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Title: Processing and characterisation of aluminium metal matrix composites
Authors: Rajan, T P D
Pai, B C
Pillai, R M
Keywords: Metal matrix composites
HMMC
Hybrid systems
Solidification
Hybrid composites
Composite processing
Fly ash composites
Mono composites
Aluminosilicate fibres
Aluminium matrix composites
Heat treatment studies
MMC
Issue Date: Jun-2002
Publisher: Regional Research Laboratory(CSIR), Thiruvananthapuram.
Citation: Ph.d Thesis, University of Kerala, Regional Research Laboratory(CSIR), Thiruvananthapuram, India; xx+ 235 pp
Abstract: Metal Matrix Composites(MMC) are an important class of high performance advanced materials with potential engineering applications especially in the areas of aerospace, defence and automotive industries. The advantages of MMC over traditional alloys are their high specific strength and modulus, increased wear resistance, high temperature strength, low coefficient of thermal expansion and good damping capacity. The metal matrix composites currently used are monocomposites, wherein only one type of reinforcement is used. Hybrid metal matrix composites are the new type of MMC where more than one type of reinforcements are used. These materials provide better properties compared to conventional mono composites. Among the various matrix material available, aluminium and its alloys are widely used as the matrix in the fabrication of MMC. Discontinuosly reinforced aluminium matrix composites are most commonly studied due to their isotropic properties, wide application, cost effectiveness, simple fabrication techniques and feasibility of large scale production. Stir casting technique is the simplest and economical processing route. The present investigation is on the processing and characterisation of mono and hybrid composites based on cast Al-7Si-0.35Mg [Al(356)] matrix alloy reinforced with silicon carbide particles, graphite particles, carbon short fibres, aluminosilicate short fibres and flyash particles. The composites fabricated are evaluated with respect to structural, interfacial, solidification physical and mechanical characteristics. Correlations are made on the effect of various processing parameters on the structure and properties of the composites. The utilization of the indigenously available low cost aluminosilicate fibres and the fine spherical flaysh particles as reinforcements for the fabrication of aluminium matrix composites are assessed. The study also compares the effect of reinforcement with different morphology such as carbon in the form of graphite particle and short fibre, and aluminosilicate in the form of short fibre and fly ash particle on the processing, structure and properties. The structure of the thesis and details of the studies are given below. The Chapter 1 deals with the general introduction to MMC. The Chapter 2 deals with the extensive literature survey carried out on aluminium matrix composites, their processing, properties, interfacial and solidification characteristics and applications with respect to mono and hybrid composites. The Chapter 3 gives the details of the materials and the experimental methods used in the present investigation. Liquid metal stir casting technique is used in general, while compocasting and squeeze casting are adopted for processing specific systems of the composites. The structural characteristics of all the composites systems synthesised are evaluated by optical microscopy, SEM, EDS, AFM and XRD and solidification behaviour by Thermal analysis. The physical properties such as density and electrical conductivity are measured. The mechanical properties such as hardness by Brinell hardness tester and tensile and compression strengths using Instron Universal testing machine are evaluated. The Chapter 4 deals with the processing and characterisation of SiC and Graphite reinforced mono and hybrid aluminium matrix composites. The mixed mode of particle addition during hybrid composite synthesis results in better dispersion and distribution of particles. There is no remarkable interfacial reaction at SiC - matrix interface, where as there is formation of MgAl204 spinel due to the presence of SiO2 layer over SiC. Introduction of silicon carbide and graphite reinforcements into the Al(356) matrix alloy reduces the liquidus temperature. Incorporation of additional Mg to the composite melt has multifunctional behaviour such as promotion of ceramic particle wetting with the aluminium alloy matrix, better contact at metal/mould interface, there by enhancing heat transfer rate and precipitation hardening of the matrix. The compression strength of mono and hybrid composites is higher than those of the base alloy. Hybridization with synthetic graphite particles has resulted in lower compression strength than natural graphite particles. The Chapter 5 deals with the processing and characterisation of SiC and carbon short fibre reinforced mono and hybrid composites. The surface treatments of carbon fibre are discussed. Addition of as received carbon short fibre to the matrix alloy leads to agglomeration and rejection. The sodium silicate surface treatment is effective in deflocculation of the fibres and making them free flow during the addition into the matrix. Interfacial reaction occurs at the carbon fibre - matrix interface with the formation of aluminium carbide providing better wetting, but it degrades the fibre surface. This brittle reaction products increase the hardness of the hybrid composite compared to the monocomposite. Carbon short fibres are highly reactive compared to the graphite particle. The Chapter 6 deals with the processing and characterisation Of standard and zirconia grade alumino silicate short fibre reinforced aluminium matrix composites. Among the various surface treatments studied, the ultrasonic treatment in aqueous acidic soIution of pH 4-5 range has given better dispersion, and less agglomeration and porosity in the composite castings. All the major constituents of the fibre such as Al2O3, SiO2 and ZrO2 react with the matrix elements. MgAl2O4 spinel formation is observed in both the standard and zirconia grade fibres. The interfacial reaction depletes the Mg content in the matrix, thus reducing the amounts of Mg2Si available during precipitation hardening and resulting in lower hardness for the composites after aging. The addition of aluminosilicate fibre reduces the liquidus temperature of the matrix and peak heat flux values. The Chapter 7 discusses the processing and characterisation of cenosphere or spherical type fly ash particle reinforced aluminium matrix composites. Addition of 5wt% of 13µm (average size) fly ash particles in as received condition leads to their agglomeration in 356 Al alloy composites processed by liquid metal processing technique. Surface treatment given to the fly ash particles has deflocculated them leading to better dispersion of individual particles in the composite. Further, improvement in the dispersion and distribution has been achieved by modified compocasting ie., by reheating the semisolid/compocast composite slurry just above the Iiquidus and then casting by permanent mould or squeeze casting. The interfacial reaction between the fly ash and the matrix alloy is higher with liquid metal processed composites than semisolid processed ones. In addition to giving the conclusion drawn under each chapter (4 to 7) separately, Chapter 8 summarises all of them together and also gives the significant contributions of the present investigation leading to the areas for the future work.
URI: http://hdl.handle.net/123456789/165
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