Ömer Necati Cora

Manufacturing Engineering, Engineering Education, Mechanical Engineering

Ph.D
24.63

Publications

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    Faruk Güner · Hasan Sofuoğlu · Ömer Necati Cora
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    ABSTRACT: In this study, multiparticle finite element (MPFE) approach was used to analyze contact interactions of spherical copper particles in powder compaction process. To this goal, 74 spherical copper particles of 200 μm in diameter were modeled as individual elastic-plastic bodies, and randomly filled into a die cavity. Interparticle and die-wall-particle contact interactions were investigated, and coefficients of friction were obtained using variable friction models; Wanheim-Bay's general friction model and Levanov's friction model. Variable friction models were incorporated into FE analyses through user-subroutines. It was found that the variation of contact stresses inside the die leads to different contact conditions at different zones. The range of coefficient of friction encountered in the analysis was found to be slightly higher in Levanov's friction model than that for Wanheim-Bay's general friction model. On the other hand, Levanov's model was found to be more appropriate for elevated temperature analyses.
    Full-text · Article · Dec 2015 · Tribology International
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    Faruk Güner · Necati Ömer · Cora · Hasan Sofuoğlu
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    ABSTRACT: Numerical analysis of powder compaction process requires multi-particle modeling approach as continuum models fail to simulate the nature of process (e.g. interparticle, and particle–die interactions), accurately. This study aimed for analyzing powder compaction process utilizing 3-D finite element modeling approach along with different material models including modified Cam-Clay, Mohr–Coulomb, Shima–Oyane and von-Mises. The finite element analyses were carried out by implementing multi-particle finite element method. Moreover, continuum modeling was also performed for comparison purposes. In both cases, the compaction of spherical copper particles was analyzed at room temperature conditions. The obtained FEA results were compared in terms of equivalent stress and strain, and deformed shape. Results showed that the FE models in which von-Mises and modified Cam-clay material models were used yielded results of similar magnitude while those of Shima–Oyane and Mohr–Coulomb material models resulted in equivalent stress and strain values are in close proximity. Effect of coefficient of friction on the results was also investigated by implementing three distinct coefficients of friction (μ = 0.1, 0.25, 0.4). It was noted that increasing friction resulted in elevated level of defor-mation for the particles and harsher particle–particle, and particle–die contact interactions.
    Full-text · Article · Feb 2015 · Powder Technology
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    Ömer Necati · Muammer Koç
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    ABSTRACT: Automotive industry relentlessly in a quest for higher performance of vehicles in several aspects such as increased fuel efficiency and hence reduced tailpipe emissions, improved aerodynamic and driving performance, elevated safety precautions such as crash energy absorption, etc. All these concerns should be addressed with minimal weight increase and without compromising from passenger safety. This manuscript, first gives an overview for world auto industry and projections, and then reviews benefits and problems encountered in development and implementation of high strength steels particularly in automotive industry.
    Full-text · Conference Paper · May 2014
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    Ender Dur · Necati Cora · Muammer Koç
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    ABSTRACT: Metallic bipolar plate (BPP) with high corrosion and low contact resistance, durability, strength, low cost, volume, and weight requirements is one of the critical parts of the PEMFC. This study is dedicated to understand the effect of the process sequence (manufacturing then coating vs. coating then manufacturing) on the corrosion resistance of coated metallic bipolar plates. To this goal, three different PVD coatings (titanium nitride (TiN), chromium nitride (CrN), zirconium nitride (ZrN)), with three thicknesses, (0.1, 0.5, 1 mm) were applied on BPPs made of 316L stainless steel alloy before and after two types of manufacturing (i.e., stamping or hydroforming). Corrosion test results indicated that ZrN coating exhibited the best corrosion protection while the performance of TiN coating was the lowest among the tested coatings and thicknesses. For most of the cases tested, in which coating was applied before manufacturing, occurrence of corrosion was found to be more profound than the case where coating was applied after manufacturing. Increasing the coating thickness was found to improve the corrosion resistance. It was also revealed that hydroformed BPPs performed slightly better than stamped BPPs in terms of the corrosion behavior.
    Full-text · Article · Jan 2014 · Journal of Power Sources
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    Cabir Turan · Ömer Necati Cora · Muammer Koç
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    ABSTRACT: In this study, results of an investigation on the effects of manufacturing and coating process sequence on the contact resistance (ICR) of metallic bipolar plates (BPP) for polymer electrolyte membrane fuel cells (PEMFCs) are presented. Firstly, uncoated stainless steel 316L blanks were formed into BPP through hydroforming and stamping processes. Then, these formed BPP samples were coated with three different PVD coatings (CrN, TiN and ZrN) at three different thicknesses (0.1, 0.5 and 1μm). Secondly, blanks of the same alloy were coated first with the same coatings, thickness and technique; then, they were formed into BPPs of the same shape and dimensions using the manufacturing methods as in the first group. Finally, these two groups of BPP samples were tested for their ICR to reveal the effect of process sequence. ICR tests were also conducted on the BPP plates both before and after exposure to corrosion to disclose the effect of corrosion on ICR. Coated-then-formed BPP samples exhibited similar or even better ICR performance than formed-then-coated BPP samples. Thus, manufacturing of coated blanks can be concluded to be more favorable and worth further investigation in quest of making cost effective BPPs for mass production of PEMFC.
    Full-text · Article · Dec 2013 · Journal of Power Sources
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    Ömer Necati Cora · Muammer Koç · Peter J. Blau · Kunio Namiki
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    ABSTRACT: Despite the advantages of advanced high strength steels (AHSS), their stamping into functional lightweight parts demands prolonged die life, which necessitates the use of alternative substrates, coating materials, and/or surface conditioning to minimize and delay the die wear. In order to avoid frequent die replacement and surface quality problems on the stamped parts, the metalworking industry has been investigating various approaches such as reducing/refining the carbide particles, adding alloying elements, and elevating the hardness and toughness values for both substrate materials and coatings.The objective of this work was to investigate the effects of different coatings on the wear behavior of a some selected tool steel materials (die sample of interest) against two different AHSS sheet blanks through a cylinder-on-flat type reciprocating test method. After wear tests, both die sample and sheet blank surface were microscopically examined. Wear resistance of the slider was quantified from wear scar width measurements. Results showed that TD and CVD coated die samples performed better than the two other PVD coated samples.
    Full-text · Conference Paper · Nov 2013
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    Hasan Sofuoglu · O.N.Cora · F. Guner

    Full-text · Conference Paper · Sep 2013
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    Cabir Turan · Ömer Necati Cora · Muammer Koç
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    ABSTRACT: The main purpose of this study is to understand the interfacial contact resistance (ICR) characteristics of coated metallic bipolar plates (BPP) manufactured through stamping and hydroforming. To this goal, 51 μm thick SS316L stainless steel sheet blanks were formed into BPPs using two forming techniques (stamping and hydroforming); then these formed plates were coated with three different PVD coatings (CrN, TiN, ZrN) at three different coating thicknesses (0.1, 0.5 and 1 μm). Contact resistance of the formed and coated BPP samples were measured before and after they were exposed to the proton exchange membrane fuel cells (PEMFC) operating conditions (i.e., corrosive environment). ICR tests indicated that CrN coating increased the contact resistance of the samples, unexpectedly. TiN samples showed the best performance in terms of low ICR; however, their ICR dramatically increased after short-term exposure to corrosion. ZrN coating, as well, improved conductivity of the SS316L BPP samples and demonstrated similar ICR performance before and after exposure to corrosion.
    Full-text · Article · Dec 2012 · International Journal of Hydrogen Energy
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    M. F. Peker · Ö. N. Cora · M. Koç
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    ABSTRACT: Micro-stamping, as a promising sheet metal forming process for mass production of small parts, can meet the expectations such as durability, strength, surface finish, and low cost for miniaturized metal products and features. The purpose of this research was set to investigate surface interactions during mass manufacturing of micro-stamped sheets, and its consequences; then establish correlations (if any) between surface interactions vs. corrosion and contact resistance of bipolar plates (BPPs) to be used in proton exchange membrane fuel cells (PEMFC).In experimental part of this study, 10,000 SS316L sheet blanks were micro-stamped using a stamping die set with 750 μm-deep micro-channels under 200 kN stamping force, and with a constant stamping speed of 1mm/s. Surface inspections (surface roughness and micro-channel height measurements), corrosion and contact resistance tests were carried out on BPPs. Analysis of variance (ANOVA) technique was utilized to investigate the significance of surface roughness, channel heights, corrosion and contact resistance variations for BPPs. Moreover, three-dimensional (3D) finite element models of micro-stamping process were established to approximate the stress and strain levels as well as coefficient of friction value experienced at contact interface.The results revealed that the roughness values for micro-stamping dies and BPPs followed similar trends during 10,000 micro-stampings. Since surface defects trigger corrosion, the correlation between surface roughness and corrosion resistance of BPPs was found to be direct. Increasing number of surface irregularities (asperities) lowered contact surface area that resulted in increased contact resistance. Finally, comparison of experimental and numerical channel height values showed that the coefficient of friction did not change considerably during the mass production of BPPs, at least within the 10,000 stamping cycle.
    Full-text · Conference Paper · Nov 2012
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    Ömer Necati CORA · Ali AĞCAYAZI · Kunio NAMIKI · Hasan SOFUOĞLU · Muammer KOÇ
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    ABSTRACT: This study intended to investigate the wear performance of different coatings (two different PVD, a CVD, and a TD coating) applied onto variety of substrate materials (DC 53, SKD 11, DRM 3, DRM 51) against AHSS sheet blanks. A non-reciprocating, CNC-based, slider type of tester was employed in wear tests. In effect of coating study, it was found that TD coated samples performed slightly better than the other samples. The substrate material effect study, on the other hand, revealed that the TD coated DRM 3 and DRM 51 die samples attained the lowest specific wear rate.
    Full-text · Article · Apr 2012 · Tribology International
  • M. Koç · O.N. Cora · R. Snell · R. Decker · J. Huang
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    ABSTRACT: Deformation behavior and formability limits of thixo-molded Mg AZ61L alloy and regular Mg AZ61 alloy sheets (with similar chemical compositions except for Mn content) were experimentally investigated at elevated temperature. Tensile (uniaxial), hydraulic bulge (biaxial) tests as well as closed-die hydroforming tests were conducted to understand the material behavior at temperatures ranging from 25°C to 300°C and strain rates at 0.0013, 0.013 and 0.13 s -1. It was found that flow stress and the maximum plastic strain increased with increasing temperature and decreasing strain rate. Closed-die wann hydroforming tests were also performed to determine the process window for the sheet alloy. Die cavity filling ratios and thinning of the sheet blanks were measured with non-contact optical photogrammetry. Results indicated that lower strain rates and higher temperatures increase formability, particularly above temperatures of 200°C.
    No preview · Article · Jan 2012
  • Ömer Necati Cora · Muammer Koç

    No preview · Conference Paper · Jan 2012
  • Source
    Mevlut Fatih Peker · Ömer Necati Cora · Muammer Koç
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    ABSTRACT: Tribological variations, surface conditions (roughness, hardness, coating) and surface interactions between micro-stamping dies and bipolar plate blanks play a critical role in determining the surface quality, channel formation and precision of bipolar plates. This study is aimed to understand the cause, mechanism and consequences of interactions between micro-stamping process conditions and bipolar plate quality. A total of 2000 repeated micro-stamping of 51 μm-thick uncoated and 1 μm-thick ZrN coated SS316L sheet blanks into an array of 750 μm micro-channels were performed using 175–220 kN force levels with constant stamping speed of 1 mm/s. Microscopic examinations were conducted periodically on both die and coated & uncoated plate surfaces to observe topographic variations. In addition, corrosion and contact resistance tests were carried out in the same intervals. Analysis of variance (ANOVA) technique was used to determine the significance of the process parameters on channel height, roughness, corrosion and contact resistance differences. The results revealed similar roughness trends for die and plate surfaces during 2000 micro-stampings. ZrN coating with 1 μm thickness dramatically improved corrosion and contact resistance behavior of plates.
    Full-text · Article · Nov 2011 · Fuel and Energy Abstracts
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    Cabir Turan · Necati Cora · Muammer Koç
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    ABSTRACT: a b s t r a c t In this study, metallic bipolar plate (BPP) samples manufactured with stamping and hydroforming under different process conditions were tested for their electrical contact resistance characteristics to reveal the effect of manufacturing type and conditions. Punch speed and force in stamping, and pressure and pressure rate in hydroforming were selected as variable process parameters. In addition, two different channel sizes were tested to expose the effect of BPP micro-channel geometry and its consequences on the contact resistance. As a general conclusion, stamped BPPs showed higher contact conductivity than the hydroformed BPPs. Moreover, pressure in hydroforming and geom-etry had significant effects on the contact resistance behavior of BPPs. Short term corrosion exposure was found to decrease the contact resistance of bipolar plates. Results also indicated that contact resistance values of uncoated stainless steel BPPs are significantly higher than the respective target set by U.S. Department of Energy. Conforming to litera-ture, proper coating or surface treatments are necessary to satisfy the requirements.
    Full-text · Article · Aug 2011 · International Journal of Hydrogen Energy
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    Ender Dur · Necati Cora · Muammer Koç
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    ABSTRACT: Corrosion Coating Hydroforming Stamping a b s t r a c t Bipolar plates (BPPs) made of stainless steels preferred in PEM Fuel Cell (PEMFC) applica-tions due to their high electrical conductivity, low material and production costs, low weight and mechanical strength. However, their corrosion resistances are not at desired levels for real PEMFC working conditions. To overcome this issue, different coating types are suggested. In this study, corrosion resistance behavior of 51 mm-thick SS316L metallic bipolar plates that were coated with the three different PVD coatings (TiN, CrN, and ZrN) at three thicknesses (0.1 mm, 0.5 mm, and 1 mm), and then were formed with two different manufacturing processes (stamping and hydroforming) investigated. Potentiodynamic and potentiostatic corrosion experiments were performed on the coated-formed SS316L plates, and coated-unformed blanks. Corrosion test results indicate that 1 mm ZrN coating demonstrated the highest corrosion resistance among the tested cases regardless of the manufacturing process employed. Moreover, hydroformed bipolar plates exhibited higher corrosion resistance than the stamped BPPs, but lower than the blank samples. Hardness measurements were also performed on the coated samples and resulted in higher corro-sion resistance for harder surfaces.
    Full-text · Article · May 2011 · International Journal of Hydrogen Energy
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    Hasan Gedikli · Ömer Necati Cora · Muammer Koç
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    ABSTRACT: This study aimed to determine the proper combinations of numerical modeling conditions (e.g. solver, element type, material model) for warm hydroforming of AA5754-O aluminum alloy sheets. Assessment of finite element analyses (FEA) is based on comparison of numerical results and experimental measurements obtained from closed-die forming, hydraulic bulge and tensile tests at different temperature (25–300 °C) and strain rate (0.0013–0.013 1/sec) levels. Thinning (% t) and cavity filling ratios (CFR) on the formed parts were taken as comparison parameters. Several numerical analyses employing different element types, solution methods and material models were performed using the commercially available FEA package LS-Dyna to determine the best combination of modeling options to simulate the actual warm hydroforming operation as accurately as possible. Analyses showed that relatively better predictions were obtained using isotropic material model, shell elements and implicit solution technique when compared with experimental results.
    Full-text · Article · May 2011 · Materials and Design
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    Hasan Gedikli · Ömer Necati Cora · Muammer Koç
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    ABSTRACT: This study numerically investigated the effects of process parameter variations such as blank holder forces (800kN-1200kN), strain rates (0.0013/sec, 0.013/sec, 0.13/sec), coefficient of friction (0.05-0.15), temperature (150 degrees C, 260 degrees C) and apex angles (0 degrees, 60 degrees, 90 degrees,120 degrees) on warm hydroforming of AA 5754-O sheet blanks. Warm hydroforming process was simulated through hydraulic bulge test with circular and elliptical die openings. Dome height and sheet thinning were selected as control parameters for formability of AA 5754-O sheet blanks. Results showed that the dome height and formed blank thicknesses did not change significantly with the variation of coefficient of friction and blank holder force. Moreover, increasing forming temperature and non-isothermal conditions yielded slightly better formability. On the other hand, increase in strain rate, and elliptical type of bulge test cavity led to significant decreases in dome height and formed part thinning. Another significant finding was that the elliptical bulge test model and isothermal analyses did not reveal the effect of anisotropy for the sheet material concerned.
    Full-text · Article · Mar 2011 · Key Engineering Materials
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    Ender Dur · Ömer Necati Cora · Muammer Koç
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    ABSTRACT: Metallic bipolar plates are one of the promising alternatives to the graphite bipolar plates in proton exchange membrane fuel cell (PEMFC) systems. In this study, stainless steel (SS304, SS316L, and SS430), nickel (Ni 270), and titanium (Grade 2 Ti) plates with an initial thickness of 51 μm were experimented as bipolar plate substrate materials in corrosion resistance tests. In addition to unformed blanks, SS316L plates were formed with stamping and hydroforming processes to obtain bipolar plates under different process conditions (stamping force, hydroforming pressure, stamping speed, hydroforming pressure rate). These bipolar plates, then, were subjected to corrosion tests, and the results were presented and discussed in detail. Potentiodynamic polarizations were performed to observe corrosion resistance of metallic bipolar plates by simulating the anodic and cathodic environments in the PEMFC. In order to determine the statistical significance of the corrosion resistance differences between different manufacturing conditions, analysis of variance (ANOVA) technique was used on the corrosion current density (Icorr, μA cm−2) values obtained from experiments. ANOVA for the unformed substrate materials indicated that SS430 and Ni have less corrosion resistance than the other substrate materials tested. There was a significant difference between blank (unformed) and stamped SS316L plates only in the anodic environment. Although there was no noteworthy difference between unformed and hydroformed specimens for SS316L material, neither of these materials meet the Department of Energy‘s (DOE) target corrosion rate of ≤1 μA cm−2 by 2015 without coating. Finally, stamping parameters (i.e. speed and force levels) and hydroforming parameters (i.e. the pressure and pressure rate) significantly affected the corrosion behavior of bipolar plates.
    Full-text · Article · Feb 2011 · Journal of Power Sources
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    Öztürk B. · Cora Ö. N. · Koç M.
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    ABSTRACT: Porous metallic surfaces are used in wide range of applications including energy conversion systems, heat exchangers (for heat and mass transfer) and biomedical implants (for osseointegration) etc. This study proposes a methodology for manufacturing of 3-d gradient porous surfaces that can be utilized for both advanced heat transfer products and solid oxide fuel cell (SOFC) interconnects applications. Annealed spherical copper powders up to 106 micron in diameter were compacted onto thin copper substrates. Manufacturing parameters such as temperature, pressure, and powder size were the variables in the experimentation. In addition, manufactured samples were sintered at different temperatures (600 and 900 degrees C) to reveal the effect of sintering temperature on porosity and microhardness. Analyses showed that both manufacturing parameters and sintering temperatures are effective on the porosity and microhardness.
    Full-text · Conference Paper · Jan 2011
  • Peker M. F. · Cora Ö. N. · Koç M.
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    ABSTRACT: Micro-stamping offers high productivity, and accuracy at low-cost. Thus, it is considered as a prominent method for fabricating bipolar plates (BPP) with micro-channel arrays on large metallic surfaces that to be used in Proton Exchange Membrane Fuel Cells (PEMFC). Tribological variations, surface conditions (e.g. roughness, hardness, and coating) and surface interactions between micro-stamping dies and blank metal plates play a critical role in determining the surface quality, channel formation and dimensional precision of bipolar plates. This study aimed to understand the effect of die surface topography on the manufactured BPP surfaces and correlate, if exists, bipolar plate surface quality (surface topography) to bipolar plate performance (corrosion resistance). A total number of 2000 micro-stampings were performed with 51 mu m-thick uncoated SS 316L and 1 mu m-thick ZrN coated SS316L sheet blanks. Optical and scanning electron microscopy examinations were conducted to observe topographic variations. In addition, corrosion resistance tests were carried out at certain intervals. Analysis of variance (ANOVA) technique was used to determine the significance of the process parameters on roughness and corrosion resistance values. The results revealed similar roughness trends for die and plate surfaces. ZrN coating found to be improving the corrosion resistance behavior of plates substantially.
    No preview · Conference Paper · Jan 2011

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