Application de l’approche de mécanique de la rupture pour la tolérance aux dommages et la prédiction de la vie en fatigue de composants obtenus par fusion sélective au laser

Abstract

In the past years, metal additive manufacturing processes are gaining significant interest for many industrial applications due to the capacity to build complex components in a near net shape. Among these processes, laser powder bed fusion (LPBF) is particularly promising due to the capacity to manufacture a wide range of alloys with mechanical properties on par with their wrought counterparts. However, the fatigue properties are less appealing due to the presence of a significant amount of processing-induced defects. In this regard, in order to broaden the range of usage of LPBF built components in applications subjected to cyclical loading, there is a need to develop a better understanding and mitigate the impact of process specific defects on their fatigue behavior. In this thesis, three scientific papers are presented. They constitute a procedure that allows relating the cyclic loading to be applied on LPBF built alloys while avoiding fatigue failure with respect to their process-induced defects. To this end, in the first scientific paper, the fracture mechanics approach is employed to characterize the fatigue behavior of Inconel 625 alloy processed by LPBF. Additional wrought specimens of the same alloy are also tested. In addition, this paper investigates the effect of microstructural anisotropy resulting from the process. The fatigue crack propagation resistance is shown to be build-orientation dependant but similar to the wrought alloys in the worst case. In the second paper, non-optimal laser exposure parameter sets are developed to manufacture Inconel 625 coupons with various levels of typical processing-induced defects. To this end, the laser scanning speed is modified and porosity typical of low-energy processing, known as lack-of-fusion defects, are generated (0.1 to 2.7%). The impact of such levels of porosity on the quasi-static mechanical behavior and on the fatigue crack propagation behavior is studied. The experimental results enlighten the limited impact of such porosity on the elastic properties, while the ductility of the material was strongly impacted by their presence. In addition, the reduction in ductility is found to be strongly build-orientation dependant. When it comes to the fatigue crack propagation behavior, the impact of defects is very limited in the near-threshold region. However, as the stress intensity factor range increases, the interaction of the porosity with the propagating crack becomes significant and strongly build-orientation dependant; the crack is retarded when propagating in the direction perpendicular to the build plane and accelerated when propagating in the parallel plane. In the third paper, specimens are manufactured for conducting stress-controlled fatigue tests with porosity induced in their gauge section using the aforementioned laser exposure parameter sets (4 porosity levels ranging from 0.1 to 2.7%). Defects were characterized via computed tomography on a restricted number of specimens (3 per porosity level) to measure the pore size distribution using Murakami’s parameter. The larges defects found for each porosity level were employed to predict the fatigue strength using El-Haddad’s formulation of the Kitagawa Takahashi diagram. Stress-controlled fatigue testing was conducted on the specimens to evaluate their fatigue strength and experimental results were found in good agreement with the model’s predicted fatigue strength, with a maximal error of 7%. Moving forward, we propose to apply this method to specimens with different gauge sections using the statistics of extreme to predict the defect size for various volumes of material.

Date	13 Jan 2020
Original language	French
Awarding Institution	École de technologie supérieure
Supervisor	Vladimir Brailovski (Supervisor) & Patrick Terriault (Co-supervisor)