Effect of in situ laser shock forging on residual stress field induced by selective laser melting

Selective laser melting (SLM) is an additive manufacturing method that generates much residual tensile stress during processing. Laser shock forging (LSF) is a surface treatment technology that can reduce residual stress on the metallic surface. At present, there are no reports on in situ LSF (ILSF) during the SLM process. The effect of ILSF on the residual stress field induced by SLM was systemically investigated using the finite element method (FEM), and the parameters were optimized. Optimized finite element simulation calculated the stress field after the ILSF impact. The results show that the SLM deposition layers have significant residual tensile stress, with a maximum range of 152 μm and extremely inhomogeneous distribution. LSF can convert SLM-induced tensile stress into compressive stress. The two most important ILSF parameters are peak pressure (Pmax) and initial temperature (Ti). The range and magnitude of the residual compressive stress field increases with Pmax and decreases with the initial temperature. The sufficiently large LSF Pmax is essential in the LSF process. When Pmax is large enough, the effect of the multiple shots on the magnitude and depth of compressive stress is insignificant. The high Ti will significantly reduce the effect of ILSF, even if residual compressive stress is not obtained. Ti influences the stress field through an inhomogeneous temperature gradient. The temperature gradient is strongly correlated with the residual stress variation, and the fitted equation shows that the stress field is a power function of the temperature gradient. This paper provides an essential guide to improving the residual stress field in additive manufacturing processes.