THE EFFECT OF LASER SHOCK PEENING ON BACK STRESS OF ADDITIVELY MANUFACTURED STAINLESS STEEL PARTS


 This work studies the use of laser shock peening (LSP) to improve back stress in additively manufactured 316L parts. Grain morphologies and irregular dislocation distribution resulting from crystallization schemes that occur during printing cause anisotropic mechanical properties in additively manufactured metal. Tension-compression asymmetry caused by back stress has been observed as one of these anisotropic properties. Such asymmetry complicates application of 3D-printed metal for mechanical design because 3D-printed parts will behave differently than traditionally manufactured parts under the same loading conditions. While heat treatments such as annealing and hot-isostatic pressing are shown to eliminate back stress, they do not also impart additional beneficial properties such as compressive residual stress. LSP is known to improve fatigue life by inducing compressive residual stress and has been applied with promising results to additive manufactured metal parts. It is here demonstrated that LSP may also be used as a tool for mitigating tensile back stress in 3D printed parts. Hysteresis tensile testing measures the back stress from additively manufactured dog bone samples built in both XY and XZ directions. Both LSPed and as-built conditions are tested and compared, showing that LSPed samples exhibit a reduction to back stress that varies for differing build directions. Electron backscatter diffraction performed under these conditions elucidates how grain morphologies and texture contribute to the observed improvement in back stress. Crystal plasticity finite element modelling develops insights as to the mechanisms by which this reduction is achieved in comparison with EBSD results.