Laser Welding of Nickel-Titanium and Stainless Steel Wires:Processing, Metallurgy and Properties

The biomedical industry has an increasing demand for dissimilar metal joining processes, which are used for complex configuration designs, such as guidewires and other intravascular interventional devices. Their production becomes more and more challenging as they decrease in size to reach components at the micron range. Nickel-titanium alloys are commonly used for their shape memory and biocompatibility properties, but are difficult to combine with other biocompatible metals, especially ferrous alloys such as stainless steels. Laser welding is a promising technique to achieve such small and complex shape joints. Indeed the laser high energy density reduces the size of the heat affected zone and the high cooling rate can avoid unwanted phase formation, especially in the particular case of dissimilar joining. Moreover, the high versatility of the technique allows to change the dilution factor in the weld pool in order to carefully select the joint microstructure. In this thesis, the laser welding process has been applied to superelastic nickel-titanium (NiTi) joining to stainless steel (SS) in the case of submillimetric diameter wires. The welded couple strength and microstructure have been optimized by investigating the influence of the laser parameters of both pulsed and continuous laser welding modes, to achieve sound welds. First, the NiTi-SS system has been studied using controlled speed solidification experiments that were performed to characterize the solidification path and its resulting microstructure according to the dilution factor of the base materials. Bridgman and infrared furnace experiments were correlated to the ternary Ni-Ti-Fe phase diagram to identify the possible phases that might form during laser dissimilar welding. Then, the laser welding process was optimized according to the previous results using several parameters to modify the solidification interval, dilution factor and cooling rate in particular. The weld quality was characterized by tensile testing and fracture surface analyses, in order to select the welding parameters leading to repeatable sound welded couples. Finally, the fracture behaviour of the welded couples was carefully investigated to understand the limitation of the tensile strength by the NiTi superelastic stress. In situ tensile experiments, mechanical property characterization and modelling were performed to determine the fracture mechanism occurring at the NiTi-weld interface during testing. Based on these observations, a simple composite model was designed to explain this precise fracture location and the upper limit, which is equal to the superelastic stress. Moreover, perspectives were detailed in order to possibly avoid this mechanical strength issue. This thesis has also emphasized the need to connect several complementary techniques, such as mechanical properties investigations, solidification path characterization and modelling to tackle complex materials science issues, such as dissimilar laser welding

Intraoperative MRI for the microsurgical resection of meningiomas close to eloquent areas or dural sinuses: patient series

BACKGROUND: Meningiomas are the most commonly encountered nonglial primary intracranial tumors. The authors report on the usefulness of intraoperative magnetic resonance imaging (iMRI) during microsurgical resection of meningiomas located close to eloquent areas or dural sinuses and on the feasibility of further radiation therapy. OBSERVATIONS: Six patients benefited from this approach. The mean follow-up period after surgery was 3.3 (median 3.2, range 2.1–4.6) years. Five patients had no postoperative neurological deficit, of whom two with preoperative motor deficit completely recovered. One patient with preoperative left inferior limb deficit partially recovered. The mean interval between surgery and radiation therapy was 15.8 (median 16.9, range 1.4–40.5) months. Additional radiation therapy was required in five cases after surgery. The mean preoperative tumor volume was 38.7 (median 27.5, range 8.6–75.6) mL. The mean postoperative tumor volume was 1.2 (median 0.8, range 0–4.3) mL. At the last follow-up, all tumors were controlled. LESSONS: The use of iMRI was particularly helpful to (1) decide on additional tumor resection according to iMRI findings during the surgical procedure; (2) evaluate the residual tumor volume at the end of the surgery; and (3) judge the need for further radiation and, in particular, the feasibility of single-fraction radiosurgery

Three-dimensional chemical analysis of laser-welded NiTi–stainless steel wires using a dual-beam FIB

The biomedical industry has an increasing demand for processes to join dissimilar metals, such as laser welding of NiTi and stainless steel wires. A region of the weld close to the NiTi interface, which previously was shown to be prone to cracking, was further analyzed by energy dispersive spectrometry (EDS) extended in the third dimension using a focused ion beam. As the spatial resolution of EDS analysis is not precise enough to resolve the finest parts of the microstructure, a new segmentation method that uses in addition secondary-electron images of higher spatial resolution was developed. Applying these tools, it is shown that this region of the weld close to the NiTi interface does not comprise a homogeneous intermetallic layer, but is rather constituted by a succession of different intermetallics, the composition of which can be directly correlated with the solidification path in the ternary Fe-Ni-Ti Gibbs simplex

Mechanical and microstructural integrity of nickel-titanium and stainless steel laser joined wires

The biomedical industry shows increasing interest in the joining of dissimilar metals, especially with the aim of developing devices that combine different mechanical and corrosive properties. As an example, nickel titanium shape memory alloys joined to stainless steel are very promising for new invasive surgery devices, such as guidewires. A fracture mechanics study of such joined wires was carried out using in situ tensile testing and scanning electron microscopy imaging combined with chemical analysis, and revealed an unusual fracture behaviour at superelastic stress. Nanoindentation was performed to determine the mechanical properties of the welded area, which were used as an input for mechanical computation in order to understand this unexpected behaviour. Automated image correlation allowed verification of the mechanical modelling and a reduced stress strain model is proposed to explain the special fracture mechanism. This study reveals the fact that tremendous property changes at the interface between the NiTi base wire and the weld area have more impact on the ultimate tensile strength than the chemical composition variation across the welded area. (C) 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved

Le management post-crise: Vers un nouveau paradigme en sciences de gestion ?

International audienc

Executive facing the undergone transition : identity restoration and imaginary challenge- in outplacement

Dans quelle mesure une transition subie force un dirigeant licencié à innover et « se réinventer » ? Le contexte actuel cristallise des enjeux économiques et sociétaux cruciaux, du fait des mutations de l’emploi et de la généralisation de la mobilité. Les cadres dirigeants -loin de l’orientation taylorienne- en subissent les conséquences : ils doivent se défaire d’un script d’ascension et de stabilité de carrière. Cette recherche qualitative se focalise sur le parcours transitionnel de cadres dirigeants, en explore les enjeux, contradictions et potentiels comme savoir expérientiel ; elle vise à mieux comprendre son lien dans la pratique. A travers une étude cas, cette thèse examine le défi anthropologique représenté par une question telle que celle de la transition pour un dirigeant, lorsque cette norme n’est pas voulue mais subie au nom d’un réalisme économique et imposée par des politiques d’entreprise, que l’acteur dirigeant a pourtant consenti à servir volontairement, sans ménagement. Cette thèse souhaite contribuer à la production de savoirs permettant d’éclairer le débat économique et social, en établissant un observatoire clair et intelligible en direction des acteurs concernés, mais aussi de celles et ceux qui gèrent leur régulation sur le marché- plus particulièrement les cabinets d’outplacement. Pour ce faire, elle croise une anthropologie clinique de la transition, outils et pratiques de gestion et le courant critique. Cette recherche est principalement clinique, en ce qu’elle se fonde sur une recherche de sens co opérée, qui conduit le chercheur à s’approcher au plus près du vécu, des affects, des émotions et de la subjectivité d’une situation pour déterminer ce qui s’y joue. Pour rendre visibles des forces invariantes, cette étude croise : analyse de corpus de presse, ethnographie en terrain gestionnaire de transition -l’outplacement- et entretiens qualitatifs avec des cadres dirigeants licenciés, pris en charge ou non par le dispositif gestionnaire dédié. Y sont explorés avec eux les marqueurs socio-historiques de leur destin et leur évolution, les apprentissages qu'ils ont réalisés au cours de leur transition, ainsi que les conditions de leur réinvention : entre restauration identitaire et défi imaginaire. Sont étudiés aussi comment les médias encapsulent le destin à part des dirigeants et continuent par une série de transpositions, d’embarquer l’imago de leur vie de héros dans l’imaginaire contemporain. En dernier lieu, cette thèse cherche à relier les ressorts d’une transition locale d’acteurs majeurs du monde productif aux enjeux plus larges de transition civilisationnelle, actuels.To what extent does an unplanned transition force a fired executive to innovate and “reinvent himself”? The current context crystallises crucial economic and societal issues, due to changes in employment and generalized mobility. Executives -far from the Taylorian orientation- suffer the consequences and have to get rid of a script of career ascension and stability. This qualitative research focuses on the transitional path of executives, explores its issues, contradictions, and potentials as experiential knowledge and aims to better understand its link to practice. It examines the anthropological challenge represented by an issue such as transition for an executive, when this change is not wanted but undergone in the name of economic realism, and imposed by corporate policies, which he or she had previously agreed to serve voluntarily, with dedication This thesis aims to contribute to a production of knowledge that will enlighten the economic and social debate, by establishing a clear and intelligible observatory for the actors concerned, but also for those who manage their regulation on the market - more particularly outplacement firms.To do this, it crosses a clinical anthropology of transition, management tools and practices and critical studies. This research is mainly clinical, in that it is based on a co-operated search for meaning, which leads the researcher to get as close as possible to the experience, affects, emotions, and subjectivity of a situation in order to determine what is at stake. In order to make invariant forces visible, this study combines analysis of a press corpus, ethnography in the field of transition management - outplacement - and clinical interviews with executives who have been fired, whether or not they have been taken in charge by the dedicated management system. It explores with them the socio-historical markers of their destiny and their evolution, the learning they have achieved during their transition, as well as the conditions of their reinvention: between identity restoration and imaginary challenge. We will also study, through the analysis of a press corpus, how the media encapsulate the leaders' unique destiny and continue, through a series of transpositions, to convey the imago of their lives as heroes in the contemporary imagination. Finally, this thesis seeks to link the ins and outs of a local transition of major actors of the productive world to current broader issues of a civilizational transition

Nd:YAG laser joining between stainless steel and nickel-titanium shape memory alloys

Nickel-Titanium (NiTi) shape memory alloys are often used in medical component devices, for instance as guide wires for neurological surgery applications. The manufacture of such devices becomes more and more challenging, especially considering the need to join them with other metals. Laser welding is a promising technique to realize and to guaranty the mechanical stability of dissimilar metal welds, although inherent differences in chemical compositions, absorption, physical and thermo-mechanical properties can lead to severe problems, in particular fracture of the weld due to the possible formation of brittle intermediate phases. Laser welds of NiTi - stainless steel (SS) pieces have been made with a Nd:YAG laser and the weld microstructure have been studied by Scanning Electron Microscopy (SEM). The phases and defects in these welds have been compared with those observed in autogenous NiTi welds and SS welds. They have been put into relation with the Fe-Ni-Ti ternary phase diagram. In addition, Differential Thermal Analyses (DTA) of NiTi-SS alloys and NiTi-SS diffusion couple experiments have been performed in order to gain a better understanding of the phases and reactions occurring during laser welding. This diffusive couple experiments have been analyzed by Energy Dispersive X-ray Spectroscopy (EDX) It appeared that although experiments were performed under controlled atmosphere, oxide layers have restrained the chemical diffusion of concerned elements. Further diffusion couple experiments will be realized with stainless steel welded caps to avoid oxygen contamination during heating