Investigation on the multiscale multiphysics based approach to modelling and analysis of precision machining of metal matrix composites (MMCs) and its application perspectives

Abstract

Over the last two decades or so, metal matrix composites (MMCs) have been drawing the attention of the industry due to their potentials in fulfilling demands for high performance industrial materials, products and advanced engineering applications. On the other hand, the high precision machining is becoming one of the most effective methods for enabling these difficult-to-machine composites to be applied particularly in precision engineering. Therefore, in-depth scientific understanding of MMC precision machining is essential and much needed so as to fulfil the gap between fundamental issues in precision machining of MMCs and their industrial scale applications. This thesis focuses on development of a multiscale multiphysics based approach to investigating the machinability of particulate MMCs and the machining process optimisation. In order to investigate the surface generation in relation to the process variables, this PhD study covers the key fundamental issues including chip formation process, dynamic cutting force, cutting temperature partition and tool wear by means of combining modelling, simulation and experimental study. The chip formation mechanisms and the minimum chip thickness in precision machining of SiCp/Al and B4Cp/Al MMCs by using PCD tools are investigated through a holistic approach. Minimum chip thickness (MCT) value is firstly identified based on the modified mathematical model. The certain threshold of the uncut chip thickness, i.e. chips starting to form at this chip thickness point, is then established. The chip formation process including the matrix material breakage, particles fracture, debonding, sliding or removal and their interfacial interactions are further simulated using finite element analysis (FEA). The minimum chip thickness and chip formation simulations are evaluated and validated via well-designed experimental trials on a diamond turning machine. The chips and surface profiles formed under varied process variables in periodic material removals are inspected and measured in order to obtain a better understanding on MMC chip formation mechanisms. The improved dynamic cutting force model is developed based on the micro cutting mechanics involving the size effect, undeformed chip thickness effects and the influence of cutting parameters in the micro scale. Cutting process variables, particle form, size and volume fraction at different scales are taken into account in the modelling. The cutting force multiscale modelling is proposed to have a better understanding on the MMCs cutting mechanics and to predict the cutting force accurately. The cutting forces are modelled and analysed in three cutting regimes: elastic recovery zone, ploughing zone and shearing zone. A novel instantaneous chip thickness algorithm including real chip thickness and real tool trajectory is developed by taking account of the tool runout. Well-designed cutting trials are carried out under varied process variables to evaluate and validate the force model. In order to obtain the actual cutting forces accurately, transfer function technique is employed to compensate the measured cutting forces. The cutting force model is further applied to correlate the cutting tool wear and the prediction of the machined surface generation. Multiphysics coupled thermal-mechanical-tribological model and FE analysis are developed to investigate the cutting stress, cutting temperature, tool wear and their intrinsic relationships in MMCs precision machining process. Heat generation, heat transfer and cutting temperature partition in workpiece, chips and cutting tool are simulated. A modified tool wear rate model is proposed, tool wear characteristics, wear mechanisms and dominate tool wear are further investigated against the real machining process. Cutting tool wear is monitored and assessed offline after machining experiments. The experimental study on the machined surface generation is presented covering cutting force, tool wear, tool life, surface roughness and machining efficiency. Process optimisation is explored by considering the variation of cutting parameters, cutting tool conditions and workpiece materials in order to achieve the desired outcomes and machinability.