Multiscale characterization of Ti-induced grain refinement in additively manufactured austenitic stainless steel
Mohammad Reza Jandaghi, Hesam Pouraliakbar, Sang Hun Shim, Leonardo Iannucci, Prithwish Tarafder, Justinas Pališaitis, Sun Ig Hong, S.Y. Persaud, Vahid Fallah, Mattias Calmunger, Johan Moverare
Abstract
• Mn-aided Ti inoculation refines grains and mitigates columnar growth in AM 316L. • Ti addition promotes formation of TiO, FeTi, and C14 Laves phases, affecting microstructure. • Annealing redistributes phases, enhancing ductility and homogenizing hardness. • TiMn co-inoculation improves strength but reduces corrosion resistance in 316L. In-situ inoculation of grain-refining elements suppresses columnar grain growth and reduces mechanical anisotropy in additively manufactured metals, while enhancing strength via the Hall-Petch effect. However, the refinement mechanism of Ti in austenitic stainless steel remains unclear. This study investigates Mn-assisted Ti inoculation in 316L stainless steel (SS316L), followed by annealing. Despite near-full densification, localized Ti enrichment formed coarse, brittle FeTi and C14 Laves intermetallic clusters, surrounded by ultrafine ferritic grains within an austenitic matrix. Elevated annealing temperatures dissolved Laves phases and promoted Ti diffusion, resulting in dispersed TiO particles and ferritic domains. Refined Laves phases were redistributed to grain boundaries and triple junctions. Mechanical testing showed improved ductility with increasing annealing temperature: ultimate tensile strength decreased from 650 MPa to 610 MPa, while elongation rose from 13 % to 38 %. Hardness mapping revealed more uniform distribution, though overall hardness dropped from 370 HV to 210 HV. Electrochemical corrosion tests in saline solution indicated that phase transformations induced by Ti-Mn co-inoculation compromised corrosion resistance, increasing susceptibility to degradation in aggressive environments.