Gradient Characteristics of Surface Metamorphic Layer Microstructure Induced by Longitudinal Torsional Ultrasonic-Assisted Milling Ti-6Al-4 V Alloy

Surface metamorphic layer formed in milling has an obvious effect on the fatigue properties and reliability of critical components in different fields. However, the microstructural evolution of the surface metamorphic layer induced by milling Ti-6Al-4 V, especially for longitudinal torsional ultrasonic-assisted milling (LTUAM), has not been investigated in-depth, which will affect its application. In this study, the microstructure evolution law under ultrasonic vibration was analyzed based on modified Kocks-Mecking model (KM model). Meanwhile, milling experiments and finite element simulations at different amplitudes were performed for Ti-6Al-4 V. Afterward the microstructural evolution in the sound field was examined via analysis of the grain boundaries, phase transitions, and cutting process variables. Experimental results show the surface metamorphic layer under ultrasonic vibration presents stronger plastic deformation which differs from conventional milling (CM). Workpiece surface showed obvious simple harmonic vibration microscopic features, and a metamorphic layer of about 70 mu m arose after LTUAM. Moreover, different degree of refinement occurs with a declining degree of about 35% on average grain size. Dislocation slip and dynamic recovery dominate the microstructural evolution during LTUAM, which underwent a gradual transition process in surface metamorphic layer.