Kissing bond defects in inertia friction welds of IN718 alloys: Experiment and prediction

Different from many solid-state-based manufacturing processes, much less attention has been paid to kissing bond (KB) defects for inertia friction welding (IFW) despite its crucial impact on weld performance. Herein, for the first time, the KB defects in inertia friction welds are experimentally investigated, and the formation mechanism is elucidated based on well-validated numerical simulation. Microstructural analysis and tensile tests reveal the occurrence of KB defects at the periphery of an inertia friction weld of IN718 alloy rods. The KB defects appear as discontinuous polygonal voids chain in microns at the interface, leading to premature fracture along the welding interface and thus reduced ductility. To predict the formation of KB defects, a novel two-step approach, in which thermo-mechanical simulation is integrated with an analytical solid-state bonding model, is proposed. The validated thermo-mechanical simulation confirms similar temperature profiles at the interface, while the central region shows much higher compressive normal stress than the periphery. Using the thermo-mechanical history data, the material bonding behavior, i.e., how the material become bonded from separated, is predicted. The prediction indicates complete bonding across most of the interface, except at the periphery where KB defects are observed. Further discussion attributes KB defects to the insufficient interfacial compressive normal stress, providing critical insights into their formation mechanism and potential mitigation strategies. The proposed approach provides a generic methodology for analyzing the interfacial bonding behavior under highly transient thermo-mechanical condition.