Simulation and Experimental Study on Serrated Chip Formation in Torsional Ultrasonic Milling of Titanium Alloys

The Ti-6Al-4V titanium alloy has found extensive application in aerospace engineering due to its elevated specific strength and corrosion resistance properties. Notably, the material exhibited low thermal conductivity but high chemical reactivity, which often resulted in the formation of irregular serrated chips during the processing stage. This in turn will affect the processing efficiency. Conventional chip suppression methods present challenges in regulating the chip segmentation frequency. Longitudinal torsional vibration assisted milling (LTUAM) offers a novel approach to enhancing chip morphology through high-frequency intermittent cutting. In this paper, we present a two-dimensional milling force-chip morphology coupled finite element model. This model is established by constructing a longitudinal and torsional composite ultrasonic kinematics model combined with the modified Johnson-Cook damage criterion. We then carry out relevant tests to verify the simulation. The experimental validation demonstrated that the relative errors between the simulated and measured values of F _x and F _y are less than 10%. Furthermore, the geometric features obtained from the simulation, such as tooth pitch and serration frequency, meet the requirements for machining accuracy. It has been determined that the ultrasonic amplitude exerts a substantial nonlinear modulation effect, leading to a 34.01% and 30.82% reduction in the main milling forces, F _x and F _y , respectively, when compared to the conventional process at an amplitude of 4 µm. Concurrently, The frequency of chip serration is increased by 171%. The frequency is 56 times per millimeter, and the pitch of the teeth is reduced by 28%. When the amplitude is more than 4 micrometers, the cutting force is rebounded due to the decrease in the proportion of the tool-workpiece effective separation time. The amplitude of 6 µm triggers the inertial fracture of the chip, which causes the tooth pitch to rebound. This results in a 67.5% reduction in the degree of serration compared to conventional milling. This study unveils the nonlinear regulatory mechanism of longitudinal torsional ultrasonic amplitude on the cutting process of Ti-6Al-4V, establishing a process parameter optimization system with the coupling relationship of ultrasonic amplitude-chip morphology as the core. The process is supported by a new theory and method, which makes it possible to machine titanium alloy components at both high efficiency and low damage.