Microstructural evolution and mechanical behavior of additive manufactured 17-4 PH steel with a periodic layer structure

Materials produced by additive manufacturing experience a unique thermal history, and particularly, proper laser remelting (LR) can introduce additional austenite in the laser direct metal deposited 17-4 PH steels. Taking advantage of the LR process and subsequent aging, we designed periodic-layered 17-4 PH steels with different austenite distributions in this study. The evolution of microstructural heterogeneity during LR process and aging treatment was investigated by microstructural characterization and thermodynamic calculations. The interrupted tensile tests and cyclic loading-unloading-reloading tensile tests were utilized to explore the mechanical behaviors of deposited 17-4 PH steels. The relationship between the heterogeneous microstructures and mechanical properties was clarified in terms of the mechanisms of strain partitioning, transformation-induced plasticity (TRIP) and back stress hardening. Results reveal that during LR process, both the temperature rise and compositional change account for the introduction of austenite, and further aging can flexibly tailor the distribution of Cu nanoprecipitation between the LR and non-LR layers. The synergism of back stress hardening and TRIP effect is responsible for the excellent strength-ductility combination of the periodic-layered samples, showing extra strain hardening persisting to a large strain. The strength and ductility are simultaneous improved by increasing the strain partitioning interface density, and the optimum mechanical property is the yield strength of 1308 MPa together with uniform elongation of 8.5%.