Effect of build orientation and heat treatment on the microstructure, mechanical and corrosion performance of super duplex stainless steels fabricated via laser powder bed fusion

Abstract

In this study, the effect of build orientation (0°, 45° and 90° from a build platform) on microstructural response as well as mechanical and corrosion properties was investigated by comparing laser powder bed fusion-produced samples in the as-built and solution-annealed states. By increasing build orientation, Widmanstätten γ-austenite formation was lowered because of faster cooling and shorter melt tracts, whilst retaining similar δ-ferrite/γ-austenite phase fractions. This is correlated with improved corrosion performance in the 90° orientation from chemically homogeneous grain boundary γ-austenite. The prevailing δ-ferrite as-built samples exhibit a strong 〈001〉 δ-ferrite crystallographic texture in the normal direction across all orientations together with greater hardness and mechanical strength in comparison to solution-annealed samples by virtue of less slip systems in the BCC δ-ferrite structure and fine cellular solidification structure. The 45° build orientation exhibits a greater Widmanstätten γ-austenite content and periodic recrystallisation between scan checkers, contributing to improved mechanical strength and ductility. Solution annealing softened structures, from an increase in the γ-austenite content, via intergranular nucleation or through prior grain boundaries and Widmanstätten needles. The underlying δ-ferrite grain structure and crystallographic texture relationship is retained, although weakened from the recrystallisation process. Tensile strength is reduced compared to the as-built structures and worsened in the 90° orientation due to few Widmanstätten needles, although elongation is significantly increased, and pitting corrosion performance is improved by the removal of stresses and the equilibrium microstructure.