Functionally Graded Materials(FGM) are characterized as a material whose material composition or structure gradually changes depending on the location. FGM has a characteristic that mechanical properties or chemical properties change gradually depending on the composition and structure according to location. In the nuclear industry, materials required complex properties such as strength, corrosion resistance, and radiation resistance. Ferritic steel has high strength and workability, and stainless steel has good corrosion resistance, toughness, and heat resistance. In the nuclear industry, a material that secures both materials’ properties by cladding stainless steel on ferritic steel is being applied. When welding ferritic steel and stainless steel, residual stress is generated at the dissimilar metal joint due to the difference in strength and composition of the two materials, which causes stress corrosion cracking during operation. In addition, due to the difference in the coefficient of thermal expansion, there is a possibility that fatigue cracks may occur at the joint due to the generation of stress due to temperature change during operation. It is necessary to study to solve the delamination caused by the difference in properties between different materials in dissimilar welds. Recently, a DED 3D printer that can gradually change material components has been developed. Using this, it is possible to manufacture an FGM in which the composition of steel and stainless steel changes gradually, and it is possible to improve the physical properties of dissimilar metal joints.

In this study, P21 ferrite steel and STS316L FGM materials were manufactured using a multi-cartridge DED 3D printer and evaluated. By laminating in 50 layers with a thickness of 0.3 mm, an FGM block that gradually changed from P21 100% and STS 0% material to P21 0% and STS 100% was fabricated, and microstructure and mechanical properties were analyzed. Through OM analysis, various shapes of solidified tissues such as linear, point, and cell were observed depending on the position. Through EBSD analysis, it was confirmed that the ferrite was gradually replaced with austenite depending on the composition, and the phase separation internal and external of dendrite during solidification was also observed. In the hardness profile, a region with a higher hardness than P21, a strong material, was observed in the material mixture region. In the tensile test, the strain was concentrated in the weak area, and the strength and elongation of the area were selectively measured. It was confirmed that the impact toughness was also selectively measured for the characteristics of the notch position.