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

The material properties of duplex stainless steels (DSS) are based on the duplex microstructure, which can be divided into an equal share of the austenite and ferrite phase, which are responsible for a good corrosions resistivity and good mechanical properties at the same time. When processing DSS using laser-based additive manufacturing technologies like laser beam melting (PBF-LB/M), a ferrite-dominant microstructure is formed. For maintaining the PBF-LB/M-specific properties of the steel, adjusted processing strategies are required for the in-situ generation of the duplex structure. Therefore, the effect of processing conditions on the phase ratio and the corresponding material properties will be examined. In addition to PBF-LB/M-specific parameters, the nitrogen concentration of the shield gas will be modified. These processing conditions result in a varying thermal history for every point inside the manufactured specimen. To assess and understand the influence of the processing conditions on this thermal history, a simplified model based on the three-dimensional heat conduction equations and an adequate replacement source will be developed for predicting the temperature development inside the manufactured specimen. By correlating the spatial and temporal temperature development inside the part during PBF-LB/M with the formed phases, adjusted processing strategies can be derived for achieving a duplex microstructure with an austenite-to-ferrite ratio of 50 %.A second approach for adjusting the microstructure and grain size is provided by the addition of nanoparticles that act as nucleation agents. Goal is to investigate to which extend the addition of nanoparticles in concentrations below 0.1 wt.-% affects the microstructure and the solidification behavior of DSS.To meet and resolve the thermally induced stresses in PBF-LB/M-manufactured parts, adjusted heat treatment strategies will be developed which simultaneously maintain the phase distribution and the process-specific grain size. Parallel to all these presented investigations, macroscopic material properties like hardness, tensile strength and corrosion resistivity will be determined and correlated with the obtained microstructure and phase formation.Based on the presented investigations, a fundamental understanding on the correlations between processing conditions, microstructure formation, and the resulting material properties of PBF-LB/M parts made from DSS is generated. Through adjusting process conditions shielding gas atmosphere in PBF-LB/M, nanoparticle concentration, and post-process heat treatment strategy, a high degree of control will be obtained for generating DSS parts using the PBF-LB/M process.

• DFG Programme: Research Grants