Serrated chip formation mechanism analysis using a modified model based on the material defect theory in machining Ti-6Al-4 V alloy

The serrated chip is a remarkable characteristic in the machining of the Ti-6Al-4 V alloy, but its formation mechanism is not completely understood. Finite element modeling is an effective method for studying the complex thermo-mechanical problems that occur during the machining process. The constitutive model plays a key role in the simulation, and Johnson-Cook (JC) model is the most commonly used model. However, the JC model neglects the coupled effects of the strain, strain rate, and temperature, and the material is assumed to be homogeneous. Thus, the JC model is applicable for studying macroscale machining. In fact, cutting is a complex thermo-mechanical process, and there are material defects in the Ti-6Al-4 V alloy. First, a modified JC model is proposed based on the THAN law, which is derived from material defects considering the coupled effects of the strain, strain rate, temperature, and strain softening. Then, the serrated chip morphology and cutting force predicted by the JC-THAN model are compared with experimental data. Finally, the serrated chip formation mechanism from macroscale machining to microscale machining is analyzed considering the material defects based on the JC-THAN model, and the dominated formation mechanism at each scale is found.