Production of cast aluminium-graphite particle composites using a pellet method

Copper- and nickel-coated graphite particles can be successfully introduced into aluminium-base alloy melts as pellets to produce cast aluminium-graphite particle composites. The pellets were made by pressing mixtures of nickel- or copper-coated graphite particles and aluminium powders together at pressures varying between 2 and 20 kg mm^–2. These pellets were dispersed in aluminium alloy melts by plunging and holding them in the melts using a refractory coated mild steel cone, until the pellets disintegrated and the powders were dispersed. The optimum pressure for the preparation of pellets was 2 to 5 kg mm^–2 and the optimum size and percentage of aluminium powder were 400 to 1000m and 35 wt% respectively. Under optimum conditions the recovery of the graphite particles in the castings was as high as 96%, these particles being pushed into the last freezing interdendritic regions. The tensile strength and the hardness of the graphite aluminium alloys made using the pellet method are comparable to those of similar composites made using gas injection or the vortex method. The pellet method however has the advantage of greater reproducibility and flexibility. Dispersion of graphite particles in the matrix of cast aluminium alloys using the pellet method increases their resistance to wear.Formerly with Indian Institute of Science, Bangalore, India.     

Effective properties of biopolymer composites: A three-phase finite element model

The effective Young’s modulus of starch–zein biopolymer composite is studied using a finite element model. This model handles the fact that starch–zein interface is not perfect in the sense that elasticity properties across the interface are altered. The motivation here is to predict the right modulus of elasticity near the interface for any zein content. In that way, the finite element model requires zein, starch and interphase properties to be implemented for the computation of the composite Young’s modulus as a function of zein content. The main variables are the interphase thickness and modulus. The comparison between the predicted and experimental results shows, firstly, that the effective composite properties are not correctly estimated using standard models with perfect interface hypothesis. Secondly, the three-phase model is able to suggest optimal interphase parameters in order to fit the experimental data obtained using three-point bending test. Finally, the optimal interphase parameters (thickness and modulus) vary as a function of zein content.     

Highly conductive graphene-based segregated composites prepared by particle templating

We use capillary-driven particle level templating and hot melt pressing to disperse few-layer graphene flakes within a polystyrene matrix to enhance the electrical conductivity of the polymer. The conducting pathways provided by the graphene located at the particle surfaces through contact of the bounding surfaces allow percolation at a loading of less than 0.01 % by volume. This method of distributing graphene within a matrix overcomes the need to disperse the sheet-like conducting fillers isotropically within the polymer, and can be scaled up easily.     

Identification of the mechanical behaviour of biopolymer composites using multistart optimisation technique

This paper aims at identifying the mechanical behaviour of starch–zein composites as a function of zein content using a novel optimisation technique. Starting from bending experiments, force–deflection response is used to derive adequate mechanical parameters representing the elastic–plastic behaviour of the studied material. For such a purpose, a finite element model is developed accounting for a simple hardening rule, namely isotropic hardening model. A deterministic optimisation strategy is implemented to provide rapid matching between parameters of the constitutive law and the observed behaviour. Results are discussed based on the robustness of the numerical approach and predicted tendencies with regards to the role of zein content.     

Production of copper-matrix composites by in situ processing

Reaction synthesis routes were studied as a means of production of copper-matrix composites. Routes for providing fine and uniform dispersions of TiC, TiB₂ and WC in a copper-rich matrix were identified. The reasons for achieving good dispersions of TiC and TiB₂ in the matrix were explained. The merits of each method were assessed in terms of the potential advantages offered.     

Cold roll bonding of multi-layered bi-metal laminate composites

Multi-layered laminate composites of dissimilar metals have assumed importance industrially. Cold roll bonding can produce multi-layered sheet composites. Study of the effect of rolling and material variables on the bonding characteristics needs to be studied in order to predict the optimum bonding conditions and the final composition of the laminate sheets. In this work, cold roll bonding of multi-layered bimetals has been modeled using the slab method. The effect of anisotropy has been included. Effects of different process and material variables are analyzed. Novel experiments were performed on multilayered Ti–Al system and the numerical results from the model were compared with the experimental results. A good agreement was observed between the model and experimental results.     

Hysteresis heating based induction bonding of thermoplastic composites

The bonding of polymer matrix composites using magnetic particulate susceptor materials for hysteresis induction heating is investigated in this study. Hysteresis heating is tailored through careful design of the microstructure of magnetic particulate polymer films. The bond strength of hysteresis-welded materials is comparable to that of autoclave-welded materials while offering an order of magnitude reduction in cycle time. The relative contribution of the intimate contact and healing mechanisms to fusion bonding process indicates that it is intimate contact controlled. The macroscopic failure modes of hysteresis bonded specimens include adhesive composite/film, cohesive film and cohesive composite. Inspection of the microscopic failure at the nickel particle/polymer interface in the film indicates quasi-brittle failure mode. The XPS peaks confirm nickel oxide in the form of NiO on the failure surface indicating adhesive failure at the particle/polymer interface. The area fraction of adhesive failure is found to increase with decreasing particle size and increasing particle volume fraction.     

颗粒增强铜基复合材料的选区激光烧结制备

利用选区激光烧结制备了亚微米WC-10%Co颗粒增强Cu基复合材料。利用X射线衍射仪、 扫描电镜及原子力显微镜表征了激光烧结试样的显微组织。结果显示: WC增强颗粒或部分熔化且圆滑化, 或完全熔化且原位析出; 与基体具有连续相容的冶金结合界面。 研究了工艺参数(激光功率、 扫描速率、 铺粉厚度)对烧结试样组织及性能的影响。结果表明, 增加激光功率能改善增强颗粒与基体的界面结合性能。激光扫描速率大于0.05m/s 时, 能提高增强颗粒分散均匀性。铺粉厚度降至0.30mm以下, 有利于提高烧结成形致密度。      

Aluminium alloy-solid lubricant talc particle composites

This paper describes the method of synthesizing cast aluminium alloy talc particulate composites and their mechanical and wear properties. Talc particles were characterized using X-ray diffraction, infrared spectroscopy and differential thermal analysis techniques. Composites with two Al-Si alloys (LM 13 and LM 6) as matrices were prepared by heating the molten alloys to 750° C and adding the preheated talc powder (–150 + 50 m size) after creating a vortex by mechanically stirring the melt. Simultaneous addition of 2 wt% Mg was found to facilitate the introduction and dispersion of talc particles in molten Al-Si alloys. Composites containing 2.8 wt % talc in LM 13 and 2 wt % talc in LM 6 have been prepared. Optical micrographs of composites revealed uniform distribution of talc particles. Hardness and tensile strength of LM 13+2.8% talc were 85 BHN and 126 MPa, respectively. After suitable heat treatment hardness and strength were increased to 125 BHN and 211 MPa respectively. Wear rates of LM 13+2.8 wt% talc and LM 6+2 wt % talc composites were found to be 22 to 30% less than the wear rates of corresponding base alloys without any dispersions.     

Poisson's ratio of hollow particle filled composites

The present study is focused on the experimental measurement of the Poisson's ratio of glass hollow particle filled composites called syntactic foams. The effect of hollow particle wall thickness and volume fraction on the Poisson's ratio of the composites is investigated. Results show that the Poisson's ratio of the composites is lower than that of the neat vinyl ester resin used as the matrix material. Despite being a fundamental elastic constant, a reliable value of the Poisson's ratio of the various types of composites is not readily available. These experimental results can be useful in benchmarking available theoretical models and guiding modeling efforts on functionally graded composites.     

Preparation of cast aluminium alloy-mica particle composites

The optimum conditions for producing cast aluminium alloy-mica particle composites, by stirring mica particles (40 to 120 m) in molten aluminium alloys (above their liquidus temperatures), followed by casting in permanent moulds, are described. Addition of magnesium either as pieces along with mica particles on the surface of the melts or as a previously added alloying element was found to be necessary to disperse appreciable quantities (1.5 to 2 wt.%) of mica particles in the melts and retain them as uniform dispersions in castings under the conditions of present investigation. These castings can be remelted and degassed with nitrogen at least once with the retention of about 80% mica particles in the castings. Electron probe micro-analysis of these cast composites showed that magnesium added to the surface of the melt along with mica has a tendency to segregate around the mica particles, apparently improving the dispersability for mica particles in liquid aluminium alloys.The mechanical properties of the aluminium alloy-mica particle composite decrease with an increase in mica content, however, even at 2.2% the composite has a tensile strength of 14.22 kg mm^–2 with 1.1% elongation, a compression strength of 42.61 kg mm^–2, and an impact strength of 0.30 kgm cm^–2. The properties are adequate for certain bearing applications, and the aluminium-mica composite bearings were found to run under boundary lubrication, semi-dry and dry friction conditions whereas the matrix alloy (without mica) bearings seized or showed stick slip under the same conditions.     

Failures analysis of particle reinforced metal matrix composites by microstructure based models

This paper discusses the methodology of microstructure based elastic–plastic finite element analysis of particle reinforced metal matrix composites. This model is used to predict the failure of two dimensional microstructure models under tensile loading conditions. A literature survey indicates that the major failure mechanism of particle reinforced metal matrix composites such as particle fracture, interfaces decohesion and matrix yielding is mainly dominated by the distribution of particles in the matrix. Hence, analyses were carried out on the microstructure of random and clustered particles to determine its effect on strength and failure mechanisms. The finite element analysis models were generated in ANSYS, using scanning electron microscope images. The percentage of major failures and stress–strain responses were predicted numerically for each microstructure. It is evident from the analysis that the clustering nature of particles in the matrix dominates the failure modes of particle reinforced metal matrix composites.     

Control of percolation curve by filler particle shape in Cu- SBR composites

Abstracts are not published in this journal This revised version was published online in November 2006 with corrections to the Cover Date.     

Fracture toughness of Kevlar-epoxy composites with controlled interfacial bonding

The fracture toughness of Kevlar-epoxy resin composites with intermittent fibre bonding of a silicone vacuum fluid (SVF-200) and a polyurethane varnish (Estapol 7008) have been studied over the temperature range –60 to 40° C and strain rates 0.03 to 5000 min^–1. Whilst both coating materials give similar tensile strengths their effects on toughness are very different. As far as toughening is concerned Estapol 7008 is more effective than SVF-200. The toughening effect increases with increasing intermittent lengths of the Estapol-7008 coating, i.e. coating parameterC, increasing temperature and decreasing strain rate. At low strain rates and high temperatures, forC=1, the toughness increase is some 200 to 300% compared to the uncoated composites. Some initial work has also been conducted for hygrothermally aged uncoated and coated fibre composites. The SVF-200 coated composites do not show any toughness degradation compared to the dry control samples. However, both the uncoated and Estapol-7008 coated composites suffer some toughness loss. Even so, the toughness of the fully coated aged specimens is as good as the uncoated dry controls. A fracture analysis is presented which gives reasonable agreement between predicted fracture toughness values and experimental measurements. It is shown that fibre pull-out toughness and fibre fracture work are the main contributors to the total fracture toughness of these fibre composites; their relative significance being dependent on the type of coating material, the temperature and strain rate of testing.     

Production of SiC particulate reinforced aluminium composites by melt spinning

Melt spinning is successfully used for the preparation of a rapidly solidified SiC particle reinforced AlSi7Mg0.3 alloy. The composites are prepared by introducing SiC particles in a semi-solid matrix slurry (SiC volume fractions up to 0.15, particle size 10 or 20 m). Duralcan material (SiC volume fraction 0.20, particle size 12 m) was also used. After stirring in the semi-solid state the composites are heated above the liquidus temperature and subsequently melt-spun. Featureless, columnar and dendritic zones can be identified in the ribbons. A finer dendritic structure is found around the SiC particles. The SiC particles tend to segregate to the air side of the ribbons and the segregation effect is influenced by particle size and volume fraction. As interface velocities are higher than the critical velocities predicted by models on interface pushing, it is concluded that fluid flow in the melt puddle is responsible for the segregation effect.