Description du sujet de thèse

Context, objectives, methodology
The aim of this project is to use cooling systems initially developed at IMN to increase the productivity of additive manufacturing processes for the purpose of controlling the microstructure developed during the manufacture of a part via arc-wire processes with high-property alloys. The alloy chosen for this study - 17-4-PH - owes its high mechanical properties to its complex microstructure, and is therefore subject to numerous metallurgical phenomena that can be controlled via the thermal path followed during part manufacture.
The development of wire-arc additive manufacturing (WAAM) has highlighted the growing need for thermal process management. Controlling thermal phenomena is key to enhancing the mechanical performance of products (particularly through microstructure) and increasing process productivity.
Precipitation-hardened martensitic stainless steels, such as 17-4-PH and 15-5-PH for example, develop a wide range of microstructures with very different properties, depending on the thermal paths traversed during additive manufacturing. These microstructural elements and the solid-phase transformations that give rise to them include the amount of residual δ ferrite, the martensitic transformation of austenite, copper precipitation and the reversion of martensite to austenite.
To the best of our knowledge, the scarce literature on additive manufacturing processes on precipitation-hardened martensitic stainless steels most often concerns powder-bed laser processes, is mostly limited to noting the presence of δ-ferrite and performing a final precipitation treatment. The effect of interpass time and post-fabrication heat treatments on the microstructure and properties of 17-4-PH in WAAM has been studied in the literature. No control of cooling rate via active systems has so far been reported in the literature.
The main aim of this thesis project is to use thermal path management during additive manufacturing (via cooling systems and by playing on interpass times), not as a means of increasing process productivity but as a means of controlling the microstructure obtained in order to get as close as possible to the microstructures and properties obtained by conventional manufacturing processes (thermomechanical treatments not usable in additive manufacturing). This fine management of microstructures means that, for example, the δ → γ transformation can be advanced more effectively, and precipitation treatment can be carried out during manufacture in areas that have undergone martensitic transformation, without being able to take place in the rest of the material; even paving the way for the manufacture of parts with a microstructure gradient.

Contexte de travail

Environment : The candidate will join the Institute of Matérials of Nantes Jean Rouxel (Chantrerie site), on the Polytech Nantes site. With over 120 researchers, the IMN offers a resource-rich environment, including state-of-the-art equipment for materials deposition and characterisation. During the course of the thesis, some characterisation on large instruments will be envisaged. Candidate profile: The candidate should hold an engineering degree or a master's degree in materials science at the start of the thesis. He or she will have knowledge of materials science, particularly metallurgy, and a keen interest in implementing experimental projects. The candidate will be able to carry out bibliographical research, mainly in English, set up experimental protocols in collaboration with the laboratory's management and technical staff, demonstrate rigour, organisation and autonomy as well as curiosity and initiative. You will also be expected to be able to analyse, summarise and communicate both orally and in writing - in French and English.

Le poste se situe dans un secteur relevant de la protection du potentiel scientifique et technique (PPST), et nécessite donc, conformément à la réglementation, que votre arrivée soit autorisée par l'autorité compétente du MESR.

Contraintes et risques

The doctoral student must comply with the laboratory's internal rules.