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    The effect of austempering on the microstructure and mechanical properties of PM Fe-0.8c steel aloyed with copper and nickel
    (Carl Hanser Verlag, 2019) Gokmese, Hakan; Saricicek, Nurullah; Gural, Ahmet
    The effects of austempering heat treatment on the microstructure and mechanical properties of sintered steel containing 3 % Cu and 3 % Ni powders with addition of 0.8 % graphite powders were examined. For this purpose, the powder mixtures containing 3 % Cu, 3 % Ni, and 0.8% graphite powder were compacted under 700 MPa of pressure within molds in accordance with the standard tensile and impact test specimen dimensions. All of the green compacts were sintered under an argon (99.99 %) gas atmosphere for 20 min at 1 150 degrees C. The austempering heat treatments were applied to the sintered specimens for different periods ranging from 15 to 120 min in a salt bath at 350 degrees C in order to obtain a bainitic structure after the austenitization process at 850 degrees C. The hardness, tensile, and charpy impact tests were carried out at room temperature. It was found that there was a decrease in the hardness value depending on the austempering conditions; moreover, a significant increase was observed in the tensile and impact toughness values. Remarkable strength and ductility values were obtained with the austempering applied for 60-120 min.
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    Effect of Various Spheroidizing Methods on Microstructure-Mechanical Properties and Wear Performance of PM High-Carbon Steel
    (Springer India, 2022) Gokmese, Hakan; Karadag, Hakan Burak; Tasdogen, Hacer
    In high-carbon unalloyed steels produced by the powder metallurgy method, the effect of different spheroidizing heat treatments on microstructure, mechanical properties, and wear performance was studied. For this purpose, 1.2 wt% graphite powders were added to Fe powders, and a powder composition was formed. The prepared powder mixtures were pressed at room temperature under 850 MPa and then sintered at 1180 degrees C for 20 min. After sintering, sample phase compositions were determined by the XRD analysis. Then, three different spheroidizing heat treatments were applied. First, the conventional spheroidizing was used for 5-10-20-40 h at a constant temperature of 700 degrees C. Secondly, for 5-10-20-40 h, cyclical spheroidizing was involved in the temperature range of 700 degrees C and 740 degrees C. A third heat treatment (post-quenching spheroidizing), which represented the study's original aspect and was an alternative to these known spheroidizing methods, was applied. Fully austenitized work-pieces were quenched in water at room temperature. Consequently, the martensitic structure was formed. These samples were over-annealed for 5-10-20 h at 700 degrees C. Even after a short period, such as 5 h, homogeneous and fine spherical cementite was formed in the microstructure. Depending on the spherical cementite morphology and distribution, impact toughness and hardness values were evaluated. Besides, steels' wear characteristics were determined using the block-on disc device, and three-dimensional worn surface morphologies were examined with the profilometer device. With the scanning electron microscope (SEM), microstructures formed due to different heat treatments were displayed. Also, the fractured and worn surfaces of the samples were examined. It was determined that with the post-quenching spheroidizing treatment, which may be an alternative to known and widely used spheroidizing treatments, the desired microstructure and properties such as machinability and ductility could be achieved in a much shorter period.
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    Fabrication and characterization of nanoceramic particle Al2O3/B4C composite by mechanochemical approach
    (Taylor & Francis Inc, 2017) Gokmese, Hakan; Bostan, Bulent; Baris, Mustafa
    Al2O3/B4C ceramic phase composite structure was produced by solid-state method. Transformations of ceramic phases, micromorphology were determined by employing X-ray diffraction, FT-IR, scanning electron microscopy and transmission electron microscopy (TEM). Transformation of ceramic phases was completed in a short time (6 h). Formation of Al2O3/B4C ceramic-phase powder mixtures was specified after leaching. As a result of the increase of mechanical energy and the amount of ceramic phase, reduction in powder particle size (50-350 nm) and increase in agglomeration tendency were observed. The impurities were removed after leaching and the distribution in which the ceramic phases are effective was obtained with the TEM study.
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    Fabrication and characterization of nanoparticle MgO/B4C composite by mechanochemical method
    (Sage Publications Ltd, 2017) Gokmese, Hakan; Bostan, Bulent
    In this study, nanoparticle boron carbide was produced after the reduction of boron oxide with magnesium under the presence of carbon by using mechanochemical synthesis method. During the synthesis processing, microstructure and phase transformations of powders were performed by using X-ray diffraction (XRD), Fourier-transform infrared spectrum (FT-IR), scanning electron microscope (SEM), and high-resolution transmission electron microscopy (HRTEM). At the end of 2h, reaction products (MgO, B4C) were determined by XRD and FT-IR examinations. Nanoparticle B4C single-phase crystalline structure from the MgO/B4C composite powder mixture, which was the reaction product, was obtained by leaching process. With the increase in the synthesis period, starting material peaks diminished and Fe peak intensity became evident. After the HRTEM examinations, it was determined that synthesized B4C particles were within range of 10-200 nm especially in the activation of leaching process. Additionally, by using spot pattern analysis method, planes represented by B4C particles were calculated.
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    Mold Modeling of Metallic Tension Bar in AA 7075 Aluminium Alloy Casting
    (Kaunas Univ Technol, 2018) Gokmese, Hakan
    In this paper, the design and application of permanent mold casting technology reserving a significant place in the light alloy casting technology was conducted due to its convenience for mass production and industrial need. The metal tensile bar mold design, mold preheat temperature (100 degrees C-150 degrees C-200 degrees C), thermal stress, fatigue and damage analyses were examined to be used in the casting experiments by using the finite element analysis. As well as the casting temperature and solidification time, the material flow analyses were conducted by using a computer-aided program. By producing the mold following the modeling and analyses studies, AA7075 aluminum alloy was melted at 800 degrees C and casted in the permanent mold. Following the casting procedure, the tensile bars of aluminum alloys were prepared in conformity with the standards and their tensile and hardness tests were conducted. The general and fracture surface microstructure differences of experiment specimens were examined. As a result of modeling and analysis studies made in the computer environment, the surface stresses of specimen and runner regions which were significant in the mold design were calculated at 100, 150 and 200 degrees C preheating temperatures. The most convenient mold lifetime was calculated by making fatigue calculations for permanent mold at 800 degrees C.
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    Production of AA 2014 Matrix Wollostanite/Wood Ash Solid Waste Particle Reinforced Hybrid Composite with Powder Metallurgy and Investigation of Its Structural Properties
    (Springer, 2024) Gokmese, Hakan; Bulbul, Saban
    In this study, using AA 2014 aluminum alloy, AA2014/wollastanite, AA2014/wood ash, and AA2014/wollastanite/wood ash composite hybrid materials were produced by powder metallurgy method. The powders, whose mixing ratios were determined by weight, were mixed in a 3-dimensional mixer for 30 min. The prepared mixture powders were pressed uniformly under a pressure of 700 MPa, and cylindrical materials with a diameter of 10 mm were obtained. Then, the materials were sintered for 1 h under argon gas flow at a temperature of 600 degrees C. The density, microstructure, hardness, and wear behavior of the materials were examined. It was observed that the density of the composite materials increased relatively compared to AA 2014 alloy. As the particle reinforcement ratios increased, porosity amounts also increased in the composite and especially in the hybrid materials. In the wood ash-reinforced composite materials, the particles were distributed more homogeneously in the microstructure. The highest hardness value obtained was 70.65 HV in AA2014/15% wollastanite composite material. The weight loss increased in the materials created by particle reinforcement, and the lowest weight loss (170 mg) was determined in this material.
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    PRODUCTION OF ELECTRO PORCELAIN REINFORCED WITH BORIC ACID AND INVESTIGATION OF ITS PROPERTIES
    (Gazi Univ, Fac Engineering Architecture, 2015) Bilgin, Aziz Emre; Gokmese, Hakan; Bostan, Bulent
    The need of electrical energy is raised by the development of industry and that required to transportation of energy to the more distant points. All these developments reveal the need of cheaper insulator production and the need of enhancement of insulators' technical specifications and mechanical resistance against the increasing mains voltage and external factors. In the study, three different prescriptions, contain 0.1 %, 0.3 % and 1% H3BO3 additive, were prepared. Then, prepared samples were shaped on vacuum presses in the laboratory. After samples were dried, they were sintered at 1255 degrees C for 2 hours in air. Particle size distribution, dry refinement, fuchsine and bending three point tests were applied to the samples. Characterizations of microstructure were performed by XRD, SEM and EDS analyses. At the end of the experimental studies, it was seen that depending on additive percentage, the reduction of particle size distribution is up to 0.3% and particle size is increasing with an increase in additive ratio. The maximum dry reduction amount was seen on the sample that contains 1 % boric acid. In general, it was determined that the sample, which contains 0.3 % boric acid, gives closer results to operation conditions. Therefore, it was observed that the boric acid could be an alternative raw material source, if it is incorporate into electro-ceramic at a certain ratio.