Induced back stress hardening and strengthening effect by repetitive progressive tensile loading of laser-powder bed fused 316L stainless steel

Laser-powder bed fusion (L-PBF) processing of 316L stainless steel is allowing the fabrication of high-performance components combined with the highest degree of freedom in defining the geometry to be produced. However, similarly to other recent technologies, the manufacturing of L-PBF requires a proper tuning of the process parameters, which vary with the application for which the manufactured component is intended. Therefore, it is fundamental to characterize the mechanical performance of functional products, especially for parts designed to resist to time-varying loading conditions. In this framework, the present work presents results about the back stress hardening and strengthening effect induced in L-PBF-fabricated samples for various combinations of building orientation and process parameters. At first, the work presents the response of the L-PBF-manufactured samples to a monotonic tensile load. Then, it follows the presentation of their hardening and strengthening back stress response to a repetitive progressive loading condition. Quasi-static tensile tests show that L-PBF specimens have lower elastic modulus but higher ultimate and yield strength than the original bulk material, whereby the results evidence a strong anisotropy related to the building orientation, especially for the ultimate tensile strain. Porosity and building orientation are found to strongly affect both the elastic and the plastic response of the samples, with a back stress hardening effect which is much less pronounced when compared to the bulk material, while the opposite occurs for the back stress strengthening. As a result, the plastic strain, therefore the ductility, and the yield strength are increased, while the stiffness is reduced.