Opportunities for new graphitic aluminium metal matrix composite

Abstract

Nickel coated graphite particles have been incorporated into aluminium with a second particulate phase to produce graphitic aluminium metal matrix composites (Gr A-Ni) with improved processing, wear, and scuffing resistance. Excellent wear behaviour is provided by a combination of solid lubrication by graphite as well as high temperature strengthening of the matrix alloy by nickel present as Al₃ Ni intermetallics. Applications being developed include cylinder liners, pistons, connecting rods, various types of brakes, air diffusers and bushings. Neutral buoyancy of two particles, one of which is lighter and the other heavier than the aluminium matrix alloy, makes this a readily sand and die castable material. The presence of graphite and Al₃Ni intermetallics reduces the amount of ceramic particulate required to achieve the desired wear properties, with resulting improved machinability. The composition of the material can be tailored to the application. All these factors influence the finished part cost.     

Transient liquid phase bonding of 2124 aluminium metal matrix composite

Abstract

The transient liquid phase (TLP) bonding of particle reinforced aluminium metal matrix composites (MMCs) using copper interlayers often results in the segregation of SiC particles to the bond region, and this has the effect of producing bonds with poor mechanical strengths. In this preliminary study, the TLP bonding of a 2124 aluminium alloy MMC is investigated using nickel interlayers, and the initial results show that good bonds are produced with no effect on the SiC dispersion in the matrix. The absence of segregation is attributed to the high diffusivity of the nickel in the aluminium MMC, which produces rapid isothermal solidification at the bonding temperature. Bond shear tests show that near parent metal strengths are possible when thin nickel interlayers are used, and failure occurs at the MMC/bond interface. When thick interlayers are used, failure is predominately through the centre of the bondline.     

Quasi-static and dynamic compression behaviour of an FPTM alumina-reinforced aluminium metal matrix composite

An aluminium metal matrix composite reinforced with continuous unidirectional -alumina fibres has been compression tested at quasi-static and dynamic strain rates. In the transverse direction, the composite showed increasing flow stress (at 5% strain) and maximum stress within the studied strain rates, 10^–3–3 × 10³ s^–1. In the longitudinal direction, the maximum stress of the composite increased similarly with increasing strain rates within the range 10^–5–7 × 10² s^–1. It is shown that, if brooming of the sample ends can be suppressed, the failure stress of the composite in longitudinal compression increases significantly. Metallographic observations reveal the typical modes of damage initiation in the composite.     

界面对颗粒增强金属基复合材料强化性能的影响

为提高颗粒增强金属基复合材料的力学性能,采用基于微观组织的胞元模型建模方法,并利用有限元软件ABAQUS着重分析了界面层厚度以及界面层强度对复合材料性能的影响,通过对复合材料中各组成部分的应力、应变云图的获取,形象地说明了各部分的变形规律.研究结果表明,在弱界面层下,随着界面层厚度的增加,复合材料的强化效果并不显著,而在强界面层下,随着界面层厚度的增加,强化效果非常明显;就界面层强度来说,界面越强,所表现出的强化效果就越明显,但当界面层强度比基体大得多时,随着界面层强度的增加,虽然复合材料的强化呈递增趋势,但是递增的幅度已逐渐降低.     

Recycling of aluminium based metal matrix composite using disintegrated melt deposition technique

Abstract

In the present study, the feasibility of recycling a silicon carbide particulate reinforced aluminium composite was investigated. The composite was synthesised and recycled using an innovative disintegrated melt deposition tech nique. Microstructural characterisation studies revealed a marginal decrease in porosity, reinforcement content and size, no change in reinforcement distribution pattern, and improved interfacial integrity between matrix and reinforcement following recycling. Microhardness measurements revealed an increase in the hardness of the interfacial region in the recycled specimens. Results of physical and mechanical property characterisation revealed an increase in elastic modulus, 0·2% yield strength, ultimate tensile strength, and ductility, and a reduction in the coefficient of thermal expansion of the recycled specimens when compared with the parent composite. These properties were rationalised in terms of the microstructural characteristics associated with the disintegrated melt deposited composite specimens. Particular emphasis is placed on the study of the effect of recycling on the microstructure and properties of the composite.     

Synthesis of nanostructured aluminium matrix composite (AMC) through machining

In recent years, the nanostructured materials played a vital role in materials science engineering. Because of a reduction in the characteristic length such as the grain size or the cluster of molecular size, the normal properties of materials are drastically changed. By large strain plastic deformation of materials the grain sizes are refined in the range of submicrometer level. In this research Aluminium Metal Matrix composites (AMC) casting consisting of 85%Vol. Aluminium alloy (LM25) and 15%Vol. Silicon carbide (SiC) particle has been prepared through stir casting. A preliminary study has been carried out in oblique cutting conditions so as to obtain a high strain rate. The casted AMC was machined using coated tungsten carbide cutting tools and the collected chips were characterized using SEM and XRD. The result shows that machined chip microstructures are refined in the submicrometer level due to large strain deformation imposed by the cutting tool. Crystalline sizes and microhardness values have also been studied.     

Laser beam processing of a SiC particulate reinforced 6061 aluminium metal matrix composite

SiC particulate reinforced 6061 Al metal matrix composites were laser beam cut using a 3kW continuous wave CO2 laser. The influence of laser processing parameters such as cutting speed, laser power, and shielding gas on the quality of the cuts were investigated. Optical microscopy, scanning electron microscopy and X-ray diffraction were used to analyse the laser treated zone. Experimental results show that 6061 Al metal matrix composites can cut be successfully using laser. A number of Al4C3/Al4SiC4 plates were formed in the heat affected zones due to a chemical reaction between Si and Al that occurred during the laser processing. This revised version was published online in November 2006 with corrections to the Cover Date.     

Microstructural characterization of a silicon carbide whisker reinforced 2124 aluminium metal matrix composite

The microstructure of a silicon carbide whisker (SiC_w) reinforced 2124 aluminium metal matrix composite was characterized using scanning transmission electron microscopy (STEM). The SiC whiskers ranged in length from approximately 2 to 10 µm, and demonstrated good bonding to the aluminium matrix. In a few cases, the interface between SiC whiskers and the aluminium matrix exhibited wavy characteristics. The size of subgrains in the aluminium matrix was found to be dependent upon that of SiC whiskers. In addition, two types of intermetallic compounds were observed in the composite.     

Kinetics of transient liquid phase diffusion bonding process for joining aluminium metal matrix composite

Abstract

The mechanism and kinetics of the transient liquid phase diffusion bonding process in a 6061–15 wt-%SiCp composite at 570°C, 0·2 MPa, with 200 μm thick copper foil interlayer, has been investigated by microstructural characterisation of the bond region using optical microscopy, scanning electron microscopy and electron probe microanalysis. The kinetics of isothermal solidification, representing the displacement of the solid/liquid interface y (in micrometres) as a function of time t (in seconds), followed a power law relationship y?=?157t^0·07. According to this kinetic equation, the effective diffusivity of copper in the composite system was found to be ～10⁶ times higher than the lattice diffusivity, indicating the dominance of short circuit diffusion through the defect rich particle/matrix interface.     

Wear behaviour of an aluminium matrix composite

The wear behaviour of a composite material consisting in AS12UNG alloy reinforced with 15% short fibres of alumina has been studied. The material composition and the wear test conditions were defined in order to evaluate the potential performance of automotive pistons produced with such composite composition. As initially expected, the results indicate that an increase in the sliding velocity lead to higher wear rates in the stationary stages, and higher applied loads also induced acceleration in the wear process. Also, reciprocating sliding movement is clearly more damaging than the circular. However, results have shown that wear rates at 150 °C are lower than those recorded at room temperature representing a promising result for the use of this material in components that operate in this condition. This advantageous behaviour is lost at temperatures near to 300 °C, when a marked increase in the wear rate and a signification contribution of adhesive wear were observed.     

Wear behaviour of aluminium matrix composite materials

Wear behaviour of aluminium matrix composites is characterized by the dry spindle wear test under various conditions (volume fractions of reinforcements, sliding distances and speeds). Wear resistance of composites is improved due to the presence of reinforcements, but no noticeable improvements are observed in the wear resistance with more than 20% addition of reinforcements. To analyse wear mechanisms, wear surfaces are examined by scanning electron microscopy (SEM). The major wear mechanisms of discontinuous metal matrix composites (MMC)s are strongly dependent on sliding speeds. Dominant mechanism is the adhesive-abrasive wear at low and intermediate sliding speeds, and melt wear at high sliding speeds. Weight loss is linearly increased with the sliding distance. The effect of reinforcements' orientations on wear behaviours is also discussed.     

Thermal and grinding induced residual stresses in a silicon carbide particle-reinforced aluminium metal matrix composite

The residual stresses in a silicon carbide particle-reinforced aluminium (SiC_p/Al) metal matrix composite (MMC) were measured using the X-ray diffraction method. The thermal residual stresses induced by annealing were found to be hydrostatic tension for the Al matrix and hydrostatic compression for the SiC reinforcement. After grinding treatment, the force equilibrium between these hydrostatic stresses was disturbed and compressive stresses were measured in both constituents. The effect of grinding extended into the bulk, and depth profiles of the residual stresses in both constituents were obtained by layer removal. The behaviour exhibited in these depth profiles is explained and their usefulness is indicated.     

Machining of aluminium based metal matrix composites

Although metal matrix composites (MMCs) are generally regarded as extremely difficult to machine, it is also acknowledged that their machining behaviour is not fully understood. The work reviewed here confirms this widely held view but also suggests that the machinability of these materials can be improved by appropriate selection of the reinforcing phase, its volume fraction, size, and morphology as well as the composition and hardness of the matrix material. Cemented carbide tools can be used to machine some of the less abrasive materials at slow speeds but if higher production rates are required or the more abrasive materials are to be machined, polycrystalline diamond tooling is required.     

氮掺杂石墨烯的制备及其电催化氧气还原性能

通过改良Hummers法制备氧化石墨(Graphite oxide,GO),采用爆炸辅助还原法将GO还原剥离并原位掺杂得到氮掺杂石墨烯(Nitrogen-doped graphene,N-RGO)。采用TEM、SEM、FI-IR、XPS、XRD及Raman等分析手段对N-RGO的形貌、组成以及结构进行了表征,利用旋转环盘电极技术测试了其电催化氧气还原活性。TEM和SEM结果表明,爆炸条件下GO被很好地剥离开来,得到只有几层厚度的石墨烯;FI-IR及XPS结果表明,GO中大部分含氧官能团被脱除,C/O原子比达到26.2,是目前所得GO还原程度非常高的方法之一,且氮元素成功掺杂进石墨烯晶格中,掺杂氮的原子质量分数约为2.11%;电化学测试结果显示,氧气还原的极限扩散电流由非氮掺杂石墨烯(Reduced graphene oxide,RGO)的0.24mA提高到N-RGO的0.49 mA,尽管爆炸辅助还原得到的RGO对氧气还原也显示出较好的催化活性,但掺杂之后的N-RGO具有更高的催化活性。     

A low cycle fatigue model of a short-fibre reinforced 6061 aluminium alloy metal matrix composite

A low cycle fatigue model has been developed to predict the fatigue life of both the unreinforced aluminium alloy and the short-fibre reinforced aluminium alloy metal-matrix composites based solely on crack propagation from microstructural features. In this approach a crack is assumed to initiate and grow from a microstructural feature on the first cycle. The model assumes that there is a fatigue-damaged zone ahead of the crack tip within which the actual degradation of the material takes place. The low-cycle fatigue crack growth and the condition for failure are controlled by the amount of cyclic plasticity generated within the fatigue-damaged zone ahead of the crack tip and by the ability of the short fibres to constrain this cyclic plasticity. The fatigue crack growth rate is directly correlated to the range of crack-tip opening displacement. The empirical Coffin–Manson and Basquin laws have been derived theoretically and applied to compare with total-strain controlled low-cycle fatigue life data obtained on the unreinforced 6061 aluminium alloy at 25 °C and on the aluminium alloy AA6061 matrix reinforced with Al₂O₃ Saffil short-fibres of a volume fraction of 20 vol.% and test temperatures from −100 to 150 °C. The proposed model can give predicted fatigue lives in good agreement with the experimental total-strain controlled fatigue data at both high strain low-cycle fatigue and low strain high-cycle fatigue regime. It is remarkable that the addition of high-strength Al₂O₃ fibres in the 6061 aluminium alloy matrix will not only strengthen the microstructure of the 6061 aluminium alloy, but also channel deformation at the tip of a crack into the matrix regions between the fibres and therefore constrain the plastic deformation in the matrix. The overall expected effect is therefore the reduction of the fatigue ductility.     

Crack and wear behavior of SiC particulate reinforced aluminium based metal matrix composite fabricated by direct metal laser sintering process

In this investigation, crack density and wear performance of SiC particulate (SiCp) reinforced Al-based metal matrix composite (Al-MMC) fabricated by direct metal laser sintering (DMLS) process have been studied. Mainly, size and volume fraction of SiCp have been varied to analyze the crack and wear behavior of the composite. The study has suggested that crack density increases significantly after 15 volume percentage (vol.%) of SiCp. The paper has also suggested that when size (mesh) of reinforcement increases, wear resistance of the composite drops. Three hundred mesh of SiCp offers better wear resistance; above 300 mesh the specific wear rate increases significantly. Similarly, there has been no improvement of wear resistance after 20 vol.% of reinforcement. The scanning electron micrographs of the worn surfaces have revealed that during the wear test SiCp fragments into small pieces which act as abrasives to result in abrasive wear in the specimen.