Materials Science Forum Vols. 706-709

Abstract: ODS steels (Oxide Dispersion Strengthened) are candidate materials for fuel cladding in Sodium Fast Reactors (SFR). These materials have good mechanical properties at high temperature due to a dispersion of nanometer-sized oxides into the matrix. Previous studies have shown that melting can induce a decrease of the mechanical properties at high temperatures due to modifications of the nanometer-sized oxide dispersion. Therefore the fusion welding techniques are not recommended and the solid state boundings has to be evaluated. This study is focused on resistance upset welding. Welding experiments and numerical simulations are coupled. The numerical simulation is developed in order to have a better understanding of the thermal and the mechanical phenomena occurring during the welding process. The simulation shows that the welding steps can be divided in two stages. First, the temperature of the contact between pieces increases. Second, the heat generation is mainly located in the cladding leading to the collapse and forging the pieces. The microstructural observations confirm that the major deformation is located in the cladding. Oxide dispersion modification and dynamical recrystallisation has been found for welds achieved with a non optimized process parameter set. The deformation and the temperature seem to be of prime importance in the modification of the oxide dispersion.

Abstract: A detailed examination of the texture evolution occurring within a friction stir weld inAA2195 reveals features that are not consistent with current models of material ow in frictionstir welds. While the deposited weld is dominated by a B ideal shear texture component, thistexture periodically alternates with a B texture, indicating a reversal in the sense of the sheardeformation. In addition, the observed rotation of the orientation of these shear textures, andtheir replacement by a distinct, new shear texture orientation to correct for the misorientationthat develops, reveals a heretofore unobserved characteristic of friction stir welds that providesfurther details about material ow during friction stir welding.

Abstract: Bobbin FSW technique is assessed in terms of process productivity and robustness considering the metallurgical and mechanical qualities of the joint. Several different designed bobbin tools were applied to aluminium A6xxx alloy extrusions, at feed rate of 400-1000mm/min with tool rotation speed of 600-1000rpm. In case of A6068-T6, a joint efficiency of 68% was achieved at 600mm/min, which is almost equivalent to the productivity and quality of the conventional FSWed joints, hence the potential of bobbin FSW technique was suggested. The bobbin tool design was further optimized considering the practical process robustness to the part fit issue, and defect free welds have been achieved for the joints with a set gap up to 1mm. However, the oxide remnant, so called “lazy S” was observed in a joint made with an inadequate designed tool, which significantly deteriorates the mechanical properties.

Abstract: The authors have developed a new friction-stir welding (FSW) tool that enables to weld high-softening-temperature materials (HSTMs), such as steels, titanium and zirconium alloys. The new tool is made of a Co-based heat-resistant alloy strengthened by precipitating intermetallics, Co₃(Al,W), with a L1₂ structure at high temperatures. The Co-based alloy tool exhibits yield strengths higher than 500 MPa at 1000 deg C, so it might have a great potential as a tool material for FSW of HSTMs. In this study, the feasibility of using the Co-based alloy tool with various HSTMs was examined. Changes in the tool shape during FSW and the weld appearances produced with the Co-based alloy tool will be briefly shown.

Abstract: Friction Stir Processing (FSP) has attracted much interest as a tool for refining grain size and achieving high angle boundary misorientation in magnesium alloys. These characteristics have a great influence in key engineered properties such as strength and ductility, which could be markedly improved by means of this technique. The main objective of this work is to study the microstructural modifications produced when FSP is applied to homogenized cast AZ91 and wrought AZ61 magnesium alloys. Several attempts were made for achieving a homogenous microstructure without defects and enhancing the refinement of the grain size in the stir zone. It was revealed that is of great importance to break the initial microstructure, of coarse grains unfavourably oriented for deformation, in order to facilitate the process, particularly in the case of cast AZ91 alloy. It is highlighted that, after breaking up the initial microstructure, is possible to process the material, in subsequent passes, Furthermore, the use of different backing materials as heat sink and a previous heating treatment of the sample were evaluated. Changing the backing plate can improve more the reduction of the grain size during a second pass. Using a copper plate instead of a steel one can promote a refinement up to 700 nm in AZ91 and 1 μm in AZ61. A coolant agent can be used for inhibiting the grain growth causing a little more reduction of the grain size.

Abstract: Friction stir welding is conceptually simple but metallurgically highly complex due to thecombination of severe deformation and high temperature. This is particularly true in the case ofprecipitation strengthened alloys, such as high strength aerospace aluminium alloys, where theheat and deformation of FSW lead to profound changes in both grain structure and precipitatedistribution that ultimately determine weld performance.

Abstract: The aim of the present paper is to report on a three-dimensional (3D) finite element modelling (FEM) of fabrication of multifilamentary composites using a restack bundle drawing process. The challenge arises from the complexity of the wire architecture. The reliability of the predictions ensues from the determination of the necessary mechanical data by means of the most realistic mechanical tests including billets drawing experiments. The simulations allow to determine response surfaces comparing the effects of the numerous fabrication parameters on the drawing force, the local stress triaxiality and the uniformity of deformation. The results agree with both experimental and theoretical data.

Abstract: Electrospinning is a straightforward and low cost method for producing carbon nanofiber (CNF) webs that have interrelated pores with high surface area. The process begins with electrospinning of polyacrylonitrile (PAN) on a Cu target collector. In current production methods, the PAN nanofiber web is taken off from the collector. But in order to omit extra stages of taking off the web from a conductive collector and later putting it back on, we will try to keep the web remained on the Cu collector plate through the carbonizing heat treatment and the electrodeposition, to later use the plate as the current collector of a LIB anode. This facilitates the handling of CNFs throughout the entire process that is now much more suitable for commercialization. This unique structure is very suitable for anode materials (AMs) of Lithium Ion Batteries (LIBs). It improves the kinetics of charge/discharge cycles by reducing lithium transport paths, and creates more stable electrochemical performance by providing space for volume expansions of lithium insertions in charging cycles. CNF webs can be used as AMs, demonstrating these advantages over conventional carbonaceous materials that have long been used as the preferred choice-in spite of having a comparatively low theoretical capacity. In this study we use the CNF web as a template for electrodepositing Sn-Sb alloy, to create the mentioned structural characteristics in a coated layer of an alloy with a higher capacity. The resulting composite is shown to have a higher capacity than the substrate CNF and a good cycling performance.

Abstract: Silicon can be considered one of the most promising anode materials for lithium ion batteries (LIBs) as it has a low charge/discharge voltage at room temperature and exhibits the highest known theoretical lithiation capacity in combination with good safety characteristics. However, the silicon anode experiences enormous volume changes during repeated lithium insertion/extraction processes in every charge/discharge cycle. This leads to severe particle pulverization and results in a quick electrode structure failure. Moreover, the Si has to be amorphous, as crystallized Si can hardly alloy with lithium at room temperature. Amorphous Si thin films are currently manufactured by techniques such as chemical vapor deposition, that usually, are quite expensive, have complex equipment design and have slow rate of deposition. Electrodeposition of Si can offer a suitable alternative method of producing a thin film of amorphous Si by much easier and lower cost means. In this study electrodeposited amorphous silicon on carbon nanofibers (CNFs) have been investigated as the anode material for LIBs. Electrically conductive CNFs are produced by electrospinning and later heat treating of polyacrylonitrile (PAN) precursor. It is shown that the macromorphology of electrospinning-derived CNFs template provides enough space to accommodate the silicon volume expansions. By controlling electrodeposition parameters, cycle-ability of the anode material was optimized. It is envisaged that exceptional characteristics of CNFs and the electrodeposited Si will make the composite material an ideal candidate for the anode material of high-power LIBs.

Abstract: Tape casting is widely used in industrial scale for production of multilayer ceramic capacitors or substrates for different applications. In 2009, it was successfully introduced as standard shaping technology for 3 (BSCF) are shown. The entire scope from the preparation of the used powders, the different manufacturing steps and their optimization potential up to the final tape-cast product will be discussed. The influence of the use of pore forming agents, heat treatment or other parameters during processing will be described in detail. Finally, the option of sequential tape casting of different materials for graded structures as a future step in shaping technology will be presented for different applications.