Optimisation des Procédés d'Elaboration par Refusion (Arc-Slag) – OPERAS

The aim of the project OPERAS is the development of numerical models for simulating the ESR (Electro Slag Remelting) and VAR (Vacuum Arc Remelting) processes. Both processes are based on the remelting of a consumable electrode of the required grade and subsequent solidification of the ingot which results from the remelted metal. They are used to process very high added-value material products for critical application in domains such as energy, aerospace or nuclear power plants. Three companies, which are currently running the remelting processes, have decided to participate, along with LSG2M (Laboratory of Materials Science and Engineering and Metallurgy at Nancy School of Mines), to build comprehensive and validated models for these processes: - Aubert&Duval, which belongs to the ERAMET group, produces steels and superalloys, - CEZUS (AREVA group) produces zirconium alloys, - TIMET Savoie, French part of the TIMET group, produces titanium alloys. The research team « Materials Processing » has been developing various numerical models, in particular for ESR and VAR processes. Indeed, mathematical modelling and numerical simulations are of great interest for the producers, since experimental studies performed on full-scale plants are very expensive and difficult to achieve. A numerical model enables linking the operating parameters (ingot size, melting rate, cooling circuit) to the local solidification conditions which are responsible for the quality of the final product. A new situation (need for a loss of weight of steel structure components in the transportation industry, especially aerospace, new competitors – China, India, Eastern countries – on the reactive metals market, more restrictive environmental regulations) requires making a step forward in the process modelling activity, as well as the experiments on industrial furnaces. The project OPERAS is aimed to help the producers to consolidate their position of leader in their field of activity. From an academic viewpoint, the project contributes to the synergy between 'Materials Processing' and 'Solidification' research groups. The project has a 48 month duration. It is composed of 3 thematic directions and 9 interconnected research operations, which are detailed in the scientific proposal, and associates: - development and optimization of the numerical models, - characterization of the remelted alloys, in terms of solidification structure and microsegregation, - full-scale plant experiments (electric arc behaviour, lateral cooling measurements), - validation of the models, by comparing the simulation results with the observation of steel, superalloy, titanium and zirconium ingots specifically processed to that purpose. Among the most important scientific developments planned in the project, we can highlight a detailed model of the genesis and growth of solidification structures and associated segregation, and its implementation in the numerical codes previously created. Achievement of novel in situ experiments, designed to precisely characterize the structure and dynamic behaviour of the electric arc, is also fully representative of the project multi-scale approach. OPERAS will give the opportunity for two PhD theses. The first one (a CIFRE convention between LSG2M and CEZUS) will start right from the beginning of the project. The PhD student will introduce the modelling of solidification structures in the computational code SOLAR, which is a macroscopic simulation model of the VAR process previously developed at LSG2M). The growth of the structures will be simulated with an averaging method, in a way similar to the numerical software SOLID, also developed at LSG2M a few years ago. In addition to the numerical part, the student will participate to experiments at CEZUS, aimed to determine the structures of remelted alloys, and actual remeltings designed for the validation of the final model. A second PhD student has the main task of implementing in SOLECS (SOLAR counterpart for Electro Slag Remelting) the explicit calculation of thermal transfer in the electrode, precise simulation of the melting of the immersed tip, and electromagnetic calculations in the crucible. In parallel, he/she will participate to industrial melt trials at Aubert&Duval's plant in Les Ancizes, in order to validate the new subroutines.