Effect of Chemical Etching Time on the Fatigue behaviour of Ti-6Al-4V produced by Laser Powder Bed Fusion

This study focuses on the evolution of the fatigue strength of L-PBF produced Ti-6Al-4V as a function of the chemical etching finishing process. The aim is to identify the critical fatigue crack initiation mechanisms and the transitions between them in terms of the evolution of the surface micro-geometry caused by the finishing process. This has been done using three different geometries and six different surface states, including the machined reference surface state. The evolution of the crack initiation mechanisms, identified via SEM observations of the fatigue failure surfaces, is then used to explain the evolutions of the fatigue strength and the fatigue scatter, in both the finite and the high cycle fatigue-life domains. As expected, chemical etching effects the fatigue life via a polishing effect on the micro-geometry, which directly influences both the finite and the high cycle fatigue domains. It is shown that chemical etching makes it possible to obtain fatigue strengths that are almost similar to those of the machined surface, despite the fact that the roughness is higher, even after an optimal etching time. However, it is also observed that etching cannot fully counter the effects of large surface connected porosities (typically lack-of-fusion) that lead to large surface cavities. Some minor scale or size effects can also be noted between the different coupon geometries. This does not modify the conclusions concerning the initiation mechanisms and their effect on the fatigue life.