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A study ofthe surface microstructure and tool wear of titanium alloys after ultrasonic longitudinal-torsional milling
Journal of Manufacturing Processes 53 (2020) 1–11
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A study ofthe surface microstructure and tool wear of titanium alloys after ultrasonic longitudinal-torsional milling
T
Peng Chen, Jinglin Tong*, Junshuai Zhao, Zhiming Zhang, Bo Zhao School of Mechanical and Power Engineering, Henan Polytechnic University, Jiaozuo, 45400, Henan, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Ultrasonic longitudinal-torsional milling Titanium alloy Micro-texture Friction properties Tool wear
In order to obtain a better surface micro-morphology on the difficult-to-machine materials such as titanium alloys, the influence of ultrasonic parameters and cutting parameters on the tool path was analyzed theoretically under the ultrasonic longitudinal-torsional vibration. Ultrasonic longitudinal-torsional assisted milling (ULTAM) of titanium alloy was performed, based on a 20° slope. The effects of the ultrasonic parameters and cucuttintting parameters were studied on the surface morphology and surface roughness. The machined surface was examined using scanning electron microscopy (SEM) and white light interferometry. The characterization parameters of the three-dimensional surface micro-morphology including the arithmetic mean height (Sa), the surface skewness (Ssk), and the density of summits (Sds), were obtained under the corresponding cutting parameters. It was found that the ultrasonic amplitude exhibited a significant influence on the surface micro-morphology, followed by the spindle speed n, while the effect of the depth of cut (ap), and the feed amount per tooth (fz) was relatively less. Under the same cutting parameters for the surface, a comparative analysis for the friction and wear testing was performed between the ultrasonic milling and conventional milling (CM). The results showed that the ULTAM transformed the original ravines and anisotropic surface imperfections into an uniform and orderly scaly micro-texture, thus forming an isotropic surface texture in all directions. Furthermore, ULTAM transformed the surface into a fish-scale micro-texture, and thereby improved the surface quality. The fine and smooth surface morphology was conducive by shortening the running-in time, allowing the stable state of friction to be reached quickly, and thereby offering a higher stability in the friction process. `Compared to CM, the tool wear was reduced by 30%, due to the unique tool-work separation characteristics of the ultrasonic longitudinal-torsional assisted milling. Moreover, the unique intermittent cutting characteristics of the ultrasonic longitudinal and torsional milling not only reduced the cutting force and tool wear, but also inhibited the generation of the burr, thus improving the surface quality.
1. Introduction Structural parts that are manufactured from titanium alloys have a high specific stiffness, strength, fatigue and resistance, and are lighter in weight, which makes them popular in the aerospace industry. However, the titanium alloy itself is a difficult material to process, as it is prone to a rapid heat generation, requires a large cutting force and produces a severe tool wear during machining [1,2]. The precision and processing quality of the manufactured parts used in aerospace applications are relatively strict; hence, a considerable volume of research has been undertaken on the machining of titanium alloy for aerospace applications. Luo et al. [3] studied the tool wear during the milling of titanium alloys, where in the influence of milling parameters on tool wear was analyzed and applied to the practical production. In order to
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reduce the surface roughness of titanium alloy during milling and to reduce the formation of burrs, Thepsonthi et al. [4] used the multiobjective particle swarm optimization to simulate and experiment, and eventually determined the optimal process parameters. Tan et al. [5] used the central composite response surface method to study the surface roughness prediction model in the investigation of the titanium alloy milling, and analyzed the influence of milling parameters on surface roughness, residual stress, and micro-hardness. In recent years, the development of ultrasonic vibration cutting has made a significant progress, and the ultrasonically-assisted cutting has become one of the most effective processing of the difficult-to-machine materials like the titanium alloy [6]. Ultrasonically-assisted machining is basically based on the traditional machining, but adds an ultrasonic signal to make the tool or
Corresponding author. E-mail address: [email protected] (J. Tong).
https://doi.org/10.1016/j.jmapro.2020.01.040 Received 21 June 2019; Received in revised form 18 January 2020; Accepted 22 January 2020 1526-6125/ © 2020 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.
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workpiece vibrate at a high frequency along a certain direction. Ultrasonic vibration cutting technology can change the traditional continuous cutting into an intermittent cutting, and the size and direction of the cutting speed changes periodically, which changes the force situation of the whole processing system [7]. In a vibration period, the tool gets contacted intermittently with the workpiece and the chips, and the net cutting time becomes shorter, which reduces the cutting force. Because ultrasonic machining is a periodic process, the generated cutting heat changes gets presented in the form of a pulse. Due to the fast cutting process, the heat cannot reach deep into the interior of the metal, thereby improving the cutting conditions and avoiding the damage and thermal deformation caused due to the excessive cutting heat [8], besides reducing the tool wear [9]. At the same time, the ultrasonic vibration cutting reduces the chances of the formation of the score marks, and instead produces a ‘fish scale’ effect. At each moment when the blade contacts the workpiece and generates the chip, the position of the blade follows the insensitive vibration cutting mechanism, which increases the rigidity of the processing system, and decreases the deformation of the workpiece. The degree of the deformation is only 1/3rd to 1/10th that of the normal cutting deformation, which makes precision machining possible [10]. Therefore, the ultrasonically-assisted processing has received an extensive attention from researchers at home and abroad. Yaet al. [11] analyzed the trajectory of the abrasive particles in the rotary ultrasonic machining tools while studying the crack propagation by using the fracture mechanics theory, and proposed a mathematical model of the material removal rate that provided a theoretical basis for rotary ultrasonic machining. In order to solve the problems of large cutting force and low machining precision in the titanium alloy processing, Jiang et al. [12] used the high linear velocity and the high frequency interrupted cutting characteristics of the ultrasonic elliptical vibration to carry out the titanium alloy milling tests. It was observed that the milling ability of the cutter was enhanced and the milling force was reduced by 50%. Furthermore, the precision of machining was improved significantly. Liu et al. [13] applied the ultrasonic elliptical vibration on the high-speed milling and carried out cutting experiments under different parameters. The results showed that the separation characteristics of the ultrasonic elliptical vibration cutting were fairly achieved using the high-speed milling by setting reasonable parameters, and smaller chips were obtained. Furthermore, the tool wear was slow and stable, thereby prolonging the tool life. Although many researchers have carried out ultrasonic-assisted milling tests, most of them are based on the one-dimensional ultrasonic vibration and only evaluated the surface roughness. Furthermore, the ultrasonic longitudinal-torsional vibration and three-dimensional surface morphology has not been studied extensively. Three-dimensional surface micro-morphology can reflect the surface generation mechanisms more objectively and can affect directly the functional characteristics of the surface. Since, the same surface roughness value may have different surface textures and different surface waviness, it is inappropriate to use surface roughness solely to evaluate the surface micro-morphology [14]. Hence, it is necessary to use the relevant characterization parameters to evaluate the three-dimensional surface morphology. Surface quality also has a certain effect on the surface friction and wear properties. Friction and wear are inevitable as a result of the relative movement of the mechanical parts, and a reduction in the wear, extends the service life of the parts. One way for improving the friction and wear properties of workpiece is to optimize the surface morphology [15]. Additionally, tool wear is an important factor affecting the surface quality of hard-to-machine materials [16]. In the present study, the tool path of ultrasonic longitudinal-torsional assisted milling (ULTAM) was analyzed theoretically in relation to a certain tool inclination. With continuous changes in the cutting angle of a ball-end milling cutter during the finishing and surface processing, several advantages are offered, including a more stable processing condition, less flutter, and a better surface finish after processing. ULTAM tests on the
Fig. 1. Solution of the maximum effective milling radius of a ball-end milling cutter.
machining of titanium alloy were carried out by changing the ultrasonic and cutting parameters, and its effects were studied on the micromorphology of the surface. The friction and wear properties of the machined surface were analyzed and the effects of surface micro-texturing were studied in the improvement of the friction and wear properties of the machined surface. Furthermore, the tool wear was analyzed by changing the milling area and comparison with the results from conventional milling (CM). 2. Analysis of the trajectory of the ball-end milling cutter The formation of the surface topography is closely related to the tool path, and a difference in the tool trajectories can lead to different surface topographies. Also, the influence of the ultrasonic and milling parameters on the tool trajectory affects the surface topography. The top-down milling method was adopted for the ULTAM of ball-end milling cutter, as shown in Fig. 1. The maximum effective milling radius r of the vertical cross-section in the direction of the cutter was as follows [17]:
r= R·sin ⎛−θ + arccos ⎝
R − ap
⎜
⎞ ⎠ ⎟
R
(1)
where, SACE is the residual cutting area, arc AB is the contact area between tool and workpiece, ap is the cutting depth, θ is the processing inclination angle, and R is the tool radius. The motion equation of the cutting edge was established using Eq. (2) at point B of the maximum effective milling radius.
⎧ x (t ) = vf ·t + r·sin (φ) y (t ) = r·cos (φ) ⎨ z ⎩ (t ) = A·sin (2πft )
(2)
where, vf is the feed speed, r is the maximum effective milling radius, A is the amplitude of longitudinal vibration in longitudinal-torsional composite ultrasonic vibration, f is the frequency of longitudinal-torsional composite ultrasonic vibration, and φ is the actual turning angle of the tool which can obtained from Eq. (3).
n φ = 2π ⎛ ·t ⎞ + α ⎝ 60 ⎠
(3)
where, n is the spindle speed, and α is the torsional vibration angle in the longitudinal-torsional composite ultrasonic vibration that can be obtained from Eq. (4).
α= B·cos(2πft + β )
(4)
where, B is the amplitude of torsional vibration in longitudinal-torsional composite ultrasonic vibration. β is the phase difference between longitudinal vibration and torsional vibration. It should be noted that when the torsional radian was very small, the arc length of torsion could be approximated to a straight line and regarded as the amplitude of torsional vibration. It can be seen from Eq. (3) that both the ultrasonic and the cutting parameters exhibited an influence on the tool trajectory, and thereby on the surface micromorphology after machining. The path of the milling edge was plotted 2
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workpieces, cylindrical titanium alloy workpieces with a diameter D = 90 mm and thickness H = 30 mm were employed in the milling process. The material parameters of the titanium alloys are shown in Table 2. It is known from the literature [18] that the surface roughness obtained by milling titanium alloy under CM condition was better at an inclination angle of a ball-end milling at 10° - 30°. Hence, the inclined plane with an inclination angle of 20° was used to install the workpiece during the machining tests. The machine tool for milling was performed by a VMC850E vertical machining center. The whole set of ultrasonic devices used for machining was developed by the project team. Fig. 6 shows the displacement vector diagram of the ultrasonic longitudinal and torsional horn in an unconstrained state, as developed in ANSYS. The figure shows that the displacement of the flange position of the horn was almost zero. The input end of the horn provided the longitudinal vibration. After the oblique groove, a composite vibration of the longitudinal and torsional direction appeared at the tool position, which shows that the horn could realize the combined vibration of longitudinal and torsional ultrasonic. The vibration of the horn from the input end to the output end is illustrated in Fig. 7. It can be seen that the output vibration displacement was the largest, and the ratio of the longitudinal vibration to the torsional vibration was about 1:1.8. Before the tests, the amplitudes of the longitudinal and torsional directions under the same vibration frequency were measured to determine the longitudinal-torsional ratio. The longitudinal amplitude was used to express the amplitude of the longitudinal and torsional vibration, and the torsional vibration amplitude was obtained by the longitudinal and torsional ratio. A KEYENCE laser displacement sensor (LK-G10) was used to measure the ultrasonic amplitude. A Talysurf CCI6000 white light interferometer and a Zeiss scanning electron microscope (SEM) was employed for the evaluation of the surface micro-morphology. The surface micro-morphology was measured using the arithmetic mean height (Sa), surface skewness (Ssk), and the density of summits (Sds). ULTAM of titanium alloy was carried out using the CNC machining center. The test set-up for the ultrasonic longitudinal and torsional assisted milling is shown in Fig. 8. The ultrasonic device employed included a bespoke wireless transmission system and an ultrasonic horn device. After the ultrasonic equipment was turned on, a water spray was applied on the tool. Coolant atomization around the tool is shown in Fig. 8, which indicates that the tool was undertaking ultrasonic longitudinal/torsional compound vibration at that moment. The effects of ultrasonic amplitude A, spindle speed n, feed rate per tooth fz and milling depth ap (where the milling depth is normal tangent depth) on the micro-morphology of the machined surface of titanium alloy were studied using a single factor test method. The milling test parameters are shown in Table 3. Dry milling was adopted and the milling step was fixed to 0.35 mm.
Fig. 2. Contrast of tool trajectory between CM and ULTAM.
and analyzed by using MATLAB software, as shown in Fig. 2. As shown in Fig. 2, the tool path in ULTAM was regressive, which resulted in the periodic separation and contact between the tool and the workpiece. This changed the traditional continuous processing state, offered an easy breaking of the chip, and reduced the cutting force and heat generation. It was found that the tool wear was closely related to the cutting force and heating condition, so the tool path played an active role in reducing the tool wear and improving the surface quality. Fig. 3 shows that if there is only an ultrasonic longitudinal vibration, the change of the ultrasonic longitudinal vibration amplitude does not change the tool-chip separation. However, the ultrasonic high-frequency vibration played a certain role in the modification of the machined surface. It can be seen from Fig. 4 that a change in the ultrasonic torsional amplitude affected the tool-chip separation. The larger the torsional amplitude, the more the cutter retreats, and more discrete was the tool-chip separation phenomenon. Furthermore, when the ultrasonic longitudinal vibration and the torsional vibration acted simultaneously, a change in the spindle rotational speed exhibited a little effect on the ultrasonic longitudinal vibration, but it had a great influence on the ultrasonic torsional vibration, as seen in Fig. 5. With an increase in the spindle speed, the ultrasonic torsional vibration was weakened and the unique phenomenon of ultrasonic torsion gradually disappeared. This further led to the weakning of the tool-chip separation phenomenon. Therefore, the effect of the ultrasonic was not evident in highspeed machining. 3. Equipmentand materials The tool employed for the ULTAM test was a carbide ball-end milling cutter. The tool parameters are shown in Table 1. In order to improve the rigidity of the whole system and facilitate clamping of the
Fig. 3. Effect of longitudinal amplitude on the trajectory of ULTAM. 3
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Fig. 4. Effect of torsional amplitude on the trajectory of ULTAM.
4. Analysis of test results
Table 1 Cutting tool parameters.
Surface micro-morphology is one of the important evaluation indicators of the surface integrity. At the same time, the surface conditions are known to exhibit a great influence on the mechanical properties and performance of the parts, which directly affects the wear resistance, corrosion resistance and fatigue strength of a component [19]. The same surface roughness values may have different surface micro-morphology, and there are many characterization parameters of surface micro-morphology, such as height parameters, spatial parameters, and mixing parameters [20]. According to the ultrasonic parameters and milling parameters presented in Table 3, the machining tests were carried out on the ULTAM titanium alloy. Subsequently, the processed workpiece was evaluated using the relevant instruments.
Tool material
Number of teeth Z
Tool diameter (mm)
Tool helix angle B (°)
Total length of tool L(mm)
Cemented carbide
2
8
35
60
Table 2 Workpiece material parameters.
4.1. Effect of ultrasonic and cutting parameters on surface microstructure The effects of ultrasonic parameters and milling parameters on the arithmetic mean height (Sa), the density of summits (Sds), and skewness (Ssk) [21] are shown in Figs. Fig. 99, Fig. 1010 and Fig. 1111, respectively. Sa represents the average value of the distance from each point on the measured surface to the reference datum, which is used to measure the overall flatness of the surface micro-morphology. Sds represents the number of vertices contained in the unit sample area, which is used to measure the presence of defects such as score marks, burrs, and other incomplete processing. Ssk is used to measure the shape of the contour concave convex amplitude distribution curve, which represents the surfaces with fewer peaks or valleys. In general, the surface with a good friction performance has a negative skewness
Workpiece material
Density ρ (g/cm3)
Modulus of elasticity E(MPa)
Yield strength σ (MPa)
Poisson ratio μ
Ti-6Al-4V
4.62
117.6
930
0.36
[22].The following observations can be made from Fig. 9, 10, and 11: (1) With an increase in A, there was an initial increase and a subsequent decrease in Sa, Sds, and Ssk. When the amplitude was A = 5 μm, both Sa and Sds were the least, indicating that the surface at this time was relatively flat, and there was no peak of surface scoring, burrs or incomplete machining. It can also be seen from the surface micromorphology that there was only a very shallow feed tool mark. This was due to the addition of the longitudinal amplitude during ULTAM, so that the tool not only participated in the cutting, but also had the effect of ironing and trimming the workpiece. Torsional vibration achieved the knife-to-chip separation, and continuous cutting became an
Fig. 5. Effect of spindle speed on the trajectory of ULTAM. 4
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Fig. 6. Simulation of the ultrasonic longitudinal and horn. Table 3 ULTAM test parameters. Spindle speed n(r/min)
Feed per tooth fz(mm)
Cutting depth ap(mm)
Amplitude A(μ m)
0.05
0.5
5
B
1600, 1800, 2000, 2200, 2400 2000
0.5
5
C
2000
0.04, 0.05, 0.06, 0.07, 0.08 0.05
5
D
2000
0.05
0.4, 0.5, 0.6, 0.7, 0.8 0.5
A
1, 3, 5, 7, 9
phenomenon at the edge, which increased the peak value for the surface and a rapid increase of Sds. Moreover, there was an increase in the area of the surface bumps, resulting in the deterioration of the surface micro-morphology. As the peaks increased, Ssk rapidly increased, which had a harmful effect on the surface friction performance. The ultrasonic amplitude improved the surface smoothness. From the tool path, it can be seen that the ultrasonic vibration produced a periodic high-frequency vibration in the cutting depth direction, generated an ironing and finishing effect on the surface, and made a periodic separation and contact of the tool and workpiece. When the ultrasonic amplitude was small, the ultrasonic longitudinal vibration and torsional vibration were relatively small. When the ultrasonic amplitude became larger, it can be seen from the tool path that a larger ultrasonic amplitude had a greater impact on the surface, which exhibited a harmful impact on both the tool and the surface. If the amplitude was too large, the ultrasonic dressing effect began to weaken, along with an increase in the score marks and burrs, and a deterioration of the surface flatness. (2) With the increase in the speed n, a corresponding decreasing trend was observed in Sa, Sds, and Ssk. When the speed n = 1600 r/ min, it was observed that many small score marks were produced on the machined surface. Although the peaks were not high, the area was wide, the number of peak points in the sampling area of the machined surface increased, and Sds was larger. This was due to the fact that when the rotation speed was low, the material was removed slowly by the cutting edge, but such difficult-to-machine alloys were prone to incomplete material removal, which caused the surface to be uneven.With the increase in the rotational speed, the milling speed increased, the temperature in the milling zone increased to a certain extent, and the material in the cutting layer softened and was easily removed. Moreover, the phenomenon of friction and slip was improved with the effect of the ultrasonic vibration, and it was easier to produce a
Fig. 7. Ultrasonic longitudinal and torsional torsional amplitudes.
Fig. 8. ULTAM test equipment.
intermittent cutting, which reduced the cutting force and cutting heat generation, thereby improving surface quality. However, as the ultrasonic amplitude increased, both Sa and Sds began to increase gradually, and Sds increased rapidly. It can be seen from the three-dimensional micro-morphology that when the amplitude was A = 9 μm, the surface not only exhibited score marks, but there was also an obvious residual 5
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Fig. 9. Effects of ultrasonic parameters and milling parameters on Sa.
fine and uniform textured morphologyof the surface. However, as the rotational speed continued to increase, the reduction in Sa and Sds was not very obvious. This is because, when the milling speed reached 60 m/min, it was close to the critical milling speed of titanium alloy [23]. A large rotating speed can affect the ultrasonic torsional vibration and lead to the weakening of the ultrasonic separation phenomenon. Although high-speed milling reduced the milling force and inhibited the formation of score marks, the ultrasonic milling effect of low-speed was
more obvious in ULTAM. When the critical milling speed was exceeded, the separation of the tool and the workpiece gradually disappeared, and the ULTAM effect was weakened, which detracted the quality of the surface micro-morphology. (3) With the increase of the feed rate fz, there was a gradual increase in the Sa, Sds, and Ssk. However, the increasing trend of Sds and Ssk was not very obvious. When fz was small, the volume was removed per unit time, the milling resistance of the tool was small, and the material
Fig. 10. Effects of ultrasonic parameters and milling parameters on Sds. 6
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Fig. 11. Effects of ultrasonic parameters and milling parameters on Ssk.
was easily cut from the workpiece. At the same time, the feed rate was slower, and the ultrasonic vibration repeatedly pressed and trimmed the milled surface to make the machined surface more fine and uniform, so as to obtain a better surface micro-morphology. With an increase in fz, there was a corresponding increase in the material removal per unit time, which increased the material removal rate and the feed residue after milling. At the same time, it was easy to sustain the incomplete material removal, resulting in an increase in surface peaks. It can be seen from the three-dimensional micro-morphology that the increase of feed did not affect the ultrasonic dressing, and a fish-scale texture was formed. The peak value in the sampling area was very large, but the number of samples was not large. Therefore, the increase in feed rate exhibited a significant effect on Sa, but not on Sds and Ssk. (4) With the increase of the depth of cut ap, a corresponding increasing trend in Sa, Sds, and Ssk was observed. When ap was small, the material removal per unit time was small, and the extrusion and tearing phenomena did not occur. With the effect of ultrasonic longitudinal vibration on the surface, the surface was relatively flat, the Sa value was low, and the surface micro-morphology was better. As ap increased, the three-dimensional surface topography showed that the score marks and burrs began to appear on the surface of the workpiece. Especially, when ap = 0.8, the probability of occurrence of score marks and burrs greatly increased, but the surface peaks were not very high, and the peak point in the sampling area was not large. Although Sa and Sds increased, they were not particularly noticeable, and their influence on the surface condition was relatively small. A smaller depth of cut was advantageous to produce uniformity of the machined surface, but a change in depth of cut presented a relatively small effect on the surface texture.
Fig. 12. Comparison of surface friction properties in CMand ULTAM (×400).
of marks with different depths in the feed direction. The presence of the knife marks created a rugged gully on the surface, and the surface was anisotropic, which resulted in a poor surface micro-morphology, thereby affecting the surface friction properties [24]. However, in ULTAM, the feed tool marks were ironed and flattened due to the effect of the ultrasonic longitudinal torsional vibration. A relatively uniform scaly micro-texture replaced the uneven gullies. Under the repeated pressing by the ultrasonic vibration, a uniform and dense fish-scale texture was formed on the surface. High-frequency vibration changed the surface texture from an anisotropic state to isotropic, forming a uniform texture in all directions, thereby improving the friction performance of the contact surface. When the mechanical parts are in direct contact, the relative motion of the mechanical surface produces a friction and wear. Under normal working conditions, there is a ‘running-in’ stage, a stable wear stage and then a severe wear stage. Mechanical wear is unavoidable, and the surface texture of the same material affects the friction and wear properties of the surface [25]. In the present work, the milled workpiece was subjected to a friction test on an MMW-1 microcomputercontrolled vertical universal friction wear machine (Maximum test force 1 K N), and the static pressure was set to 10 N, the motion speed of friction pin was 10 mm/s, and the resultsare shown in Fig. 13.
4.2. Comparison of surface friction properties in CM and ULTAM Comparative tests of CM and ULTAM were carried out with cutting parameters of n = 2000 r/min, fz = 0.05 mm, ap = 0.4 mm, and ultrasonic parameters A = 5um, and then the friction and wear tests were carried out. After milling, the workpiece was examined using SEM (shown in Fig. 12). It can be clearly seen from Fig. 12 that the surface in ULTAM exhibited a uniform scaly micro-texture, which was not present in CM. A large amount of scoring was evident on the surfacein CM, leaving a lot 7
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friction pin and reduced the suspension phenomenon of the frictional contact surface caused by the fluctuations of the microscopic surface contours, which in-turn reduced the amount of wear on the workpieceduring the running-in period [26]. The friction test was more conducive in shortening the running-in time, allowing it to reach the friction stable state quickly, and the friction process was more stable. 5. Analysis of tool wear for CM and ULTAM Tool wear affects not only processing efficiency, but also changes the surface quality of the workpiece. With the continuous increase of cutting area, the cutting edge gets extruded and scrubbed for a longer time, which inevitably leads to the wear and tear at the cutting edge. With the increase in wear, there is an increase in thecutting force and cutting temperature, further affecting the processing quality and efficiency [27]. Hence, the study of tool wear is of great significance in relation to the surface quality. In theory, any difference between the surface of the used tool and that of a new tool, belongs to the scope of tool wear. However, on comparing the test results of ULTAM of the titanium alloy with those from the ball-end milling cutter, it was found that the wear location of the cutter was mainly in the two areas of the flank and the vertex. When the cutting depth was large or the processing inclination was zero, the wear at the vertex was relatively large. However, in actual processing, the ball-end milling cutter is seldom used at zero inclination. At the same time, ULTAM is mainly used for finishing, and its cutting depth is small, so the significance of tool vertex wear is relatively small. In addition, flank wear affects directly the quality of a machined surface, so flank wear is considered to be the main method of evaluation. In order to analyze the tool wear, a comparative trial of CM and ULTAM machining was carried out by changing the milling area with cutting parameters n = 2000 r/min, fz = 0.05 mm, ap = 0.4 mm, ultrasonic frequency f = 35KHz and amplitude A = 5 μm. Each group of tests were carried out twice, and the flank wear value (VB) was measured respectively and the average value was taken. The test results are shown in Table 4. From Fig.15, it can be seen that the wear value of the flank surface increased with an increase in the removal area for both CM and ULTAM. However, the wear value of the flank surface in ULTAM was significantly smaller than that in CM, when the removal area was same. During the early stages of wear, the wear rate of the flank surface in ULTAM was significantly lower than that with CM. With an increase in the milling area, the wear rate of the flank surface in ULTAM was almost the same as that in CM, but the wear value of the tool in ULTAM was about 30% lower than that for CM. With the increase in milling area, the tool cutting edge showed different degrees of wear, as shown in Fig.15. In Fig.16, A and B are the tool wear diagrams for CM and ULTAM, respectively, with a milling area of 6356 mm2. It can be clearly seen that, under CM, the tool wear band appeared wider and the tool edge exhibited a significant wear gap,along with the damage of the tool coating. Compared to CM, the tool flank wear of ULTAM was uniform and only a small wear band was evident. Although there was a minor wear at the edge of the tool, there was no larger gap. This was due to the fact that CM includes a continuous cutting mechanism, where the
Fig. 13. Surface friction and wear test of CMand ULTAM machined surfaces.
It can be seen from Fig.13 that, under the same cutting parameters, the average friction coefficient of the ultrasonically-milled surface after the friction test finally reached the stable wear stage, which was smaller than that obtained from CM. Compared with CM, the average friction coefficient of ULTAM was reduced by about 5.1–18.3 %. The findings also show that ULTAM improved the dimensional accuracy of parts and the quality of machined surfaces. In the friction and wear test, it was found that both ULTAM and CM went through the running-in stage first, and then gradually reached the stable state of friction, but the time spent in the running-in stage was quite different. In the friction and wear test, the surface after CM first underwent a slow running-in transition period, about 310 s and then gradually reached the state of friction stability. However, the surface running-in transition period after ULTAM was very short, about 150 s, and then quickly reached the state of friction stability. Compared with CM, the running-in time was reduced by about 51.6 %. In combination with the surface micromorphology and surface roughness test curve, the workpiece during the friction and wear was modeled, as shown in Fig. 14. After CM, due to the presence of more score marks and burrs on the surface, the presence of the feed tool marks increased the surface gully, the depth was different, and the surface texture was anisotropic, as shown in Fig.14a. In the friction and wear test, the actual contact area between the friction pin and the workpiece surface was small, and the friction pin rotated more times to remove the surface gully. Hence, a greater time was required to achieve the friction stability. However, after ULTAM, there were fewer score marks on the surface, and the feed tool marks disappeared after repeated pressing and trimming by the ultrasonic vibration. Instead, uniform and dense fish-scale micro-texture was generated. The high-frequency ultrasonic vibration transformed the original anisotropic surface into an isotropic finish and presented a uniform texture in all directions, as shown in Fig.14b. This fine and smooth surface morphology enlarged the actual contact area between
Fig. 14. Friction states of workpiece surfaces after CM and ULTAM. 8
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contacted periodically by the workpiece. The unique intermittent cutting characteristics of ULTAM made the chips separate and reduced the cutting force and heat generation. Moreover, it inhibited the generation of score marks, delayed the tool wear, and improved the machining efficiency and the surface quality.
Table 4 Variation of milling area and flank wear maximumwidth. Milling area(S)/ mm2
CM
ULTAM
VB-1/μm
VB-2/μm
Average value
VB-1/μm
VB-2/μm
Average value
908 1816 2724 3632 4540 5448 6356
39.25 70.22 100.37 107.52 116.56 118.35 121.82
55.02 74.69 98.83 109.75 118.28 130.94 143.61
47.135 72.455 99.6 108.635 117.42 124.645 132.715
28.62 36.79 43.30 61.90 65.39 68.36 82.57
40.52 47.26 48.35 69.75 87.83 102.21 104.07
34.57 42.025 45.825 65.825 76.61 85.285 93.32
6. Conclusions In the present work, a theoretical analysis of the tool path during ULTAM was performed, and the influence of ultrasonic parameters and milling parameters on the tool path were analyzed using MATLAB software. Milling tests of the titanium alloy with ball-end milling cutter under ULTAM were carried out by using an NC machining center. The surface morphology and tool wear after the tests were observed and analyzed, and the following conclusions were obtained:
accumulation of cutting heat is not easy to absorb, and it is easy then to produce bond wear on the cutting edge, which causes the cutting edge to collapse. In ULTAM, the cutting regime between the tool and the workpiece gets transformed into a periodic intermittent mode, which results in a periodic separation and contact between tool and workpiece, thereby reducing the production of the cutting heat and also the required cutting force. To a certain extent, the periodic tool-chip separation in the rotation direction plays an active role in reducing tool wear, which is consistent with the previous theoretical analysis of the tool path. With the increase in the milling area, the tool wear increased gradually, which affected the milling force and the quality of the machined surface. Fig.17. shows the variation trend in the milling force (F) and the surface morphology (Sa) with the effect of milling area for CM and ULTAM. It can be seen that with an increase in the tool wear, the value of ULTAM was always smaller than that of CM, regardless of the milling force or Sa. In the early stages of tool wear, the milling force and Sa increased gradually, with an increase in the milling area. Once the tool wear became severe, the milling force and Sa increased rapidly, especially during CM. However, in ULTAM, the change in milling force and Sa was not significant due of the occurance of less wear. After the tool wear, there was a change in the cutting environment. The most direct change was that the cutting edge was no longer sharp, which affected the contact area between the cutting edge and the workpiece in the cutting process. Morover, it also increased the cutting force and destroyed the protective coating of the tool. This led to the direct contact between the tool matrix and the workpiece, which speeded the tool wear. In ULTAM, the tool flank and cutting edge were separated and
• The ultrasonic amplitude significantly influenced the surface micro-
•
•
•
morphology. Smaller ultrasonic amplitudeswere unable to exploit the advantages of the ultrasonic dressing. In contrast, the larger the ultrasonic amplitude led to a greater impact acceleration, which damaged the micro-morphology of the surface. ULTAM with a suitable ultrasonic amplitude reduced the formation of score marks and burrs, and ultrasonically pressed and trimmed the surface to improve the surface micro-morphology. During ULTAM, the spindle speed exhibiteda significant effect on the Sa, Sds and Sk values, followed by feed rate fz and cutting depth ap. A higher spindle speed produced a better surface micro-morphology. However, under ULTAM, when the critical cutting speed was exceeded, tool/chip separation was weakened and resulted in a less improvement in surface quality. Compared with CM, the ultrasonic longitudinal amplitude produced the effects of ironing and dressing of the surface, and the ultrasonic torsional amplitude changed the continuous cutting into intermittent cutting, which played an important role inthe cutter/chip separation cycle. The simultaneous action of the ultrasonic longitudinal amplitude and torsional amplitude reduced the formation of score marks and burrs, and produced a uniform dense ‘fish-scale’ micro-texture on the surface, thereby improving the surface micromorphology. Tool wear analysis showed that,compared to CM, the unique intermittent cutting characteristics of the tool in ULTAM made the tool and workpiece cycle separate and contact, thereby improving the
Fig. 15. Wear value of flank face after CM and ULTAM. 9
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Fig. 16. Toolflank wearresulting from CM and ULTAM. [2] Qingcun X, Ge S, Wei Z. Cutting technology development of titanium alloy aircraft structural part. Aeronaut. Manuf. Technol. 2013;14:42–7. [3] Luo M, Wang J, Wu B, et al. Effects of cutting parameters on tool insert wear in end milling of titanium alloy Ti6Al4V. Chinese J. Mech. Eng. 2017;30:53–9. [4] Thepsonthi T, Özel T. Multi-objective process optimization for micro-end milling of Ti-6Al-4V titanium alloy. InJ. Adv. Manuf. Technol. 2012;63:903–14. [5] Liang T, Dinghua Z, Chang feng Y. Effect of high-speed milling parameters on surface metamorphic layer of TC17 titanium alloy. J. Aeronaut. Mater. 2017;37(6):75–81. [6] Shanxiang F, Huiling Z, Qing jian Z. The application status and development trends of ultrasonic machining technology. J. Mech. Eng. 2017;53(19):22–32. [7] Jian jian W, Jian fu Z, et al. Advances in rotary ultrasonic elliptical machining of advanced aviation materials. Aeronaut. Manuf. Technol. 2018;61(21):30–7. [8] Yundian Z. Ultrasonic Processing and Its Application. Beijing: National Defence Industry Press; 1995. [9] Tong J, Feng Z, Jiao F, Zhao B. Tool wear in longitudinal-torsional ultrasonic vibration milling of titanium alloys. Surf. Technol. 2019;48(3):297–303. [10] Kumabe J. The Foundation and Application of Precision Vibration Cutting. Beijing: Machinery Industry Press; 1985. [11] Ya G, Qin HW, Yang SC. Analysis of the rotary ultrasonic machining mechanism. J. Mater. Process Technol. 2002;129:182–5. [12] Xinggang J, Haitong L, Huimin L, Deyuan Z. Investigation of ultrasonic elliptical vibration milling of thin-walled titanium alloy parts. Acta. Armamentarii 2014;35(11):1891–7. [13] Liu J, Jiang X, Zhang D. Research on the characteristics of chips and tool flank wear in high-speed rotary ultrasonic elliptical machining for side milling of Ti-6Al-4V. J. Mech. Eng. 2019:1–9. [14] Tong J, wei G, Zhao L, Wang X, Ma J. Surface microstructure of titanium alloy thinwalled parts at ultrasonic vibration-assisted milling. Int. J. Adv. Manuf. Technol. 2018;3005–7:1–15. [15] Podgornik B, Hogmark S. Surface modification to improve friction and gallingproperties of forming tools. J. Mater. Process Technol. 2006;174:334–41. [16] Hocheng H, Kuo KL. On-line tool wear monitoring during ultrasonic machining using tool resonance frequency. J. Mater. Process Technol. 2002;123(1):80–4. [17] Chuang F. Research on The Tool Wear Mechanism of Cemented Carbide Ball end Mill Machining Titanium Alloy. Harbin, China: Harbin University of Science and Technology; 2016. [18] Houwei Z, Song Z, Gaoqi W, et al. Effects of machining inclination angle of ball-nose end mill on surface topography. Comput. Integr. Manuf. Sys. 2013;19(10):2438–44. [19] Ulutan D, Ozel T. Machining induced surface integrity in titanium and nickel alloys: A review. Int. J. Mach. Tools Manuf. 2011;51:250–80. [20] Bo L. Multi-Scale Characterization and Simulation of The Milling Surface Topography. Zhejiang University; 2015. [21] Xiaojuan Z. Surface Topography Analysis and Process Optimization For Five-Axis High-Speed Milling. South China University of Technology; 2015. [22] Xiaojuan Z. Surface Topography Analysis and Process Optimization For Five-Axis High-Speed Milling. South China University of Technology; 2015. [23] Guocan T. Study on The Forming Mechanism and Surface Properties of Ultrasonic Vibration Assisted Milling for Squamous Surfaces. Shandong university; 2016. [24] Sun J, Guo YB. A comprehensive experimental study on surface integrity by end milling Ti–6Al–4V. J. Mater. Process Technol. 2009;209(8):4036–42.
Fig. 17. Effects of CM and ULTAM on milling force and the surface morphology.
cutting environment. This effectively reduced the cutting force and the tool wear, and inhibited the generation of score marks, thus improving the surface quality. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This research was financially supported by the HenanNatural Science Foundation (162300410120) and the National Joint Fund (U1604255). References [1] Jinfa Y, Jun Z. Research on processing technology of aviation difficult machining materials. Metal Working (Metal Cutting) 2012;21:11–3.
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[27] Peng Y, Liang Z, Wu Y, et al. Effect of vibration on surface and tool wear in ultrasonic vibration-assisted scratching of brittle materials. Int. J. Adv. Manuf. Technol. 2012;59(1–4):67–72.
[25] Chao M, Jianhua Z, Guocan T. Wear and friction properties of titanium alloy surface subject to ultrasonic vibration assisted milling. Surf. Techno.l 2017;46(8):115–9. [26] Lu Zhang. Study on Surface Morphology and Friction Characteristics of Aluminum Alloy by Ultrasonic Vibration Milling. Henan Polytechnic University; 2018.
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Journal of Manufacturing Processes journal homepage: www.elsevier.com/locate/manpro
A study ofthe surface microstructure and tool wear of titanium alloys after ultrasonic longitudinal-torsional milling
T
Peng Chen, Jinglin Tong*, Junshuai Zhao, Zhiming Zhang, Bo Zhao School of Mechanical and Power Engineering, Henan Polytechnic University, Jiaozuo, 45400, Henan, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Ultrasonic longitudinal-torsional milling Titanium alloy Micro-texture Friction properties Tool wear
In order to obtain a better surface micro-morphology on the difficult-to-machine materials such as titanium alloys, the influence of ultrasonic parameters and cutting parameters on the tool path was analyzed theoretically under the ultrasonic longitudinal-torsional vibration. Ultrasonic longitudinal-torsional assisted milling (ULTAM) of titanium alloy was performed, based on a 20° slope. The effects of the ultrasonic parameters and cucuttintting parameters were studied on the surface morphology and surface roughness. The machined surface was examined using scanning electron microscopy (SEM) and white light interferometry. The characterization parameters of the three-dimensional surface micro-morphology including the arithmetic mean height (Sa), the surface skewness (Ssk), and the density of summits (Sds), were obtained under the corresponding cutting parameters. It was found that the ultrasonic amplitude exhibited a significant influence on the surface micro-morphology, followed by the spindle speed n, while the effect of the depth of cut (ap), and the feed amount per tooth (fz) was relatively less. Under the same cutting parameters for the surface, a comparative analysis for the friction and wear testing was performed between the ultrasonic milling and conventional milling (CM). The results showed that the ULTAM transformed the original ravines and anisotropic surface imperfections into an uniform and orderly scaly micro-texture, thus forming an isotropic surface texture in all directions. Furthermore, ULTAM transformed the surface into a fish-scale micro-texture, and thereby improved the surface quality. The fine and smooth surface morphology was conducive by shortening the running-in time, allowing the stable state of friction to be reached quickly, and thereby offering a higher stability in the friction process. `Compared to CM, the tool wear was reduced by 30%, due to the unique tool-work separation characteristics of the ultrasonic longitudinal-torsional assisted milling. Moreover, the unique intermittent cutting characteristics of the ultrasonic longitudinal and torsional milling not only reduced the cutting force and tool wear, but also inhibited the generation of the burr, thus improving the surface quality.
1. Introduction Structural parts that are manufactured from titanium alloys have a high specific stiffness, strength, fatigue and resistance, and are lighter in weight, which makes them popular in the aerospace industry. However, the titanium alloy itself is a difficult material to process, as it is prone to a rapid heat generation, requires a large cutting force and produces a severe tool wear during machining [1,2]. The precision and processing quality of the manufactured parts used in aerospace applications are relatively strict; hence, a considerable volume of research has been undertaken on the machining of titanium alloy for aerospace applications. Luo et al. [3] studied the tool wear during the milling of titanium alloys, where in the influence of milling parameters on tool wear was analyzed and applied to the practical production. In order to
⁎
reduce the surface roughness of titanium alloy during milling and to reduce the formation of burrs, Thepsonthi et al. [4] used the multiobjective particle swarm optimization to simulate and experiment, and eventually determined the optimal process parameters. Tan et al. [5] used the central composite response surface method to study the surface roughness prediction model in the investigation of the titanium alloy milling, and analyzed the influence of milling parameters on surface roughness, residual stress, and micro-hardness. In recent years, the development of ultrasonic vibration cutting has made a significant progress, and the ultrasonically-assisted cutting has become one of the most effective processing of the difficult-to-machine materials like the titanium alloy [6]. Ultrasonically-assisted machining is basically based on the traditional machining, but adds an ultrasonic signal to make the tool or
Corresponding author. E-mail address: [email protected] (J. Tong).
https://doi.org/10.1016/j.jmapro.2020.01.040 Received 21 June 2019; Received in revised form 18 January 2020; Accepted 22 January 2020 1526-6125/ © 2020 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.
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workpiece vibrate at a high frequency along a certain direction. Ultrasonic vibration cutting technology can change the traditional continuous cutting into an intermittent cutting, and the size and direction of the cutting speed changes periodically, which changes the force situation of the whole processing system [7]. In a vibration period, the tool gets contacted intermittently with the workpiece and the chips, and the net cutting time becomes shorter, which reduces the cutting force. Because ultrasonic machining is a periodic process, the generated cutting heat changes gets presented in the form of a pulse. Due to the fast cutting process, the heat cannot reach deep into the interior of the metal, thereby improving the cutting conditions and avoiding the damage and thermal deformation caused due to the excessive cutting heat [8], besides reducing the tool wear [9]. At the same time, the ultrasonic vibration cutting reduces the chances of the formation of the score marks, and instead produces a ‘fish scale’ effect. At each moment when the blade contacts the workpiece and generates the chip, the position of the blade follows the insensitive vibration cutting mechanism, which increases the rigidity of the processing system, and decreases the deformation of the workpiece. The degree of the deformation is only 1/3rd to 1/10th that of the normal cutting deformation, which makes precision machining possible [10]. Therefore, the ultrasonically-assisted processing has received an extensive attention from researchers at home and abroad. Yaet al. [11] analyzed the trajectory of the abrasive particles in the rotary ultrasonic machining tools while studying the crack propagation by using the fracture mechanics theory, and proposed a mathematical model of the material removal rate that provided a theoretical basis for rotary ultrasonic machining. In order to solve the problems of large cutting force and low machining precision in the titanium alloy processing, Jiang et al. [12] used the high linear velocity and the high frequency interrupted cutting characteristics of the ultrasonic elliptical vibration to carry out the titanium alloy milling tests. It was observed that the milling ability of the cutter was enhanced and the milling force was reduced by 50%. Furthermore, the precision of machining was improved significantly. Liu et al. [13] applied the ultrasonic elliptical vibration on the high-speed milling and carried out cutting experiments under different parameters. The results showed that the separation characteristics of the ultrasonic elliptical vibration cutting were fairly achieved using the high-speed milling by setting reasonable parameters, and smaller chips were obtained. Furthermore, the tool wear was slow and stable, thereby prolonging the tool life. Although many researchers have carried out ultrasonic-assisted milling tests, most of them are based on the one-dimensional ultrasonic vibration and only evaluated the surface roughness. Furthermore, the ultrasonic longitudinal-torsional vibration and three-dimensional surface morphology has not been studied extensively. Three-dimensional surface micro-morphology can reflect the surface generation mechanisms more objectively and can affect directly the functional characteristics of the surface. Since, the same surface roughness value may have different surface textures and different surface waviness, it is inappropriate to use surface roughness solely to evaluate the surface micro-morphology [14]. Hence, it is necessary to use the relevant characterization parameters to evaluate the three-dimensional surface morphology. Surface quality also has a certain effect on the surface friction and wear properties. Friction and wear are inevitable as a result of the relative movement of the mechanical parts, and a reduction in the wear, extends the service life of the parts. One way for improving the friction and wear properties of workpiece is to optimize the surface morphology [15]. Additionally, tool wear is an important factor affecting the surface quality of hard-to-machine materials [16]. In the present study, the tool path of ultrasonic longitudinal-torsional assisted milling (ULTAM) was analyzed theoretically in relation to a certain tool inclination. With continuous changes in the cutting angle of a ball-end milling cutter during the finishing and surface processing, several advantages are offered, including a more stable processing condition, less flutter, and a better surface finish after processing. ULTAM tests on the
Fig. 1. Solution of the maximum effective milling radius of a ball-end milling cutter.
machining of titanium alloy were carried out by changing the ultrasonic and cutting parameters, and its effects were studied on the micromorphology of the surface. The friction and wear properties of the machined surface were analyzed and the effects of surface micro-texturing were studied in the improvement of the friction and wear properties of the machined surface. Furthermore, the tool wear was analyzed by changing the milling area and comparison with the results from conventional milling (CM). 2. Analysis of the trajectory of the ball-end milling cutter The formation of the surface topography is closely related to the tool path, and a difference in the tool trajectories can lead to different surface topographies. Also, the influence of the ultrasonic and milling parameters on the tool trajectory affects the surface topography. The top-down milling method was adopted for the ULTAM of ball-end milling cutter, as shown in Fig. 1. The maximum effective milling radius r of the vertical cross-section in the direction of the cutter was as follows [17]:
r= R·sin ⎛−θ + arccos ⎝
R − ap
⎜
⎞ ⎠ ⎟
R
(1)
where, SACE is the residual cutting area, arc AB is the contact area between tool and workpiece, ap is the cutting depth, θ is the processing inclination angle, and R is the tool radius. The motion equation of the cutting edge was established using Eq. (2) at point B of the maximum effective milling radius.
⎧ x (t ) = vf ·t + r·sin (φ) y (t ) = r·cos (φ) ⎨ z ⎩ (t ) = A·sin (2πft )
(2)
where, vf is the feed speed, r is the maximum effective milling radius, A is the amplitude of longitudinal vibration in longitudinal-torsional composite ultrasonic vibration, f is the frequency of longitudinal-torsional composite ultrasonic vibration, and φ is the actual turning angle of the tool which can obtained from Eq. (3).
n φ = 2π ⎛ ·t ⎞ + α ⎝ 60 ⎠
(3)
where, n is the spindle speed, and α is the torsional vibration angle in the longitudinal-torsional composite ultrasonic vibration that can be obtained from Eq. (4).
α= B·cos(2πft + β )
(4)
where, B is the amplitude of torsional vibration in longitudinal-torsional composite ultrasonic vibration. β is the phase difference between longitudinal vibration and torsional vibration. It should be noted that when the torsional radian was very small, the arc length of torsion could be approximated to a straight line and regarded as the amplitude of torsional vibration. It can be seen from Eq. (3) that both the ultrasonic and the cutting parameters exhibited an influence on the tool trajectory, and thereby on the surface micromorphology after machining. The path of the milling edge was plotted 2
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workpieces, cylindrical titanium alloy workpieces with a diameter D = 90 mm and thickness H = 30 mm were employed in the milling process. The material parameters of the titanium alloys are shown in Table 2. It is known from the literature [18] that the surface roughness obtained by milling titanium alloy under CM condition was better at an inclination angle of a ball-end milling at 10° - 30°. Hence, the inclined plane with an inclination angle of 20° was used to install the workpiece during the machining tests. The machine tool for milling was performed by a VMC850E vertical machining center. The whole set of ultrasonic devices used for machining was developed by the project team. Fig. 6 shows the displacement vector diagram of the ultrasonic longitudinal and torsional horn in an unconstrained state, as developed in ANSYS. The figure shows that the displacement of the flange position of the horn was almost zero. The input end of the horn provided the longitudinal vibration. After the oblique groove, a composite vibration of the longitudinal and torsional direction appeared at the tool position, which shows that the horn could realize the combined vibration of longitudinal and torsional ultrasonic. The vibration of the horn from the input end to the output end is illustrated in Fig. 7. It can be seen that the output vibration displacement was the largest, and the ratio of the longitudinal vibration to the torsional vibration was about 1:1.8. Before the tests, the amplitudes of the longitudinal and torsional directions under the same vibration frequency were measured to determine the longitudinal-torsional ratio. The longitudinal amplitude was used to express the amplitude of the longitudinal and torsional vibration, and the torsional vibration amplitude was obtained by the longitudinal and torsional ratio. A KEYENCE laser displacement sensor (LK-G10) was used to measure the ultrasonic amplitude. A Talysurf CCI6000 white light interferometer and a Zeiss scanning electron microscope (SEM) was employed for the evaluation of the surface micro-morphology. The surface micro-morphology was measured using the arithmetic mean height (Sa), surface skewness (Ssk), and the density of summits (Sds). ULTAM of titanium alloy was carried out using the CNC machining center. The test set-up for the ultrasonic longitudinal and torsional assisted milling is shown in Fig. 8. The ultrasonic device employed included a bespoke wireless transmission system and an ultrasonic horn device. After the ultrasonic equipment was turned on, a water spray was applied on the tool. Coolant atomization around the tool is shown in Fig. 8, which indicates that the tool was undertaking ultrasonic longitudinal/torsional compound vibration at that moment. The effects of ultrasonic amplitude A, spindle speed n, feed rate per tooth fz and milling depth ap (where the milling depth is normal tangent depth) on the micro-morphology of the machined surface of titanium alloy were studied using a single factor test method. The milling test parameters are shown in Table 3. Dry milling was adopted and the milling step was fixed to 0.35 mm.
Fig. 2. Contrast of tool trajectory between CM and ULTAM.
and analyzed by using MATLAB software, as shown in Fig. 2. As shown in Fig. 2, the tool path in ULTAM was regressive, which resulted in the periodic separation and contact between the tool and the workpiece. This changed the traditional continuous processing state, offered an easy breaking of the chip, and reduced the cutting force and heat generation. It was found that the tool wear was closely related to the cutting force and heating condition, so the tool path played an active role in reducing the tool wear and improving the surface quality. Fig. 3 shows that if there is only an ultrasonic longitudinal vibration, the change of the ultrasonic longitudinal vibration amplitude does not change the tool-chip separation. However, the ultrasonic high-frequency vibration played a certain role in the modification of the machined surface. It can be seen from Fig. 4 that a change in the ultrasonic torsional amplitude affected the tool-chip separation. The larger the torsional amplitude, the more the cutter retreats, and more discrete was the tool-chip separation phenomenon. Furthermore, when the ultrasonic longitudinal vibration and the torsional vibration acted simultaneously, a change in the spindle rotational speed exhibited a little effect on the ultrasonic longitudinal vibration, but it had a great influence on the ultrasonic torsional vibration, as seen in Fig. 5. With an increase in the spindle speed, the ultrasonic torsional vibration was weakened and the unique phenomenon of ultrasonic torsion gradually disappeared. This further led to the weakning of the tool-chip separation phenomenon. Therefore, the effect of the ultrasonic was not evident in highspeed machining. 3. Equipmentand materials The tool employed for the ULTAM test was a carbide ball-end milling cutter. The tool parameters are shown in Table 1. In order to improve the rigidity of the whole system and facilitate clamping of the
Fig. 3. Effect of longitudinal amplitude on the trajectory of ULTAM. 3
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Fig. 4. Effect of torsional amplitude on the trajectory of ULTAM.
4. Analysis of test results
Table 1 Cutting tool parameters.
Surface micro-morphology is one of the important evaluation indicators of the surface integrity. At the same time, the surface conditions are known to exhibit a great influence on the mechanical properties and performance of the parts, which directly affects the wear resistance, corrosion resistance and fatigue strength of a component [19]. The same surface roughness values may have different surface micro-morphology, and there are many characterization parameters of surface micro-morphology, such as height parameters, spatial parameters, and mixing parameters [20]. According to the ultrasonic parameters and milling parameters presented in Table 3, the machining tests were carried out on the ULTAM titanium alloy. Subsequently, the processed workpiece was evaluated using the relevant instruments.
Tool material
Number of teeth Z
Tool diameter (mm)
Tool helix angle B (°)
Total length of tool L(mm)
Cemented carbide
2
8
35
60
Table 2 Workpiece material parameters.
4.1. Effect of ultrasonic and cutting parameters on surface microstructure The effects of ultrasonic parameters and milling parameters on the arithmetic mean height (Sa), the density of summits (Sds), and skewness (Ssk) [21] are shown in Figs. Fig. 99, Fig. 1010 and Fig. 1111, respectively. Sa represents the average value of the distance from each point on the measured surface to the reference datum, which is used to measure the overall flatness of the surface micro-morphology. Sds represents the number of vertices contained in the unit sample area, which is used to measure the presence of defects such as score marks, burrs, and other incomplete processing. Ssk is used to measure the shape of the contour concave convex amplitude distribution curve, which represents the surfaces with fewer peaks or valleys. In general, the surface with a good friction performance has a negative skewness
Workpiece material
Density ρ (g/cm3)
Modulus of elasticity E(MPa)
Yield strength σ (MPa)
Poisson ratio μ
Ti-6Al-4V
4.62
117.6
930
0.36
[22].The following observations can be made from Fig. 9, 10, and 11: (1) With an increase in A, there was an initial increase and a subsequent decrease in Sa, Sds, and Ssk. When the amplitude was A = 5 μm, both Sa and Sds were the least, indicating that the surface at this time was relatively flat, and there was no peak of surface scoring, burrs or incomplete machining. It can also be seen from the surface micromorphology that there was only a very shallow feed tool mark. This was due to the addition of the longitudinal amplitude during ULTAM, so that the tool not only participated in the cutting, but also had the effect of ironing and trimming the workpiece. Torsional vibration achieved the knife-to-chip separation, and continuous cutting became an
Fig. 5. Effect of spindle speed on the trajectory of ULTAM. 4
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Fig. 6. Simulation of the ultrasonic longitudinal and horn. Table 3 ULTAM test parameters. Spindle speed n(r/min)
Feed per tooth fz(mm)
Cutting depth ap(mm)
Amplitude A(μ m)
0.05
0.5
5
B
1600, 1800, 2000, 2200, 2400 2000
0.5
5
C
2000
0.04, 0.05, 0.06, 0.07, 0.08 0.05
5
D
2000
0.05
0.4, 0.5, 0.6, 0.7, 0.8 0.5
A
1, 3, 5, 7, 9
phenomenon at the edge, which increased the peak value for the surface and a rapid increase of Sds. Moreover, there was an increase in the area of the surface bumps, resulting in the deterioration of the surface micro-morphology. As the peaks increased, Ssk rapidly increased, which had a harmful effect on the surface friction performance. The ultrasonic amplitude improved the surface smoothness. From the tool path, it can be seen that the ultrasonic vibration produced a periodic high-frequency vibration in the cutting depth direction, generated an ironing and finishing effect on the surface, and made a periodic separation and contact of the tool and workpiece. When the ultrasonic amplitude was small, the ultrasonic longitudinal vibration and torsional vibration were relatively small. When the ultrasonic amplitude became larger, it can be seen from the tool path that a larger ultrasonic amplitude had a greater impact on the surface, which exhibited a harmful impact on both the tool and the surface. If the amplitude was too large, the ultrasonic dressing effect began to weaken, along with an increase in the score marks and burrs, and a deterioration of the surface flatness. (2) With the increase in the speed n, a corresponding decreasing trend was observed in Sa, Sds, and Ssk. When the speed n = 1600 r/ min, it was observed that many small score marks were produced on the machined surface. Although the peaks were not high, the area was wide, the number of peak points in the sampling area of the machined surface increased, and Sds was larger. This was due to the fact that when the rotation speed was low, the material was removed slowly by the cutting edge, but such difficult-to-machine alloys were prone to incomplete material removal, which caused the surface to be uneven.With the increase in the rotational speed, the milling speed increased, the temperature in the milling zone increased to a certain extent, and the material in the cutting layer softened and was easily removed. Moreover, the phenomenon of friction and slip was improved with the effect of the ultrasonic vibration, and it was easier to produce a
Fig. 7. Ultrasonic longitudinal and torsional torsional amplitudes.
Fig. 8. ULTAM test equipment.
intermittent cutting, which reduced the cutting force and cutting heat generation, thereby improving surface quality. However, as the ultrasonic amplitude increased, both Sa and Sds began to increase gradually, and Sds increased rapidly. It can be seen from the three-dimensional micro-morphology that when the amplitude was A = 9 μm, the surface not only exhibited score marks, but there was also an obvious residual 5
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Fig. 9. Effects of ultrasonic parameters and milling parameters on Sa.
fine and uniform textured morphologyof the surface. However, as the rotational speed continued to increase, the reduction in Sa and Sds was not very obvious. This is because, when the milling speed reached 60 m/min, it was close to the critical milling speed of titanium alloy [23]. A large rotating speed can affect the ultrasonic torsional vibration and lead to the weakening of the ultrasonic separation phenomenon. Although high-speed milling reduced the milling force and inhibited the formation of score marks, the ultrasonic milling effect of low-speed was
more obvious in ULTAM. When the critical milling speed was exceeded, the separation of the tool and the workpiece gradually disappeared, and the ULTAM effect was weakened, which detracted the quality of the surface micro-morphology. (3) With the increase of the feed rate fz, there was a gradual increase in the Sa, Sds, and Ssk. However, the increasing trend of Sds and Ssk was not very obvious. When fz was small, the volume was removed per unit time, the milling resistance of the tool was small, and the material
Fig. 10. Effects of ultrasonic parameters and milling parameters on Sds. 6
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Fig. 11. Effects of ultrasonic parameters and milling parameters on Ssk.
was easily cut from the workpiece. At the same time, the feed rate was slower, and the ultrasonic vibration repeatedly pressed and trimmed the milled surface to make the machined surface more fine and uniform, so as to obtain a better surface micro-morphology. With an increase in fz, there was a corresponding increase in the material removal per unit time, which increased the material removal rate and the feed residue after milling. At the same time, it was easy to sustain the incomplete material removal, resulting in an increase in surface peaks. It can be seen from the three-dimensional micro-morphology that the increase of feed did not affect the ultrasonic dressing, and a fish-scale texture was formed. The peak value in the sampling area was very large, but the number of samples was not large. Therefore, the increase in feed rate exhibited a significant effect on Sa, but not on Sds and Ssk. (4) With the increase of the depth of cut ap, a corresponding increasing trend in Sa, Sds, and Ssk was observed. When ap was small, the material removal per unit time was small, and the extrusion and tearing phenomena did not occur. With the effect of ultrasonic longitudinal vibration on the surface, the surface was relatively flat, the Sa value was low, and the surface micro-morphology was better. As ap increased, the three-dimensional surface topography showed that the score marks and burrs began to appear on the surface of the workpiece. Especially, when ap = 0.8, the probability of occurrence of score marks and burrs greatly increased, but the surface peaks were not very high, and the peak point in the sampling area was not large. Although Sa and Sds increased, they were not particularly noticeable, and their influence on the surface condition was relatively small. A smaller depth of cut was advantageous to produce uniformity of the machined surface, but a change in depth of cut presented a relatively small effect on the surface texture.
Fig. 12. Comparison of surface friction properties in CMand ULTAM (×400).
of marks with different depths in the feed direction. The presence of the knife marks created a rugged gully on the surface, and the surface was anisotropic, which resulted in a poor surface micro-morphology, thereby affecting the surface friction properties [24]. However, in ULTAM, the feed tool marks were ironed and flattened due to the effect of the ultrasonic longitudinal torsional vibration. A relatively uniform scaly micro-texture replaced the uneven gullies. Under the repeated pressing by the ultrasonic vibration, a uniform and dense fish-scale texture was formed on the surface. High-frequency vibration changed the surface texture from an anisotropic state to isotropic, forming a uniform texture in all directions, thereby improving the friction performance of the contact surface. When the mechanical parts are in direct contact, the relative motion of the mechanical surface produces a friction and wear. Under normal working conditions, there is a ‘running-in’ stage, a stable wear stage and then a severe wear stage. Mechanical wear is unavoidable, and the surface texture of the same material affects the friction and wear properties of the surface [25]. In the present work, the milled workpiece was subjected to a friction test on an MMW-1 microcomputercontrolled vertical universal friction wear machine (Maximum test force 1 K N), and the static pressure was set to 10 N, the motion speed of friction pin was 10 mm/s, and the resultsare shown in Fig. 13.
4.2. Comparison of surface friction properties in CM and ULTAM Comparative tests of CM and ULTAM were carried out with cutting parameters of n = 2000 r/min, fz = 0.05 mm, ap = 0.4 mm, and ultrasonic parameters A = 5um, and then the friction and wear tests were carried out. After milling, the workpiece was examined using SEM (shown in Fig. 12). It can be clearly seen from Fig. 12 that the surface in ULTAM exhibited a uniform scaly micro-texture, which was not present in CM. A large amount of scoring was evident on the surfacein CM, leaving a lot 7
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friction pin and reduced the suspension phenomenon of the frictional contact surface caused by the fluctuations of the microscopic surface contours, which in-turn reduced the amount of wear on the workpieceduring the running-in period [26]. The friction test was more conducive in shortening the running-in time, allowing it to reach the friction stable state quickly, and the friction process was more stable. 5. Analysis of tool wear for CM and ULTAM Tool wear affects not only processing efficiency, but also changes the surface quality of the workpiece. With the continuous increase of cutting area, the cutting edge gets extruded and scrubbed for a longer time, which inevitably leads to the wear and tear at the cutting edge. With the increase in wear, there is an increase in thecutting force and cutting temperature, further affecting the processing quality and efficiency [27]. Hence, the study of tool wear is of great significance in relation to the surface quality. In theory, any difference between the surface of the used tool and that of a new tool, belongs to the scope of tool wear. However, on comparing the test results of ULTAM of the titanium alloy with those from the ball-end milling cutter, it was found that the wear location of the cutter was mainly in the two areas of the flank and the vertex. When the cutting depth was large or the processing inclination was zero, the wear at the vertex was relatively large. However, in actual processing, the ball-end milling cutter is seldom used at zero inclination. At the same time, ULTAM is mainly used for finishing, and its cutting depth is small, so the significance of tool vertex wear is relatively small. In addition, flank wear affects directly the quality of a machined surface, so flank wear is considered to be the main method of evaluation. In order to analyze the tool wear, a comparative trial of CM and ULTAM machining was carried out by changing the milling area with cutting parameters n = 2000 r/min, fz = 0.05 mm, ap = 0.4 mm, ultrasonic frequency f = 35KHz and amplitude A = 5 μm. Each group of tests were carried out twice, and the flank wear value (VB) was measured respectively and the average value was taken. The test results are shown in Table 4. From Fig.15, it can be seen that the wear value of the flank surface increased with an increase in the removal area for both CM and ULTAM. However, the wear value of the flank surface in ULTAM was significantly smaller than that in CM, when the removal area was same. During the early stages of wear, the wear rate of the flank surface in ULTAM was significantly lower than that with CM. With an increase in the milling area, the wear rate of the flank surface in ULTAM was almost the same as that in CM, but the wear value of the tool in ULTAM was about 30% lower than that for CM. With the increase in milling area, the tool cutting edge showed different degrees of wear, as shown in Fig.15. In Fig.16, A and B are the tool wear diagrams for CM and ULTAM, respectively, with a milling area of 6356 mm2. It can be clearly seen that, under CM, the tool wear band appeared wider and the tool edge exhibited a significant wear gap,along with the damage of the tool coating. Compared to CM, the tool flank wear of ULTAM was uniform and only a small wear band was evident. Although there was a minor wear at the edge of the tool, there was no larger gap. This was due to the fact that CM includes a continuous cutting mechanism, where the
Fig. 13. Surface friction and wear test of CMand ULTAM machined surfaces.
It can be seen from Fig.13 that, under the same cutting parameters, the average friction coefficient of the ultrasonically-milled surface after the friction test finally reached the stable wear stage, which was smaller than that obtained from CM. Compared with CM, the average friction coefficient of ULTAM was reduced by about 5.1–18.3 %. The findings also show that ULTAM improved the dimensional accuracy of parts and the quality of machined surfaces. In the friction and wear test, it was found that both ULTAM and CM went through the running-in stage first, and then gradually reached the stable state of friction, but the time spent in the running-in stage was quite different. In the friction and wear test, the surface after CM first underwent a slow running-in transition period, about 310 s and then gradually reached the state of friction stability. However, the surface running-in transition period after ULTAM was very short, about 150 s, and then quickly reached the state of friction stability. Compared with CM, the running-in time was reduced by about 51.6 %. In combination with the surface micromorphology and surface roughness test curve, the workpiece during the friction and wear was modeled, as shown in Fig. 14. After CM, due to the presence of more score marks and burrs on the surface, the presence of the feed tool marks increased the surface gully, the depth was different, and the surface texture was anisotropic, as shown in Fig.14a. In the friction and wear test, the actual contact area between the friction pin and the workpiece surface was small, and the friction pin rotated more times to remove the surface gully. Hence, a greater time was required to achieve the friction stability. However, after ULTAM, there were fewer score marks on the surface, and the feed tool marks disappeared after repeated pressing and trimming by the ultrasonic vibration. Instead, uniform and dense fish-scale micro-texture was generated. The high-frequency ultrasonic vibration transformed the original anisotropic surface into an isotropic finish and presented a uniform texture in all directions, as shown in Fig.14b. This fine and smooth surface morphology enlarged the actual contact area between
Fig. 14. Friction states of workpiece surfaces after CM and ULTAM. 8
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contacted periodically by the workpiece. The unique intermittent cutting characteristics of ULTAM made the chips separate and reduced the cutting force and heat generation. Moreover, it inhibited the generation of score marks, delayed the tool wear, and improved the machining efficiency and the surface quality.
Table 4 Variation of milling area and flank wear maximumwidth. Milling area(S)/ mm2
CM
ULTAM
VB-1/μm
VB-2/μm
Average value
VB-1/μm
VB-2/μm
Average value
908 1816 2724 3632 4540 5448 6356
39.25 70.22 100.37 107.52 116.56 118.35 121.82
55.02 74.69 98.83 109.75 118.28 130.94 143.61
47.135 72.455 99.6 108.635 117.42 124.645 132.715
28.62 36.79 43.30 61.90 65.39 68.36 82.57
40.52 47.26 48.35 69.75 87.83 102.21 104.07
34.57 42.025 45.825 65.825 76.61 85.285 93.32
6. Conclusions In the present work, a theoretical analysis of the tool path during ULTAM was performed, and the influence of ultrasonic parameters and milling parameters on the tool path were analyzed using MATLAB software. Milling tests of the titanium alloy with ball-end milling cutter under ULTAM were carried out by using an NC machining center. The surface morphology and tool wear after the tests were observed and analyzed, and the following conclusions were obtained:
accumulation of cutting heat is not easy to absorb, and it is easy then to produce bond wear on the cutting edge, which causes the cutting edge to collapse. In ULTAM, the cutting regime between the tool and the workpiece gets transformed into a periodic intermittent mode, which results in a periodic separation and contact between tool and workpiece, thereby reducing the production of the cutting heat and also the required cutting force. To a certain extent, the periodic tool-chip separation in the rotation direction plays an active role in reducing tool wear, which is consistent with the previous theoretical analysis of the tool path. With the increase in the milling area, the tool wear increased gradually, which affected the milling force and the quality of the machined surface. Fig.17. shows the variation trend in the milling force (F) and the surface morphology (Sa) with the effect of milling area for CM and ULTAM. It can be seen that with an increase in the tool wear, the value of ULTAM was always smaller than that of CM, regardless of the milling force or Sa. In the early stages of tool wear, the milling force and Sa increased gradually, with an increase in the milling area. Once the tool wear became severe, the milling force and Sa increased rapidly, especially during CM. However, in ULTAM, the change in milling force and Sa was not significant due of the occurance of less wear. After the tool wear, there was a change in the cutting environment. The most direct change was that the cutting edge was no longer sharp, which affected the contact area between the cutting edge and the workpiece in the cutting process. Morover, it also increased the cutting force and destroyed the protective coating of the tool. This led to the direct contact between the tool matrix and the workpiece, which speeded the tool wear. In ULTAM, the tool flank and cutting edge were separated and
• The ultrasonic amplitude significantly influenced the surface micro-
•
•
•
morphology. Smaller ultrasonic amplitudeswere unable to exploit the advantages of the ultrasonic dressing. In contrast, the larger the ultrasonic amplitude led to a greater impact acceleration, which damaged the micro-morphology of the surface. ULTAM with a suitable ultrasonic amplitude reduced the formation of score marks and burrs, and ultrasonically pressed and trimmed the surface to improve the surface micro-morphology. During ULTAM, the spindle speed exhibiteda significant effect on the Sa, Sds and Sk values, followed by feed rate fz and cutting depth ap. A higher spindle speed produced a better surface micro-morphology. However, under ULTAM, when the critical cutting speed was exceeded, tool/chip separation was weakened and resulted in a less improvement in surface quality. Compared with CM, the ultrasonic longitudinal amplitude produced the effects of ironing and dressing of the surface, and the ultrasonic torsional amplitude changed the continuous cutting into intermittent cutting, which played an important role inthe cutter/chip separation cycle. The simultaneous action of the ultrasonic longitudinal amplitude and torsional amplitude reduced the formation of score marks and burrs, and produced a uniform dense ‘fish-scale’ micro-texture on the surface, thereby improving the surface micromorphology. Tool wear analysis showed that,compared to CM, the unique intermittent cutting characteristics of the tool in ULTAM made the tool and workpiece cycle separate and contact, thereby improving the
Fig. 15. Wear value of flank face after CM and ULTAM. 9
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Fig. 16. Toolflank wearresulting from CM and ULTAM. [2] Qingcun X, Ge S, Wei Z. Cutting technology development of titanium alloy aircraft structural part. Aeronaut. Manuf. Technol. 2013;14:42–7. [3] Luo M, Wang J, Wu B, et al. Effects of cutting parameters on tool insert wear in end milling of titanium alloy Ti6Al4V. Chinese J. Mech. Eng. 2017;30:53–9. [4] Thepsonthi T, Özel T. Multi-objective process optimization for micro-end milling of Ti-6Al-4V titanium alloy. InJ. Adv. Manuf. Technol. 2012;63:903–14. [5] Liang T, Dinghua Z, Chang feng Y. Effect of high-speed milling parameters on surface metamorphic layer of TC17 titanium alloy. J. Aeronaut. Mater. 2017;37(6):75–81. [6] Shanxiang F, Huiling Z, Qing jian Z. The application status and development trends of ultrasonic machining technology. J. Mech. Eng. 2017;53(19):22–32. [7] Jian jian W, Jian fu Z, et al. Advances in rotary ultrasonic elliptical machining of advanced aviation materials. Aeronaut. Manuf. Technol. 2018;61(21):30–7. [8] Yundian Z. Ultrasonic Processing and Its Application. Beijing: National Defence Industry Press; 1995. [9] Tong J, Feng Z, Jiao F, Zhao B. Tool wear in longitudinal-torsional ultrasonic vibration milling of titanium alloys. Surf. Technol. 2019;48(3):297–303. [10] Kumabe J. The Foundation and Application of Precision Vibration Cutting. Beijing: Machinery Industry Press; 1985. [11] Ya G, Qin HW, Yang SC. Analysis of the rotary ultrasonic machining mechanism. J. Mater. Process Technol. 2002;129:182–5. [12] Xinggang J, Haitong L, Huimin L, Deyuan Z. Investigation of ultrasonic elliptical vibration milling of thin-walled titanium alloy parts. Acta. Armamentarii 2014;35(11):1891–7. [13] Liu J, Jiang X, Zhang D. Research on the characteristics of chips and tool flank wear in high-speed rotary ultrasonic elliptical machining for side milling of Ti-6Al-4V. J. Mech. Eng. 2019:1–9. [14] Tong J, wei G, Zhao L, Wang X, Ma J. Surface microstructure of titanium alloy thinwalled parts at ultrasonic vibration-assisted milling. Int. J. Adv. Manuf. Technol. 2018;3005–7:1–15. [15] Podgornik B, Hogmark S. Surface modification to improve friction and gallingproperties of forming tools. J. Mater. Process Technol. 2006;174:334–41. [16] Hocheng H, Kuo KL. On-line tool wear monitoring during ultrasonic machining using tool resonance frequency. J. Mater. Process Technol. 2002;123(1):80–4. [17] Chuang F. Research on The Tool Wear Mechanism of Cemented Carbide Ball end Mill Machining Titanium Alloy. Harbin, China: Harbin University of Science and Technology; 2016. [18] Houwei Z, Song Z, Gaoqi W, et al. Effects of machining inclination angle of ball-nose end mill on surface topography. Comput. Integr. Manuf. Sys. 2013;19(10):2438–44. [19] Ulutan D, Ozel T. Machining induced surface integrity in titanium and nickel alloys: A review. Int. J. Mach. Tools Manuf. 2011;51:250–80. [20] Bo L. Multi-Scale Characterization and Simulation of The Milling Surface Topography. Zhejiang University; 2015. [21] Xiaojuan Z. Surface Topography Analysis and Process Optimization For Five-Axis High-Speed Milling. South China University of Technology; 2015. [22] Xiaojuan Z. Surface Topography Analysis and Process Optimization For Five-Axis High-Speed Milling. South China University of Technology; 2015. [23] Guocan T. Study on The Forming Mechanism and Surface Properties of Ultrasonic Vibration Assisted Milling for Squamous Surfaces. Shandong university; 2016. [24] Sun J, Guo YB. A comprehensive experimental study on surface integrity by end milling Ti–6Al–4V. J. Mater. Process Technol. 2009;209(8):4036–42.
Fig. 17. Effects of CM and ULTAM on milling force and the surface morphology.
cutting environment. This effectively reduced the cutting force and the tool wear, and inhibited the generation of score marks, thus improving the surface quality. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This research was financially supported by the HenanNatural Science Foundation (162300410120) and the National Joint Fund (U1604255). References [1] Jinfa Y, Jun Z. Research on processing technology of aviation difficult machining materials. Metal Working (Metal Cutting) 2012;21:11–3.
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