Experimental study on portable pneumatic milling of aircraft titanium alloy skin for war injury

Portable cutting technology is urgently needed in the structural repair of modern war-injured aircraft. Pneumatic metal milling has the advantages of portability, explosion-proof and easy access to power source, which is an ideal method to achieve rapid and safe cutting of titanium alloy skin damage. However, there are few researches on portable milling equipment, pneumatic milling parameters and tool lubrication methods at the damaged site, which restricts the popularization and application of this technology. In this paper, a portable pneumatic cutting device for titanium alloy AMS 4911 Titanium Sheet skin damage in situ was designed and developed by combining pneumatic milling and Combined mist jet (CMJ) lubrication technology. A set of aircraft skin simulation cutting test platform was established, and simulation milling and in-situ cutting tests of thin-wall titanium alloy skin were carried out. The test results show that when the titanium alloy TI6AL4V sheet skin with a thickness of 1.5mm is milling, the air motor can operate in the best power range by using the inverse milling method, the diameter of φ10mm milling cutter and the milling width of ae is 1~3mm. Using CMJ lubrication, after each cutting, maintain the original milling width and then cut 1~2 times, can improve the tool life and incision shape accuracy. The portable pneumatic milling device and the technological parameters developed in this paper have successfully realized the circular hole cutting for repairing the damaged part of the broken hole on a certain airframe, which provides a simple and practical new technology for aircraft Titanium Alloy Sheet damage repair.

Military aircraft will suffer a variety of blows in combat, and the implementation of rapid (24~48h) and effective (recovery or partial recovery of flight and combat capability) battle injury repair for aircraft assessed as repairable damage is the best way to "regenerate" damaged aircraft, and has extremely important military significance for maintaining continuous combat capability in the air. Although many war-injured accessories can be repaired by replacement parts, the structural damage of various tearing belts with sharp holes in the body skin must be repaired quickly at the damaged site. It is a common method to cut the broken holes into regular shape with various cutting tools, and then restore or partially restore the structural strength by means of gluing and riveting. With the large number of applications of difficult-to-process titanium alloy in modern military aircraft skin, the existing aluminum alloy skin cutting technology can not be applied to titanium alloy skin, and laser, shaped blasting and other high-energy technology can not ensure the safety of the aircraft and related components. At present, the difficulty of rapid cutting of titanium alloy skin damage in situ has become the technical bottleneck of aircraft titanium alloy skin battle injury repair, especially in the conflict between Russia and Ukraine, military aircraft with high intensity low-altitude penetration are more susceptible to various penetrating injuries, and the application value of titanium alloy skin battle injury repair technology is great, which will directly affect the attendance rate of aircraft.

Pneumatic cutting has the advantages of portability, explosion-proof, easy access to power source, and large output torque of equal volume. It is an ideal and feasible technical method to solve the problem of aircraft AMS 4911 Titanium Alloy sheet skin damage cutting. However, titanium alloy is a typical difficult material to process, and the following key problems must be solved to achieve this technology: (1) weak stiffness cutting. From the aspects of cutting device structure and operation mode, the problem of insufficient stiffness of portable pneumatic cutting process system should be overcome to achieve effective cutting of titanium alloy skin. (2) Cutting parameters of the reasonable working section of the air motor. Different from the motor, the instantaneous power and torque of the air motor are easily affected by the external load, so that the parameters of the air cutting are interwoven and always in change. The appropriate cutting parameters and technological measures should be selected to give full play to the cutting ability of the air motor and improve the cutting quality. (3) body cutting lubrication. Safe and effective in situ cutting lubrication of the body can reduce tool wear and improve the efficiency of emergency repair. Although there are many studies on Titanium Alloy Aerospace cutting under machine tool conditions, no portable cutting device directly targeting at titanium alloy skin holes has been reported, nor has there been research on the pneumatic milling process of titanium alloy skin under in-situ conditions of aircraft, which limits the application of portable pneumatic milling technology in the repair of titanium alloy skin damage cutting of our advanced fighter aircraft. In view of this, this paper designed and developed a portable pneumatic cutting device at the damaged spot of titanium alloy skin, integrated the self-developed Combined mist jet (CMJ) system and key cutting parameter measurement system, and developed a set of aircraft skin simulation cutting test platform. Through the experimental analysis of pneumatic milling of titanium alloy skin, the manual pneumatic milling method was optimized, and the combination of pneumatic cutting parameters to play the performance of the air motor was obtained, the effectiveness of CMJ lubrication was verified, and the measures to improve the cutting shape accuracy were given. Finally, the in-situ verification cutting of the body skin was realized. The cutting device developed in this paper and the technological parameters obtained by experiments have great practical value and popularization prospect in the field of emergency repair of titanium alloy structure of our military aircraft.