Microstructure evolution characterization of GH4169 superalloy under ultrasonic high-frequency vibration energy

Ultrasonic vibration is widely used for nanocrystal surface modification and components performance enhancement due to its significant effects on grain refinement and microstructure during casting, additive manufacturing, cold rolling and cutting processes. The microstructure evolution characterization and dislocation–twin grain refinement mechanism of GH4169 superalloy under ultrasonic high-frequency vibration energy (UHFVE) condition in ultrasonic vibration-assisted side milling (UVASM) were investigated. Meanwhile, the conventional side milling (CSM) was conducted as comparison experiments to highlight and demonstrate the effect of ultrasonic vibration in machining. The UHFVE of the ultrasonic vibration system (UVS) acting on the material surface was theoretically derived by the one-dimensional fluctuation equation. The effects of UHFVE on the average grain diameter (AVG), kernel average misorientation (KAM), geometrically necessary dislocation (GND) density and grain boundary orientation angle were investigated. The higher GND density and lower AVG were due to the accumulation of UHFVE on microstructure in UVASM, which were 24.8% higher and 12.6% lower compared to those of CSM, respectively. The increase of UHFVE from 1.57 × 105 J to 4.01 × 105 J resulted in more refined grains with a 12.5% reduction in the AVG. The mechanism of dislocation-twin grain refinement dominated by UHFVE was revealed. The intervention of UHFVE strengthens the dislocation accumulation and fragmentation of twins and grain boundaries in the dislocation–twin grain refinement mechanism, resulting in a thorough grain refinement. In addition, the microhardness and residual compressive stress improvement in the surface was attributed to the increase of dislocation density and grain refinement in UVASM, which is beneficial to improve surface strengthening and fatigue resistance.