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RESEARCH PRODUCT

Additive manufacturing, new elaboration technique of metallic materials : impact of the microstructure on high temperature durability of AISI 316L steel

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Additive Manufacturing (AM) processes are able to elaborate, in a single step, metallic parts with very complex geometry, as close as possible to their final dimensions, based on 3D CAD model. Despite a very large number of studies on AM parameters, very few studies report on the high temperature behavior of the metallic materials produced by these processes. This thesis work evaluates the impact of the Selective Laser Melting (SLM) process on the high temperature durability of the austenitic stainless steel AISI 316L, commercial grade widely used in the industry. For this purpose, the reactivity of SLM samples was studied in different conditions (atmosphere, temperature, isothermal or cycling) and compared to that of the wrought 316L, used as reference.SLM steel has better resistance to high temperature oxidation (700-1000°C), in all the considered experimental conditions. The better behavior was related to a higher Cr content available at the metal surface, high enough to maintain the formation of protective Cr2O3 layer, unlike the wrought material. Several hypotheses were discussed in order to explain the better diffusion of Cr from bulk to the surface of SLM material. The better behavior of SLM steel was explained through its microstructure, typical of laser melting AM processes. Indeed, it contains a high number of dislocations and sub-grain boundaries that favor the diffusion. The presence of oxide nano-inclusions also has positive impact on behavior of the SLM steel over long ageing periods at high temperature.