Papers by Author: Roland Taillard

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: The current work focuses on the particular case of dissimilar 6082 Al alloy/pure copper butt-friction stirred joints. It takes advantage of voluntarily non optimized welding conditions in order to test the potential of an original approach of identification of the welding defects by means of a single tensile test. The sequence and mechanism of the fracture events arise from their localization on the fracture surfaces thanks to strain maps obtained by digital image correlation. This technique of flaws identification is proved to be particularly efficient at least with the present highly damaged welds.

Abstract: Effect of the third alloying element Mn on Cu-precipitation was studied in a binary Fe-1.3% Cu alloy. Precipitation in both the alloys was investigated after homogenization treatment and subsequent artificial aging. Advanced characterization techniques such as Positron Annihilation Spectroscopy (PAS) and Tomographic Atom Probe (TAP) were used to establish the chemical composition, morphology, size and number density of the Cu-rich phases. Combined results of PAS and TAP were particularly useful in order to follow the Cu precipitation in the binary alloy. At short aging times, addition of Mn significantly increased the kinetics of hardening while its effect on the magnitude of precipitation strengthening is only marginal. It further increases the over-aging kinetics.

Abstract: The field of application of composites is ever-growing because of their unrivalled combinations of functional and structural properties. Such associations seem the more improvable as there are at once manifold possibilities to modify the architecture of composites, their route of elaboration, an ever-increasing scientific potency for both experimental investigations and modelling, and always more demanding materials performances in projects. This presentation wants to illustrate all these aspects by the example of metal matrix composites (MMCs) mostly elaborated by severe plastic deformation. Emphasis is given to results dealing with the effects of composite design and conditions of metalworking on the success of the fabrication process that depends on the uniformity of deformation and/or on the quality of the interfaces. At last, the consequences of all these data, and the need of complementary work, for the continuous improvement of the properties MMCs are delineated.

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.

Abstract: Composites made of niobium filaments within an oversaturated α-CuSn bronze matrix were processed by restack bundle drawing. The differences in mechanical behaviour of the various components entail an heterogeneity of deformation. Such a feature is mostly unfavourable not only for the phase transformations that govern the microstructure and the electromagnetic properties but also for the composite mechanical integrity. The obtained results finally demonstrate the chief importance of the selection of both the design of the composite and the processing steps.