Local microstructure engineering of super duplex stainless steel via dual laser powder bed fusion – An analytical modeling and experimental approach

Laser powder bed fusion is a metal additive manufacturing technique, valued for its ability to produce near-net-shaped components with high precision. Its layer-by-layer approach and localized melting create complex temperature cycles, allowing for potential in-situ microstructure modifications. Recently, the productivity of laser beam-based additive manufacturing processes has been increased substantially by the introduction of multiple beams that operate in a parallel way, e.g. building at different locations on the same build platform. However, two laser beams can also be operated in tandem, i.e. using an additional laser beam as a trailing laser that follows the primary melting laser, enabling in-situ heat treatment and local microstructure control. This study investigates the application of dual laser powder bed fusion to locally tailor the microstructure of super duplex stainless steel, a material characterized by a dual-phase microstructure composed of δ-ferrite and γ-austenite. The phase ratio of ferrite and austenite is highly sensitive to the thermal trajectory experienced by the fabricated part, particularly in the critical temperature range of 800–1200 °C, where austenite nucleation and growth from the primary solidified δ-ferrite can occur. An analytical modeling approach, utilizing the thermal field solution based on a moving Goldak heat source, was employed to optimize the parameters of the second laser beam to maximize the residence time within the critical temperature range, thereby enhancing austenite formation. The modeling insights were then qualitatively compared through a dual-laser single-track campaign before being applied to bulk samples. This approach successfully produced specimens with varying austenite contents, ranging from 0 % under high-speed single-laser conditions to 48 % using optimized dual-laser settings. These results demonstrate that careful tuning of laser parameters enables exceptional local microstructure control along both the build and scan directions, i.e. in full 3D. On the other hand, achieving this optimal microstructure required a low scanning speed of 15 mm/s, which reduced the build rate to about 0.07 mm³/s, approximately an order of magnitude lower than the one achieved with higher-speed parameters. Although this demonstrates potential for precise 3D microstructure control, it also underscores a significant trade-off with productivity, presenting a practical limitation for industrial applications.