Defect of functionally graded material of inconel 718 and STS 316L fabricated by directed energy deposition and its effect on mechanical properties

For multi-materials having different compositions, delamination or crack may occur at the interface of two dissimilar materials due to differences in a lattice structure, residual thermal stress, and coefficient of thermal expansion. To solve such problems, functionally graded material (FGM) that gradually changes the properties of the interfacial boundary has been attracting attentions. Since the chemical compositions of FGM materials are gradually changed according to position, inter-layer residual stress between two heterogenic materials can be relieved. In this study, a functionally graded material (FGM) containing Inconel 718 and stainless steel (STS) 316L was fabricated by directed energy deposition (DED), with STS 316L volume fraction changing in the range of 0 wt% to 100 wt%. The fabricated FGM includes depositions of 25% graded (Graded (25)) and 10% graded (Graded (10)) materials. In non-graded samples, cracks were observed at the interface of Inconel 718 and STS 316L. In all three conditions, cracks in the vertical direction were observed in specific regions, and these cracks were caused by precipitation, inclusion, and columnar-to-equiaxial transition (CET). As far as hardness is concerned, a smooth decrease in hardness was observed in the graded-materials, but the hardness decreased rapidly at the interface of the non-graded sample. The highest tensile strength and elongation were obtained in the Graded (25) material. Microstructural and mechanical properties indicated that mechanical properties vary depending on the gradient composition, and it is essential to choose the appropriate gradient composition of FGM fabricated by DED.