David Mellé obtained his engineering degree from the Ecole Nationale Supérieure de Mécanique et d’Aérotechnique (ENSMA) in 2009. After an almost ten years of experience in design and R&D activities of aircraft wheels and brakes, he joined the Safran group research center, Safran Tech, in 2019. As a research engineer of the Material and Processes department, his activities are focused on the impact of the surface integrity of additively manufactured components on their fatigue life. Supported by experimental results for the Ti-6Al-4V titanium alloy his work deals with the impact of finishing processes, especially chemical etching, on the fatigue behaviour of additively produced components. He is currently studying the effects of these treatments on the fatigue life, crack initiation mechanisms and the population of surface micro-geometric features in order to propose the best approaches to accurately design safe components by additive manufacturing and to be able to efficiently specify the required surface state. David Mellé is one of Safran Tech’s specialists on the fatigue life of metallic materials.
Producing structurally loaded components by additive manufacturing processes is still a challenge that is partly due to the high surface roughness that is generally obtained. This is true despite increasing efforts in the development of finishing processes to improve the surface state.
Different studies concerning the fatigue strength of additively produced Ti-6Al-4V components, especially by laser powder bed fusion, are available in the scientific literature. The crack initiation mechanisms encountered, including after finishing processes, are also well documented. However, in order to correctly design highly loaded components, it is important to be able to identify the critical porosity, or critical surface feature, in the population of surface features.
Fatigue tests have been conducted on plane bending coupons with a rectangular section in the high cycle fatigue domain. The complete scanning of the tested surface, before and after testing, made it possible to both identify the population of surface micro-geometric features and determine the critical porosity in this population. Classical approaches (especially Kitagawa-Takahashi diagram) have been applied. Several parameters have been compared to determine the ideal parameter to evaluate the criticity of the surface features and to predict the most critical one. Even though the Murakami √area parameter establishes a consistent link between critical porosity and the fatigue strength, it is more ambiguous concerning the identification of the killer porosity from the population.