The influence of Laves phases on the room temperature tensile properties of Inconel 718 fabricated by powder feeding laser additive manufacturing

In this paper, a powder feeding laser additive manufacturing technology has been used for fabricating the Inconel 718 super-alloy. Laves phases of different sizes and morphologies have been obtained by using three types of heat treatments. The influence of Laves phases on the room temperature tensile properties of laser additive manufactured Inconel 718 has been investigated. The results show that small and granular Laves phase can be gained after heat treatment at 1050 °C for 15 min (S-15 sample). When the holding time extends to 45 min, the morphologies of Laves phases basically remain unchanged while its volume fraction further decreases (S-45 sample). Nevertheless, irregular and long-striped Laves phases still exist in the samples only after direct aging heat treatment (DA sample). The room temperature tensile results reveal that the S-15 samples have better tensile strength and ductility than that of S-45 samples. Besides, the DA samples with irregular and long-striped Laves phases show the lowest tensile strength and ductility. Hence, a certain amount of small and granular Laves phases are presumably beneficial for the room temperature tensile properties of Inconel 718. Moreover, a model has been established to describe the fracture of the Laves phase. On the basis of the fracture model, the critical stress needed for the fracture of long-striped Laves phases is lower than that needed for the fracture of granular Laves phases. Therefore, the former generally suffered internal fracture while the latter often fail by interfacial decohesion. Through influencing the volume fraction, the size and the distribution of γ” phase, the effect of the Laves phases on the room temperature tensile property is achieved. Furthermore, a yield strength model has been developed to reveal this influence in terms of numbers. The yield strength increments caused by grains, solution elements and γ′ phase are almost the same for the three kinds of samples. The differences of the yield strength are mainly caused by γ” phase. In addition, in terms of ductility, granular Laves phases are more favorable than long-striped Laves phases.