Papers by Author: Konstantinos D. Bouzakis

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Authors: Konstantinos D. Bouzakis, Stefanos Gerardis, G. Katirtzoglou, S. Makrimallakis, G. Skordaris, K. Efstathiou
Abstract: The effect of the developed chip length on the coated tool’s cutting performance was investigated. Milling experiments at various cutting speeds and chip lengths were performed, which resulted to different tool wear developments. To explain these results, a FEM simulation of the cutting process was conducted and the related chip geometries were predicted and compared to the corresponding experimental ones. Based on these results, the Coulomb friction coefficient between chip and tool rake was appropriately adjusted to achieve a sufficient correlation between experimental and computational data. By additional FEM calculations, the mechanical and thermal loads of the cutting edge were estimated and insight was provided concerning the effect of chip length on coated tool stresses and film fatigue fracture. The obtained results revealed that the chip length reduction improves the cutting performance of coated tools and a significant increase of the removed material volume and material removal rate as well can be achieved.
Authors: Konstantinos D. Bouzakis, G. Skordaris, Emmanouil Bouzakis, Eleftheria Lili
Abstract: If the film of a coated component deforms plastically because of operational loads, residual stresses are developed in the coating material after the load removal. In this way, material mechanical properties changes occur due to endogenous reasons i.e. induced by the coating crystalline structure plastic deformation. In this case, the determination of the effective film mechanical properties has been introduced in a recent publication [1]. Moreover, if the loading conditions lead only to a substrate plastic deformation, the coating remains elastically deformed during the relaxation, due to the substrate residual stresses. Thus, the associated material mechanical properties changes are caused by exogenous parameters related to the permanent substrate deformation. In the present paper, a novel experimental-analytical method based on FEM calculations is introduced to determine the effective film mechanical properties when the coating is stressed elastically due to a plastic substrate deformation. The perpendicular impact test is a convenient experimental procedure to investigate such an effect because under appropriate loading conditions, the substrate deforms plastically and the coating elastically. The pristine constitutive law of the applied PVD film was determined by nanoindentation and FEM supported results evaluation. Impact tests were conducted at various loads and loading cycles. The impact test was simulated by a two dimensional FEM model. Additionally, the developed elastic residual stress fields in the coating and the plastic ones of the substrate in the imprint were determined. In these calculations, a rate-independent anisotropic plasticity with kinematic hardening material law was considered and the film as an anisotropic material with variable mechanical properties in three main directions. Finally, by a FEM supported simulation of the nanoindention, the coating’s load-displacement behaviour in various areas of the impact imprint were predicted and the effective coating mechanical properties as well.
Authors: Nikolaos Michailidis, Konstantinos D. Bouzakis, Fritz Klocke, Martin Witty, Eleftheria Lili, Stefanos Gerardis, Maria Pappa
Abstract: A methodology based on FEM-calculations and experimental tests for predicting coated tool efficiency in milling Ti6Al4V by coated cemented carbide inserts is introduced. The used coatings were: (Ti,Al,Si)N and (Ti,Al)N films. The stress-strain curves and the fatigue critical loads of the coatings were determined by nanoindentations and impact tests respectively at various temperatures, employing FEM-supported procedures developed for results evaluation. The milling investigations were conducted at various cutting speeds. The stress and temperature fields in the cutting edge region were obtained by FEM calculations of the milling process. These results facilitated the explanation of the coated inserts’ cutting performance versus the cutting speed. The cutting tests at various cutting speeds and the impact tests at ambient and elevated temperatures revealed that the tool life and the film impact resistance versus the temperature are not linear. Moreover, a sufficient correlation of the coatings’ impact resistance at various temperatures with their cutting performance at corresponding cutting speeds was revealed. In this way, an adaption of the cutting conditions to the films’ temperature-dependent strength can lead to a considerable cutting performance improvement.
Authors: Konstantinos D. Bouzakis, M. Batsiolas, G. Malliaris, Maria Pappa, Emmanouil Bouzakis, G. Skordaris
Abstract: In the paper, innovative methods for characterizing coatings’ properties at ambient and elevated temperatures are introduced based on various experimental procedures. Nanoindentation results, which were obtained at elevated temperatures, are evaluated by FEM algorithms, rendering possible the determination of temperature dependent coating mechanical properties. Impact tests conducted on coated specimens revealed a non-linear film impact resistance versus the temperature. The latter results were evaluated by appropriate FEM supported procedures, to predict the coating fatigue endurance stress versus the temperature. In these investigations, the impact load was induced electromagnetically for a duration of ca. 1ms, depending on the force amplitude. To change the impact load characteristics, such as frequency, impact duration etc., a new test device has been developed, employing a piezoelectric actuator. This device enables the investigation of the impact time effect on the dynamic response of coated surfaces and on the coating fatigue endurance stress. Finally, diffusion phenomena in coatings were examined by a developed convenient experimental setup. A specimen is pressed onto a coated surface at adjustable high temperature and pressure in an inert atmosphere. After this test, the diffusion of characteristic elements into the coating and vice versa is detected by EDX-microanalyses. These results contribute, among others, to the description of diffusion phenomena between coatings and various materials.
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