Effects of laser powder bed fusion parameters on the delta-ferrite to austenite phase transformation in duplex stainless steels

Abstract

Laser powder bed fusion (LPBF) of 2205 duplex stainless steels is attractive for making complex shaped engineering parts for applications requiring unique combinations of strength-toughness-corrosion properties. However, as-built parts possess highly non-equilibrium microstructures (>98 % δ-ferrite). The desirable balanced austenite/δ-ferrite microstructure can be recovered after a brief heat treatment, potentially achieving much finer duplex microstructures than in wrought counterparts. However, systematic understanding of how LPBF parameters control the microstructural characteristics of the parent δ-ferrite and, hence, the transformed austenite product, is currently missing. We aim to close this gap by establishing the process-microstructure-property relationship using multi-scale characterization and nano-indentation. We compare as-built and heat-treated conditions fabricated with eight different combinations of laser power and scan speed. We show how variations in residual stress, texture, and characteristics of dislocations and inclusions in the parent δ-ferrite control the phase fraction, morphology, grain size, texture, and variant selection of the daughter austenite. A higher dislocation density in δ-ferrite is found to be associated with lower laser power and/or higher scan speed, contributing to higher hardness in as-built δ-ferrite and smaller intragranular austenite grain sizes in heat-treated conditions. Higher residual stress and/or stronger δ-ferrite texture contribute to higher austenite phase fractions. Intragranular austenite variant selection is revealed to be related to the complexity of δ-ferrite dislocation structures, and results in significant hardness increments in the heat-treated conditions. These findings highlight the capability of microstructural engineering via adjusting LPBF parameters and controlling materials performance through manipulating solid-state phase transformations.