of the unreinforced alloys. Only a high-temperature broad peak was observed during the DSC scanning. On the hardness-ageing curves no hardening phenomena were detected, but rather a softening phenomenon occurred in the 30% or 40% SiCp composites, suggesting that during ageing an exothermic dislocation recovery softening process coexists with precipitation hardening. A model was introduced by dividing the matrix of the composite into Region I (normal precipitation) and Region II (particular precipitation). The precipitation of GP zones is completely suppressed and the precipitation of η′ phase is accelerated in Region II. The matrix of the low volume fraction SiCp composite comprises Regions I and II, whereas that of the high volume fraction SiCp composite comprises only Region II. The ageing behaviour and DSC scans of the composites can be fully explained by this model. This revised version was published online in November 2006 with corrections to the Cover Date.     

Production and properties of micro‐porous glas bubble zinc and aluminium composites

Micro‐porous syntactic foams were produced by means of integration of glass bubbles into aluminium and zinc matrices. Preforms of glass bubbles were pressure infiltrated with the alloys AlSi9Cu9 ans ZnAl4Cu using squeeze casting. The preforms were sintered thermically without the use of bonding agents. Using the combination of different sintering steps syntactic foams with locally different densities could be produced. The mechanical properties of the foams were tested indicating a high compression strength of the foams and a very good compression energy absorption. Furthermore, corrosion behaviour and behaviour at higher temperatures were investigated.     

The fabrication and properties of metal-matrix composites based on aluminium alloy infiltrated alumina fibre preforms

Composites have been produced by infiltrating a porous preform of semi-random short alumina fibres with liquid aluminium alloys, using squeeze-casting techniques. The fibres were wetted by pure aluminium and several different alloys, and sound castings produced. Full infiltration was observed to take place very rapidly when moderate pressures were applied.

The composites produced have been shown to possess enhanced stiffness which is maintained at useful levels at temperatures approaching 400°C; at ambient temperature they were brittle, but at elevated temperatures the strength was significantly enhanced.     

Reactivities of aluminium and aluminium-magnesium alloy powders in polymeric composites

Polymeric compositions containing Al-Mg alloys show higher reactivities, in comparison with similar compositions containing aluminium. This is observed irrespective of the amount of oxidizer, type of oxidizer used, type of polymeric binder, and over a range of the particle sizes of the metal additive. This is evident from the higher calorimetric values obtained for compositions containing the alloy, in comparison to samples containing aluminium. Analysis of the combustion residue shows the increase in calorimetric value to be due to the greater extent of oxidation of the alloy. The interaction between the polymeric binder and the alloy was studied by coating the metal particles with the polymer by a coacervation technique. On ageing in the presence of ammonium perchlorate, cracking of the polymer coating on the alloy was noticed. This was deduced from differential thermal analysis experiments, and confirmed by scanning electron microscopic observations. The increase in stiffness of the coating, leading to cracking, has been traced to the cross-linking of the polymer by magnesium.     

Preparation and characterization of aluminium alloy sheet Aramid fibre-laminated composites

Laminated composites consisting of alternate layers of aluminium alloy sheets and unidirectional Kevlar-49 fibre epoxy composites were prepared using two different aluminium alloys DTD 687 and aluminium-lithium alloy. Tensile, compressive and interlaminar shear strengths of the laminates were measured. The residual stresses in the aluminium alloy sheets arising out of thermal mismatch between aluminium alloys and aramid fibres were also measured. It is found that the laminates have lower density, higher tensile strength and marginally lower Young’s modulus as compared with monolithic alloy sheets.     

Threshold creep behaviour of discontinuous aluminium and aluminium alloy matrix composites: An overview

Several sets of creep data for aluminium and aluminium alloy matrix composites reinforced by silicon carbide particulates, silicon carbide whiskers or alumina short fibres are analysed. It is shown that for this class of discontinuous composites the threshold creep behaviour is inherent. Applying the concept of threshold stress, the true stress exponent of minimum creep strain rate of approximately 5 follows from the analysis even when the matrix solid solution alloy exhibits Alloy Class creep behaviour, for which the value of 3 for the true stress exponent is typical. The creep strain rate in the discontinuous aluminium and aluminium alloy matrix composites is shown to be matrix lattice diffusion controlled. The usually observed high values of the apparent stress exponent of creep strain rate and the high values of the apparent activation energy of creep are then rationalized in terms of the threshold creep behaviour. However, the origin of the threshold stress decreasing with increasing temperature but not proportional to the shear modulus in creep of discontinuous aluminium and aluminium alloy matrix composites is still awaiting identification. The creep-strengthening effect of silicon carbide particulates, silicon carbide whiskers and alumina short fibres is shown to be significant, although the particulates, whiskers and short fibres do not represent effective obstacles to dislocation motion.     

Extrusion characteristics of aluminium alloy/SiCpmetal matrix composites

Abstract

Systematic extrusion studies have been carried out on aluminium alloy 2124/SiC_p metal matrix composites. Effects of various extrusion process parameters, such as die design, ram speed, extrusion ratio, reheat temperature, and lubrication, on the pressure requirement and surface quality of the as extruded circular rods have been investigated. Different volume fractions of SiC particles (10, 15, and 20 vol.-%) were used for the synthesis of metal matrix composite billets. These composites were synthesised using two different techniques, namely, stir casting and powder metallurgy. These billets were then hot extruded on a laboratory scale 500 ton vertical hydraulic press. The significance of specially designed dish shaped dies, avoiding the dead metal zone, has also been highlighted. The results indicated that the best extrusion was possible when powder metallurgical processed billets were extruded. Volume fraction analysis of ceramic reinforcement in the extruded rod (typically 2 m long) and in the extruded discard showed no appreciable backward migration of these particles during extrusion.     

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.     

Fracture of alumina particulate reinforced aluminium alloy matrix composites

Fracture toughness tests were performed on two aluminium alloy matrices, 2014-0 and 2024-0 reinforced with alumina particulates of different volume fractions and particulate sizes so as to investigate the fracture mechanisms operative in such composites and to determine how microstructural parameters such as volume fraction, particulate size and interparticle spacing affect the fracture toughness. The results indicate that fracture occurred by a locally ductile mechanism. The fracture toughness increased with increasing particle spacing provided that the particle size was less than a limiting value, above which unstable crack growth occurred and the toughness lowered.     

Structure and properties of boron/aluminium composites

The influence of heat treatment on the structure and strength of boron/aluminium composites has been studied. Matrix material around a fibre is assumed to be hardened by dispersed particles of boron-containing compounds, presumably magnesium borides. Such a material is expected to have higher yield strength and greater brittleness compared to the original matrix material. A fibre and its surrounding zone is a prospective site of microcracking under loading. These zones grow as heat treatment proceeds. When neighbouring zones become linked to each other, the size of a possible brittle microcrack is doubled and a fall in the composite strength follows. A corresponding microstructural model of the composite failure is constructed.

Without having direct observation of magnesium borides, the model of composite behaviour is supported by (i) dependence of the composite strength on the size of the influence zone; (ii) dependence of the strength on the effective time, the latter being determined by the activation energy of magnesium diffusion in aluminium; (iii) observation of failure surfaces of composites subjected to various heat treatments; and (iv) a correlation of changes of matrix state detected by changes in X-ray diffraction patterns with a hypothetical picture of the development of the influence zones.     

Influence of microstructure and texture on mechanical properties of aluminium alloy 2124 + 5%SiC particulate composites

Abstract

A study of texture, microstructure, mechanical properties, and crack propagation mechanisms was carried out on aluminium alloy 2124 reinforced with 5 vol.-%SiC particles (3 μm). Three fabrication techniques have been used to produce the composites. composite I wasfabricated by blending followed by hot isostatic pressing. composite II was fabricated by mechanical alloying followed by hot isostatic pressing. composite III wasfabricated by agglomeration of aluminium powder by mechanical alloying followed by blending with SiC and hot isostatic pressing. All three composites were hot rolled to nominally 12.5 mm thick plate. Similar textures were observed for all composites. A model of the observed texture is {001} (211), {111} (211), and {211} (111) for rolling, side, and transverse planes respectively. Composite I showed a homogeneous distribution of SiC particles. Transmission electron micrographs of composite I showed good interface bonding, stacking faults present in SiC particles, and segregation of aluminium, oxygen, copper, and magnesium to the interface. composites II and III showed an inhomogeneous distribution of SiC particles. The elastic modulus was slightly higher in the (211) direction than in the (111) direction. The fracture toughness of composite I was higher in the (211) crack direction whereas that for composite II and composite III was higher in the (111) crack direction. Secondary crack propagation modes follow the crystallographic orientations of {100} and {111} planes.     

Cast aluminium alloy composites containing copper-coated ground mica particles

An electroless method of coating copper on ground mica particles using copper sulphate solution is described. The effects of time of sensitization, PdCl₂ concentration and time of stirring the activated particles in electroless solution, on the extent of copper coating on mica particles are reported. Using this method it is possible to deposit up to about 35 wt% copper on mica particles. A process for making cast aluminium alloy-mica particle composite alloys using these coated particles is also described. The process involves stirring the copper-coated mica particles into liquid alloys using an impeller, and casting the melts containing suspended mica particles in suitable permanent moulds. Coating of copper on mica particles makes possible the dispersion of ground mica particles in molten aluminium alloys with high recoveries which is otherwise difficult, even when magnesium is added to the surface of the melts. Copper coating on ground mica particles masks the basal planes, and apparently increases their wettability with aluminium alloy melts. Recoveries of ground mica particles in composite castings made using copper-coated mica particles are as high as 80%, which is three times higher than the corresponding recoveries in the castings made using uncoated particles. The mechanical properties of cast aluminium mica composites made using copper-coated ground mica powders are adequate for a variety of bearing and antifriction applications.     

Failure behaviour of particulate-reinforced aluminium alloy composites under uniaxial tension

Tensile tests were carried out at room temperature on 6061-aluminium alloy reinforced with SiC and Al₂O₃ particulates. Although a significant increase in strength could be achieved by introducing ceramic reinforcements into the aluminium alloy matrix, it is associated with a substantial decrease in fracture strain. In order to understand the reason for the inferior ductility of such composites, analytical solutions were obtained using a simple composite model. SEM studies were carried out on the side surfaces of the fractured specimens to verify the proposed failure behaviour. Failure modes observed to operate in such composites under uniaxial tension are described.     

Preparation and properties of cast aluminium alloy-granite particle composites

With a view to develop light weight, low cost and abrasion resistant material cast aluminium alloy composites dispersed with granite particles were prepared and their properties were evaluated. Natural mineral granite was crushed and treated prior to its incorporation in the aluminium alloy. Liquid metallurgy techniques was used to prepare composites involving the following steps: melting of aluminium alloy in graphite crucible, stirring of the melt, addition of granite particles and reactive metal in the melt and pouring the composite melt into permanent moulds. Physical, mechanical, tribological and metallographic properties of composites were studied. It was observed that there was reasonably uniform dispersion of granite particles in the matrix. Hardness and tribological (abrasive wear) properties of the base alloy improved considerably due to addition of the granite particles into it. This clearly indicates that these cast aluminium alloy based composites can be used as wear resistant materials.