M/F - PhD on the study of Aluminum Alloys Produced by a Friction Stir Additive Manufacturing Process

Job Description

Thesis Subject

Additive friction stir deposition (AFSD) is currently a promising emerging process for the production of medium- and large-sized metal parts. AFSD is based on the principle of severe plastic deformation induced by friction, without a change in the material's state. The filler material, typically in the form of a solid rod, is brought into contact with the substrate and rotated by the deposition tool. The material undergoes a temperature increase—below its melting point—which induces a paste-like behavior and enables both deposition and mixing of the material. This technique enables the production of dense deposits with deposition rates in the range of 1–7 kg/h for aluminum alloys that are free of porosity, exhibit a finely recrystallized microstructure, and possess isotropic mechanical properties.
Like all additive manufacturing technologies, AFSD reduces the amount of raw material needed to manufacture parts and also increases productivity. However, despite this potential, AFSD is still not well understood scientifically. The physical phenomena involved—heat generation by friction, material flow, mechanical mixing, formation of multi-material interfaces, and dynamic recrystallization—are complex, strongly coupled, and difficult to model. However, understanding these mechanisms is essential to ensure the quality of the deposits, optimize operating parameters and microstructures, predict in-service performance, and explore critical industrial applications. The economic and societal implications are also significant.
This PhD is part of the ASTRID FALbALA project funded by the ANR. The consortium consists of ONERA (project coordinator), LMS (the thesis's host laboratory), MATEIS, CETIM, KNDS France, and Constellium. The main objective of the FALbALA project is to establish the relationship between the manufacturing parameters of the AFSD process, the microstructure induced in the samples produced by this process, and their mechanical behavior, in the case of two aluminum alloys: one that hardens structurally (7020) and the other through work hardening (5083).

The objective of this thesis is to establish a relationship between the thermal fields induced by the AFSD process and the metallurgical state at every point within the material, as well as the mechanical behavior of parts manufactured by AFSD, using an aerospace aluminum alloy (grade 7020).
The first step is to establish a detailed understanding of the metallurgical state of the deposited material. Indeed, the locally obtained precipitation state is both characteristic of the thermal profile as experienced by the material and, to a large extent, responsible for the level of local hardening achieved. Characterizing the precipitation state is therefore one of the primary challenges of this thesis. We will rely on several complementary characterization techniques: local measurements using differential scanning calorimetry (DSC) and scanning and transmission electron microscopy will be compared with more global characterizations obtained through thermoelectric power (TEP) measurements and X-ray diffraction (XRD). The next step will be to precisely link the impact of thermal fields to the precipitation state at every point on the wall, not only at the time of material deposition but also following thermal cycling caused by the deposition of a new layer on top of the previously deposited material. The use of the PreciSo mean-field model, developed at MATEIS by Michel Perez, will enable the simulation of the growth of hardening precipitates based on thermal fields. These simulations can be compared with local measurements of precipitate sizes and volume fractions. This task serves a dual purpose: first, to characterize the strong microstructural gradients in the direction of precipitation, as observed in previous studies [Girault, 2026 and Puybras, 2024]; and second, to validate the accuracy of the predictions based on thermal and even thermomechanical measurements and simulations carried out by the ANR project partners.
The objective of the second stage is to link the precipitation state and its evolution to local mechanical properties. Particular attention will be given to describing the (visco)plastic behavior of the material, and specifically to the yield strength and work hardening parameters under thermomechanical stresses, which can induce changes in the precipitation state. The modeling approach will initially be macroscopic; that is, the dependence on the precipitation state will be explicitly incorporated through the spatial and temporal evolution of the behavior law parameters. This microstructure dependence has already been introduced in this way in previous work [Girault, 2026] in the case of the elastic limit, yielding promising results that justify such an approach. The goal of this thesis is to extend this approach to the entire viscoplastic behavior. Mechanical characterization tests, such as cyclic strain hardening and relaxation tests, will be performed on samples taken from different locations within the specimens manufactured by AFSD, using various process parameters.

The final stage of this thesis will consist of demonstrating the feasibility and relevance of the developed model with regard to accounting for an interface in samples fabricated using the AFSD process. The focus will be on the transition between the substrate and the deposited material, which represents a textbook example for studying repair solutions using the AFSD process for damaged parts. It will therefore be necessary to account for variations in grain morphology and size between the substrate—typically a thick, pre-rolled aluminum plate characterized by coarse, elongated grains in the rolling direction—and the deposited material, which is characterized by small, recrystallized, equiaxed grains free of defects. The interface corresponding to the mixture of materials, for its part, consists of a complex population of deformed and partially recrystallized grains. To determine the impact of variations in grain morphology and size, it is therefore necessary to employ a so-called micromechanical approach, which involves characterizing the behavior of the interface at the microstructural scale—that is, at the scale of a grain aggregate. To this end, we will draw on the expertise of LMS and ONERA in conducting micromechanical tests to perform tensile tests at the interface between the substrate and the deposited material on micromechanical specimens, thereby enabling tensile testing to be conducted within a scanning electron microscope (SEM). It will then be possible to characterize the morphology and grain size using electron backscatter diffraction (EBSD) analysis of the sample surface prior to testing, and subsequently to determine the evolution of local strain fields through the use of digital image correlation, performed in situ under SEM during mechanical loading. These experimental strain fields can be compared to crystalline plasticity simulations, and here again, it will be possible to draw on the expertise of both the LMS and ONERA, as well as on previous work that has demonstrated the relevance and feasibility of this approach. Since the necessary numerical and experimental methodologies and tools have already been developed and successfully applied in Girault's dissertation [Girault, 2026], the next step will be to adapt them and apply them to the 7020 alloy. The major benefit of this final stage lies in the ability to compare the results obtained at the grain aggregate scale—which explicitly account for the specific characteristics of the microstructure—with those obtained using the macroscopic approach.

References :
[Girault, 2026] F. Girault, Réparation par fabrication additive des pièces endommagées d'un lanceur réutilisable, thèse de doctorat de l'Institut Polytechnique de Paris (2026).
[Puybras, 2024] M. Puybras, Development of 5xxx Aluminium Alloys for Additive Friction Stir Deposition, thèse de l'INSA Lyon (2024).

Your Work Environment

The doctoral student's primary host laboratory will be the LMS, but since the LMS and ONERA sites in Paris are located close to one another, the student will conduct most of the planned experimental characterizations at ONERA. In addition, it should be noted that part of the modeling work will be carried out at the MATEIS laboratory, under the supervision of Michel Perez. Travel to Lyon will be funded by the project.

Constraints and risks

No specific risk associated to the proposed work

Compensation and benefits

Compensation

2300 € gross monthly

Annual leave and RTT

44 jours

Remote Working practice and compensation

Pratique et indemnisation du TT

Transport

Prise en charge à 75% du coût et forfait mobilité durable jusqu’à 300€

About the offer

About the CNRS

The CNRS is a major player in fundamental research on a global scale. The CNRS is the only French organization active in all scientific fields. Its unique position as a multi-specialist allows it to bring together different disciplines to address the most important challenges of the contemporary world, in connection with the actors of change.

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