Defect analysis and fatigue design basis for Ni-based superalloy 718 manufactured by selective laser melting

It is well known that high strength metallic materials with Vickers hardness HV > 400 are very sensitive to small defects. This paper discusses fatigue properties of a Ni-based Superalloy 718 with HV = similar to 470 manufactured by additive manufacturing (AM). The advantage of AM has been emphasized as the potential application to high strength or hard steels which are difficult to manufacture by traditional machining to complex shapes. However, the disadvantage or challenge of AM has been pointed out due to defects which are inevitably contained in the manufacturing process. Defects of the material investigated in this study were mostly gas porosity and those made by lack of fusion. The successful application of the root area parameter model was confirmed. Although the statistics of extremes analysis is useful for the quality control of AM, the particular surface effect on the effective value of defect size must be carefully considered. Since the orientations of defects in AM materials are random, a defect in contact with specimen surface has higher influence on fatigue strength than an internal defect and has the effective larger size termed as root area eff than the real size, root area, of the defect from the viewpoint of fracture mechanics. The guide for the fatigue design and development of higher quality Ni-based Superalloy 718 by AM processing based on the combination of the statistics of extremes on defects and the root area parameter model is proposed.