Effect of the deposition strategy and endogenous defect pattern on the plastic deformability and the fracture mechanism of 316L stainless steel obtained using material extrusion

Abstract

In the metal additive manufacturing (MAM) scenario, metal-polymer-based systems such as material extrusion (MEX) and binder jetting (BJ) are gaining increasing interest as they are expected to provide production cost savings alongside increased productivity compared with well-established powder bed fusion (PBF) and direct energy deposition (DED) technologies. However, metal-polymer-based systems are multistep processes characterized by internal porosity which is strictly dependent on the adopted manufacturing technology in terms of size, shape and distribution. This study examined the impact of such endogenous defects on the tensile behavior of 316L stainless steel manufactured with two different deposition strategies, i.e. the conventional ± 45° and an experimental strategy borrowed from PBF processes, that consists of 67° rotation of each layer, namely k67°. The microstructure consisted mostly of equiaxed austenitic grains with segregation of δ-ferrite at the grain boundaries in all the studied conditions, as expected. The overall volumetric porosity was close to 2 % for both deposition strategies, but the morphology of macro-defects was significantly different. The distribution of defects assessed using micro computational tomography (micro-CT) showed a periodic, stacked, and continuous macro-defect pattern for ± 45° strategy, whereas a more distributed, fragmented and discontinuous pattern was observed for the k67° strategy. The tensile tests curves highlighted a broad and homogeneous plastic deformation without any noteworthy evidence of necking. The fracture morphologies were characterized by a preferential fracture propagation path driven by porosity pile-up for both infill strategies. This resulted in higher deformations at break for the k67° strategy specimens than for the conventional ± 45°. The δ-ferrite regions found at the austenitic grain boundaries functioned as preferential sites for crack initiation, while propagation was almost ductile in bulk.