Étude de l’effet des conditions thermiques de fabrication sur la vie en fatigue à grand nombre de cycles de l’alliage AlSi7Mg produit par fabrication additive

RESUME Ce memoire presente une etude experimentale de l’effet de la microstructure et des discontinuites de solidification issus du procede de mise en forme par fabrication additive (FA) de l’alliage d’AlSi7Mg sur la vie en fatigue. De ce fait, des echantillons produits par la methode de fusion selective par laser (de l’anglais Selective Laser Melting - SLM) ont ete eprouves dans le regime de fatigue a grand nombre de cycles (de l’anglais High-Cycle Fatigue - HCF). Cette methode de mise en forme est une des candidates potentielles pour la fabrication des pieces de geometries complexes dans les industries du transport, plus specifiquement dans le domaine aeronautique. La revue de litterature demontre que les alliages d’Al-Si-Mg ont une microstructure dendritique tres fine generee par le taux de refroidissement rapide lors de la fabrication additive. La temperature de la plateforme de fabrication peut influencer ce taux de refroidissement et consequemment la taille de la microstructure obtenue. De plus, le traitement de durcissement structural T5 a ete identifie comme etant efficace pour augmenter les resistances mecaniques des metaux ayant une microstructure fine, comme ceux produits par FA. D’un autre cote, la litterature revele que les defauts de solidification peuvent avoir un effet dominant sur la resistance en fatigue, masquant les benefices d’un traitement thermique de durcissement. En revanche, certaines etudes recentes effectuees sur des pieces saines montrent que la microstructure de solidification peut avoir une influence significative sur la vie en fatigue. Le but de ce travail est donc d’explorer s’il est possible d’ameliorer la tenue en fatigue d’un alliage d’aluminium produit par FA en optimisant les conditions thermiques de fabrication. Pour atteindre ce but, la vie en fatigue de specimens ayant des microstructures generees a partir d’une fabrication sur des plateformes a differentes temperatures et ayant suivi un traitement T5 a ete quantifiee. Le travail experimental a ete realise sur des specimens de fatigue en AlSi7Mg produits selon deux orientations de fabrication (X et Z). Un premier lot (#1) a ete produit dans une plateforme de fabrication a 200 °C et 3 autres lots (#2, #3 et #4) ont ete produits a 35 °C, 60 °C et 80 °C. La moitie des echantillons des lots #2, #3 et #4 ont ete exposes a un traitement T5. Des essais de fatigue flexion rotative (R=−1) ont ete conduits a 40 Hz. L’etude de vie en fatigue a ete effectueea partir des essais de fatigue a une amplitude de contrainte constante jusqu’a la rupture des specimens ou jusqu’a 5 ×106cycles. ---------- ABSTRACT: This project presents an experimental study of the effect of microstructure and discontinuities from additively manufactured (AM) AlSi7Mg alloy on fatigue life. Therefore, samples produced by the Selective Laser Melting (SLM) method have been tested on the High-Cycle Fatigue regime (HCF). This manufacturing process is one of the potential candidates for the production of parts with complex geometries for the transport industries, pointedly in the aeronautical field. The literature review demonstrates that Al-Si-Mg alloys have a fine microstructure, due to the rapid cooling rate from the additive process. The temperature of the manufacturing platform can influence the cooling rate and consequently th esize of the microstructure. In addition, the T5 microstructural hardening treatment has been identified in the literature as being effective in increasing mechanical strengths of metals having a fine microstructure, such as those produced by AM. On the other hand, the literature reveals that solidification defects can have a dominant effect on fatigue strength, obscuring the benefits of T5 heat treatment. Although, recent studies carried out on healthy parts, show that the microstructure and the Si precipitation can have a significant influence on the fatigue life. The aim of this work is therefore to explore whether it is possible to improve the fatigue life of an aluminum alloy produced by AM by optimizing thermal manufacturing conditions. To achieve this goal, the fatigue life of specimens produced on platforms at different temperatures and post-treated T5 conditions was quantified. The experimental work was carried out from specimens produced according to two building orientations (X and Z). A first batch (# 1) was produced on a platform at 200 °C and 3 other batches (# 2, # 3, and # 4) were produced at 35 °C, 60 °C, and 80 °C. Half of the samples from batches # 2, # 3, and # 4 were submitted to a T5 heat treatment. Rotating-bending fatigue tests (R=−1) were carried out at 40 Hz. The fatigue life was studied at two constant stress amplitudes up to the failure or up to5 x 106cycles.The microstructures obtained differ in terms of dendrites and the presence of silicon precipitates. Higher platform temperature results in a lower cooling rate, so the heating of the platform significantly increases the size of the dendritic structure (0.45 μm for the platform at 35°C versus 0.64 μm for the platform at 200 °C). Also, silicon precipitates could be observed in the aluminum matrix on the specimens T5 heat treated. For specimens produced in the X orientation at 35 °C, 60 °C and 80 °C, specimens T5 treated obtained higher fatigue life in 3.17 (6.84 x 105cycles), 2.98 (7.16 x 105cycles), and 2.19 (8.84 x 105cycles) times, respectively, when compared to those tested as as-built. For specimens produced in Z orientation and tested at T5 condition, the fatigue life is about 1.01 times higher for manufacturing at 35 °C, 2.56 times at 60 °C, and 2.83 times at 80 °C. The results are consistent with the previous work that rapid cooling during solidification refines the size of the dendrites which is beneficial in increasing fatigue life. In addition, the results also confirm the beneficial effect of the T5 post-treatment on fatigue life. Therefore, the research question is assertively answered. The optimization of thermal manufacturing conditions increased fatigue life of AlSi7Mg specimens produced by additive manufacturing. From the manufacturing conditions specific to this project, it is possible to increase the fatigue life of the alloy studied by a production at usually called by cold-platforms (35 °C, 60 °C, and 80 °C), and an addition of a T5 post-treatment. Finally, from the manufacturing point of view, it would be relevant to do a deep study at other platform temperatures between 80 °C and 200 °C. In addition, it is proposed to study these variables on the as-built condition. Eventually, characterizations at several stress amplitudes and ratios would lead to a more deeply understanding ofthe fatigue behavior of this alloy.