Thermally driven changes in the microstructure and mechanical properties of martensitic 15-5 precipitation-hardened stainless steel during directed energy deposition

Abstract

The effect of thermal history induced by different idle times between subsequent layer depositions on the microstructural evolution and mechanical properties of martensitic precipitation-hardened 15-5 stainless steel during laser-based directed energy deposition with powder flow is investigated. Samples deposited with short and long idle times had martensite matrix with retained/reverted austenite. A short idle time contributed to increased Ni, Cu and Nb microsegregation in the interdendritic region owing to a low cooling rate and the formation of martensite without Cu precipitation. A long idle time allowed the temperature of the deposited layer to drop below the martensite transformation start (Ms) temperature after the deposition of each layer, triggering martensitic transformation and Cu precipitation during deposition. Atom probe tomography analyses revealed that Cu precipitation occurred during deposition and subsequent aging. The exact fractions of each phase, including martensite, and austenite were obtained through synchrotron-based X-ray diffraction. Furthermore, long idle time contributed to improved yield strength and elongation, which was attributed to the effects of austenite, transformation-induced plasticity, and precipitation strengthening in the martensite. However, deposition with a short idle time required subsequent aging to form Cu-rich precipitates in order to increase yield strength.