Effects of manufacturing defects and microstructure on the tensile and low cycle fatigue behavior of selective laser melting IN718 TPMS structures

This study investigates the tensile performance and low cycle fatigue (LCF) behavior of Triply Periodic Minimal Surface (TPMS) structures fabricated by Selective Laser Melting (SLM). Due to complex lattice geometry and inherent poor thermal stability of the SLM process, surface roughness of TPMS structures is increased. Micro-CT analysis reveals defects, such as pores and gas voids, within TPMS structures, less common in traditional plate samples. The defect porosity in TPMS structures is 325 times greater than in plate specimens. Although increasing pore spacing to 0.25 mm improves tensile performance and fatigue life, tensile strength of TPMS structures remains lower than Inconel 718 plate samples due to lower relative density and higher defect sensitivity. Both finite element analysis and experimental results confirm significant stress concentrations in TPMS structures, particularly around defects serving as potential crack initiation sites. In contrast, plate samples exhibit more uniform stress distribution and superior mechanical performance. EBSD analysis shows grains in plate samples are primarily uniformly distributed equiaxed fine grains, while TPMS structures contain larger grains in the central region, with fine grains concentrated at the edges. Moreover, dislocation accumulation occurred at TPMS thin-wall edges, and recrystallized grains increased significantly. High dislocation density becomes a weak point under long-term fatigue, leading to crack formation. Additionally, distinct subgrains observed after fatigue deformation indicate original equiaxed grains fragmented, exacerbating deformation. This caused coarse-grained regions to undergo substantial plastic deformation, generating numerous voids. These microstructural differences likely significantly influence the mechanical performance of TPMS structures.