A New Grain Refinement Route for Duplex Stainless Steels: Micro-Duplex Stainless Steel Matrix Composites Processed by Laser Powder Bed Fusion

Abstract

Duplex stainless steels (DSSs) with high corrosion resistance and good mechanical properties have been widely applied in various engineering fields. To enhance their application potential and prolong their service life, the employment of grain refinement methods can be regarded as a favorable strategy. However, the studies of grain refinement have been mainly limited to thermomechanical processing and severe plastic deformation in the case of DSSs. In this work, a new grain refinement route was successfully developed via laser powder bed fusion (LPBF) of micro-duplex stainless steel (SS) matrix composites along with a subsequent quenching process. For as-built composites, TiCxNy nanoparticles were first formed in-situ via the introduction of submicron/micron-sized TiC particles with low cost into 2205 DSSs, and their microstructure is almost composed of fine equiaxed high-temperature ferrite (δ-ferrite) grains by utilizing the ultra-high cooling rate of LPBF and heterogeneous nucleation sites provided by in-situ formed TiCxNy nanoparticles. The existence of TiCxNy nanoparticles prohibited the epitaxial growth of δ-ferrite grains, showing no obvious textures in as-built composites. Subsequently, fine austenite grains were produced at grain boundaries of fine unrecrystallized δ-ferrite by subsequent quenching treatment of composites, which enabled them to achieve an ensemble of outstanding ultimate tensile strength and uniform elongation. The δ-ferrite recrystallization was mainly inhibited by the existence of TiCxNy nanoparticles. This study can provide a groundwork for designing grain refinement methods to produce high-performance DSS parts with complex geometry based on LPBF technique.

Keywords: Additive Manufacturing, Laser Powder Bed Fusion, duplex stainless steels, Grain refinement, metal matrix composites