Tensile and ductile fracture properties of as-printed 316L stainless steel thin walls obtained by directed energy deposition

Abstract

Mechanical properties of as-printed 316L stainless steel thin-walled structures obtained by directed energy deposition are investigated. In-situ tensile and fracture tests are performed on small samples obtained from a additively manufactured square section tube and extracted with three different orientations with respect to the part build direction. Despite a strongly oriented microstructure resulting from the process, as-printed specimens exhibit a reduced anisotropy in comparison with thick or polished samples commonly reported in the literature. Moreover, it is shown using a simple model that the reduced dentified anisotropy can be explained by considering the material thickness variation pattern only, resulting from the layer stacking process. Fracture tests are analyzed using an adapted digital image correlation procedure that evaluates the specimen fracture toughness from experimentally computed J-integrals. Using time reversal, strain fields in regions close to the crack path are identified. Stress fields are then computed from the constitutive behavior identified in tensile tests. A regularization procedure is proposed to enforce the stress equilibrium. Finally, the J-integral is computed using various integration contours in order to validate its path-independance. On this basis, a nearly isotropic fracture toughness is identified. Additional scanning electron microscope observations show that fracture surface features are independent from specimen orientation. This apparent isotropy is explained by the isotropic distribution of lack-of-fusion defects driving crack initiation and propagation.

Mechanical and Microstructural Anisotropy of Laser Powder Bed Fusion 316L Stainless Steel

“…Figure 13 shows the dimple morphology of a metallic fracture at higher magnification. The equivalent diameter of dimples was below 1 um on average, that was lower than originally expected based on Margerit et al [32]. This is also one of the reasons why there were no noticeable differences among fracture appearance among the samples.…”

“…Figure 13 shows the dimple morphology of a metallic fracture at higher magnification. The equivalent diameter of dimples was below 1 um on average, that was lower than originally expected based on Margerit et al [32]. This is also one of the reasons why there No obvious signs of oxidation were observed on powder particles by SEM or on particles trapped on metallographic sections, despite the less protective atmosphere generated by the nitrogen generator.…”

“…Instead of polygonal equiaxed grains of conventional material, columnar anisotropic "weld-like" grains with an internal dendritic structure should be expected. These microstructural features have led to higher strength and lower ductility (elongation) when compared to conventional 316L SS in the vast majority of studies [7,10,11,32]. In this context, Tolosa et al [33] found higher yield strength in 316L stainless steel samples processed by SLM in contrast to cast and rolled material, which can be attributed to the fine-grained microstructure.…”

Comparison of Different Additive Manufacturing Methods for 316L Stainless Steel

“…This allows DLD 316L steels to usually show a higher hardness and elastic yield compared to conventional ones, but with values that are lower than those of other AM techniques. In the same sense, these materials have a relatively high fracture strain [ 23 ]. On the other hand, SLM 316L steels show a much finer microstructure with a clear directional morphology.…”

“…In the case of DLD, the processing window has not been established with the same methodology, but using these normalized parameters provides values in the range of 1–3 for E* min ; larger values lead to melting of the manufactured structures and lower ones lead to very porous structures [ 23 ]. In DLD, due to the thickness of the walls, the inverse of the relative layer height is very high, as minimal overlapping is used between adjacent layers, so E* 0 cannot be established for some conditions.…”