Design optimization for defect-free AISI 316 L/IN718 functionally graded materials produced by laser additive manufacturing

Additive manufacturing (AM) of functionally graded materials (FGM) has become an important growing trend in the research and development of various sectors. The main challenging issue is alloy design for the transition zone between terminal alloys so that the expected performance is achieved with minimal risk of defects. In this study, the transition zone design in AISI 316 L-IN718 FGM via laser-directed energy deposition (L-DED) is investigated concerning susceptibility to cracking defect, its formation mechanisms, as well as the design effect on mechanical properties. Consequently, the compatible design of the AISI 316 L-IN718 graded materials is established for laser-based AM. The evaluation of the gradient structures showed that the 75 % AISI 316 L-25 % IN718 region, with the continuous evolution of low-melting eutectic compounds caused by the formation of crystallographic texture in the build direction, is highly sensitive to liquation cracking, which occurs in the presence of thermal stresses arising from the deposition of subsequent layers. In addition, defects of porosity and oxide micro-inclusions intensify the discontinuity and cracking in the remarked composition region. Finally, it was demonstrated that the improvement of the dissimilar structure of AISI 316 L and IN718 alloys can be purposefully fulfilled by designing a compositional gradient in the form of a 50 wt% mixed region between the base alloys (FGM-50 %) resulted in the maximum mechanical properties (tensile strength 540 ± 10 MPa, elongation 52 ± 2 %, and toughness 24 ± 1.4 kJ/mm3).