Fatigue Crack Propagation in As-Built and Aged Additively Manufactured 18Ni300 Maraging Steel

Abstract Since its inception, additive manufacturing (AM) has progressed substantially in delivering high-performance components in the aerospace, tooling and medical industries, among other applications. Directed energy deposition (DED), one of the predominant technologies within metallic AM, has established itself as a valid technological process both in repairing damaged components, as well as coating parts such as shafts, gears and high-pressure vessels. One of the types of metallic alloys that simultaneously possesses high yield strength and toughness while retaining good weldability and printability are Maraging steels, particularly 18Ni300 (C300) Maraging steel. In this work, 18Ni300 Maraging steel produced via DED was characterised in regards to its fatigue crack propagation behaviour using standardised compact tension (CT) tests at a constant stress intensity ratio of R=0.1, both in its as-built and heat-treated conditions, for two distinct configurations: the loading direction parallel to the building direction (LS), translating into the crack propagation along the same layer interface; loading direction perpendicular to the building direction (SL), resulting in the crack propagation through different layer interfaces. Additionally, bimetallic specimens comprised of AM’ed material (18Ni300) and the deposition substrate (AISI 1045), in which the material interface is placed along the initial notch, were tested in order to analyse the crack propagation along the heat affected zone (HAZ) and diffusion zone. Crack deviation was observed in the As-Built SL and bimetallic specimens, due to the presence of columnar dendritic structures at angles relative to the primary crack growth direction which favoured kinking. The Paris region of crack growth remained somewhat similar in terms of da/dN along different configurations, with the aged samples displaying less crack tip plasticity due to its loss of ductility. The stress intensity factor threshold, obtained through the delta-K decreasing method at constant stress ratio, barely oscillated around the same value of 5.8 MPa m0.5 for different specimens.