Fatigue crack nucleation and growth in laser powder bed fusion AlSi10Mg under as built and post-treated conditions

Abstract

Numerous efforts have been devoted to produce reliable additive manufactured (AM) materials for structural applications. However, the critical fatigue issue poses a significant hurdle in relation to the nature of this production method. Despite the relative flexibility of the laser powder bed fusion (LPBF) AM process, there exists a limitation in the possibility of tailoring parameters to obtain a satisfactory combination of porosity and surface roughness for adequate fatigue resistance. This quandary arises interest for post-treatments that could help overcome this limitation. Friction stir processing was proved to be a viable solution to improve fatigue behaviour of LPBF AlSi10Mg in the present work. Indeed, thanks to this post-process that reduces overall porosity and eliminates critical defects while keeping a satisfactory fine microstructure and static mechanical behaviour, fatigue resistance of the material was significantly enhanced. In contrast, the other studied post-process, stress relieve heat treatment, did not reduce porosity, which remained the main factor behind fatigue crack nucleation, cyclic life of the material being thus little affected with respect to the as built state. Furthermore, both post-treatments decreased fatigue crack growth rate by an order of magnitude. This is interpreted as being due to plasticity induced crack closure and crack branching.