Publications

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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.
    Journal of Power Sources 01/2014; 246:788-799. · 5.26 Impact Factor
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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.
    Tribology International 04/2012; 52:50-60. · 1.54 Impact Factor
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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.
    International Journal of Hydrogen Energy 01/2012; 37:18181-18204. · 3.55 Impact Factor
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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.
    International Journal of Hydrogen Energy 08/2011; 36:12370-12380. · 3.55 Impact Factor
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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.
    International Journal of Hydrogen Energy 05/2011; 36:7162-7173. · 3.55 Impact Factor
  • 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.
    Materials & Design. 05/2011; 32(5):2650–2662.
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    Muammer Koç, Eren Billur, Ömer Necati Cora
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    ABSTRACT: An in-depth understanding and full characterization of mechanical behavior for sheet materials are required since it is critical to establish the highly reliable material models over a broad range of strain levels for accurate modeling and analysis of sheet material deformation processes such as stamping, hydroforming, deep drawing, etc. Hydraulic bulge testing of sheet materials has been known to provide flow stress properties at higher strain levels compared to commonly used tensile tests mainly due to the fact the tested specimens are strained under biaxial loading conditions. However, analysis of the hydraulic bulge test data has not been standardized yet as there have been numerous approaches developed and adopted throughout the years. In this study, different approaches for the analysis of hydraulic bulge were compared with experimental results to determine the best combination in obtaining accurate flow curves models at room and elevated temperature conditions for different lightweight materials of interest for several industrial applications (AA5754 and AISI 201). It was determined that Panknin’s bulge radius and Kruglov’s thickness calculation approaches are the best combination to accurately obtain the flow curves at both cold and elevated temperature conditions.
    Materials & Design. 01/2011;
  • 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.
    Fuel and Energy Abstracts 01/2011; 36(23):15427-15436.
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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.
    Journal of Power Sources 01/2011; 196(3):1235-1241. · 5.26 Impact Factor
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    Ömer Necati CORA, Dahye Min, Muammer Koç, Massoud Kaviany
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    ABSTRACT: Themanufactureofmicroscale-modulatedcoatings(periodic,porousstackswithathinbaselayer)undermass-productionconditionswasinvestigatedexperimentallytoestablishrobustprocessparameters.Copperpowders(averagediameterof100μm)werecompactedonacoppersubstrateundercontrolledpressure,temperature,surfacegeometryandprocessingsequences(e.g.,sinteringbeforeandaftercompaction).Porosity,stackheight,pitchandbasethicknessofcoatingsweremeasured.Resultsshowthataminimumtemperatureof350 • Candpressureof25MPaarerequiredforpermeablecoatings,whensinteringiscarriedoutaftercompaction.When'sinteringbeforecompaction'isfollowed,pressurevaluesinexcessof100MPaareneededandsurfacesfromthisapproachblocktheporesanddiminishpermeability.Poolboilingheattransferexperimentswereconductedonselectedcoatings,showingthatthebestenhancementisbylow-pressurecompationcoatings.
    Journal of Micromechanics and Microengineering 01/2010; 20:35020-9. · 1.79 Impact Factor
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    Journal of Micromechanics and Microengineering 01/2010; 20:035020. · 1.79 Impact Factor
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    Sasawat Mahabunphachai, Ömer Necati Cora, Muammer Koç
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    ABSTRACT: Metallic bipolar plates in PEM fuel cells offer low-volume, low-mass and low-cost stack fabrication in addition to superior durability when compared to composite bipolar plates, which suffer due to their much higher thickness and less durability. This study aims to address the formability and surface topography issues of metallic bipolar plates fabricated by stamping and hydroforming technologies. Particular emphasis was given to process repeatability, surface topology, and dimensional quality of bipolar plates that would greatly affect the corrosion and contact resistance characteristics. Thin metal sheets of several alloys (i.e., SS304, SS316L, SS430, Ni270, Ti grades 1 and 2) were used in the fabrication experiments. SS304 and SS316L were shown to possess better formability when compared to other alloys that were used in this study, while SS430 and Ti grade 2 demonstrated the worst among all. Channel formability was observed to be greatly affected by the hydroforming pressure, while it does not differ much above certain level of stamping force. The confocal microscopy analyses showed that surface roughness values of the formed samples were altered significantly when compared to the initial flat blanks. In general, increasing hydroforming pressure and stamping force yielded higher surface roughness values at channel peaks. In addition, the surface topography was shown to be influenced mainly by the pressure level rather than the pressure rate in hydroforming process.
    Journal of Power Sources 01/2010; 195(16):5269-5277. · 5.26 Impact Factor
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    Ömer Necati CORA, Muammer KOÇ
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    ABSTRACT: a b s t r a c t Newer sheet alloys (such as Al, Mg, and advanced high-strength steels) are considered for automotive body panels and structural parts to achieve lightweight construction. However, in addition to issues with their limited formability and high springback, tribological conditions due to increased surface hardness and work hardening necessitate the use of alternative microstructurally improved die materials, coatings, and lubricants to minimize the wear-related die-life issues in stamping of advanced high-strength steel grades. This study aims to investigate and compare the wear performances of seven different, uncoated die materials (AISI D2, Vanadis 4, Vancron 40, K340 ISODUR, Caldie, Carmo, 0050A) using a newly developed wear testing device. DP600 AHSS (advanced high-strength steel) sheets were used in these tests. It was concluded that the tested die materials demonstrated higher wear resistance performance when compared to the conventional tool steel AISI D2 die material.
    International Journal of Machine Tools and Manufacture 08/2009; 49:897-905. · 2.26 Impact Factor
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    Ömer N Cora, Yusuf Usta, Muammer Koç
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    ABSTRACT: Integrated and compact products necessitate the use of advanced thermal management systems with reduced footprint and cost as well as increased efficiency. Micro-scale, porous and modulated (i.e. channels, pyramids, etc) surfaces offer increased surface area for a given volume and lead to two-phase heat transfer conditions with efficiency enhancements up to 300%. Such surfaces made of copper powders were demonstrated to be quite effective by several researchers after they were produced in controlled lab environments. Similar surfaces made of high temperature resistant materials such as stainless steel, nickel and titanium can also be used in fuel processor, SOFC and PEM fuel cell applications as bipolar/interconnect plates. However, their fabrication under mass-production conditions for marketable and cost-effective products requires well-established process parameters. In this study, warm compaction of copper powders onto thin copper solid substrates was experimented with under different compaction pressure (15–50 MPa), temperature (350–500 °C) and surface geometry (flat, large and small channeled) parameters using a design of experiment (DOE) approach to determine the proper process conditions. Porosity and bonding strength of compacted samples were measured to characterize their feasibility for compact and/or micro-scale heat/mass transfer applications. Results showed that a minimum 350 °C temperature and 15 MPa pressure level is necessary to obtain sound porous and micro-channeled surface layers. It was also found that at higher pressure levels (50 MPa), fabrication of micro-scale surface structures is highly repeatable with enhanced bonding strength characteristics. DOE findings will be used to establish proper process conditions to produce such porous surfaces using a continuous roll compaction process in the future.
    Journal of Micromechanics and Microengineering 03/2009; 19(4):045011. · 1.79 Impact Factor
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    ABSTRACT: A new fabrication method (hot-powder compaction) is developed to readily make 2-D and 3-D modulated coatings for enhanced pool-boiling performance. The modulated coatings are 2-D and 3-D stacks with different height, width, and pitch (modulation wavelength), and made with different particle diameters and porosities. The maximum measured critical heat flux (qCHF) of 2-D and 3-D modulated coatings are 3.3 and 2.0 times that of the surface without coatings (plain). As expected from the hydrodynamic stability theory, the experimental results for 2-D coatings are similar to 3-D coatings and show that qCHF strongly depends on the modulation wavelength, while particle diameter and porosity have a little effects. The results also suggest that a stack aspect ratio (height to pitch) larger than unity is needed for modulation enhancement of the critical heat flux.
    International Journal of Heat and Mass Transfer - INT J HEAT MASS TRANSFER. 01/2009; 52(11):2607-2613.
  • Ö. N. Cora, M. Akkök, H. Darendeliler
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    ABSTRACT: In metal forming operations, the friction between die and workpiece is a complex phenomenon and has a strong influence on the metal forming parameters and the product quality. This study aims to investigate the effect of friction in cold forging operations using different friction models. In the analyses, the forging operations are simulated using the Coulomb and constant friction models, and the results are compared with the simulation results obtained by using the variable friction models developed by Wanheim—Bay (general friction model) and Levanov, which are integrated into the program. Cylinder upsetting and bolt head forging are numerically simulated and the cylinder upsetting results are compared with the experimental data. The results showed that depending on the workpiece geometry and the contact pressure, the coefficient of friction may change significantly during the process and the constant coefficient of friction may not simulate the actual friction condition in the cold forging process.
    ARCHIVE Proceedings of the Institution of Mechanical Engineers Part J Journal of Engineering Tribology 1994-1996 (vols 208-210) 01/2008; 222(7):898-908. · 0.63 Impact Factor
  • Ömer N. Cora, Muammer Koç
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    ABSTRACT: ABSTRACT
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    Ömer Necati CORA
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    ABSTRACT: Friction is one of the important parameters in metal forming processes since it affects metal flow in the die, forming load, strain distribution, tool and die life, surface quality of the product etc. The range of coefficient of friction in different metal forming applications is not well known and the factors affecting variation are ambiguous. Commercially available FEA packages input the coefficient of friction as constant among the whole process which is not a realistic approach. In this study, utility of user-subroutines is integrated into MSC SuperForm v.2004 and MSC Marc v.2003 FEA packages, to apply a variable coefficient of friction depending on the contact interface conditions. Instead of using comparatively simple friction models such as Coulomb, Shear (constant) models, friction models proposed by Wanheim-Bay and Levanov were used to simulate some cold forging operations. The FEA results are compared with the experimental results available in literature for cylinder upsetting. Results show that, large variation on the coefficient of friction is possible depending on the friction model used, the part geometry and the ratio of contact normal pressure to equivalent yield stress. For the ratio of contact normal pressure to equivalent yield stress values above 4, coefficient of friction values are approximately same for both friction models.
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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.
    Journal of Power Sources · 5.26 Impact Factor

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