1. Introduction
Carbon fiber-reinforced plastic/titanium alloy (CFRP/Ti) stacks are widely used as joint components with a high load-bearing capacity in the wings and fuselages of aircrafts [
1,
2,
3,
4]. CFRP/Ti stacks possess not only the advantages of CFRP [
5], such as high specific strength, good vibration damping, and good fatigue resistance, but also the advantages of titanium alloys [
6], such as superior strength, low density, excellent hardness, good corrosion resistance, extreme temperature resistance, etc. As a result, with the effects of improving the overall strength and achieving the lightweight demand, CFRP/Ti stacks have gradually replaced traditional alloy materials in the advanced aircraft manufacturing industry.
Laminated material components are usually fastened via bolt holes, which determine the assembly strength and become vital to flight safety [
7]. Aircraft assembly requires extremely high-quality bolt holes, particularly holes that cannot be damaged. As both CFRP and titanium alloys are typical difficult-to-machine materials, the laminated materials prepared from them will face new challenges when machining. CFRP is highly susceptible to defects such as delamination and tearing with conventional drilling [
8]. In contrast, titanium alloys are prone to defects such as chipping at the entrance and burrs at the exit. The hole quality from conventional drilling cannot satisfy aircraft assembly requirements. Therefore, reaming finishing of bolt holes is indispensable [
9,
10].
Reaming is the process of removing a very thin layer of material from the hole wall with a reamer to improve dimensional accuracy and surface quality. However, the bolt holes of CFRP/Ti stacks made by the existing reaming process still cannot reach the excellent tolerance requirement [
11,
12,
13,
14,
15,
16]. Moreover, other problems also exist. The dimension of the machined diameter and roundness is unstable. The hole entrance of CFRP materials is prone to burrs, while residual damage may appear at the hole exit. This can harm the stack’s mechanical integrity with negative effects such as the loosening of bolted connections.
Ultrasonic-assisted drilling has been widely focused on by researchers in recent years due to its advantages of low thrust forces and enhanced surface quality. Cong et al. [
17] drilled CFRP/Ti stacks with variable feed rate machining and concluded that drilling stacks with variable feed rates (low feedrate should be used for machining of Ti, and machining of CFRP could be conducted using feedrate 10 times higher) at different ultrasonic output powers resulted in lower axial forces than with fixed feed rates. In addition, the cycle time using variable feedrate was shorter than that using fixed federate, which led to lower tool wear. Gao et al. [
18] carried out an experimental study of the ultrasonic vibration-assisted helical milling drilling process for laminated composites and found that ultrasonic vibration-assisted helical milling reduces thrust forces, hole exit tears, burrs, and other defects. Ma et al. [
19,
20,
21] drilled CFRP with ultrasonic longitudinal and torsional vibration, which reduced the thrust forces and hole exit burrs and reduced the surface roughness of the hole wall due to the torsional vibration of the side edge of the drill in longitudinal, and torsional vibration helps strengthen the effect of resin adhesion. It has been proven that the variable stress state makes the fibers more susceptible to shear fracture by kinematic characteristic analysis. Geng et al. [
22] introduced ultrasonic elliptical vibration-assisted reaming into finish drilling CFRP/Ti stacked bolt holes and showed that it can reduce axial forces and torques, where the average axial force can be reduced by 30%, and the geometric accuracy of the hole is better compared to conventional reaming (CR). In dry reaming, it is of great value to select ultrasonic power and cutting parameters quickly for efficient practical machining. Research shows that ultrasonic-assisted machining can reduce axial force and improve machining quality in reaming, but the effect of reducing axial force with different output power is not specified, which of guiding significance for industrial practice. In CR, the influence rule of different cutting conditions on dry reaming is that it is advantageous for the operator to choose the appropriate processing parameters, which is also the same in ultrasonic reaming. There are now some guiding machining parameters with cutting fluids added, but there is no guiding conclusion on the cutting parameters with dry conditions. However, during the reaming of CFRP/Ti stacked holes, there is little research on the influence of CR and UVR on the axial force and surface quality during reaming. Furthermore, there has been no systematic study on the impact of cutting conditions on the quality of reaming in CR and UVR.
In this paper, based on the motion characteristics analysis of ultrasonic vibration reaming (UVR), UVR experiments are conducted on CFRP/Ti stacks to explore the thrust forces, hole wall roughness, hole entrance and exit surface topography, hole diameter, and roundness with the variation of tool speed. In addition, it is verified that ultrasonic reaming can reduce the thrust forces and improve the hole quality, which can provide guidance for high-quality hole-making of CFRP/Ti stacks.
2. Experiments
The experimental setup diagram of UVR for CFRP/Ti stacks is shown in
Figure 1a. The adopted experimental platform is a 5-axis vertical machine center (KMC600SU, Kede Machine Tool Co., Ltd., Dalian, China), which includes the UVR unit and the thrust forces measurement unit, as shown in
Figure 1b. The UVR unit consists of an ultrasonic generator, a UAR tool holder, and a carbide reamer. The ultrasonic generator converts conventional (220 v, 50 Hz AC) power into high-frequency electrical energy, which will supply the piezoelectric transducer. The piezoelectric transducer converts the high-frequency electrical energy into mechanical vibrations. The ultrasonic vibrations from the transducer are amplified and transmitted to the reamer, which causes the reamer to vibrate axially at high frequency. It has been proven that the greater the output power, the greater the amplitude. The amplitude of the ultrasonic vibrations can be adjusted by varying the output power of the power supply.
The experimental materials are CFRP (T700-12K/AG80) and titanium alloy (Ti6AI4V) stacks, where CFRP plate and titanium alloy plate are glued together, as shown in
Figure 1c. CFRP is at the top and titanium is at the bottom. This stacking method is a common stacking sequence in industry. When the CFRP is at the bottom and the titanium alloy is at the top, the tool passes through the titanium alloy at a high temperature and cokes the CFRP at the bottom. According to the company’s requirements, the top CFRP is being placed and the bottom titanium alloy is being placed. This is a commonly used direction for titanium alloys in industry. The thicknesses of the CFRP and the titanium alloy are 7 mm and 8 mm, respectively. The reason for the different thicknesses is that they are bonded with glue in the middle, close to 1 mm, similar to the CFRP bonding method, which is essential to ensure the same thickness of the two layers. The fiber laying direction of CFRP is 0° and 90°, and the stacking order is 0/90/0
5/90/0
2s. The main component of CFRP is carbon fiber, and its resin matrix is epoxy resin. The CFRP plate is placed on the top of the titanium alloy plate, and their compositions and performance parameters are shown in
Table 1,
Table 2,
Table 3 and
Table 4.
A 4-fluted carbide reamer (
Figure 2) with a diameter of D 7.8 mm ± 0.003 mm and a helix angle of 18° was used in the experiment. Titanium alloys have high chip hardness and slenderness, making it very easy to scratch the reamed CFRP holes. In order to avoid secondary scratches on CFRP holes caused by chips discharged during reaming, only the left helical-edge reamer can be used to discharge chips under the holes. The reamer parameters are shown in
Table 5. The pilot holes were drilled by two-flute uncoated tungsten carbide twist drills with a diameter of 7.65 ± 0.003 mm. The wet machining was employed in the single-shot drilling to reduce the heat and mechanical damage to the pilot holes. The tolerances of the pilot holes were evaluated after drilling and strictly constrained between IT9 to IT10. The pre-drilling conditions are shown in
Table 6.
In the preliminary experiment, the thrust forces and tool wear of CFRP and titanium alloys under different feeds were compared at 200 rpm. According to [
22], the feed rate range of CFRP and the titanium alloy is 5, 10, 20, and 25 mm/min. Under these parameters, the CFRP is suitable for 25 mm/min feed and the titanium alloy is suitable for 10 mm/min feed. The feed speed of the two materials is more than three times different. In the case of a similar feed speed, only the smaller one can be used, which will reduce the efficiency of processing. It has been shown that a low feedrate should be used for UVR of Ti, but high feedrate could be used for UVR of CFRP [
17]. In addition, using a variable feedrate led to a lower cutting force. Therefore, the experiments were carried out using different cutting parameters according to the different material properties, and lower thrust forces and higher machining efficiency were achieved compared to machining using the same parameters. In this experiment, a faster speed of 0.41 mm/s was used to ream holes in CFRP for machining and a slower speed of 0.16 mm/s for machining titanium alloys. Near the end of CFRP machining, the feedrate changes from 0.41 mm/s to 0.16 mm/s. The pattern of variation of feedrate with feed depth is shown in
Figure 3. The other parameters used in the experiments are described in
Table 5 and
Table 6. The error bars were based on three repetitions in each of the following groups.
A multicomponent dynamometer (9139AA, Kistler Instrumente AG, Winterthur, Switzerland) was used to measure the thrust force in experiments. The thrust force was converted into electrical signals by dynamometer, which were processed by a multichannel charge amplifier (5080A, Kistler Instrumente AG, Switzerland), converted, and transmitted to a computer through an analog to digital converter card (5697A1, Kistler Instrumente AG, Winterthur, Switzerland).
The hole quality was analyzed after reaming from aspects of geometric parameters (hole diameter, roundness), internal hole surface defects, the roughness of CFRP and titanium alloy holes, CFRP hole entrance, and exit morphology. The measurements of CFRP and titanium alloy hole diameter and roundness were performed using a coordinate measuring machine (PRISMO12020222, ZEISS, Oberkochen, Germany). CFRP and titanium alloy hole walls are measured, respectively. When measuring, 50 points are taken every week to fit the circle at the hole position every 1 mm. Average values are taken to obtain the hole diameter and roundness. The surface roughness of holes was measured with the Tyler Hopson surface profiler (CLI2000, Taylor Hobson Ltd., Leicester, UK). The roughness of CFRP and titanium alloys is measured separately. For the reliability of the results, the probe of the roughness analyzer runs through almost the entire pore size. The CFRP test length is 5.6 mm, and the titanium alloy test length is 6.6 mm. The optical microscope (OM; VHX-600E, KEYENCY, Osaka, Japan) was applied to observe the hole exit morphology. The inner wall morphology of the pores was observed with a scanning electron microscope (SU5000, Hitachi-hightech, Tokyo, Japan). The laser displacement sensor (OPTEX) is used to measure ultrasonic amplitude.