Search

Hole-making for Carbon Fiber Reinforced Plastics (CFRP)/Ti-6Al-4 V stacks is crucial for the assembling strength of aircraft structure parts. This work carried out experimental work for helical milling (HM) of the stacks with sustainable cooling/lubrication (dry, MQL and cryogenic) conditions. Cutting forces and temperatures at the CFRP layer, Ti-6Al-4 V layer and the interface of stacks were obtained by a developed measuring system. The temperatures in CFRP machining at cryogenic condition varied from −167 °C to −94 °C, which were much lower than those at dry and MQL conditions. The maximum temperature near the interface of stacks for the ninth hole was higher than 240 °C due to heat conduction from Ti-6Al-4 V layer. The hole quality, hole diameter and tool wear mechanism at different cooling/lubrication conditions were presented and discussed. MQL condition generated mainly extrusion fracture for the fibers, due to the reduced friction effect compared with dry condition. MQL was helpful to reduce the feed mark at the hole surface of Ti-6Al-4 V alloy. The flank wear of cutting edge at MQL condition was better than those at dry and cryogenic conditions. Cryogenic cooling contributed to better CFRP surface with smaller delamination and hole entrance damage due to the increased resin strength and fiber brittleness. The damage near the entrance of CFRP were analyzed by the contact state of cutting edges and fibers. Additionally, hole diameters near the exit of CFPR layer were larger than other test positions. This work provided feasible processes for improving hole quality and tool life in hole-making of CFRP/Ti-6Al-4 V stacks.

Helical milling (HM), axial ultrasonic vibration helical milling (A-UVHM) and longitudinal and torsional ultrasonic vibration helical milling (LT-UVHM) were developed for hole-making of Carbon Fiber Reinforced Plastic (CFRP) composites. Cutting trajectory, cutting speed, acceleration and effective rake angle of peripheral edge in HM and UVHM processes were modeled. Compared with HM, axial forces with UVHM processes were reduced by 9.0 %–35.7 %. The burrs at entrance and exit with UVHM were also reduced. In A-UVHM and LT-UVHM processes, an effect of shear with variable direction which is good to fracture fibers was presented, according to the relative movement of peripheral edge and fibers. Additionally, the variations of cutting speed and acceleration in A-UVHM and LT-UVHM are also helpful to fracture fibers, due to the wide range of speed and impact effect. Smooth surface with regular fiber fracture morphology and tiny fragments was generated in UVHM due to the effect of shear along variable direction. Compared with HM, the maximum effective rake angle of peripheral edge in UVHM was increased by 112.1 %–192.6 %, which is helpful to fracture fibers by sharper cutting edge.

Compared with conventional hole-making process, rotary ultrasonic machining (RUM) is commonly believed to improve the cutting performance, such as low cutting force, good hole quality, long tool life, and high material removal rate. However, this work proposes an interesting observation on hole-making process using longitude-torsional ultrasonic assisted helical milling (LT UAHM). Due to the special kinematic motion in LT UAHM, the trajectory of the cutting tool has complex characteristics which may generate a negative effect on machining performance. In this study, a comparative experiment between LT UAHM and conventional helical milling (HM) was carried out. The results showed that the axial cutting force is not always reduced, the machined hole quality is also not improved consistently in LT UAHM. To comprehend the cutting mechanism of LT UAHM incisively, a kinematic trajectory model was proposed. By the theoretical kinematic trajectory model, LT UAHM can generate the ultrasonic elliptical cutting in the process, and the phase shift is a critical factor that can affect the elliptical trajectory shape even the vibration cutting direction of the tool’s cutting edge. The equivalent cutting angle and direction of friction between the rake face and chip in processing change accordingly. As a result, the magnitude of cutting force and machined hole quality in terms of burr height in this process will be significantly affected.

A fuel cell-electrolyzer series device with monoethanolamine (MEA) as fuel in the fuel cell and MEA-containing electrolyte in the electrolyzer is developed. The fuel cell stack directly powers the electrolyzer. As a result, MEA is degraded electrochemically in the anode sides of both fuel cell and electrolyzer, while high value-added H2 is produced in the cathode side of electrolyzer with a little supplied power or even without any. This work firstly screens out Au as the best-performing electrocatalyst for MEA electrooxidation. Following this, Au–Ni bimetals coated on nickel foam (Au–Ni@Ni foam) are prepared by the facile electrodeposition. An Au–Ni@Ni foam electrode with the optimized electrodeposition time of 30 min exerts a considerable area current of 27.2 mA cm−2, Au mass current of 58.1 A g−1 and onset potential of −0.44 V vs. Ag/AgCl in 1.0 mol L−1 KOH and 0.05 mol L−1 MEA. The electrochemical tests companied with the physico-chemical characterizations elucidate the monometallic-state Au, rather than its alloy-state, benefits the MEA electrooxidation. Tests finally show such fuel cell-electrolyzer series device equipped with Au–Ni@Ni foam anodes operates well in bench scale, which accordingly proves the device to be a tenable and smart strategy for simultaneous MEA degradation and resource recovery.

Surface integrity of hole-making for Ti-6Al-4 V alloy is very important due to its application in aviation industry. Geometrical and mechanical behaviors of machined holes by helical milling (HM) and ultrasonic vibration helical milling (UVHM) were investigated. The textures of machined holes by the two processes were analyzed and the distances of adjacent textures were calculated according to the movement of peripheral edges. Due to the different surface geometrical textures, the hole-diameter error and surface roughness in UVHM are smaller than those of HM. A plastic deformation layer with thickness of 4–6 μm was observed by the distorted β phase in the cross-section of hole-surface in UVHM by SEM results. Compared with HM process, UVHM generates larger hardness at the subsurface of machined holes (20–200 μm) due to the material deformation caused by ultrasonic vibration. Compared with HM, the compressive residual stress of hole surface was increased larger than 63.5% by UVHM. The compressive stresses in UVHM are larger than 150 MPa at the depth less than 10 μm, which is consistent with the microstructure evolution caused by mechanical deformation.