LAUSR

Abstract The usage of an appropriate shield gas for laser beam powder bed fusion (LB-PBF) of the popular 17−4 precipitation hardened (PH) stainless steel (SS) has long been debated in the additive manufacturing (AM) community. While the inertness of Ar is often desirable as the shield gas from unwanted chemical reactions, its low solubility combined with the violent nature of the melt pool inevitably result in the formation of defects that are detrimental to the fatigue performance of the alloy. On the other hand, although N2 is reactive with some alloys (e.g. titanium), it may have remarkable favorable effects on the mechanical properties and fatigue performance of 17−4 PH SS. In this article, the advantageous effects of N2 as an in-process micro-/defect-structure refiner of LB-PBF 17−4 PH SS, and in turn, an enhancer of its mechanical/fatigue properties are demonstrated. It is shown that, compared to Ar, the use of N2 shield gas induces more retained austenite, refines grains, makes the melt pools deeper, produces fewer and smaller pores, and as a result, slightly improves tensile properties, moderately improves crack growth resistance, and considerably improves axial fatigue properties in the high cycle regime.