Effect of Ni Addition on the Phase Balance and Grain Boundary Character Distribution in 2507 Super Duplex Stainless Steel Fabricated via LPBF

Abstract

Super duplex stainless steels (SDSSs) can be effectively fabricated via Laser Powder Bed Fusion (LPBF), yet achieving the necessary phase balance remains a critical metallurgical challenge. The rapid solidification rates inherent to the LPBF process typically result in a predominantly ferritic microstructure. Since CSL boundaries-specifically high-symmetry & sum;3 twins-form preferentially in the austenite phase, achieving a high fraction of these boundaries in the ferritic as-built LPBF state remains a significant challenge. To address this limitation, we implemented a feedstock modification strategy by mechanically blending 2507 SDSS powder with 3 and 6 wt.% elemental nickel prior to LPBF processing. The microstructural evolution, phase distribution, and boundary character were comprehensively evaluated using Electron Backscatter Diffraction (EBSD). Analysis revealed that the addition of nickel did not compromise densification, with all samples achieving relative densities exceeding 99.2%. While the base alloy remained 98.5% ferritic, the addition of 6 wt.% Ni successfully promoted the formation of approximately 31.1 wt.% austenite, characterized by intragranular laths formed via a massive-like transformation mechanism6. Crucially, despite the theoretical increase in Stacking Fault Energy (SFE) associated with high nickel content, the restored austenite phase exhibited a significant fraction of high-symmetry CSL & sum;3 twin boundaries (rising to 7.05%). These findings demonstrate that compositional modification can overcome the kinetic limitations of the LPBF process, facilitating the development of a favorable Grain Boundary Character Distribution (GBCD).