The present thesis focuses on characterisation of microstructure and the resulting mechanical and tribological properties of CVD and PVD coatings used in metal cutting applications. These thin and hard coatings are designed to improve the tribological performance of cutting tools which in metal cutting operations may result in improved cutting performance, lower energy consumption, lower production costs and lower impact on the environment.  In order to increase the understanding of the tribological behaviour of the coating systems a number of friction and wear tests have been performed and evaluated by post-test microscopy and surface analysis. Much of the work has focused on coating cohesive and adhesive strength, surface fatigue resistance, abrasive wear resistance and friction and wear behaviour under sliding contact and metal cutting conditions. The results show that the CVD deposition of accurate crystallographic phases, e.g. α-Al2O3 rather than κ-Al2O3, textures and multilayer structures can increase the wear resistance of Al2O3. However, the characteristics of the interfaces, e.g. topography as well as interfacial porosity, have a strong impact on coating adhesion and consequently on the resulting properties.  Through the deposition of well designed bonding and template layer structures the above problems may be eliminated. Also, the presence of macro-particles in PVD coatings may have a significant impact on the interfacial adhesive strength, increasing the tendency to coating spalling and lowering the surface fatigue resistance, as well as increasing the friction in sliding contacts. Finally, the CVD-Al2O3 coating topography influences the contact conditions in sliding as well as in metal cutting. In summary, the work illuminates the importance of understanding the relationships between deposition process parameters, composition and microstructure, resulting properties and tribological performance of CVD and PVD coatings and how this knowledge can be used to develop the coating materials of tomorrow.

Orientador: Amauri Hassui / Dissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecânica / Made available in DSpace on 2018-08-16T19:30:43Z (GMT). No. of bitstreams: 1 Ventura_CarlosEijiHirata_M.pdf: 5734965 bytes, checksum: 2b91710d6113e0caaa9313bbc9b54acf (MD5) Previous issue date: 2010 / Resumo: A manufatura de um grande número de componentes na indústria depende de moldes e matrizes para processos como forjamento, metalurgia do pó, estampagem, fundição e injeção de plástico. Uma das principais operações para a fabricação destas peças corresponde à abertura da cavidade inicial, a partir da qual geometrias mais complexas são usinadas a fim de se obter o negativo do produto final. Sendo esta uma operação de desbaste, verificam-se altos esforços de corte e desgaste significativo da ferramenta. A fim de se reduzir tais grandezas, além de melhorar o acabamento (diminuindo-se o tempo necessário para semi-acabamento e acabamento final), diversas estratégias de entrada da ferramenta e de corte interno têm sido propostas. Neste contexto, o trabalho irá avaliar três diferentes estratégias para abertura de cavidades pelo método HSM em um aço AISI D2, sendo cada uma delas caracterizada pela ferramenta utilizada (I: fresa de alto avanço, II: fresa toroidal e III: fresa de mergulho), parâmetros de corte, estratégia de entrada (I: rampa, II: hélice e III: mergulho) e de corte interno (I: espiral, II: ziguezague, III: não há). Em cada uma delas serão analisados estática e dinamicamente os esforços de corte em cada trecho da cavidade e a vida da ferramenta. Além disso, é proposto ainda um modelo de forças para as diferentes formas de entrada da ferramenta na peça, que leva em conta as peculiaridades de cada estratégia. Verificou-se que o principal mecanismo de desgaste associado a todas as estratégias corresponde à adesão, fenômeno intimamente ligado à vibração do processo, excitado principalmente nos trechos em que a ferramenta tangencia as paredes menos rígidas da cavidade. Já em relação ao modelo, comprovou-se sua utilidade na avaliação prévia dos esforços médios / Abstract: The manufacturing of a great number of components in the industry depends on molds and dies used in processes like forging, powder metallurgy, deep drawing, casting and plastic injection. One of the main operations for manufacturing of these parts corresponds to the opening of the initial pocket, from which more complex geometries are machined in order to obtain the negative face of the final product. As this is a rough operation, it is characteristic of the process high cutting forces and a significant tool wear. Aiming to reduce such variables and to improve the finishing (decreasing the time for semi- and end finishing), many entry- and internal cutting strategies have been proposed. In this context, the work will evaluate three different strategies for pocket milling by HSM method in AISI D2 steel, which are characterized by the used tool (I: high-feed mill, II: torus mill and III: plunge mill), cutting parameters, entry strategy (I: ramp, II: helical and III: plunge) and internal cutting strategy (I: spiral, II: zigzag and III: no cutting). In each of the mentioned strategies not just the tool life, but also the cutting forces in each interval of the pocket will be analyzed statically and dynamically. Moreover, it is also proposed a cutting force model for the different entry strategies, taking into account their peculiarities. It was noted that the main wear mechanism associated with all strategies corresponds to adhesion, which is closely related to the vibration in the process, excited mainly in the intervals where the tool touch the less rigid walls of the pocket. Considering the model, its usefulness in the evaluation of the average cutting forces was proved / Mestrado / Materiais e Processos de Fabricação / Mestre em Engenharia Mecânica

Please read the abstract in the section 00front of this document / Dissertation (M Eng (Chemical Engineering))--University of Pretoria, 2005. / Chemical Engineering / unrestricted

A friction stir weld (FSW) is a solid state joining operation whose processing parameters are currently determined by lengthy trial and error methods. To implement FSWing rapidly in various applications will require an approach for predicting process parameters based on the physics of the process. Based on hot working conditions for metals, a kinematic model has been proposed for calculating the shear strain and shear strain rates during the FSW process, validation of the proposed model with direct measuring is difficult however. Since the shear strain and shear strain rates predicted for the FSW process, are similar to those predicted in metal cutting, validation of the FSW algorithms with microstructural studies of metal chips may be possible leading to the ability to predict FSW processing parameters.

In metal cutting applications, development of coatings to reduce friction between tool and chip and also enhance wear resistance of the tool is an important objective. The effectiveness of such coatings is ultimately evaluated through metal cutting trials; however, bench-scale tests can play a role in predicting some aspects of a candidate coating’s performance. This dissertation further develops the concept of an on-axis rotation pin-on-disc tribometer for the evaluation of friction coefficient between tool and work material pairs under temperature and stress conditions similar to those experienced between tool and chip in metal cutting. Firstly, the characteristics of the imprint formed by the spherical-tipped pin in the disc during tribometer tests are studied. Specific focus is given to the growth of the imprint during the rotating stage of the test; the severity of pile-up of work material around the periphery of the imprint; different zones of contact at the imprint surface; and evidence of (or lack thereof) of bulk shear in the surrounding work material below the surface of the disc. The importance of estimating the actual temperature at the pin-disc interface (inaccessible for direct measurement) is also raised. Evidence is presented that suggests the pin-disc interface is higher for tests involving coatings with higher electrical resistivity, despite exhibiting similar temperatures 2 mm above the interface (accessible for direct measurement). A numerical model is developed in an effort to estimate the pin-disc interface during stationary specimen tests for specific pin and disc materials under controlled conditions. An empirical relationship is also established to express the variation of electrical resistivity with temperature for cemented tungsten carbide (6% cobalt content). Finally, coefficient of friction results for coated and uncoated cemented carbide pins in contact with AISI 1045 steel discs are related to short duration turning trials involving the same material pairs. Coatings exhibiting low friction coefficient result in appreciably lower cutting forces, reduced built-up edge intensity and more tightly curled chips. The possibility that the low thermal conductivity of such coatings could be producing similar effects by forcing more heat into the chips is also explored. / Dissertation / Doctor of Philosophy (PhD) / This dissertation further develops the concept of a pin-on-disc apparatus for evaluating the friction coefficient between materials under temperature and stress conditions similar to those experienced in metal cutting. Firstly, characteristics of the imprint formed by the pin in the disc during tests with the apparatus are studied. Specific focus is given to the growth of the imprint during the rotating stage of the test and different zones of contact at the imprint surface. Secondly, the importance of estimating the actual temperature at the pin-disc interface, inaccessible for direct measurement, is raised and a numerical model developed to aid in its estimation. Finally, coefficient of friction results generated on the apparatus are correlated to the magnitude of forces measured and other observations made during metal cutting trials involving the same material pairs.

Any machining operation induces significant deformation and associated stress states within the component being machined. Once the component has been finished and is removed from the machining tool, a portion of these stresses remain within the finished component, and are termed residual stresses. These stresses have a significant effect upon the performance of the final component. However, despite their importance there is no accurate and cost effective method for measuring residual stresses. For this reason predicting these stresses without the need for measurement is highly desirable. The focus of this thesis is on advancing the development and implementation of finite element models aimed at predicting residual stresses induced by metal cutting operations. There are three main focus areas within this research, the first of which is concerned with predicting residual stresses when small feed rates are used. It is shown that in the existing cutting models residual stress prediction accuracy suffers when feed rates are small. A sequential cut module is developed, which greatly increases the accuracy of the predicted residual stress depth profiles. A second area of focus concerns the influence of friction models on predicted residual stresses. A detailed set of simulations is used to elucidate the effect of friction not only for sharp tools, but also for tools which have accrued wear. It is shown that whilst friction is not of critical importance for new tools, as tools continue to wear the choice of friction model becomes significantly more important. The third area of focus is on phase transformations, induced by the cutting process. A decoupled phase transformation module is developed in order to predict the depth, if any, of a phase transformed layer beneath the newly machined surface. Furthermore, the effect of this layer on the residual stress depth profile was also studied. All three focus areas present new and novel contributions to the field of metal cutting simulations, and serve to significantly increase the capabilities of predictive models for machining. / Thesis / Doctor of Philosophy (PhD)

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published_or_final_version / Mechanical Engineering / Master / Master of Philosophy

This project has as main goal to study the influence of the coating thickness, deposited by arc-PVD- technology, on the wear resistance of coated cemented carbide inserts in three different turning operations. An additional effort has also been made to develop a new test method for evaluating flank wear resistance. Three different coating types (Coating A, Coating B and Coating C) have been studied in three distinctive thicknesses (2, 4 and 6 μ m) for each type. For two of the coating types (Coating A and B) special studies has been done with the thickest coating, creating two additional coating versions of the thickest coating, by changing various deposition parameters, with the aim to enhance the cohesive properties of the coating and lower the residual stresses at the cutting edge. The results show increased crater wear resistance with increased coating thickness for all coating types, but the amount of increase changes with coating type (Coating A>B>C). Flaking wear resistance decreased with increased coating thickness for all coating types. Further the high temperature version of Coating A, showed a significant increase in the flaking wear resistance compared to the standard version of Coating A. The flank wear test showed an increased resistance with thicker coatings in all cases except for the 6 μ m version of Coating C. The flank wear resistance of the most flank wear resistant coatings (Coating B, C) was also successfully examined in a new developed test method. The method suppressed excessive crater wear on the rake face and presented a high abrasive wear rate on the flank and some flaking on the rake face. The amount of flaking is judged not to influence the testing of the flank wear resistance. For all the tested coatings in the new test an increase in the coating thickness resulted in better flank wear resistance.