Thèse Comportement en Fatigue des Assemblages Soudés en Alliages Métalliques Recyclés Effet des Contraintes Résiduelles et de l'Hétérogénéité Microstructurale H/F - Doctorat.Gouv.Fr

Établissement : Université Toulouse II Jean Jaurès École doctorale : MEGEP - Mécanique, Energétique, Génie civil, Procédés Laboratoire de recherche : ICA - Institut Clément Ader Direction de la thèse : Adriana SOVEJA ORCID 0009000672926752 Début de la thèse : 2027-09-01 Date limite de candidature : 2026-11-23T23:59:59 L'objectif principal de ce travail de thèse est de caractériser et de modéliser l'effet de l'hétérogénéité microstructurale induite par le recyclage sur la soudabilité et le comportement en fatigue d'assemblages soudés en alliages métalliques légers, afin d'établir les conditions scientifiques nécessaires à une réintégration fiable de ces matériaux dans des structures soudées soumises à des sollicitations cycliques. Pour atteindre cet objectif trois axes d'étude seront explorés :
1) la caractérisation fine de la variabilité chimique et microstructurale d'alliages recyclés représentatifs et l'évaluation de leur soudabilité selon le procédé de soudage choisi afin d'identifier les mécanismes d'apparition de défaut associés ;
2) la modélisation thermomécanique des contraintes résiduelles générées par le soudage de matériau hétérogène issus du recyclage ;
3) l'étude du comportement en fatigue des assemblages obtenus afin de comprendre les mécanismes d'amorçage/propagation de fissure et formuler des leviers de correction/contrôle du procédé de soudage permettant de compenser les effets délétères de l'hétérogénéité sur la durée de vie en fatigue.
The metallurgical industry faces a growing tension between two objectives that are difficult to reconcile: reducing its environmental footprint by relying more heavily on metal recycling, while maintaining strict mechanical performance requirements for the resulting structures. Solid-state recycling, in particular, is an alternative route that has been studied intensively in recent years [1, 2, 3, 4], as has the valorisation of titanium scrap for new forming processes [5, 6]. The recycling of aluminium and titanium thus offers an already well-demonstrated energy benefit [7], but this second life of the material comes with more pronounced chemical and microstructural heterogeneity than in the case of a conventional alloy, notably through an impurity content, such as iron, that is more difficult to control [8] in recycled alloys. This heterogeneity raises a specific scientific issue as soon as these materials need to be joined by welding. The weldability of a recycled alloy is not necessarily degraded in appearance, but specific defects appear under process conditions that are narrower than for a conventional material [9, 10, 11].
The influence of this residual microstructural heterogeneity inherited from recycling raises a scientific question that remains, to date, largely open: its effect on the fatigue life of welded structures made from a recycled alloy. On bulk material, the observation is unambiguous. While the tensile strength and hardness of recycled alloys deviate only marginally from those of the conventional material, fatigue life can be reduced by a factor close to an order of magnitude [12, 13]. This phenomenon, relatively well characterised on bulk material, has however never been studied in the case of a welded joint, a configuration in which the microstructural gradients and residual stresses specific to the welding process are superimposed on top of it. This scientific territory therefore remains virtually unexplored, even though it directly governs the reliability and durability of welded structures made from recycled alloys.
This gap can in fact be explained by a discontinuity between two scientific fields that have so far been treated separately: on the one hand, studies on the weldability of recycled alloys, focused on the microstructural characterisation and mechanical properties of the joint at the quasi-static scale [9, 10, 11]; on the other hand, studies on the fatigue behaviour of recycled materials, carried out on bulk material outside any welded configuration. The characterisation and modelling tools for fatigue and residual stresses in welded assemblies have only been established for conventional alloys [14, 15, 16]; their validity remains to be demonstrated for a base material exhibiting the uncontrolled microstructural heterogeneity characteristic of recycling routes. Finally, the link between the choice of welding process and the objectives of industrial transition is identified as an emerging and still poorly structured research direction [17, 18].
This thesis therefore proposes to study the effects of the microstructural variability inherited from recycling and of the residual stresses generated by welding on the fatigue life of the resulting welded joint. This work is directly in line with the challenges of industrial transformation and sustainable management of metallic resources, at the heart of the transitions expected across industrial sectors. Ensuring the reliability of welded joints in recycled alloys, such as aluminium, constitutes a major scientific barrier to the integration of recycled material into structures with high safety requirements, starting with the aeronautics and space sectors, where lightweighting, durability and the reduction of the environmental footprint of metallic materials are among the strategic priorities of upcoming industrial transformations. This study aims to overcome the major scientific barrier to the use of recycled metallic alloys in high-performance welded structures, typically in the aeronautics and space sectors, by investigating the still poorly controlled coupling between the microstructural heterogeneity inherited from recycling and the thermomechanical heterogeneity induced by welding. To this end, the study proposes to develop the characterization, modelling, and process control tools required to ensure the fatigue performance of welded assemblies despite this dual heterogeneity, thereby contributing to reducing the material footprint of the sector without compromising structural reliability. 1) Systematic literature review conducted throughout the thesis.
2) Selection and microstructural/chemical characterisation of representative recycled alloys.
3) Production of welds using one or more processes to be defined with ISIM, and characterisation of the weld zone and the heat-affected zone (HAZ).
4) Numerical modelling of residual stresses and experimental validation (XRD).
5) Fatigue testing and fractographic analysis of fracture surfaces.
6) Cross-analysis of results toward a predictive life model and design/process-control recommendations.
7) Scientific dissemination (publications, conference communications) and prospects for transposition to other alloys/processes.

Master 2 ou diplôme d'ingénieur en mécanique, science des matériaux ou génie mécanique.
Compétences scientifiques attendus : connaissance des procédés de soudage, connaissance des mécanismes de fatigue et de la mécanique de la rupture ; compréhension des contraintes résiduelles, modélisation numérique élément fini.
Qualités transversales : autonomie, capacité d'adaptation, goût pour la collaboration internationale.

Application link: https://edd-projets.utoulouse.fr/