Tensile and Low-Cycle Fatigue Behavior of Dissimilar Friction Welds of Alloy 718/Alloy 720Li

This study investigates the microstructural and mechanical properties of dissimilar alloy 718/720Li joints, contemplated for use in hot-end applications of aircraft engine components with service temperatures of up to 730°C. Microstructural characterization, microhardness, and room-temperature tensile tests were performed, followed by high-temperature tensile tests (650°C) on both as-welded and post-direct aging heat-treated conditions to analyze the performance of weld joints at high temperature. Results indicate that the friction welded joint interface in its as-welded condition exhibits lower hardness (240 HV at the heat affected zone) than the alloy 718 base material due to the dissolution of strengthening precipitates. However, post-weld direct aging (DA) with an alloy 718 aging cycle has improved the microhardness to be comparable to the parent materials. The peak hardness value of 478 HV achieved at the interface region is 26% higher than the as-welded condition. The dissimilar 718/720Li FRW joints in direct aging condition show room and high-temperature tensile strengths of 1374 and 1172 MPa, respectively. Similarly, during low-cycle fatigue testing, 718/720Li FRW joints in direct aging condition withstand up to 3648 cycles, comparable to the low-cycle fatigue life of the alloy 718 parent material, with failures away from the FRW weld interface and into the alloy 718 base material. The cyclic softening and hysteresis trend obtained during low-cycle fatigue tests of dissimilar 718/720Li friction welded joints are comparable with alloy 718 parent material. The dissimilar 718/720Li friction welded joints have a peak stress of 1097 MPa, which falls between low-strength alloy 718 and high-strength alloy 720Li. These findings confirm that friction welding of dissimilar nickel-based superalloys, such as alloy 718 with alloy 720Li, is promising for adaptation in aero engine applications.