This study reports the comprehensive effects of building direction and scanning speed on the fatigue performance of 304L austenitic stainless steel (SS) fabricated by selective laser melting (SLM) through a series of detailed microstructural characterizations, including optical microscopy (OM), scanning electron microscopy (SEM), Transmission electron microscopy (TEM), electron backscatter diffraction (EBSD), acoustic emission (AE), and thermal imager (TI). The fatigue tests were performed at a single stress level (90% of yield strength). The correlation analyses between fatigue life and the factors of surface roughness, porosity, and residual stress were performed to elucidate the comprehensive influence of these factors on fatigue lifetime. The combination of fractography analysis, AE signals, and TI signals were used to reveal the underlying microstructural mechanisms during cyclic deformation. Direct evidence is offered to show that the process of fatigue crack initiation promoted by cyclic deformation occupied most of the fatigue history. The synergistic effects of part densification, residual stress, and microstructure variable dominate the fatigue performance of SLM 304L SS. Clear evidence is shown that the higher density of high angle grain boundaries (HAGBs) on the top surface can hinder dislocation flow rate, promoting the dislocation piling-up at HAGBs, thus weakening the fatigue performance. In addition, the stronger <101> texture on the top surface can improve the twinning behavior, strengthening the fatigue crack initiation resistance.

本研究通过一系列详细的微观结构表征，包括光学显微镜 (OM)、扫描电子显微镜，报告了构建方向和扫描速度对选择性激光熔化 (SLM) 制造的 304L 奥氏体不锈钢 (SS) 疲劳性能的综合影响。 (SEM)、透射电子显微镜 (TEM)、电子背散射衍射 (EBSD)、声发射 (AE) 和热成像仪 (TI)。疲劳测试在单一应力水平（屈服强度的 90%）下进行。通过疲劳寿命与表面粗糙度、气孔率、残余应力等因素的相关性分析，阐明这些因素对疲劳寿命的综合影响。断口分析、AE信号相结合，和 TI 信号用于揭示循环变形过程中的潜在微观结构机制。提供了直接证据表明循环变形促进的疲劳裂纹萌生过程占据了大部分疲劳历史。零件致密化、残余应力和微观结构变量的协同效应决定了 SLM 304L SS 的疲劳性能。明确的证据表明，顶面高角度晶界（HAGBs）密度较高会阻碍位错流动，促进HAGBs处位错堆积，从而削弱疲劳性能。此外，顶面更强的<101>织构可以改善孪晶行为，增强抗疲劳裂纹萌生能力。提供了直接证据表明循环变形促进的疲劳裂纹萌生过程占据了大部分疲劳历史。零件致密化、残余应力和微观结构变量的协同效应决定了 SLM 304L SS 的疲劳性能。明确的证据表明，顶面高角度晶界（HAGBs）密度较高会阻碍位错流动，促进HAGBs处位错堆积，从而削弱疲劳性能。此外，顶面更强的<101>织构可以改善孪晶行为，增强抗疲劳裂纹萌生能力。提供了直接证据表明循环变形促进的疲劳裂纹萌生过程占据了大部分疲劳历史。零件致密化、残余应力和微观结构变量的协同效应决定了 SLM 304L SS 的疲劳性能。明确的证据表明，顶面高角度晶界（HAGBs）的密度较高会阻碍位错流动，促进HAGBs处位错堆积，从而削弱疲劳性能。此外，顶面更强的<101>织构可以改善孪晶行为，增强抗疲劳裂纹萌生能力。SLM 304L SS 的疲劳性能主要受微观结构变量的影响。明确的证据表明，顶面高角度晶界（HAGBs）密度较高会阻碍位错流动，促进HAGBs处位错堆积，从而削弱疲劳性能。此外，顶面更强的<101>织构可以改善孪晶行为，增强抗疲劳裂纹萌生能力。SLM 304L SS 的疲劳性能主要受微观结构变量的影响。明确的证据表明，顶面高角度晶界（HAGBs）密度较高会阻碍位错流动，促进HAGBs处位错堆积，从而削弱疲劳性能。此外，顶面更强的<101>织构可以改善孪晶行为，增强抗疲劳裂纹萌生能力。