Effects of induction heating on creep resistance of Inconel 718 during laser metal deposition

Abstract

Inconel 718 (IN718) is widely used as a material for aircraft or power- generator gas turbine blades due to its excellent combination of tensile strength and creep resistance, particularly at high temperatures. Traditional manufacturing methods such as casting or forging have been the go-to methods for the production of turbine blades. However, with the recent drive for increased thermal efficiency, gas turbines are operating at higher temperatures. To prevent heat- related damage to blades used under such elevated operating temperatures, turbine blades with complex internal cooling channels are required. However, traditional manufacturing methods face limitations in producing such complex shapes. Therefore, as an alternative to traditional manufacturing methods, additive manufacturing (AM) methods that can effectively produce complex shapes are gaining popularity. AM involves melting or sintering materials using a heat source and then layering them one by one to create a component. During the AM process, IN718 exhibits varying microstructural characteristics, such as grain size or the type of secondary phases, due to the complex thermal history resulting from repetitive heating cycles. These microstructural characteristics are closely related to dislocations, which are a primary cause of creep, occurring in high-temperature environments where IN718 is commonly employed. Creep occurs due to the accumulation of dislocations around grain boundaries or Laves phases. Therefore, as the number of grain boundaries or Laves phases increases, creep resistance tends to decrease. Meanwhile, strengthening phases formed within the grain boundaries hinder the movement of dislocations, thereby enhancing creep resistance. Based on the relationship between these microstructural characteristics and creep resistance, achieving improved creep resistance requires reducing sites where dislocations accumulate, such as grain boundaries and Laves phases, and forming strengthening phases that can inhibit dislocation motion. In the past, post-heat treatment processes were carried out at specific temperatures to reduce grain boundaries and Laves phases and to promote the formation of strengthening phases. If, during the AM process, IN718 could undergo conditions similar to post-heat treatment at specific temperatures, leading to the reduction of grain boundaries and Laves phases and the formation of strengthening phases, then a novel method could be proposed to enhance creep resistance without the need for post-heat treatment processes. Therefore, in this study, the influence of the microstructural characteristics formed at different temperatures during laser metal deposition (LMD) combined with induction heating on the creep resistance of IN718 was investigated. As a result, the IN718 deposits produced under temperature-controlled conditions during the LMD process exhibited different grain sizes and secondary phases compared with those produced solely through the LMD process. Due to the differences in these microstructural characteristics, the IN718 deposits with induction heating during the LMD process exhibited improved creep resistance compared with those produced solely through the LMD process. The findings of this study suggest that it is possible to control the microstructural characteristics of various heat-resistant metal materials similar to IN718, during the AM process. This is expected to significantly contribute to the manufacturing technology of additive products with improved creep resistance in various industries where heat-resistant metal materials are used.