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Article Citation - WoS: 9Citation - Scopus: 15Free forming of locally heated specimens(Elsevier Sci Ltd, 2007) Okman, O.; Ozmen, M.; Huwiler, H.; Tekkaya, A. E.A novel manufacturing method is investigated, in which a steep temperature gradient within the workpiece is induced to facilitate material flow locally. By this method, complex shapes can be formed without complicated dies. The feasibility of the idea is analyzed experimentally and numerically. Local heating is realized either by means of induction or laser heating. Experiments using materials 16MnCr5, X5CrNi18/9, and Ti6Al4V have been conducted under various process conditions. These experiments have also been modeled by finite element method (FEM) validating the analysis procedure. Electromagnetic models are used to analyze the heat generation pattern on the workpiece by induction. It is found that the most important process parameters are the thermal diffusivity and the temperature sensitivity of the flow curve of the workpiece material. The lower the thermal diffusivity and the higher the temperature sensitivity, the more differentiated local shapes can be formed. For the analyzed geometries, induction heating has been observed to be more effective. Deformation rate and initial workpiece geometry have also a significant effect on the achievable local deformations. Various failure modes such as unintended deformations, damage by fracture, and melting of the workpiece material are described. It is concluded that the new idea of forming local shapes by local heating is a feasible and controllable manufacturing alternative. (c) 2006 Elsevier Ltd. All rights reserved.Article Citation - WoS: 5Citation - Scopus: 6Material Flow Control in High Pressure Sheet Metal Forming of Large Area Parts With Complex Geometry Details(verlag Stahleisen Mbh, 2005) Trompeter, M; Önder, E; Homberg, W; Tekkaya, E; Kleiner, MWorking media based forming processes show advantages compared to the conventional deep drawing in the range of sheet metal parts with complex geometry details. By High Pressure Sheet Metal Forming (HBU), complex parts can be formed with reduced tool costs, fewer process steps, and improved part properties, particularly by the use of high strength steels. In order to use these advantages to full capacity, the material flow into the area of the geometry details needs to be optimised. The key element for the material flow control is a multi-point blank holder. In combination with flange draw-in sensors, a closed loop flange draw-in control can be built up which guarantees a reproducible material flow and, consequently, defined part properties. Furthermore, a favourable pre-distribution of sheet metal material can be reached which leads to a widening of the process limits. Considering a large area sheet metal part with a complex door handle element as example, strategies for the material flow control will be discussed in this paper. The conclusions are based on FE-simulations as well as experimental findings.Article Citation - WoS: 6Citation - Scopus: 9Comparative Assessment of Electromagnetic Simulation Tools for Use in Microstrip Antenna Design: Experimental Demonstrations(Wiley, 2019) Bilgin, Gulsima; Yilmaz, Vadi Su; Kara, Ali; Aydin, ElifThis paper presents a better understanding of the use of finite integration techniques (FIT) and finite element method (FEM) in different types of microstrip antennas in order to determine which numerical method gives relatively more accurate results. Although the theoretical formulation based on Maxwell's equations of both FEM and FIT are approached from different aspects in the literature, there is still a lack of comparison of the same antenna type using different numerical methods employing FEM and FIT. Therefore, in this study, FEM and FIT were applied to two different types of microstrip antennas, and their simulation and experimental results was compared. For the first antenna demonstration, a multilayer structure was chosen to achieve one of the significant parameters. Then, a microstrip antenna with a compact structure was used in the second demonstration. Using these two antennas, the accuracy of FEM and FIT in different structures were compared and all simulated return loss and gain results were verified by the measured results. The experimental demonstrations show that FEM performs better for both types of microstrip antennas while FIT provides an adequate result for two-layer microstrip antennas.Conference Object Effects of Local Heating on Material Flow in Free Forming Process(2006) Okman,O.; Huwiler,H.; Özmen,M.; Tekkaya,A.E.A novel manufacturing method is investigated, in which a proper temperature gradient is created within workpiece in order to control local material flow during free forming. The main motivation is to produce complicated shapes by reducing the flow stress on the regions, where local deformation is desired to take place. A sufficient control of temperature within the material results in the required product shape even in the absence of complicated dies. Besides the lower tooling costs the process provides, the heat energy applied to the workpiece is less than that in conventional hot forming processes, which is currently a strong alternative for manufacturing of such products. In the study, heating is realized by means of induction heating and laser beam scanning. The process is investigated experimentally on circular cylinder specimens made of different materials, namely Ti6Al4V, X5CrNi18/9 and 16MnCr5. The effect of process parameters on the mode of deformation is analyzed by finite element method (FEM). The thermo-mechanical analysis of induction heating is supported by electromagnetic calculations. The two alternative heating methods are compared. Affects of heating on multiple locations is investigated for induction heating applications. A brief overview of the process is presented and conclusions are drawn on the effectiveness, limitations, failure modes and applicability of the process. Copyright © 2006 by ASME.Article Citation - WoS: 7Citation - Scopus: 7Prediction of White Layer Formation in Μ-Wedm Process of Niti Shape Memory Superalloy: Fem With Experimental Verification(Springer London Ltd, 2021) Ilkhchi, Reza Najati; Akar, Samet; Meshri, Hassan Ali M.; Seyedzavvar, MirsadeghMicroscopic changes in the surface of nickel-titanium (nitinol) shape memory alloys (SMAs) in micro-wire electro-discharge machining (mu-WEDM) due to the formation of a resolidified layer on the machined surface, called white layer, are one of the main drawbacks in the processing of such alloys. Since these changes significantly affect the shape memory and elastic recovery characteristics of these alloys, reduction of the white layer thickness (WLT) based on the selection of optimum process parameters is essential to raise the quality of the machined parts. In this regard, a finite element model (FEM) has been developed to simulate the effects of mu-WEDM process parameters, including discharge current, pulse on-time, pulse off-time, and servo voltage, on the heat distributing in Ni55.8Ti SMA to predict the WLT. The flushing efficiency of electric discharges and the effect of flow regime of the dielectric fluid on the heat distribution in the workpiece and the formation of the WLT are analyzed. Experimental data are used to verify the accuracy of the FEM. The results show that the developed model can predict the WLT in mu-WEDM process of Ni55.8Ti SMA with an average error of 14%. The effects of discharge parameters on the formation of the WLT are discussed in details based on the results of the FEM.Book Part A Numerical Approach To Simulating Oxidation in Thermal Barrier Coatings(Elsevier, 2020) Saeidi,F.; Gurses,E.; Aslan,O.Computational analysis and simulation of multi-physics phenomena taking place in coating systems is still a challenging task. Specifically, for ceramic coatings used as a system of protection for base materials against elevated temperatures, known as thermal barrier coating (TBC) systems, construction of continuum level models which can express coupled nonlinear phenomena has attracted great attention. Thermal stresses, oxidation, creep and numerous other mechanisms and phenomena makes it even harder to model and simulate the behavior of TBCs. In this article, a new numerical model which allows simulation of oxidation and thermally grown oxide (TGO) of bond-coat is presented. Phase field theory is used with finite strain formulation and implemented using user element subroutine (UEL) in ABAQUS software for finite element method. Results are compared with experimental data available for TGO in the literature. © 2020 Elsevier Inc. All rights reservedArticle Citation - Scopus: 2Simulation Trends in Quenching Technology for Automotive Components(Maney Publishing, 2014) Felde,I.; Simsir,C.Quenching technology is widely used in automotive industry from a simple immersion quenching of gears up to the complex production technology of press hardening. The selection of process parameters to develop the desired properties is challenging due to the complexity of the physical phenomena occurring during the manufacturing cycle. In the last decades several computational methods have been applied successful to optimise the heat treatment processes. This paper is focusing on some examples demonstrating the state of the art of the simulation tools, including the physical phenomena of quenching, the theoretical background of the coupled models used for estimation the microstructure, mechanical properties and deformation of heat treated automotive components. © 2014 IHTSE Partnership.Article Citation - WoS: 19Citation - Scopus: 19Modeling and Simulation of Coupled Phase Transformation and Stress Evolution in Thermal Barrier Coatings(Pergamon-elsevier Science Ltd, 2020) Sait, Ferit; Aslan, Özgür; Gurses, Ercan; Aslan, Ozgur; Sait, Ferit; Aslan, Özgür; Sait, Ferit; Mechanical Engineering; Aerospace Engineering; Mechanical Engineering; Aerospace EngineeringThe thermally grown oxide layer is known to be responsible for the failure of coating systems due to the generation of severely high stresses. In this work, oxidation induced stresses generated in thermal barrier coating (TBC) systems are investigated for high temperature isothermal oxidation. In that sense, a comprehensive model, where phase transformation is coupled with mechanics is developed for the life-time estimation of TBC systems and a modified version of the Allen-Cahn type phase field approach is adopted in order to model the generation of thermally grown oxide (TGO) in finite strain constitutive framework. The top-coat material behavior is modeled using a rate-dependent Gurson type plasticity for porous materials which also accounts for creep. The results for the isothermal phase transformation analysis and the model validation using experimental results are demonstrated. The capability of the model in predicting the local stresses which is the main variable in the analysis of possible delaminations and accurate lifetime estimation of TBC systems is shown.
