Electric field-assisted machining of aerospace-grade materials

Aerospace-grade materials are highly regarded for their exceptional mechanical properties, making them the preferred choice for use in demanding engineering applications. Engineering alloys like Ti-6Al-4V (also known as Grade 5 titanium) and silicon carbide reinforced aluminium (SiCp/Al) metal matrix composites (MMCs) possess excellent mechanical and physical properties, such as high strength-to-weight ratio, high stiffness and rigidity, high corrosion resistance, good wear resistance through their hardness and abrasion-resistant phase, high thermal stability, and good fatigue performance properties. However, certain material properties of such materials also make them challenging for machining. These include low thermal conductivity and high chemical affinity to the cutting tool material for Ti-6Al-4V, as well as high hardness and abrasive nature of the silicon carbide in the case of MMC. As a result of these factors, conventional machining techniques often face limitations due to the low material removal rates resulting from restricted cutting parameters such as low cutting speed, low feed rate, and low depth of cuts. To overcome these limitations, hybrid techniques have been developed: ultrasonically-assisted turning (UAT) is a hybrid type of machining developed to address the machining issue of difficult-to-machine materials. It uses low-amplitude and high-frequency vibration superimposed on the cutting tool movement during the material removal process, which has been shown to reduce cutting forces and improve surface quality. Electrically-assisted turning (EAT) is another hybrid machining technique introduced to improve the machinability of challenging metals and alloys through the advantage of material softening, resulting from the electroplasticity effect induced by the electric current flow. The technique involves dry cutting, at low temperatures with a fast reaction at the process zone, resulting in decreased cutting force and reduced surface roughness.

This work introduces a novel and innovative hybrid-hybrid manufacturing technique called Vibration Electrically-assisted Turning (VEAT) to machine aerospace-grade materials of Ti-6Al-4V and SiCp/Al metal matrix composites. The machining technique combines EAT and UAT to deliver pulsed current at high ultrasonic frequency, achieving the combined advantages and enhancing the machinability of difficult-to-machine materials. In addition, another machining technique, Pulsed Electrically-assisted Turning (PEAT), was also performed to deliver electic current in pulses during machining.

Prior to the machining experiments, electrically-assisted (EA) compression and tension tests were carried out on Ti-6Al-4V to study material deformation under the influence of electric current flow. The tests showed that optimal electrical parameters in terms of peak current, current density, and frequency were 200 A, 2 A/mm2, and at 100 Hz respectively. In the machining experiment, various turning techniques were employed, comparing CT, UAT, EAT, PEAT, and VEAT under different current conditions. To facilitate the experiment, several prototypes of the EA machining setup were developed to explore the optimal method of electric current delivery into the process zone while also being able to allow ultrasonic vibration of the cutting tool simultaneously. The lathe machine was modified and insulated to accommodate the electrification of the workpiece, with compressed air used to control chip evacuation and address built-up edge (BUE) formation on the cutting tool. The cutting force, surface roughness, temperature and tool wear were analysed. The experimental analysis of the VEAT technology reveals noteworthy advantages, with a significant reduction in the cutting forces and an improvement in the surface roughness.

The preferred setup for electrically and ultrasonically-assisted turning SiCp/Al 217XG and Ti-6Al-4V with cutting parameters of cutting speed of 7.5-12.5 m/min, feed rate of 0.1 mm/rev and depth of cut of 0.25 mm, was to deliver the electric current directly from the cutting tool into the workpiece. The electric pulsed current was more effective than continuous current, yielding lower cutting forces and surface roughness at increased current density (peak current 150 A), and the cutting tool’s ultrasonic vibration enhanced the surface quality of the machined surface at lower current density (peak current 50 A).