207 research outputs found

    Hydrodynamic modelling of hydrostatic magnesium extrusion

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    Wilson’s hydrodynamic model of the hydrostatic extrusion process is extended to meet the geometry found on residual billets. The transition from inlet\ud to work zone of the process is not considered sharp as in the model of Wilson but as a rounded edge, modelled by a parabolic function. It is shown that\ud this rounded edge has a considerable influence on the predicted film thickness. Furthermore, it is shown that for hydrostatic extrusion of magnesium with castor oil as pressure medium, it is not possible to generate full film lubrication in the work zone of the hydrostatic extrusion process

    The Influence of Plastic Bulk Deformation on Surface Roughness and Frictional Behavior during Deep Drawing Processes

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    In Sheet Metal Forming (SMF) processes, friction does play an important role. This with respect to the increase of product quality demands and the ability of predicting these processes by for instance finite element simulations. The existing simulation models do not contain an adequate friction model. In SMF processes different contact situations can be distinguished. As a result different coefficients of friction are locally present, which influences the forming process.\ud \ud Experiments are performed on a testing device by which it is possible to simulate the operational conditions as present in SMF processes. This test rig is a combination of a tensile tester and a friction measuring device, by which it is possible to measure the coefficient of friction as a function of the operational conditions (velocity and contact pressure) and deformation (elastic or plastic) in a well controlled way. Friction is presented in a generalized Stribeck-curve in which the different lubrication regimes can be distinguished, i.e. Boundary Lubrication (BL) and Mixed Lubrication (ML), which are also occurring during SMF processes.\ud \ud In SMF processes the sheet material deforms elastically and plastically and therefore the surface roughness will change and as a consequence will influence the frictional behavior between sheet and tool. In this paper, the influence of plastic deformation on A) the surface microgeometry and B) as a consequence of that on the frictional behavior of the sheet-tool system is studied. With the aid of a 3D-surface interference microscope, the microgeometry of the deformed material has been analyzed. The result of this investigation is that the CLA-roughness due to the deformation first decreases and then increases with increasing deformation. Furthermore, friction is hardly influenced due to the change in surface roughness. No change in the shape and the level of the generalized Stribeck curve is found

    Friction Analysis on Scratch Deformation Modes of Visco-Elastic-Plastic Materials

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    Understanding of abrasion resistance and associated surfaces deformation mechanisms is of primary importance in materials engineering and design. Instrumented scratch testing has proven to be a useful tool for characterizing the abrasion resistance of materials. Using a conical indenter in a scratch test may result in different deformation modes, like as elastic deformation, ironing, ductile ploughing and cutting. This paper presents the friction analysis of some deformation modes of visco-elastic-plastic behaving polymer materials, especially PEEK (poly etheretherketone). In general, it is accepted that the friction consist of an adhesion and a deformation component, which can be assumed to be independent to each others. During a scratch test, the friction coefficient is influenced by some parameters, such as the sharpness of indenter, the deformation modes and the degree of elastic recovery. Results show that the adhesion component strongly influences the friction in the elastic and ironing deformation mode (scratching with a blunt cone), friction for the cutting deformation mode (scratching with a sharp cone) is dominantly influenced by the deformation component. From the analysis, it can be concluded that the adhesion friction model is suitable for ironing - elastic deformation mode and the deformation friction model with elastic recovery is good for cutting mode. Moreover, the ductile ploughing mode is combination of the adhesion and plastic deformation friction model

    Stribeck Curve for Starved Concentrated Contacts

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    Effects of a second phase on the tribological properties of Al2O3 and ZrO2 ceramics

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    The tribological properties of four different materials are investigated, tetragonal zirconia (Y-ZTP), Al2O3 dispersed in Y-TZP (ADZ), ZrO2 dispersed in Al2O3 (ZTA) and Al2O3 (with 300 ppm MgO). These materials are used as a cylinder sliding against a plate of Y-TZP (TZ-3Y)). Compared to Y-TZP, the wear resistance of ADZ composites is increased by a factor of 4¿10. At a contact pressure of 230 MPa, a wear transition for Y-TZP is observed from plastic deformation to microchipping and microfracture due to the high interfacial temperature (450°C¿550°C) generated by frictional heating. Because of the higher elastic modulus, hardness and fracture toughness at high temperature, ADZ composites show better wear resistance and a higher transition contact pressure (over 400 MPa) under the present conditions. For Al2O3, the transition from mild to severe wear occurs when the contact pressure is changed from 250 to 400 MPa. For ZTA ceramics, the wear behaviour does not change because of the presence of a compressive layer due to the zirconia phase transformation during sliding.\ud \ud In water the wear resistance for ADZ and ZY5 is almost two orders of magnitude higher than the results under dry conditions. Reduction of the interfacial temperature by using water and the formation of a hydroxide layer at the contact surface by the tribochemical reaction of water with the ceramic, as observed by XPS, gives a positive effect on wear resistance
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