1. Introduction
The contradiction between machining efficiency and quality is the most prominent contradiction in today’s manufacturing industry. Research shows that we must continue to pursue the improvement of efficiency if we want to fully and effectively deal with other contradictions in the manufacturing industry. Therefore, machining efficiency is pushed to a crucial position in the development of today’s manufacturing industry. High performance machining has gradually developed into an inevitable trend of modern manufacturing technology [
1,
2].
TC25 alloy belongs to the Ti–A1–Sn–Zr–Mo–W–Si series of titanium alloys, which is an (α + β) typed heat-strength titanium alloy with good comprehensive properties. TC25 alloy also has the high thermal stability of titanium alloys BT8 and BT9. It can work for a long time in a working environment of 500~550 °C, which is an ideal metal material for aero engines and a common material for compressor parts on engines. However, literatures [
3,
4] mainly studied the microstructure and fatigue performance of TC25 alloy, while few literatures [
5] on its cutting performance, meaning there is a lack of guidance for the highly efficient machining of TC25.
There are mainly the following two problems in the actual cutting process of titanium alloys. One is the relative low machining efficiency. At present, the cutting of titanium alloys becomes very difficult when cutting speeds are over 30 m/min using high-speed steel cutting tools or over 60 m/min by using cemented carbide cutting tools [
6]. The cutting speed in the high-speed cutting of titanium alloys is generally above 100 m/min, so the cutting efficiency is lower in the case of high-speed cutting [
7]. However, the cutting speed in the high-speed milling of titanium alloy at abroad is as high as 100~200 m/min, and the application range of high-speed cutting of titanium alloy is relatively wide. Domestically speaking, the current cutting speed of titanium alloys ranges from 30 to 50 m/min, which is a big gap when comparing with foreign countries.
The second is the relatively poor surface quality because of the serious tool wear progression. The reason accounts that the low thermal conductivity of titanium alloy hinders the heat loss during the machining process. As known to all, little heat is given off by the chip and most is absorbed by the cutting tool in cutting of titanium alloys, i.e., adequate heat dissipation, which is one of the biggest problems during machining. As a result, the heat generated during cutting is mainly concentrated at the cutting edge of the cutting tool, which accelerates the wear of the cutting tool and deteriorates the surface quality of the machined surface. For this reason, scholars commonly carried out works on tool wear and surface roughness to get the relations with the cutting temperature [
8,
9]. Additionally, the cutting temperature must be one of the factors to evaluate the machinability of titanium alloys.
At present, more and more researchers focused on the high-speed and high efficiency cutting of Ti alloys. High-speed and high-efficiency cutting has the advantages of high production efficiency, good product quality, and the ability to process thin-walled parts. It can effectively reduce surface roughness and cutting force, slow down the increase trend of cutting temperature, and, in turn, reduce thermal deformation.
Yang et al. [
10] found that it was prone to produce saw-tooth-shaped cutting chips during high-speed turning of titanium alloys, resulting in high-frequency periodic fluctuations in cutting force. In addition, the low deformation coefficient and thermal conductivity of titanium alloys resulted in large specific cutting force, high cutting temperature, and serious spring-back in high-speed machining of titanium alloys, which could aggravate the progression of tool wear, and seriously deteriorate the surface integrity of the machined surface. Yang et al. [
10] concluded that using a low cutting speed, large feed per tooth, and a large cutting depth could reduce the generation of saw-tooth-shaped cutting chips. Ma et al. [
11] experimentally analyzed the stability of chip flow in cutting Ti6Al4V alloy. It was observed that the chip morphology transformed from continuous to saw-tooth-shaped and then serrated cutting chips with the increase of cutting speed. The above research fully shows that titanium alloy is a typical difficult-to-cut material, and its machinability must be improved. Gupta et al. [
12] found that the machining performance of Ti6Al4V in the turning process can be enhanced by applying cryogenic cooling environments. Pimenov et al. [
13] further reviewed the application of cooling-lubrication techniques in order to improve the machinability of Ti and its alloys. However, the use of dry cutting is more conducive to achieving sustainability and clean manufacturing.
The cutting parameters have a very significant effect on cutting force and cutting temperature. Narutaki [
14] studied the influence of cutting speed on both cutting force and cutting temperature through turning of TC4 alloy with cemented carbide cutting tools. The results showed that the cutting speed had no obvious influence on cutting force when turning titanium alloy TC4 with cutting speed
Vc = 20–200 m/min, where the cutting force was about 70% of that in turning AISI 45 steel under the same conditions. Hence, cutting force was not the main reason for aggravating tool wear. However, the cutting temperature in the turning of titanium alloy TC4 could reach 700 °C or more. Especially, the cutting temperature in the turning of titanium alloy could be higher than 1000 °C with a cutting speed of
Vc = 300 m/min, which was about 1.5 times of that in machining AISI 45 steel under the same conditions. It could be concluded that the poor machinability in the turning of titanium alloy was mainly due to the high cutting temperature as a result of adequate heat dissipation [
14]. Dewes et al. [
15] experimentally investigated the cutting temperature in high-speed milling die steel by using both the infrared camera method and artificial thermocouple method. Cotterell and Byrne [
16] further established a thermal model for the prediction of the formation of saw-tooth-shaped cutting chips during the cutting process of titanium alloy TC4, based on which the average temperature on the rake face and the main shear zone could be predicted.
The surface roughness of the machined surface is another important indicator to characterize the quality of parts. Kumar et al. [
17] investigated the effect of cutting parameters (cutting speed, feed rate and the depth of cut) on surface roughness when turning Ti6Al4V titanium alloy with medium temperature chemical vapor deposition (MT-CVD) tool insert. The results indicated that the influence of feed on surface roughness is the greatest, followed by the depth of cut and cutting speed. Oosthuizen et al. [
18] studied the effects of cutting speed and feed rate on surface roughness, microhardness and microstructure when milling Ti6Al4V. The surface roughness increases as the feed rate increases and the cutting speed decreases. Sun et al. [
19] conducted an end milling test on the titanium alloy TC4 and found that the surface roughness of the machined surface along the feed direction and perpendicular to the feed direction had different evolutional trends. Surface roughness along the feed direction increased first and then decreased with the increase of cutting speed within the range of
Vc = 50–110 m/min, while the surface roughness perpendicular to the feed direction showed a decreasing trend. Many scholars had further conducted in-depth research on how to improve the surface quality in machining titanium alloy. Sharman et al. [
20] analyzed the effect of cutting speed on surface quality within the range of
Vc = 25–40 m/min. The results showed that both the size and density of micro-cracks generated on the machined surface were decreased with high cutting speed, which was benefitted to improve the surface quality of the machined surface.
Although the machining parameters are considered in the above research, the relative position between the milling tool and workpiece is ignored. As known to all, the milling process can be divided into symmetrical milling and asymmetrical milling based on the different positions between the milling tool and workpiece when milling a plane. With respect to machinability, asymmetric milling is better than symmetric milling [
21,
22]. Asymmetric milling can be further divided into asymmetric up-milling and asymmetric down-milling.
Figure 1a, b present how the pure up-milling as the radial depth of cut is smaller than half of the tool diameter. However, the kinematics of asymmetric up-milling changes into down-milling when the radial depth of cut exceeds half of the tool diameter, as shown in
Figure 1c, d.The tool teeth cut into the workpiece from the smallest cutting thickness and is cut out from the largest cutting thickness in asymmetric up-milling, while it is just the opposite asymmetric down-milling. As a result, the tool vibration in asymmetric down-milling is greater than that of asymmetric up-milling, and the gap between the screw and the nut of the workbench should be eliminated to prevent the workbench from moving in the case of large horizontal milling force component. Favero Filho et al. [
23] indicated the machining direction (i.e., up- and down-milling) generated a much more significant influence on the machinability when compared to the cutting parameters. The better machinability of up-milling processes in relation to down-milling was also found. Hence, the asymmetric up-milling was utilized for milling TC25.
In our previous work [
5], the machinability of TC25 was investigated by comparing the differences in cutting force, cutting temperature and surface roughness between TC25 and TC4 alloys. It is found that TC25 has worse machinability than TC4, that is, larger cutting force, higher cutting temperature and worse surface roughness. Therefore, it is very necessary and urgent to study the influence of machining parameters on the machinability of TC25. In addition, empirical formula models were used to analyze the machinability of TC25 and TC4 in the literature [
5]. A more accurate numerical prediction model is needed for investigating the influence of cutting parameters on machinability. In this paper, the evolutions of cutting force, cutting temperature, and surface roughness in asymmetric milling TC25 alloy are investigated based on the Taguchi method and analysis of variance (ANOVA), aiming at exploring the optimized milling parameters for high-efficiency and high-quality milling of TC25 alloy. On the basis of this work, it is expected to provide theoretical guidance for the high-efficiency and high-quality milling of TC25, and then promote the application of TC25 for aero engines and/or compressor parts on engines.
2. Experimental Details
In this paper, asymmetric up-milling of TC25 alloy were carried out based on Taguchi’s experimental design. The milling experiments were carried out on DAEW00 AVE-V500 machining center, as shown in
Figure 2a. The size of the machined surface of the workpiece was 40 × 70 mm, which was stepped after machining to retain the relevant information of the machined surface under each group of cutting parameters. The tool insert used in the experiments was carbide insert XOMX120408TR-ME08 F40M (SECO, Sweden) with (Ti, Al)N-TiN composite coating, which is amounted on the tool holder R217.69-2525.0-12-2AN (SECO, Sweden). The diameter of the tool holder is
Φ25 mm, and the tool insert is equipped with a rake angle of 25°, relief angle of 14°, nose angle of 80°, and a nose radius of 0.8 mm. Only one insert is installed in order to avoid runout errors during the milling process. During the cutting experiment, a new insert was used for each set of cutting conditions in order to ensure the sharpness of the cutting edge. Additionally, the cutting distance of the cutting tool was very short. With these considerations, the tool wear of the cutting tool could be negligible. The cutting force was measured with the Kistler 9275B Dynameter, cutting temperature of the cutting zone was measured by the TH5104R thermal imaging camera (NEC, Japan), and surface roughness was measured by using the NT9300 optical profiler (Wyko, America). Three repetitions were performed for each trial to ensure the reliability of the results.
The Taguchi method is useful for determining the best combination of factors under desired experimental conditions [
24,
25,
26]. The Taguchi method reduces a large number of experiments that could be required in traditional experiments when the number of process parameters increases. In the Taguchi method, an orthogonal array is designed that studies the entire parameter space with a small number of experiments.
As illustrated in
Figure 2b, there are four cutting parameters, i.e., radial depth of cut
ae, axial depth of cut
ap, cutting speed
Vc (
Vc =
nπ
d/1000), and feed per tooth
fz (the displacement of the milling tool relative to the workpiece in feed direction when the milling tool rotates an inter-tooth angle in milling with multi-teeth milling tools, and
fz =
Vf/
nz, where
Vf represents the feed velocity (mm/min),
n is the rotating speed of the spindle (r/min), and
z is the number of teeth of the milling tool), for asymmetric milling. If a full factor experimental design is performed, 256 sets of experiments are required in the case of four values for each variable. The orthogonal experimental design based on the Taguchi method can reduce the number of experiments to 16, which greatly reduces the time as well as the cost of the experiment. The factors and levels, and the orthogonal experimental series L
16(4
4) (including experimental results) are shown in
Table 1 and
Table 2, respectively. In order to make the milling process cover a complete up-milling process, the maximum radial depth of cut is determined slightly larger than half of the tool diameter. After all milling experiments, mintab software was utilized for statistical analysis and model establishment.