Tensile Behavior of Additively Manufactured Inconel 718 and Stainless Steel 316L with Compositionally Graded Joints

Abstract

The microstructure and tensile behavior of laser powder bed fusion (LPBF) processed 316L austenitic stainless steel (316L) and Inconel 718 Ni-based superalloy (IN718) coupons with compositionally graded joints (CGJ), spanning lengths of 0, 10 and 20 mm, in the as built and heat-treated conditions, are investigated. In the as built condition, the microstructure of pure 316L and IN718 ligaments consist of micron-sized sub-grains present within <001> textured columnar grains, whereas CGJs contain a mixture of randomly textured columnar and equiaxed grains. Heat treatment, involving solutionizing above 1040 °C with subsequent ageing at 720 and 620 °C, leads to the recrystallization of portions with > 85 wt.% IN718 of the CGJ coupons. Higher composition gradient span, in both the as built and heat-treated states, improves the yield and tensile strengths of the specimens, but compromises ductility. Tension-compression asymmetry, which also progressively increases with increasing strain and the CGJ span, is observed in all the specimens. Simulations indicate that CGJs with shallower composition gradients have lower fluctuations in the stress triaxiality, von mises equivalent stress, and the maximum shear stress compared to those with sharper gradients. These mechanical property variations and the deformation characteristics of the CGJ specimens are analyzed in detail by considering the varying degrees of plastic constraint on the 100 wt.% 316L and the degree of interactions between strain-generated dislocations and geometrically necessary dislocations. Finally, the effectiveness of CGJ in enhancing the tensile properties of the 316L/IN718 joints and the geometrical considerations for designing such joints for different alloy combinations is discussed.