Study on Laser Scan Strategy for Correcting Anisotropic Residual Stress Distribution and Reducing Warpage in Structures Fabricated by PBF-LB/M

Abstract

Metal-based powder bed fusion with a laser beam (PBF-LB/M) can be applied to fabricate high-accuracy structures compared with other metal additive manufacturing (AM) methods. The rapid solidification of metal powder formed by laser irradiation introduces heterogeneous residual stress, which causes deformation and cracking of the structure. This, in turn, results in the deterioration of quality. In this study, the influence of the laser scan strategy on the residual stress distribution and warpage of the structure was investigated. Using maraging steel powder with an average particle size of 32.5 μm, the structures were constructed using several laser scan strategies at a wavelength of 1070 nm. The residual stress distributions on the surface of the structures were measured by the cosα method by applying X-ray diffraction (XRD). In addition, the warpage of the reverse side of the substrate as a foundation of the structure was measured by a stylus-type surface roughness measuring instrument. The results clarified that the structures constructed by unidirectional scan directions had a tensile residual stress that was generated parallel to the laser scan direction. Meanwhile, the compressive residual stress was generated perpendicular to the laser scan direction. The large warpage was aligned with the laser scan direction and tensile residual stress. When the laser scan direction was rotated by 90° for each layer, the residual stress distribution was generated with a cruciform shape. It was indicated that this residual distribution was caused by a laser scan on the top surface and a lower layer. The anisotropic residual stress distribution and reduction of warpage could be corrected by rotating the laser scan direction by 15° in each layer.