Tensile properties and microstructural evolution of 17-4 PH stainless steel fabricated by laser hybrid additive manufacturing technology

Wire-based laser directed energy deposition (LDED) technology is an attractive and efficient additive manufacturing technology, which shows great potential in the process of manufacturing high-value engineering components. However, the thermal effect will lead to a large amount of residual tensile stress in the components, and the large crystal structures after solidification seriously affects the tensile performance. The pursuit of high mechanical properties in additive manufacturing components have prompted researchers to seek new strengthening process to produce high-strength components. Therefore, in this paper, laser shock peening (LSP) composite wire-based LDED were used for the additive manufacturing of 17-4 PH stainless steel under the specified experimental conditions, and the effects of LSP on microstructure evolution, microhardness, and tensile properties of wire-based LDED components were systematically studied. Results showed that uniform grains with an average size of 2.31 mu m were generated on the top surface of specimen due to the ultra-high plastic strain induced by LSP shock wave. The size of hierarchical martensitic units shows varying degrees of reduction after LSP treatment, which is shown by the width decrease of packet, block and lath. Dynamic recrystallization caused by rearrangement and annihilation of high-density dislocations promoted the grain refinement. Meanwhile, the residual stress was completely converted from tensile state into compressive state with a maximum value of 425 MPa, and the microhardness was also enhanced to a peak value of 432 HV. In addition, we observed that the high-density dislocation environment created by LSP treatment promoted the further precipitation, and the aggregation phenomenon of precipitates was observed in the dense dislocation area. Furthermore, the tensile strength and elongation of LSP treated specimen were improved significantly, which was attributed to the combined effects of grain refinement, introduction of dislocation structures, precipitation strengthening and gradient residual compressive stress. In summary, laser hybrid additive manufacturing technology provides a new idea and method for the production of high-performance precipitation hardening stainless steel components.