Rabu, 19 Mei 2010

[France] PhD Studentship in Material Physics at UMET (Unité Matériaux et Transformations)

Abstract

Friction stir welding (FSW) is currently a growing process of solid-state joining. Under development since a decade, its main advantage lies in the absence of intrinsic defects unavoidable in usual liquid-state joining processes. However, the mechanisms of formation of friction-stirred welds still remain largely unknown, due to the complexity of the underlying phenomena:
(i) the strong spatial variations of the deformation and temperature fields,
(ii) the nature and thermomechanical behaviour of materials and tools.



The fields of application of FSW are spreading towards the joining of different materials, especially those with strong chemical affinity, and the microstructural characterization of FSW beads between aluminium alloys and pure copper recently performed in our research group has evidenced the presence, probably detrimental, of intermetallic compounds at the interface. In particular, phases with stoichiometries AlnCup(Xq) - with X an addition element - not mentioned on the equilibrium phase diagrams Al-Cu(-X), have been detected with the nugget, a vortex zone mostly deformed by the process.

In this context, the proposed study, lying within a more general framework of interpretation of the physical phenomena involved in FSW, will aim at elucidating the origin and nature of the previously mentioned AlnCup(Xq) compounds, by means of coupling between experiments and atomic-scale modelling.

The first part of the work will consist in the elaboration of AlnCup(Xq) phases by mechanical alloying of a mixture of powders, followed by their characterization (crystal structure, thermal stability…) via complementary investigations relying on microscopy, calorimetry, X-ray diffraction… The results obtained through this alternative large-strain process will enable to precise and extend the thermomechanical conditions of formation and stability of the metastable compounds induced by FSW. In parallel, numerical simulations will be carried out in the case of a model binary alloy Al-Cu. These simulations, involving ab initio calculations followed by a cluster-expansion-based thermodynamic modelling, will aim at providing a semi-quantitative measurement of the degree of metastability of the concerned phases, leading to data relevant for an improved assessment of the phase diagram. As far as possible, these results, concerning a binary alloy, will be extended via the study of the influence of addition elements (mainly X=Si) on the stability of the intermetallic compounds detected.

References
1. Structure and mechanical properties of friction stirred beads 6082-T6 Al alloys and pure copper, M. N. Avettand-Fenoel, R. Taillard, Ch. Herbelot and A. Imad, Materials Science Forum 638-642, 1209 (2010)
2. The challenge of the friction stir welding of dissimilar materials with the emphasis on the Cu-Al case, M. N. Avettand-Fenoel, R. Taillard, Ch. Herbelot and A. Imad, submitted to Science and Technology of Welding and Joining
3. Hexagonal-based ordered phases in H-Zr, L. Holliger, A. Legris and R. Besson, Phys. Rev. B 80, 094111 (2009)
4. Multiscale modeling of precipitate microstructure evolution, V. Vaithyanathan, C. Wolverton and L. Q. Chen, Phys. Rev. Lett. 88, 125503 (2002)
5. Crystal structure and stability of complex precipitate phases in Al-Cu-Mg(-Si) and Al-Zn-Mg alloys, C. Wolverton, Acta Mater. 49, 3129 (2001)

Contact: Ludovic THUINET
E-mail: Ludovic.Thuinet@univ-lille1.fr
Phone: +33 (0) 320 33 6225
Address: UMET UMR 8207 USTL bat C6 2ème étage Rue Paul Langevin 59655 Villeneuve d`Ascq cedex France

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