# Additively Manufactured Inconel 718 Low-Cycle Fatigue Performance
## 1 Introduction  
Additive manufacturing (AM) has revolutionized the production of metallic components that are nearly fully dense by employing a layer-by-layer build approach. There are several AM technologies, including powder bed fusion and direct energy deposition technologies. These technologies can vary in the energy source used to fuse the material together such as utilizing a laser or electron beam or relying on a cold spray source [1, 2, 3]. Among various AM techniques, laser powder bed fusion (L-PBF) has gained significant traction due to its ability to produce components with mechanical properties comparable to those manufactured by traditional methods [4, 5]. The L-PBF process allows for the creation of intricate geometries with minimal waste, offering enhanced design flexibility and cost efficiency, which is particularly beneficial for the aerospace and gas turbine industries [5, 6]. These benefits include the ability to increase performance, improve quality, and reducecosts. Despite these advantages, the implementation of L-PBF for critical components necessitates a thorough understanding of the material's mechanical properties, particularly under fatigue loading conditions [7]. Inconel 718, a nickel-based superalloy, is widely used in the aerospace industry due to its excellent high-temperature strength and fatigue resistance [8]. Traditionally manufactured through casting, forging, and sheet forming, Inconel 718 is well characterized for use in critical components [9]. However, the advent of AM techniques like L-PBF presents new opportunities for producing Inconel 718 with more complex geometries and reduced material waste. These now-possible complex geometries provide increased design freedom, allowing for enhanced component performance and cost-saving opportunities [5; 9; 10]. Nevertheless, the successful application of L-PBF in producing Inconel 718 components requires a complete understanding of the material's mechanical properties. One such property significantly impacted by the L-PBF build process and its parameters is fatigue performance [11; 12; 13]. Previous studies have indicated that L-PBF Inconel 718 exhibits superior fatigue performance compared to cast forms and only a minor reduction in low-cycle fatigue compared to forged materials [14; 15; 16; 17; 18]. However, a notable reduction in fatigue life is associated with as-printed surfaces, which becomes more pronounced with increasing cycle life [14; 19; 20]. L-PBF-produced surfaces typically exhibit higher surface roughness compared to conventionally machined surfaces due to the layer-by-layer fabrication process, incomplete fusion, and the presence of defects such as unmelted particles or pores [21]. This increased roughness can significantly debit fatigue life, as it introduces stress concentrators that amplify localized stresses under cyclic loading. These stress concentrators can act as initiation sites for microcracks, accelerating crack initiation and reducing the material's ability to withstand repeated stress cycles [19; 20]. In contrast, machined surfaces are generally smoother and have fewer surface defects, which minimizes stress concentration effects and enhances fatigue performance. Additionally, post-processing techniques like machining or polishing are often required for L-PBF parts to improve surface quality and restore fatigue life closer to that of traditionally manufactured components. Previous research has explored the effects of surface roughness on the fatigue performance of L-PBF Inconel 718, including notable studies by [PERSON], [PERSON], and [PERSON] [22; 23; 24]. [PERSON]'s work demonstrated the fatigue life debit associated with surface roughness over a range of 10,000 to 2 million cycles; however, testing was limited to room temperature and did not include fatigue data for very-low-cycle-fatigue (LCF) conditions, specifically below 10,000 cycles [22].