<p>Ultrasonic vibrations have been utilized extensively in numerous machining processes owing to their positive impact on the machinability of difficult-to-cut materials. Understanding the energy conversion mechanism during ultrasonic vibration-assisted machining is essential. Knowing the impact of ultrasonic energy on the workpiece material enables optimization of the energy input to improve workpiece machinability. This study investigates the energy conversion for ultrasonic vibration-assisted milling of TNM alloys. The acoustic softening effect in ultrasonic vibration-assisted milling was evaluated by investigating the influence of ultrasonic energy on the plastic deformation energy of the workpiece material. The influence of ultrasonic power on the plastic deformation released energy was evaluated by acoustic emission signals and milling forces. Meanwhile, its impact on stored energy was investigated by microstructure testing. The results indicate that an increase in the ultrasonic vibration energy density enhanced both plastic deformation released energy and stored energy within a certain range. The increase in proportion is attributed to the released energy. Energy conversion is conducive for improving the machinability of TNM alloys. Excessive or insufficient ultrasonic vibration energy density reduces the energy released during plastic deformation. This, in turn, limits improvements in TNM alloy machinability. This study has provided a reference and guidance for optimizing ultrasonic energy to improve the machinability of workpiece materials in ultrasonic vibration-assisted milling.</p>