In-situ Microstructure Modification of Duplex Stainless Steels by means of Powder Bed Fusion using Laser Beam Melting of Metals (PBF-LB/M)

• Applicant: Professor Dr.-Ing. Michael Schmidt
• Subject Area: Joining and Separation Technology ; Primary Shaping and Reshaping Technology, Additive Manufacturing
• Term: since 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 496140019

This follow-up proposal builds upon the initial funding period, which focused on investigating how processing conditions influence the phase ratio and corresponding material properties of duplex stainless steel (DSS) produced by laser-based powder bed fusion (PBF-LB/M). The initial research demonstrated that local duplex phase formation can, in principle, occur in-situ during the PBF-LB/M process without requiring post-process heat treatment, thereby significantly advancing the state of the art. However, the austenite content is still insufficient compared to an ideal duplex microstructure (a 50:50 ratio of austenite and ferrite), and remains locally variable. Therefore, the current microstructure lacks the precise phase control required to replace conventional DSS processing for complex components. The aim of this second funding period is to develop geometry-specific processing strategies that enable the production of complex geometries with a controlled duplex phase composition. These strategies build upon the knowledge established in the first funding period and aim to specifically tailor the duplex microstructure. The first processing strategy to be investigated involves the fabrication of simple DSS specimens exhibiting a uniform duplex microstructure (a phase ratio of 50:50), achieved through forced heat accumulation by combining geometric restriction with base-plate preheating to 800 °C. The second processing strategy aims to locally control phase composition through in-situ heat treatment using post-scanning with a defocused laser beam. With post-scanning, by decreasing the cooling rate, the total time above the austenite transformation temperature after solidification can be increased, enabling increased austenite precipitation. By precisely controlling key post-scanning parameters, each layer can be reheated immediately after solidification, without causing remelting, enabling targeted microstructural control. This approach will be supported by low-fidelity simulation and thermal monitoring to prevent remelting of the heat-treated area. Initially, a uniform phase composition will be established in simple cubic specimens, followed by the controlled adjustment of ferrite/austenite phase distributions through manipulation of the initial solidification and post-scanning parameters. The final objective is to demonstrate the transferability of the developed processing strategies to application-relevant parts. These parts vary in geometrical complexity and, consequently, in their potential for heat accumulation. The strategies for the targeted control of the duplex microstructure are validated using these components, and the results will be consolidated into a phase-control toolbox applicable to arbitrarily complex components.

• DFG Programme: Research Grants
• Co-Investigator: Lova Chechik, Ph.D.