Fatigue tests of cryogenic drilled specimens

摘要: Aeronautic industry encounters continuously several challenges to achieve optimum and reliable components. Owing to the high mechanical properties over a wide range of temperatures, Inconel 718 is frequently used in airplane components that are subjected to fatigue loading. In this study, manufacturers need to achieve excellent surface integrity of the machined components to ensure high fatigue performance. Most of studies in this topic are focused on milling effect and few have dealt with drilling process, even if it is widely employed for the airplane components. In this context, the aim of the present study is to understand the impact of cryogenic drilling operations on the high cycles fatigue behavior of Inconel 718. Fatigue specimens have been drilled under three different conditions: conventional (Wet) and two cryogenic cooling conditions. The two cryogenic cutting fluids used are the liquid nitrogen (LN2) and the liquid carbon dioxide (LCO2). The drilling trials were carried out using the same test configuration (cutting parameters, drill bit geometry, set-up), varying the cooling conditions. The fatigue strength at 2x106 cycles has been characterized in tension with a load ratio R= 0.1 via the staircase method. Results showed that wet condition exhibited the highest fatigue resistance compared to both cryogenic batches. In contrast, the LN2 condition disclosed the lowest fatigue resistance in comparison with the LCO2 drilled specimens. The low fatigue strength obtained under LN2 configuration could be explained by the poor surface quality generated.