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 mgrm) 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.

The UPAL process: a direct method of preparing cast aluminium alloy-graphite particle composites

A direct method of preparing cast aluminium alloy-graphite particle composites using uncoated graphite particles is reported. The method consists of introducing and dispersing uncoated but suitably pretreated graphite particles in aluminium alloy melts, and casting the resulting composite melts in suitable permanent moulds. The optical pretreatment required for the dispersion of the uncoated graphite particles in aluminium alloy melts consists of heating the graphite particles to 400° C in air for 1 h just prior to their dispersion in the melts. The effects of alloying elements such as Si, Cu and Mg on the dispersability of pretreated graphite in molten aluminium have also been reported. It was found that additions of about 0.5% Mg or 5% Si significantly improve the dispersability of graphite particles in aluminium alloy melts as indicated by the high recoveries of graphite in the castings of these composites. It was also possible to disperse upto 3% graphite in LM 13 alloy melts and retain the graphite particles in a well distributed fashion in the castings using the pre-heat-treated graphite particles. The observations in this study have been related to the information presently available on wetting between graphite and molten aluminium in the presence of different elements and our own thermogravimetric analysis studies on graphite particles. Physical and mechanical properties of LM 13-3% graphite composite made using pre-heat-treated graphite powder, were found to be adequate for many applications, including pistons which have been successfully used in internal combustion engines.

Improved damping capacity and machinability of graphite particle-aluminum alloy composites

The damping capacity of cast graphitic aluminum alloy composites has been measured using a torsion pendulum at a constant strain amplitude. It was found that flake-graphite particles dispersed in the matrix of aluminum alloys increased the damping capacity; the improvement was greater, the higher the amount of graphite dispersed in the matrix. At sufficiently high graphite contents the damping capacity of graphitic aluminum composites approaches that of cast iron. The ratio between the damping capacity and the density of graphitic aluminum alloys is higher than cast iron, making them very attractive as light-weight, high-damping materials for possible aircraft applications. Machinability tests on graphite particle-aluminum composites, conducted at speeds of 315 sfm and 525 sfm, showed that the chip length decreased with the amount of graphite of a given size. When the size of graphite was decreased, at a given machining speed, the chip length decreased. Metallographic examination shows that graphite particles act as chip breakers, and are frequently sheared parallel to the plane of the

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 1000mgrm 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.

Effect of compatibilization on mechanical and thermal properties of polypropylene–soy flour composites

A new biobased composite was developed by adding soy flour (SF) to polypropylene (PP). This composite shows an enhanced tensile strength and modulus but decrease in elongation at break. The compatibilizer (coupling agent) appears to have a synergistic effect on tensile strength. The presence of the compatibilizer improves the dispersion of SF in the PP matrix. The addition of glycerol plasticizer to the composite improves the processability resulting in improved performance, as compared to composites without glycerol plasticizer. The optimal compatibilizer content appears to be 6%.

Synthesis, sintering and microstructural characterization of nanocrystalline Hydroxyapatite Composites

Nanocrystalline hydroxyapatite (HAp) exhibits better bioactivity and biocompatibility with enhanced mechanical properties compared to the microcrystalline counterpart. In the present work, nanocrystalline hydroxyapatite was synthesized by wet chemical method. Sintering was carried out with nanocrystalline alumina as additive, the content of alumina being varied from 10 to 30 wt% in the composite. For 20 and 30 wt % Al2O3, hydroxyapatite decomposed into tricalcium phosphate (TCP) above the sintering temperature of 1100 degrees C. The fracture toughness of nano HAp-nano Al2O3 composite is anisotropic in nature and reached a maximum value of 6.9 MPa m(1/2).

Dielectric properties of (100-x)Li2B4O7-x(Ba5Li2Ti2Nb8O30) glasses and glass nanocrystal composites

Transparent glasses of various compositions in the system (100 -x)(Li2B4O7)-x(Ba5Li2Ti2Nb8O30) (5 <= x <= 20, in molar ratio) were fabricated by splat quenching technique. The glassy nature of the as-quenched samples was established by differential thermal analyses (DTA). X-ray powder diffraction studies confirmed the as-quenched glasses to be amorphous and the heat-treated to be nanocrystalline. Controlled heat-treatment of the as-quenched glasses at 500 degrees C for 8 h yielded nanocrystallites embedded in the glass matrix. High Resolution Transmission Electron Microscopy (HRTEM) of these samples established the size of the crystallites to be in the nano-range and confirmed the phase to be that of Ba5Li2Ti2Nb8O30 (BLTN) which was, initially, identified by X-ray powder diffraction. The frequency, temperature and compositional dependence of the dielectric constant and the electrical conductivity of the glasses and glass nanocrystal composites were investigated in the 100 Hz to 10 MHz frequency range. Electrical relaxations were analyzed using the electric modulus formalisms. The imaginary part of electric modulus spectra was modeled using an approximate solution of Kohlrausch-Williams-Watts relation. The frequency dependent electrical conductivity was rationalized using Jonscher's power law. The activation energy associated with the dc conductivity was ascribed to the motion of Li+ ions in the glass matrix. The activation energy associated with dielectric relaxation was almost equal to that of the dc conductivity, indicating that the same species took part in both the processes. (C) 2010 Elsevier B.V. All rights reserved.

Role of alumina in flexure of glass/epoxy composites

The influence of particulate additions of alumina on the flexural properties of glass-fabric/epoxy composites was studied. The additions improved translaminar flexural strength, while decreasing interlaminar strength. The translaminar bending modulus showed an increasing trend whereas its interlaminar value showed a decrease, up to additions of 3 vol%. The mechanisms of deformation and the fracture features have been discussed with the aid of scanning electron microscopy.

Mullite-zirconia composites

Mullite-zirconia composite powders were prepared by the combustion of an aqueous heterogeneous redox mixture consisting of Al(NO3)(3), Zr(NO3)(4)/ZrO(NO3)(2), silica fume and urea/diformyl hydrazine at 500 degrees C. X-ray diffraction data showed that a large amount of tetragonal zirconia existed in the composite powders in spite of high temperature calcination. Milled composite powders showed enhanced densification compared to the unmilled powders and the microstructure of the sintered (1600 degrees C) compacts showed the presence of spherical zirconia grains in intergranular positions along with elongated mullite grains.

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.

Study of fracture features in foam bearing glass epoxy composites subjected to repeated impacts

The paper reports the failure features observed in low mass repeatedly (pendulum) impacted glass epoxy composites with and without the mid section having either 2-layers or 3-layers of flexible foam. Features such as through width and inclined cracks as well as adhering of foam observed in the experiments are explained. The significance of the foam material in modifying the impact response of the composite is stressed.