Study on the chip formation mechanism and micro characteristics of adiabatic shear band in nickel-based single crystal superalloy

Nickel-based single crystal superalloy DD98, characterized by its unique monocrystalline structure, exhibits exceptional high-temperature performance and is extensively utilized in manufacturing critical hot-section components for aero-engines. Conventional deformation theories based on grain boundary sliding in polycrystalline materials are not applicable to single crystal machining. Furthermore, the chip formation mechanism during turning of nickel-based single crystal superalloy remains inadequately understood. To achieve high-efficiency and low-damage turning of this advanced material, this study systematically investigates the chip formation mechanism and characteristics of adiabatic shear bands(ASBs). The research methodology encompasses: macroscopic analysis of chip morphology to quantify serration frequency variations with cutting parameters; SEM-based microstructural evolution characterization of shear bands; microhardness testing to validate mechanistic interpretations; and TEM examination of subsurface nanostructures to elucidate ASB formation mechanisms. Key findings demonstrate that: Serrated chips form during turning, with plastic deformation severity quantified by serration density. Serration frequency increases with cutting speed and depth but decreases with feed rate; Thermoplastic instability induces ASB formation, accompanied by phase elongation, fragmentation, and dynamic recrystallization within shear bands; ASB regions exhibit nanocrystalline structures with high dislocation density, pronounced tangling, and widespread stacking faults/twinning. Dynamic recrystallization-generated new grains constitute the primary mechanism for ASB hardening. These results provide fundamental theoretical insights for advancing nickel-based single crystal superalloy machining technologies, particularly in overcoming current manufacturing bottlenecks.