Abstract: With the increasing requirements for stringent material performance in nuclear power equipment and other fields, preparing functionally graded material (FGM) with variable elemental composition has become a developing trend. Taking the advantage of laser additive manufacturing in controlling multiple sedimentary components, different composition gradients (10%, 20% and abrupt) with 316L stainless steel and Inconel 718 (IN718) are prepared by coaxial powder feeding directed energy deposition (DED). The microstructures and mechanical properties of different FGMs are analyzed by metallographic analysis, microstructure analysis, energy spectrum analysis, microhardness and tensile test. The results show that both the contents of Ni/Nb elements and microhardness increased with the increase of IN718. For gradient or abrupt FGM, microhardness is positively correlated with the content of IN718 based on the linear fitting of Fe and Ni elements contents. The transition zone located in the interface of abrupt FGM makes the elements diffuse sufficiently between layers. Three important factors, i.e., the large differences in materials properties, the rapid change of composition and the existence of Laves phase lead to poor interface bonding performance and cracks. The best tensile properties with the highest average ultimate tensile strength (526 MPa) are obtained by the FGM with composition gradient of 10%. The findings will be helpful to choose appropriate gradient variation and understand the forming mechanism of functionally graded materials fabricated by laser additive manufacturing.

Key words: functionally graded material, directed energy deposition, microstructure, composition gradient, mechanical properties