Influence of laser beam scanning pattern and polar angle on the fatigue behavior of additively manufactured specimens made of steel 316L

Abstract

While Laser Powder Bed Fusion (LPBF) is a popular, rapidly advancing field of research, a comprehensive or deeper understanding of the mechanical, especially fatigue, properties of steels has not yet been fully achieved. In this work, a deeper investigation on the microstructural load capacity, critical defect size distribution and defect tolerance of fatigue performance of the commonly used 316L austenitic steel were performed. The 316L parts were generated via LPBF with three different scan patterns and polar angles. Resulting porosities were measured with Archimedes' method and cross section scanning with optical microscopy. To understand the relation of process parameters, microstructure and mechanical behaviors, the microstructure and mechanical properties were studied. Fatigue damage origins and fracture mechanisms were investigated. Results showed that apart from comparable hardness to the conventional steel, tensile properties (except Young's modulus) are slightly worse than the conventional values, owing to the fact that the presence of pores and inclusions promote earlier plastic deformations and ductile fracture. The fatigue strength is inferior compared to the conventional steel and most samples failed from lack of fusion. The surfaces of lack of fusion areas, triggering fatigue failure, are either smooth or show parallel lines. The different scan strategies lead to no big difference in the microstructures except the size of the largest lack of fusion area. The polar angles determine the orientations of lack of fusion to the loading axis and result in different sizes of fatigue triggering defects and deteriorate the matrix load capacity to different extents. Spherical pores within a certain range have limited impact. Parallel simulations confirm the defect tolerance determined by the morphology and critical size on the fatigue strength.