Effect of laser defocusing on duplex stainless steels for higher scan speeds in Laser-Based Powder Bed Fusion (PBF-LB/M)

Abstract

Duplex stainless steels (DSS) employ a bi-modal microstructure consisting of equal parts ferrite and austenite. However, when processed via laser-based powder bed fusion (PBF-LB/M) the high cooling rates lead to a dominantly ferritic microstructure, thus making a post-process heat treatment necessary. This sparks the interest in accelerating the manufacturing time by increasing scan speeds. Defocusing of the laser beam offers the possibility to alter the melt pool morphology and thereby the melting mode. Therefore, this study presents the influence of the defocusing distance for PBF-LB/M manufactured DSS parts. The melt pool becomes shallower and wider as the defocus distance increases, but also has a more uniform shape at high scan speeds compared to the focused beam. Furthermore, defocusing of the laser beam results in denser parts at scan speeds of up to 1800 mm/s, potentially facilitating higher build rates. The duplex microstructure could be restored by a post process heat-treatment.