Ti-6Al-4V lattices obtained by SLM: characterisation of the heterogeneous high cycle fatigue behaviour of thin walls.

Abstract

Architectured materials are attracting increasing interest since a few years, achieving numerous excellent specific properties compared to fully dense materials. The transport industry sector see periodic assemblies of elementary cells, called lattices, as a solution to lighten structures. This is made possible by the development of Additive Manufacturing technologies (AM). Among the numerous advantages of AM, these processes allow to create a wide range of porous cell topologies, and to tailor their complex geometry with the desired properties. However, understanding their fatigue properties is essential to validate their long-term use in load-bearing parts.The FA process plays an important role in fatigue life, as it generates residual stresses, heterogeneous microstructure, as well as surface and volume defects (roughness, gaz porosities, lack of fusion porosities), which are preferred sites for fatigue crack initiation. The fatigue life of these structures also depends strongly on their geometry, which, due to their complexity, induces scale and stress gradient effects.Recent studies have recognize thin-walls TPMS (Triply Periodic Minimal Surfaces) structures as the most promising structures for fatigue resistance, especially the gyroid geometry, compared to conventional strut-lattices.The first aim of this work is to characterise the major parameters influencing fatigue life at the lattice scale. Roughness, microstructure will be discribed on a lattice specimen, and topology effect on stress distribution will be numerically quantified. In order to understand the influence of these parameters, and identify the critical region for fatigue life, this work focuses on the local thin-wall scale. Here, the aim is to characterise the high cycle (HCF) fatigue heterogeneous response of Ti-6Al-4V thin-walls manufactured, characterising localy the HCF behaviour of a TPMS lattice structure (R=0.1; uniaxial tension loading, N=1.106, f=40Hz).