Feasibility Study on Machining Super-dimensioned Aviation Deep-hole Ti6Al4V Part With Axial Ultrasonic Vibration-assisted Boring

Abstract

\n Deep-holes are typical super-dimensioned parts of aircraft structures, with associated machining technology which is recognized as challenging due to the difficult-to-machine material, the narrow and enclosed machining environment, and the weak rigidity and large deformation of the boring bar. Axial ultrasonic vibration-assisted cutting has been proved to greatly enhance machining performance, especially in the precision machining of aviation alloy. This paper focuses on the machining of a super-dimensioned titanium alloy Ti6Al4V aviation deep-hole part (aspect ratio exceeding 20) with the axial ultrasonic vibration-assisted boring (AUVB) method. First, the kinetics of the AUVB process is analyzed and a retrospective of its separation cutting feature is provided. Subsequently, a multi-step cantilever beam model of the boring bar is established to analyze its static rigidity and dynamic stability. The aperture error of bored hole is deduced, and it is found to be mainly determined by the diameter of the basic hole, the cutting depth, the boring force and the length, diameter, elastic modulus, and rotational inertia of each step. Size coefficient, rather than aspect ratio, is then put forward to represent the static rigidity of the boring bar, which is proportional to the third power of the diameter and inversely proportional to the fourth power of the diameter. In addition, two different vibration cases, namely modal-coupling vibration and regenerative vibration are considered for dynamic stability analysis. The actual dynamic rigidity of AUVB in both of these cases is larger than that of conventional boring (CB), resulting from the lower actual boing force and the larger actual cutting depth. Next, the morphology of bored surface is analyzed, and the geometric height of peaks formed by AUVB and CB are calculated. Phase shift φ= π is suggested for obtaining a better surface in AUVB. Finally, the feasibility of AUVB on machining a super-dimensioned titanium alloy Ti6Al4V aviation deep-hole part with three different sizes of boring bar (size coefficients are 7473, 4076 and 2718, respectively) is verified through systematic experiments, and compared with CB. Results demonstrate that AUVB has obvious advantages in reducing the boring force, improving boring accuracy, suppressing vibration and promoting surface quality. Furthermore, the aperture error decreases to 50% and vibration amplitudes decrease to only 20%-25%. The overall surface roughness of the deep-hole part stays below Ra=0.8μm with rotational speeds of 60r/min and 80r/min, and the surface residual stress state is transferred from the tensile state to a compressive one. As a result, not only AUVB can provide better boring accuracy and surface finish, but it also can enhance the surface fatigue properties.