L. Looney

Metal matrix composites: production by the stir casting method

Abstract Combining high specific strength with good corrosion resistance, metal matrix composites (MMCs) are materials that are attractive for a large range of engineering applications. Given the factors of reinforcement type, form, and quantity, which can be varied, in addition to matrix characteristics, the composites have a huge potential for being tailored for particular applications. One factor that, to date, has restricted the widespread use of MMCs has been their relatively high cost. This is mostly related to the expensive processing techniques used currently to produce high quality composites. In this paper, the relatively low cost stir casting technique is evaluated for use in the production of silicon carbide/aluminium alloy MMCs. The technical difficulties associated with attaining a uniform distribution of reinforcement, good wettability between substances, and a low porosity material are presented and discussed.

Particle distribution in cast metal matrix composites—Part II

Abstract In order to achieve a good distribution of reinforcement particles in a cast metal matrix composite (MMC), the stirring action must be efficient enough to disperse the particles in a homogeneous way. In normal practice stirring takes place in a closed vessel or crucible, where efficiency cannot be seen, and simulation methods are required to inform experimental research. For this research finite element analysis, employing a specialised computational fluid dynamics package, is used to simulate the fluid flow, and thus dispersion of reinforcement material in a molten matrix alloy during stirring. The emphasis is on investigating optimum stirring conditions in order to achieve effective flow patterns to disperse the solid particles in the melt, without breaking the surface layer of the melt. The simulation shows that the stirring parameters such as stirring speed, and impeller position in the crucible have a significant effect on the flow behaviour of the fluid. These parameters interact, with various combinations generating suitable conditions for composite mixing. The model was validated using a visualisation experiment which indicates that, despite some limitations arising from the simplification of the physical situation, the model is a useful tool in specifying process parameter in the production of MMCs.

The wettability of SiC particles by molten aluminium alloy

Abstract A critical step in the processing of cast, particle reinforced, metal matrix composites (MMCs) is the incorporation of the ceramic particles into the molten matrix alloy. Therefore, in a foundry MMC fabrication method, wettability of the reinforcement particles by the matrix alloy is one aspect of the process that must be optimised. In general, the reinforcement ceramic particles are very difficult to wet by a liquid metal. Information about this phenomenon is also difficult to obtain as it is widely dispersed throughout the literature. This paper presents a review of available information on factors which contribute to poor wetting between ceramic phases, and liquid metals. It particularly focuses on aluminium, a common MMC matrix material. Research on methods which can be used to improve this wettability is also detailed. This paper aims to bridge an existing information gap between the fundamentals of the wetting of solid particles, and the practice of the preparation of cast MMCs.

The enhancement of wettability of SiC particles in cast aluminium matrix composites

Abstract Many methods have been proposed to overcome the problem of poor wettability between ceramic reinforcement particles and molten aluminium for metal matrix composite (MMC) production by casting. Some of these methods are expensive and complex, and a cheap and simple technique still to be found. In this paper, an innovative approach to fabricating cast MMCs is proposed. In order to study the enhancement of wettability of SiC particles by the matrix alloy A359, a casting rig was specially designed, and tests carried out using the A359 matrix alloy, SiC particles, and magnesium (as a wetting agent). Stirring of these mixtures was performed under several different conditions, and the effect of this stirring action on the wettability enhancement was studied. The percentage of particles entrapped in the resulting composite was used as a measure of wettability, and plotted against cooling time and volume percentage of SiC particles. The use of clean SiC particles, magnesium as a wetting agent, and stirring continuously while the MMC slurry is solidifying were found to promote wettability of SiC with A359 matrix alloy. Decreasing this solidification time was also found to improve the wettability, whereas increasing the volume fraction of SiC particles present will give the opposite effect.

Selective laser sintering of hydroxyapatite/poly-ε-caprolactone scaffolds☆

Selective laser sintering (SLS) enables the fabrication of complex geometries with the intricate and controllable internal architecture required in the field of tissue engineering. In this study hydroxyapatite and poly-epsilon-caprolactone, considered suitable for hard tissue engineering purposes, were used in a weight ratio of 30:70. The quality of the fabricated parts is influenced by various process parameters. Among them Four parameters, namely laser fill power, outline laser power, scan spacing and part orientation, were identified as important. These parameters were investigated according to a central composite design and a model of the effects of these parameters on the accuracy and mechanical properties of the fabricated parts was developed. The dimensions of the fabricated parts were strongly dependent on the manufacturing direction and scan spacing. Repeatability analysis shows that the fabricated features can be well reproduced. However, there were deviations from the nominal dimensions, with the features being larger than those designed. The compressive modulus and yield strength of the fabricated microstructures with a designed relative density of 0.33 varied between 0.6 and 2.3 and 0.1 and 0.6 MPa, respectively. The mechanical behavior was strongly dependent on the manufacturing direction.

SIMULATION OF THE STIR CASTING PROCESS

Abstract Non-homogeneous particle distribution is one of the greatest problems in casting metal matrix composites (MMCs). To optimise some of the parameters for uniform particle distribution for batch compocasting the present simulation studies were conducted. The simulation involves visualisation experiments. In the visualisation experiments liquid and semisolid aluminium are replaced by other fluids with similar characteristics. SiC reinforcement particulate similar to that used in aluminium MMCs was used in the simulation fluid mixtures. Scaled-up stirring experiments were carried out in a transparent crucible with the percentage of reinforcement material being varied. Optimum conditions for photographing flow patterns were established. The dependence of the photography conditions (shutter speed, aperture control, lighting), particles dispersion and settling times and vortex height on stirrer geometry and speed was found. Results are discussed in terms of their applicability to MMC production.

HVOF system definition to maximise the thickness of formed components

Abstract The present study aims to establish the potential of producing various hard metal industrial components using the high-velocity oxy-fuel (HVOF) thermal spray process, rather than by the sintering techniques currently used. The HVOF technique is generally used as a coating process, but previously has been employed to spray-form thin tungsten carbide–cobalt (WC–Co) components, flat and cylindrical in shape [Proceedings of Advances in Powder Metallurgy and Particulate Materials, Washington DC 5 (1996) 18.41]. The present work is focused on maximising the thickness of such components. The difficulty in producing thick-formed components using this technique arises from residual stress in the sprayed material. In coatings this stress leads to adhesion loss and interlaminar debonding and, in formed components, cracking or buckling. Residual stress that arises during spraying can be reduced by limiting the rise and fluctuation of the die temperature, and this was carried out in the present study using a carbon dioxide cooling system. This enabled continuous deposition at a steady temperature, and led to the successful production of thick-formed WC–Co components.

Deposition and Characterization of HVOF Thermal Sprayed Functionally Graded Coatings Deposited onto a Lightweight Material

There is a significant interest in lightweight materials (like aluminum, magnesium, titanium, and so on) containing a wear resistance coating, in such industries as the automotive industry, to replace heavy components with lighter parts in order to decrease vehicle weight and increase fuel efficiency. Functionally graded coatings, in which the composition, microstructure, and/or properties vary gradually from the bond coat to the top coat, may be applied to lightweight materials, not only to decrease weight, but also to enhance components mechanical properties by ensuring gradual microstructural (changes) together with lower residual stress. In the current work, aluminum/tool-steel functionally graded coatings were deposited onto lightweight aluminum substrates. The graded coatings were then characterized in terms of residual stress and hardness. Results show that residual stress increased with an increase in deposition thickness and a decrease in number of layers. However, the hardness also increased with an increase in deposition thickness and decrease in number of layers. Therefore, an engineer must compromise between the hardness and stress values while designing a functionally graded coating-substrate system.

The effect of high pressure hydrogen on the creep fracture of notched ferritic-steel components

Abstract In order to study hydrogen attack under near-service conditions, an experimental technique for examining the multiaxial behaviour of model-sized tubular components was modified to allow exposure to high pressure hydrogen gas, at a temperature in the creep range for the material chosen. Components with both axial and circumferential internal notches were loaded using the internal pressure of hydrogen, at a temperature of 600°C. The material under study was a low-alloy ferritic steel, 2 1 4 Cr–Mo, used widely in pressure vessels and piping in the petro-refinery industry, where high pressure hydrogen is encountered. Analysis of the results takes the form of metallographic investigation, correlation of rupture life with base-line data, and characterisation of damage evolution. The distribution of inter-granular voids and cavities observed by S.E.M. confirms the stress-assisted nature of hydrogen-induced degradation. The hydrogen attack caused a dramatic change in the multiaxial creep deformation mechanism from von Mises control to maximum principal stress, a factor very important in the design of plant components. The effect of this change on the reference-stress calculations for both notch types is examined in terms of the stress rupture time correlation with base-line data. C* values are compared with standard data for the material. Factors incorporated in the method of C* calculation, such as the reference stress equations and Norton constants, which may influence the value of the results are considered.

Effect of vacuum mixing and manual pressurisation on residual strains in polymethyl methacrylate bone cement mantles

Residual stresses associated with the polymerisation and subsequent cooling of PMMA bone cement are suspected to be responsible for the initiation of cement cracks following total hip arthroplasty (THA). A previous study has measured temperatures and strains developed in a twin cylinder construct representing the implant and femur, during and after bone cement polymerisation, for hand mixed PMMA bone cement. In the present study, the effect of manual pressurisation of the cement mantle is experimentally investigated through stem and femur strain measurements which are directly attributable to cement stresses. Manual pressurisation of the curing cement mass had a significant effect on the measured residual strains. The mean representative femur hoop residual strain was reduced from a compressive strain of -1802 (± 449.8) μe to tensile strain of 49 (± 374.3) μe, while the mean axial residual strain reduced from -649.2 (± 196.6) μe to -271.0 (± 173.6) μe. This study demonstrates that a reduction in net residual stress may be achieved by pressurisation of the bone cement mantle.