Ömer Necati Cora

Micro-manufacturing of micro-scale porous surface structures for enhanced heat transfer applications: an experimental process optimization study

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.

Modelling of variable friction in cold forging

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.

Investigations on the Effect of Coating Material and Method on Die Wear in Stamping of Advanced High Strength Steels

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.© 2013 ASME

Finite Element Analysis and Experimental Validation of Warm Hydromechanical Deep Drawing Process

This study intended to establish finite element analysis (FEA) model of warm hydro mechanical deep drawing process (WHMD) of cylindrical cups by means of commercial FEA package Ls-Dyna The validity of established FEA model is verified by means of WHMD experiments through several studies. It was noted that the established model successfully simulated the real process leading to significant cost and time spent on trial-error stage in hydromechanical deep-drawing of lightweight alloys.

Parametric Investigation of Circular and Elliptical Bulge Tests in Warm Hydroforming Process for AA5754-O Sheet

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 °C, 260 °C) and apex angles (0º, 60º, 90º,120º) 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.

Thermal conductivity of different colored compomers

Background Compomers are mostly used in primary dentition. The thermal conductivity properties of traditional or colored compomers have not been investigated in detail so far. The aim of this in vitro study was to assess and compare the thermal conductivities of traditional and colored compomers. Method Two sets of compomers – namely, Twinky Star (available in berry, lemon, green, silver, blue, pink, gold and orange shades) and Dyract Extra (available in B1, A3 and A2 shades) –were included in this study. All of the traditional and colored compomers were applied to standard molds and polymerized according to the manufacturers’ instructions. Three samples were prepared from each compomer. Measurements were conducted using a heat conduction test setup, and the coefficient of heat conductivity was calculated for each material. The heat conductivity coefficients were statistically analyzed using Kruskal-Wallis and Duncan tests. Uncertainty analysis was also performed on the calculated coefficients of heat conductivity. Results Statistically significant differences were found (p<0.05) between the thermal conductivity properties of the traditional and colored compomers examined. Among all of the tested compomers, the silver shade compomer exhibited the highest coefficient of heat conductivity (p<0.05), while the berry shade exhibited the lowest coefficient (p<0.05). Uncertainty analyses revealed that 6 out of 11 samples showed significant differences. Conclusions The silver shade compomer should be avoided in deep cavities. The material properties could be improved for colored compomers.

Numerical investigation of coefficient of friction in copper powder compaction process at micro scale

Multi Particle Finite Element Method (MPFEM) which is a power full approach for particle systems analyzes particle interactions via different friction models. In this study, Amontons-Coulomb constant friction (ACM), Wanheim/Bay generalized friction (WBM) and Levanovs friction models (LFM) are utilized in MPFEM in order to obtain coefficient of friction at particle-particle and particle-die wall interactions in spherical copper powder compaction. Friction models are introduced into the analysis by user subroutines. Compaction processes at room temperature and at 270°C were investigated by terms of coefficient of friction, shear stress and equivalent strain. Although equivalent strain curve of WBM and LFM are in good agreement, ACM resulted in higher equivalent strain and shear stress values. Coefficient of friction those were obtained with WBM and LFM varies in a reasonable range.

Material Characterization of Ultrasonically Consolidated Laminated Ti-Al Composites (UC-LMC)

Ultrasonic consolidation (UC) is an additive manufacturing process where thin and dissimilar metallic layers are bonded through the action of ultrasonic oscillation energy with low energy consumption. The surface oxide layer and the other contaminations between two surfaces are broken up with ultrasonic oscillation improving the bonding strength. This study aimed for investigating the mechanical behavior of laminated metal composite (LMC) blanks that consist of several layers of commercially pure titanium (CP-1) and pure aluminum (AA 1100) foils with different number of layers. The LMC blanks were ultrasonically consolidated on a thick aluminum substrate with three different numbers (1, 3, 5) of bi-layers. Each bi-layer consists of UC bonded one AA 1100 and one Ti foils. Both uniaxial (tensile) and biaxial (hydraulic bulge) tests were carried out under two strain rates and four different temperature levels to reveal the mechanical response of LMCs with different conditions. Increase in number of bi-layers resulted in higher overall strength of LMC’s as titanium content in LMC is increased. Delamination of layers was observed for 1-bilayer LMC’s at room temperature while curling was noted at higher temperature tests. The results obtained from tensile and hydraulic bulge tests were compared to observe significant differences in UTS values and elongation. The effect of temperature, loading condition, and strain rate on the material responses were discussed on the basis of test results. At low temperature, the strain and strength values of bulge samples were higher than the values of tensile samples. However, at high temperatures, lower strain and lower strength were obtained from bulge test. The maximum strain of 0.46 was obtained at 300°C test temperature for 5 bi-layer both parallel rolling direction sample in the all LMCs from tensile test.Copyright

Surface Topography Evolution During Long-Run Micro-Stamping of Bipolar Plates (BPPs) and Effects on Corrosion and Contact Resistance Characteristics

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.Copyright