Materials Science Forum Vols. 638-642

Abstract: In this work, we studied a NiO/GDC-GDC-LSCF micro-tubular SOFC system using saturated hydrogen at 450-550 oC. The systems were electrochemically and mechanically tested using a number of different methodologies, and were modeled using a 2D axial symmetric steady state non-isothermal computational model incorporating radiation heat transfer, flow and species transport.

Abstract: The performance of fuel cells is largely dependent on the properties of the membrane electrode assembly (MEA) which consists of a polymer electrolyte membrane and carbon based electrodes. The aim of this work was to obtain new polymer membranes and carbon thin films (for comparison) by Plasma Enhanced Chemical Vapor Deposition (PECVD) and proceed to their physical characterization in order to be able to choose the best conditions that may lead to outstanding proton exchange membranes. The films were deposited on silicon wafers and were analyzed by Scanning Electron Microscopy and Energy Dispersive Spectroscopy (SEM/EDS), Electron Spectroscopy for Chemical Analysis and X-Ray Photoelectron Spectroscopy (ESCA-XPS), Atomic Force Microscopy (AFM), Fourier Transform Infrared Spectroscopy (FTIR) and Rutherford Backscattering Spectrometry (RBS). SEM/EDS and XPS analysis have shown that the chemical composition of the films varied as a function of the plasma (relative percentages of the reactant gases CH4 and SF6) while variation of the pressure and RF power did not have a marked effect on the topographic quality and composition of the membranes. AFM analysis, as well as SEM, made possible to observe the topography of the membranes showing that the films are very smooth with some localized defects. Futhermore, AFM micrographs have shown that the roughness (root mean square - RMS) of the membranes have peak to valley differences in the order of some nanometers. RBS and Profilometer techniques indicated that the deposition rate of the films is not linearly dependent on the plasma composition while FTIR spectroscopy has shown the presence of the perfluorinated sulphonate groups.

Abstract: The use of glucose, which is produced from the acid hydrolysis of starch and cellulose, is studied as a fuel in a low-temperature direct-mode fuel cell (LTDMFC) with an alkaline electrolyte. Glucose is regarded as being as good a fuel as bioethanol, because both the fuels give 2 electrons per molecule in the fuel cell without carbonisation problems. However, glucose can be produced with fewer processing stages from starch and cellulose than can bioethanol. In the LTDMFC the fuel and the electrolyte are mixed with each other and the fuel cell is equipped only with metal catalysts. Cellulose as a fuel is of great importance because the fuel for the energy production is not taken from food production. A description of an acid hydrolysis method for starch and cellulose is presented. Values for glucose concentrations in each hydrolysate are analysed by means of a chromatographic method. Each glucose hydrolysate was made alkaline by adding of potassium hydroxide before feed in the fuel cell. Polarisation curves were measured, and they were found to produce lower current density values when compared to earlier tests with pure glucose. The Coulombic efficiency of pure glucose electrochemical oxidation in LTDMFC, which was calculated from a ratio of detected current capacity (As) to the maximum current capacity with the release of two electrons per molecule, was also found to be very low. Concerning the hydrolysates, the glucose concentrations were found to have values that were too low when compared to the earlier tests with pure glucose in a concentration of 1 M. The further development demands for the system under consideration are indicated. The concentration of glucose in the hydrolysate is essential to achieve high enough current density values in the LTDMFC.

Abstract: Solid Oxide Fuel Cells (SOFCs) are gaining more and more importance as auxiliary power units (APU), e.g. for passenger cars, trucks and airplanes. In this context, the main challenge is the development of SOFC stacks, which fulfill the strong requirements for mobile applications. These are a low weight, a low volume, and a high power density with reformate gases but also low long term degradation rates. The paper presents results of investigations of SOFC short stacks for mobile applications. Therefore, a light weight stack design was developed in an industrial consortium in cooperation with the German Aerospace Center (DLR) in Stuttgart and the Research Center Jülich (FZJ). The SOFC stacks were operated at different temperatures, varying fuel gas compositions and different fuel gas flow rates. The short stacks were electrochemically characterized mainly by long-term measurements, by current-voltage measurements and by impedance spectroscopy. The fuel utilizations and the power densities were determined. In order to analyze the uniformity inside the stacks, the electrochemical behaviour of the individual cassettes were analyzed and compared to each other. Finally, the degradation rates and the degradation mechanisms were determined from the long-term measurements.

Abstract: The use of a double sided friction stir welding tool (known as a bobbin tool) has the advantage of giving a processed zone in the workpiece which is more or less rectangular in cross section, as opposed the triangular zone which is more typically found when conventional friction stir welding tool designs are used. In addition, the net axial force on the workpiece is almost zero, which has significant beneficial implications in machine design and cost. However, the response of these tools in generating fine microstructures in the nugget area has not been established. The paper presents detailed metallographic analyses of microstructures produced in 25mm AA6082-T6 aluminium wrought alloy, and examines grain size, texture and mechanical properties as a function of processing parameters and tool design, and offers comparison with data from welds made with conventional tools.

Abstract: As a new solid-state welding process, friction stir welding (FSW) has been successfully used for joining low melting point materials such as aluminum and magnesium alloys, but the FSW of high melting point materials such as steels and titanium alloys is still difficult to carry out because of their strict requirements for the FSW tool. Especially for the FSW of titanium alloys, some key technological issues need to solve further. In order to accomplish the FSW of titanium alloys, a specially designed tool system was made. The system was composed of W-Re pin tool, liquid cooling holder and shielding gas shroud. Prior to FSW, the Ti-6Al-4V alloy plates were thermo-hydrogen processed to reduce the deformation resistance and tool wear during the FSW. Based on this, the thermo-hydrogen processed Ti-6Al-4V alloy with different hydrogen content was friction stir welded, and the microstructural characterizations and mechanical properties of the joints were studied. Experimental results showed that the designed tool system can fulfill the requirements of the FSW of titanium alloys, and excellent weld formation and high-strength joint have been obtained from the titanium alloy plates.

Abstract: In this article, recent investigations on magnesium alloys by friction stir processing (FSP) are addressed. It indicates that remarkable grain refinement and breakup/dissolution of second-phase particles could be achieved simultaneously by FSP. High values of superplastic elongation were achieved in the FSP magnesium alloys at a wide range of strain rates and temperatures. The pinning of heat resistant particles on the grain boundaries in Mg-RE alloys stabilized the fine microstructure, leading to the occurrence of superplasticity at higher temperature and higher strain rate.

Abstract: This study was investigated the influence of preheat temperature and welding speed by a view point of microstructure of joining zone used by friction stir welding (FSW) between ductile cast irons and stainless steels. FSW conditions were carried out by change of tool rotation by 200, 400 and 600rpm, preheat temperature by non-preheat, 573 and 773K, and welding speed by 1～10mm/s. As a result, microstructure of FCD450 side became a pearlite structure changed by preheat condition. Also, stir zone (SZ) of FSW were crystallized the chill structure because of cast iron melted partially why SZ temperature exceeded the eutectic temperature. However, higher welding speed could inhibited crystallization and found the possibility of bonding between ductile cast irons and stainless steels.

Abstract: Friction stir welding (FSWing) induces residual stresses and distortions in welded structures. Such residual stresses reduce the fatigue life of welded components, while the induced distortions prevent the welding of large or thin components. In the present study, needle peening was used to induce additional residual stresses in 2.3-mm thick (FSWed) aluminum alloy (AA) 2024-T3 sheets. This was done with the objective to counterbalance the welding-induced stresses and thus reduce the overall stresses and distortions. The needle peening process, which stems from shot peening, consists of hammering a surface using cylindrical spherical ended shots sliding back and forth in a treatment head. An instrumented needle peening machine was used to carry out peening on as-received (or bare) and bead-on-plate FSWed AA2024-T3 material. In both cases, the width of the peening area corresponded to that of a typical weld. The influence of the peening process parameters such as needle size, applied power and travel speed on the surface quality and magnitude of the induced distortions were evaluated. The results indicate that, by increasing the needle diameter from 1.2 mm to 2.0 mm, the peening-induced deflection on bare sheet material increased by an average value of 27% while the roughness average, Ra, decreased by an average value of 47%. It was also found that a surface finish qualitatively similar to that of conventional shot peening could be obtained by using appropriate needle peening trajectories. Finally, needle peening with an applied power of 10% was sufficient for eliminating 37% of the welding-induced transverse curvature and 82% of the welding-induced longitudinal curvature.

Abstract: Friction stir welding (FSW) is a quite recent welding method which takes advantage of being performed in the solid state. Compared with the usual welding processes, it therefore presents many benefits such as a lower heat-input, a reduction of residual stresses and an elimination of the solidification defects etc.. Up to now, it has essentially been applied to aluminium alloys and far more recently to a small number of bimaterials. The present study deals with three kinds of beads between pure copper and a 6082 aluminium alloy. Both materials were butt joined by FSW. The welds differ by the location of the tool which was placed either at the interface between the two metals or on the copper or the 6082 side of this surface. Their structure was characterized at a multi-scale level by using a number of techniques. Tensile and microhardness tests were also performed. The tool place is shown to govern the microstructure and the ensuing mechanical behaviour of the weld. Its influence on the plastic flow with its repercussions on i) welding defects and ii) mechanical properties is going to be demonstrated. Some ways of improvement of the welding process will finally be suggested.