Dislocation cell-mediated nanosized carbide precipitation enables ultrastrong AISI D2 tool steel fabricated via laser-powder bed fusion

Abstract

AISI D2 tool steels possess considerable advantages in various industrial applications, but its fabrication through laser-powder bed fusion (L-PBF) is still challenging due to the high thermal stress and high carbon content, resulting in severe crack formations. In this study, we adopted both pre-heating and hatch spacing adjustment to overcome the crack phenomenon, and microstructures and mechanical properties were investigated for successfully fabricated specimens according to the post heat treatment. The most characteristic microstructural feature of the L-PBF D2 tool steel in its as-built sample was film-like M₇C₃ carbides decorated with dislocation cells. After the heat treatment, the morphologies of film-type carbides in the as-built sample transformed to nanosized particulates in the martensitic matrix, and the crystal structure also changed from M₇C₃ to M₂₃C₆. The L-PBF D2 tool steel showed exceptionally superior strength in both the as-built (1199 MPa) and heat-treated samples (2110 MPa) without severe sacrifice of ductility. The pre-existing carbide network structure located on the dislocation cells in the as-built sample enabled a fine distribution of nanosized carbides after heat treatment, which contributed to the significantly high strength of the present alloy. These findings imply that the dislocation cell structure-mediated precipitation successfully strengthened the L-PBF D2 tool steel, suggesting novel strategies for microstructural design of high-strength alloys utilizing L-PBF.