PSI - Issue 68

Ilia Nikitin et al. / Procedia Structural Integrity 68 (2025) 24–31

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I. Nikitin et al. / Structural Integrity Procedia 00 (2025) 000–000

1. Introduction Selective Laser Melting (SLM) is one of the modern manufacturing technologies used for producing elements with complex spatial shapes. Recently, SLM structural elements are implemented in engineering structures of various complexities. During operation, these SLM structures are subjected to loads of different natures, including mechanical cycling loading. The study of the mechanical properties of materials produced by additive technology shows sufficiently high quasi-static tensile strength, comparable to conventional technologies such as stamping or extrusion. However, the fatigue strength of the SLM materials is often lower than that of conventional products. The decrease in fatigue strength of SLM materials is related to their complex internal structure resulting from the manufacturing process. The study of very high cycle fatigue (VHCF) behavior of metal materials shows a high sensitivity of cyclic strength due to internal defects in the range of very high number of cycles (10 7 – 10 10 ), as shown by Sakai (2016) and Palin-Luc (2018). Modern engineering applications are designed for long service lives. A large amount of research is aimed at extending the in-service life of already designed engineering units. In all these cases, the acting stress level decreases, which leads to the problem of VHCF loading and internal crack initiation from microstructural defects. Due to the nature of the technological process, SLM technology naturally produces a number of internal interfaces. This significantly inhomogeneous structure is very sensitive to parameters such as laser beam settings, scanning strategy, powder quality, environment, and more. Some types of defects can be removed or improved through further treatments, but not all can be. Numerical simulations are used to predict the formation of SLM microstructure defects and their influence on fatigue strength. In this paper, the relationship between laser beam parameters, scanning strategy, and the final microstructure is experimentally investigated. The numerical simulation of fatigue fracture in the SLM material is performed based on the multi-regime two-criterion fatigue fracture model by Nikitin (2022), considering the shape and properties of typical defects. The verification of the simulated results is done by comparing the calculated fatigue life of the hourglass specimen made from SLM material with experimental results in the VHCF region at R= – 1. All fracture surfaces are examined using scanning electron microscopy (SEM), and defects at crack initiation sites are compared with the characteristics of SLM ingots. Based on this analysis, the relationship between laser beam parameters, scanning strategy, and the formation of critical defects is established. 2. Mathematical models The complex study of fatigue behavior of materials produced by SLM technology under VHCF loading is carried out by both numerical and experimental methods. The fatigue strength of the metallic materials is primarily determined by its microstructure. The VHCF behavior of the metal is very sensitive to the internal defects of the microstructure such as nonmetallic inclusions, pores, texture, and macrozone boundaries (Sakai, 2016). The SLM technology produces a large number of internal boundaries in the material and other specific types of local defects. The main objective of this study is to predict the types and spatial distribution of typical SLM-induced defects depending on the technological parameters of production, and to determine their effect on the fatigue strength of the SLM material. The fatigue strength of the material is estimated using the extended multi-regime fatigue fracture model by Burago (2024) while the specific defect distribution and shape is simulated by solving a multi-phase heat conductivity problem. Both models are presented and discussed below. 2.1. Multi regimes fatigue fracture model In the last decades, SLM technology has been incessantly developed, leading to active implementation of the SLM produced elements in modern engineering structures. SLM technology allows the design of elements with complex shapes and reduces the weight of the construction. Recent advancements in the SLM procedure have significantly increased the quasi-static mechanical properties of the material. However, the engineering structures are often subjected to cyclic loading. To design for the fatigue life of SLM structures, it is necessary to link laser beam parameters with fatigue behavior.