Investigation into micro machinability of Mg based metal matrix compostites (MMCs) reinforced with nanoparticles

PhD ThesisAs composite materials with combination of low weight and high engineering strength, traditional metal matrix composites (MMCs) with micro-sized reinforcement (micro-MMCs) have been utilized in numerous area such as aerospace, automobile, medical and advanced weapon systems in the past two decades. With the development of composite materials, metal matrix composites reinforced with small volume fraction of nano-sized reinforcements (nanoMMCs) exhibits an equivalent and even better properties than that reinforced with large volume of micro-sized reinforcement and thus receive increasing attention from academia and industries. MMCs components are typically fabricated in near net shape process such as casting. But micro machining processes are indispensable in order to meet the increasing demands on the component with high dimensional accuracy and complex shapes. However, the enhanced mechanical properties of MMCs and tool-like hardness of reinforced particles bring challenges to machining process. The deteriorative machined surface finish and excessive tool wear have been recognised as the main obstacles during machining of MMCs due to their heterogeneous and abrasive nature. In this research, the detailed material removal mechanism of nano-MMCs in terms of micro machinability, micro tool wear and simulated material removal process with finite element analysis (FEA) is investigated. The systematic experimental studies on micro machining mechanism of magnesium-based MMCs reinforced with nanoparticles (Ti, TiB2, BN, ZnO) are conducted. The cutting force, burr formation, surface roughness and morphology are characterised to investigate the micro machinability under the effect of various machining parameters, particle volume fraction and matrix/reinforcement materials using design of experiment (DoE) and analysis of variance (ANVOA) methods. The micro structure changes of Mg-MMCs by addition of nanoparticles were taken into account. In addition, surface morphology and the minimum chip thickness is obtained and characterised with the aim of examining the specific cutting energy. A comprehensive investigation of tool wear mechanisms in the micro milling of Mg-MMCs is conducted. The tool wear is characterised both quantitatively and qualitatively by observing tool wear patterns and analysing the effect of cutting parameters and tool coating on average flank wear, reduction in tool diameter, cutting forces, surface roughness, and burr formation. The main wear mechanisms at different machining conditions are determined. Finally, the tool wear phenomenon observed from experiments is explained by simulating the tool-particles interaction using finite element modelling, and hence new wear mechanisms are proposed for machining nano-MMCs. iv The two dimensional micromechanical finite element (FE) models are established to study the material removal mechanism of MMCs reinforced with micro-sized and nanoparticles in micro machining process with consideration of size effect. Two phases, namely particle and matrix are modelled in FE cutting models. Particle fracture properties are involved in micro-sized particles to study the fracture behaviours. The cutting force, tool-particles interaction, particle fracture behaviours, stress/strain distribution, chip formation process and surface morphology are investigated in the FE models. The surface defect generation mechanism is studied in details by developing the additional three dimensional (3D) FE models in machining micro-MMCs. Moreover, the cutting mechanism comparison between machining nano-MMCs and microMMCs is conducted to investigate the effect of significant particle size reduction from micro to nano-scale. The model validation is carried out by studying the chip morphology, cutting force, surface morphology obtained from machining experiments and good agreements are found with the simulation results