The surface tension of liquid pure aluminium and aluminium-magnesium alloy

This paper discusses the results of several experiments designed to further illustrate the recent findings of the present authors according to which, if a virtually leak-fee maximum bubble pressure system is used to measure the surface tension of liquid aluminium, a surface tension around 1100 mJ m^–2 is first obtained, decreasing to the oxidized value (around 865 mJ m^–2) as the experiment proceeds and oxygen enters the system mainly through the capillary walls. The peculiarities and difficulties inherent to the maximum bubble pressure method are illustrated. For instance, a study of the time needed for the surface tension to decrease to the oxidized value as a function of temperature reveals the important role played by the vapour pressure in the process. This is further illustrated by considering Al-Mg alloys, as magnesium has a different heat of vaporization and a much larger vapour pressure than aluminium at the measuring temperatures. Results for the changes in density and surface tension for the oxidized and unoxidized cases induced by magnesium (up to 8 wt%) are also presented and compared to previous data.     

Degassing and surface analysis of gas atomized aluminium alloy powders

The degassing behaviour and surface characterization of Al-Mg base alloys has been investigated using quadrupole mass spectrometry (QMS) and X-ray photoelectron spectroscopy (XPS). The alloy composition, particle size and the nature of the atomizing gas have been studied in terms of gas evolution and surface composition. XPS has been used both to measure oxide thicknesses and magnesium enrichment ratios. XPS results show that magnesium segregation increases for larger particle sizes and this is supported by QMS, with a correspondingly higher hydrogen evolution on heating being observed for the larger size fractions. High-resolution XPS of the carbon 1s photoelectron peak (C1s) indicates the presence of carbonate component on the as-received magnesium-containing powders. This component is less pronounced on degassed powders indicating the evolution of CO₂ on heating. This observation is supported by thermodynamic calculations.     

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.     

Microstructures of explosively consolidated rapidly solidified aluminium and Al-Li alloy powders

The microstructures and the characteristics of water-atomized, nitrogen gas-atomized Al powders and ultrasonic argon gas-atomized Al-Li alloy powder were investigated by means of metallography, SEM, Auger electron spectroscopy and X-ray diffraction techniques. Rapidly solidified powders were explosively consolidated into different sized cylinders under various explosive parameters. The explosively consolidated compacts have been tested and analysed for density microhardness, retention of rapidly solidified microstructures, interparticle bonding, fractography and lattice distortion. It is shown that the explosive consolidation technique is an effective method for compacting rapidly solidified powders. The characteristics of surface layers play a very important role in determining the effectiveness of the joints sintered, and the Al-Li alloy explosive compacts present an abnormal softening appearance compared to the original powder.     

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.     

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.     

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.     

Chemical segregation of solute elements in ultrasonically gas atomized aluminium alloy powders

A new experimental approach to the evaluation of chemical segregation of solute elements in ultrasonically gas atomized aluminium-alloy powders using X-ray spectral data of scanning electron microprobe analyser is described. The experimentally obtained chemical segregation data is compared with the conventional method of quantitative analysis and with theoretical predictions as determined from Scheil’s approach to the evaluation of elemental segregation during the solidification process. A comparison of experimental and theoretical predictions confirms the validity of the experimental approach in the estimation of solute segregation levels and also suggests that the solidification conditions considered for estimation of microchemical segregation can appropriately be applied to ultrasonically gas atomized powders.     

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.     

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.     

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.     

Miniature thermal cycling tests on aluminium alloy metal matrix composites

Abstract

A new miniaturised electrothermomechanical test system has been used to study the thermal cycling response of a number of aluminium alloy metal matrix composites reinforced with either Al₂0₃ or SiC particles. Tests were also performed on a monolithic 2618 aluminium alloy for comparison. The system showed good test discrimination between the different materials for both constant load-constant temperature (creep) tests and constant load-temperature cycling (50–200°C) tests. The system was also used to compare the yield behaviour at 200°C, and the thermal expansion and thermal diffusivity of several of the materials.     

Fabrication of fibre composites using an aluminium superplastic alloy as matrix

An aluminium superplastic alloy has been used as the matrix for a variety of fibre reinforcements. It is shown that, by hot pressing in the superplastic regime of the alloy, a number of different reinforcements can be incorporated into the matrix. Tensile tests on composites with up to 25 vol % of reinforcement showed good agreement with the rule of mixtures.     

Dry sliding wear of TiB2 particle reinforced aluminium alloy composites

Abstract

Al–4 wt-%Cu alloy and composites reinforced with 10 and 20 vol.-% of TiB₂ particles were prepared by powder metallurgy followed by hot isostatic pressing. The dry sliding wear behaviour of specimens of these materials was investigated. Pin-on-disc measurements showed that the wear resistance of Al–4Cu alloy can be improved dramatically by the addition of 20 vol.-%TiB₂ particles. This was due to the high hardness of the TiB₂ particles, and to strong particle–matrix bonding. The wear data were found to correlate with SEM observations.     

Sliding wear behaviour of SiC particle reinforced 2024 aluminium alloy composites

Abstract

Sliding wear tests on SiC particle reinforced 2024 aluminium alloy composites fabricated by a powder metallurgy technique were carried out, and the effects of SiC particle content, size, and the wear load on the wear properties of the composites were systematically investigated. It was found that the wear resistance of the composites was about two orders of magnitude superior to that of the unreinforced matrix alloy, and increased with increasing SiC particle content and size. Under the conditions of sliding wear used, the effect of SiC particle size on the wear resistance was more significant than that of particle content.

MST/3161