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Effect of cutting parameters on chips and burrs formation with traditional micromilling and ultrasonic vibration assisted micromilling

In the traditional micromilling (TMM) of Inconel718 alloy, due to the influence of material plasticity and size effect, relatively large burr will be produced. In order to hinder the burr forming in micromilling, ultrasonic vibration in feed direction is applied to the workpiece to complete vibration cutting. Combined with trajectory simulation and cutting experiment, the burr formation mechanism of TMM and ultrasonic vibration assisted micromilling (UVAMM) was studied. The results show that when the ratio of amplitude (A) to feed per tooth (ƒz) is greater than 0.5, continuous cutting changes to intermittent cutting in the vibration cutting process. The fractured area with dimples on the burr increases with the increase of amplitude. Compared with TMM, UVAMM improves chip breaking ability, facilitates the propagation of burr crack, and effectively inhibits the formation of burr. When the chip breaking condition is reached, the burr shape is usually tearing or flocculent. Under the conditions of low speed (n), large ƒz, and large A, the burr suppression is more obvious.


Introduction
Inconel718 alloy has excellent high-temperature strength, high-temperature oxidation resistance, and corrosion resistance. It is favored by aerospace, ship, automobile, and military industries [1][2][3]. Micromilling is an important processing method that can be used to manufacture micro-scale precision parts [4], and is also the most commonly used precision processing method for machining high-temperature nickel-based alloy materials. However, Inconel718 is easy to produce large burrs in the process of micromilling due to its large plasticity, adhesion, and chip breaking, which seriously affects the assembly accuracy and performance of parts [5].
The main factors affecting the formation of burrs are tool parameters, workpiece material, processing method, and cutting parameters, as shown in Fig. 1. Hashimura et al. [6] classified the burr types into top burr, entrance burr, exit burr, entrance side burr, and exit side burr according to the burr location on the workpiece, as shown in Fig. 2. In order to suppress the generation of burrs, some scholars have done a lot of research by optimizing processing parameters. Li et al. [7] found that burrs tend to appear on the top of the groove at lower speed (n), lower feed per tooth (ƒ z ), and higher cutting depth (a p ). Lee and Dornfeld [8,9] showed that large burrs are likely to occur when the cutting thickness is smaller than the cutting edge radius. Sun et al. [10] investigated the groove top burr of 304 austenitic stainless steel and concluded that the size of the burr on the down milling side is larger than that on the up milling side. Hajiahmadi [11] studied the influence of a p , n, and ƒ z on the size of the burr, and optimized the process parameters reasonably. Mian et al. [12] found that the ratio of ƒ z to the cutting edge radius is the most important factor for the thickness of the burr. Yao et al. [13] reported that the size of the top burr is closely related to specific cutting energy and impeded by reducing cutting energy with the cutting parameters of optimization. In addition to optimizing processing parameters, burrs can be also suppressed by changing cutting conditions. Oliveira et al. [14] found that a higher flow of cutting fluid 1 3 can effectively inhibit the formation of burrs. However, the use of cutting fluid will not only inevitably cause pollution problems but also increase the cost. Some scholars suggested the method of low temperature pre-cooling to suppress the formation of burrs [15]. Although the burrs are effectively suppressed, it also increases tool wear. In the traditional micromilling (TMM) process, surface defects and burrs are usually removed by post-processing. Schubert et al. [16] chose to use electrochemical polishing technology to remove the generated groove top burrs and found that although the burrs can be effectively removed, the machined surface suffered a height loss of 2 μm. In addition, the technology to reduce burrs includes TMM which will produce new burrs, laser technology which may burn the surface of the workpiece, micropeening which is only applied to higher hardness steel, and ultrasonic wet peening which may lead to the longer cleaning time [17,18]. Therefore, the minimization and control of burrs are the keys for micromilling to improve the surface quality of the workpieces and lower the machining cost.
Vibration assisted machining is a machining method in which high-frequency and small amplitude vibration generated by external energy is applied to the tool or workpiece to prefect the cutting process. Vibration cutting has been widely used in turning, milling, drilling, and other cutting processes. It has excellent performance in reducing cutting force and cutting temperature [19][20][21], inhibiting tool wear [22], improving surface quality [23][24][25][26], and improving surface performance [27]. Geng et al. [28] studied the effect of rotating ultrasonic ellipse machining (RUEM) on the formation and suppression of carbon fiber reinforced plastic (CFRP) delamination and found that RUEM can effectively suppress the delamination of CFRP. Gao et al. [29] investigated the chip morphology characteristics of longitudinal torsional ultrasonic assisted drilling (LTUAD), and found that LTUAD can improve the chip breaking capacity and produce more broken chips. Xiang et al. [30] used ultrasonic vibration assisted cutting (UVAC) to process Nomex honeycomb core material and found that UVAC greatly improved burrs and tearing defects of the incision. The results of these studies show that vibration cutting changes the material removal mechanism, which is beneficial to suppress the formation of burrs.
In this work, ultrasonic vibration was introduced into the micromilling process of Inconel718 to achieve the purpose of suppressing the formation of burrs. The trajectory equation of the tool tip was built and the chip interruption

Chip formation process
In micromilling processing, burrs are caused by incomplete formation or incomplete fracture of chips. As shown in Fig. 3, the formation of chips is divided into three stages, including elastic deformation stage, elastic-plastic deformation coexisting stage, and plastic deformation stage. On the up milling side, when the cutting thickness is less than the minimum thickness, the material will be elastic deformation and no chips can be formed. When the cutting thickness increases to h min or even greater, chips begin to form and the material recovery phenomenon still exists. With the increase of cutting thickness, plastic shear deformation of materials becomes the main deformation form. On the down milling side, the cutting thickness is continuously reduced. When the cutting thickness is reduced to a certain value, the chips will break or stay on the top of the groove. Therefore, the burr on the up milling side is the product of extrusion deformation, and the burr on the down milling side is the product of incomplete chip fracture. In a single cutting cycle, the condition of chip formation is that the instantaneous cutting thickness is greater than the minimum cutting thickness. The simplified diagram of chip is shown in Fig. 3. When the cutting edge radius and material are determined, the minimum cutting thickness is a constant value. The minimum cutting thickness of Inconel718 is expressed by Eq. (1) [31] and the instantaneous cutting thickness is expressed by Eq. (2).
When the instantaneous cutting thickness is equal to the minimum cutting thickness, Eq. (1) and Eq. (2) can be combined to obtain Eq. (3). The chip length is expressed by Eq. (4). According to Eq. (4), there is positive correlation between the chip length and ƒ z .
where h min is the minimum cutting thickness; h is the instantaneous cutting thickness; r e is the cutting edge radius; θ is the rotation angle; L is the chip length; r is the tool radius; and ƒ z is feed per tooth.

Trajectory equation of cutting edge
Compared with TMM, the application of ultrasonic vibration changes the trajectory of the tool tip, which affects chip formation. Figure 4 shows the trajectory of two adjacent tool tips in a motion cycle in TMM. The movement trajectory of the tool tip 1 and the tool tip 2 is represented by the solid line and the dashed line, respectively. The motion trajectory of the tool tip is a compound motion of rotation and feed motion. According to the coordinate system established in Fig. 4, the motion of the ith (i = 1, 2) tool tip can be expressed by Eq. (5) and Eq. (6).
The vibration is applied to the workpiece through the ultrasonic equipment, which can realize the ultrasonic vibration in the feeding direction. In order to express the relative positional relationship between the tool tip and the workpiece more intuitively, the vibration of the workpiece is equivalent to the sinusoidal motion of the tool in the feed direction. The motion trajectory of the ith tool tip can be expressed by Eq. (7) and Eq. (8).
where v f is the feed rate; n is the spindle speed; r is the tool radius; A is the amplitude; λ is the ratio of the vibration frequency to the spindle rotation frequency; and ɷ is the ultrasonic vibration angular frequency.

Experimental materials
The micromilling cutter is the cemented carbide doubleedged flat end mill with UT coating, as shown in Fig. 5. The tool geometry parameters are shown in Table 1. The workpiece material is Inconel718 alloy, whose chemical composition is shown in Table 2.

Experimental equipment and arrangement
The cutting experiment was carried out on a four-axis precision milling machine (UCAR-DPCNC4S) with a speed range of 500-24,000 r/min. The ultrasonic vibration device (USM-300A) provides vibration energy for the workpiece with the maximum amplitude (A) of 15 μm. The vibrator was connected through the upper connecting plate. The machine table was connected through the lower connecting plate. A new micromilling tool was used to machine a groove on the surface of the workpiece. The experimental equipment is shown in Fig. 6. The machining parameters are shown in Table 3.
The chips and burrs are investigated with a super-high magnification lens zoom 3D microscope and scanning electron microscope (SEM). In order to obtain the burr size accurately, the burr is measured by the super-high magnification lens zoom 3D microscope. The measuring method is shown in Fig. 7. The width and height of burr on both sides of the groove were measured at the different 20 positions, and the average width and height of burr were calculated according to Eqs. (9) and (10).

Trajectory of cutting edge
In the machining process of UVAMM, ƒ z and A are the most important parameters to change the formation mechanism of (10) Δh i chip and burr, so it is very important to judge the relationship between ƒ z and A by analyzing the tool tip trajectory. According to the tool tip trajectory model, the motion trajectories of the tool tips with different parameters are shown in Fig. 8. It can be seen from Fig. 8a-c that when the A is 3 μm, there are the significant intersections of tool tip trajectory between tool tip 1 and tool tip 2, which indicates that the chip breaking is the result of the combined effect of the A and ƒ z . It can be seen from the tool tip trajectory in Fig. 8d-f that when the A increases from 0 to 3 μm, the tool tip trajectory begins to intersect, and the chips may have been broken.
As the A increases from 3 to 6 μm, there are the significant intersections of tool tip trajectory between tool tip 1 and tool tip 2, which indicates that the chip is no longer continuous. According to the characteristics of the tool tip trajectory, it can be concluded that the motion trajectory of the tool tip 2 and the tool tip 1 intersects when A/ƒ z > 1/2, which can meet the chip breaking requirements. Shen et al. [20,21] also simulated the motion trajectory of the tool tip, and the results showed that when the tool tip trajectory intersects, the cutting process changes to intermittent cutting. It is also found from Fig. 8 that the origin of intersections of tool tip trajectory is close related with the ratio of A and ƒ z . The vibration frequency is not key factor for the origin of intersection. The density of the intersections of tool tip trajectory may be affected by the vibration frequency. The effect of the forming intersection on the broken chip is more significant than that of the number of intersection.

Influence of cutting parameters on chip size
Burr is the product of incomplete chip fracture. Research on chip formation is the basis for research on burr formation. Figure 9 shows the chip morphology of TMM and UVAMM under different n. It can be seen from the figure that the chip is long size and spiral shape without the ultrasonic vibration. Since it is difficult to break chip for Inconel718, and the change of n cannot change the chip formation mode, the shape of chips does not change significantly with the increase of n. After the ultrasonic vibration is applied, the spiral chip is gradually transformed into C-type chip, which is more obvious when A/ƒ z > 1/2. At this time, the cutting mode changes from continuous cutting to intermittent cutting. According to Gao et al. [29], this phenomenon can be explained as the result of chip crack promoted by vibration cutting. Figure 10 shows the variation of chip size of TMM and UVAMM under different n. The maximum chip size is 1865 μm when n is 6000 r/min. With the increase of n, the chip size first decreases and then increases, and the trends of chip size are similar for the amplitude of 0 and 3 μm. According to the simulation of tool tip trajectory, there is not intersection between tool tip 1 and tool tip 2 on the condition of the amplitude of 0 and 3 μm, which lead to the similar mode of the chip breaking. In addition, the separation between the tool tip and workpiece is beneficial for the chip breaking on the condition of the amplitude of 3 μm, which decreases the chip size. However, chip size of the amplitude of 6 and 9 μm is obviously smaller than that of the amplitude of 0 and 3 μm. There exists the significant intersection between tool tip 1 and tool tip 2 on the condition of the amplitude of 6 and 9 μm and is prone to easily break the chips. When ultrasonic vibration is applied, the chip size decreases obviously, especially at low n. The reason is that the lower n will lead to more vibration times in the same distance, so chip breaking is more frequent. With the increase of n, the times of ultrasonic vibration per unit distance decrease, which lead to decline the vibration effect of the cutting process. Figure 11 shows the chip morphology of TMM and UVAMM under different ƒ z . It can be seen from the figure that when the ultrasonic vibration is not applied, a large number of C-shaped chips or serrated chips are easily generated with a small ƒ z . However, when ƒ z is more than 5 μm/z, long and spiral chips are formed. The reason is that when ƒ z is small, the material must be accumulated to a certain thickness before it is sheared and removed [32,33]. When ultrasonic vibration is applied, high-frequency intermittent cutting changes the instantaneous cutting thickness, reduces the influence of size effect, and makes chip formation and breaking easier. It can be also seen from the figure that the number of C-type chips increases after applying vibration. When A is 3 μm and ƒ z is small, spiral chip appears. The reason is that the A is too small and the chip breaking effect is not obvious. When A is more than 3 μm, the chip is completely transformed into C-type chip. The larger the A is, the more broken the chip is. Figure 12 is the variation of chip size of TMM and UVAMM under different ƒ z . When the ultrasonic vibration is not applied, the chip size increases with the increase of ƒ z . When ƒ z is in the range of 2-4 μm/z, the chip formation is difficult and the size and quantity of chips is small due to the size effect. However, the chip size dramatically increases in TMM when ƒ z is larger than 6 μm. The lower feed, ƒ z /r e < 1, leads to the negative rake angle so that the sliding and the plowing processes govern instead of the cutting processing [9]. It is also found that the trend of chip size is similar when A is 6 and 9 μm, which indicate that the larger A is beneficial for the chip-broken. When ƒ z is larger, the chip size of UVAMM is obviously smaller than that of TMM. The larger A is, the smaller chip size is. However, the effect of ƒ z on chip size is no longer obvious in UVAMM. Ultrasonic vibration plays an important role in chip breaking. It is prone to the chip breaking of smaller n, larger ƒ z , and A in the micromilling.

Influence of cutting parameters on burr size
In order to study the influence of ultrasonic vibration on burr formation, the milling groove morphology was observed by scanning electron microscope, as shown in Fig. 13 and  Fig. 15. In the figures, the upper side of the milling groove is the down milling side, and the lower side is the up milling side. Figure 13 shows the burr morphology of TMM and UVAMM at different n. The burrs size at the down milling side is smaller than that at the up milling side, as reported in Gomes, Silva, and Duarte [34] in the micromilling of stainless steel. The burr on the down milling side is flaky, and the burr on the up milling side is long strip for TMM. When n is 6000 r/min, the flaky burr on down milling side is large, smooth, and complete. The strip burr on up milling side has large size and high density. With the increase of n, the flaky burr curls and the strip burr turns into flaky burr. This is because the plasticity of the shear surface material decreases and the brittleness increases with the increase of n. When the ultrasonic vibration is applied, the flaky burr on the down milling side is gradually transformed into tearing burr, and the strip burr on the up milling side is gradually transformed into flocculent burr. Ultrasonic vibration improves the possibility of brittle fracture of materials and provides energy for chip fracture, which inhibits the generation of burrs with complete shape and large size. Figure 14 shows the variation of burr size of TMM and UVAMM under different n. It is found that the burr size on both sides of the groove top is the largest at the spindle speed of 6000 for TMM, in which the width and height of the burr on the down milling side are 123 μm and 69 μm, respectively, and the width and height of the burr on the up milling side are 256 μm and 125 μm, respectively. With the increase of n, the width and height of the burr on the down milling side decrease first and then increase, while the width and height of the burr on the up milling side decrease. The reason is that the extrusion and friction time of the up milling side is shortened with the increase of n, and the plastic deformation is reduced, so the burr size is also reduced. However, the thermal softening effect caused by the increase of cutting temperature on the down milling side is obvious, and the chips are difficult to break and remain on the top of the groove. After the ultrasonic vibration is applied, the burr size of both the down milling side and the up milling side decreases significantly, especially at low n. Tan et al. [35] reported that ultrasonic vibration assisted cutting has lower cutting force, friction, and cutting temperature, which promotes the smooth discharge of chips. This is an important reason for the reduction of burr size. When the high-frequency chip-broken condition is reached, the burr size decreases more obviously, and the change trend is different from that of TMM. The application of ultrasonic vibration can increase the chip breaking frequency and reduce the chip size, which leads to reducing the burr size. Figure 15 shows the burr morphology of TMM and UVAMM under different ƒ z . When the ultrasonic vibration is not applied, the chip is difficult to produce because ƒ z is small. In this case, the burr on the down milling side is the result of incomplete chip fracture, and the burr on the up milling side is the result of ploughing [36]. Therefore, smaller flake burr will be produced on the down milling side, while flocculent burr will be produced on the up milling side. With the increase of ƒ z , the flake burr changes into turning burr, and the flocculent burr changes into larger strip burr. The reason is that with the increase of ƒ z , the volume of material removed per unit time increases, and the cutting force also increases, so the plastic deformation of material is more serious, leading to the increase of burr. After the vibration is applied, the chip breaking ability is improved and the extrusion effect is reduced. Therefore, the burr on the down milling side is gradually changed into tearing burr, and the burr on the up milling side is also gradually changed into flocculent burr, especially at high feed rate. The reason for this phenomenon is that vibration cutting plays a decisive role in promoting crack extension [29]. Figure 16 shows the variation of burr size of TMM and UVAMM under different ƒ z . When the ultrasonic vibration is not applied, the width and height of both sides of the groove top increase with the increase of ƒ z . After the vibration is applied, the instantaneous cutting thickness is obviously changed, the influence of size effect is reduced, the formation and separation of chips are easier, and the burr is well restrained, especially when the feed rate is large. However, when ƒ z is 8 μm/z, the influence of ultrasonic vibration on burr size is not obvious. The reason is that the larger ƒ z is, the smaller A/ƒ z is. Although A/ƒ z > 1/2, the size effect is still very important due to the influence of cutting edge radius. When ƒ z is small, ultrasonic vibration can suppress the formation of burr by reducing the influence of size effect and changing the way of chip formation. When ƒ z is larger, ultrasonic vibration can suppress the formation of burr by changing the instantaneous cutting thickness and improving the ability of chip breaking.
Combined with the formation characteristics of chip and burr, it can be found that burr and chip are closely related, and improving the ability of chip breaking plays an important role in burr suppression. Figure 17 shows the burr morphology of TMM and UVAMM. It can be seen from Fig. 17a, due to the high plasticity of the material, the material is continuously accumulated and bent under the influence of cutting force, thus forming a spiral burr with wide texture on the down milling side. Due to the influence of extrusion effect, the burr with dense texture and flaky shape is formed on the up milling side. After ultrasonic vibration is applied, due to the influence of high-frequency vibration, the burr with dense texture and tearing shape is produced on the down milling side and the burr with dense texture and flocculent is produced on the up milling side. There are the ductile surfaces and fractured surfaces with dimples on the burrs according to the fracture mechanics [6]. With the increase of A, especially when A/ƒ z > 1/2, the fractured area with dimples on the burr increases and crack propagation on the burr becomes more severe, as shown in Fig. 17b and Fig. 17c. The materials can be destructed when the impact stress exceeds its critical stress of the materials. The destruction of materials is easier with the increase of amplitude during the vibration machining process because of the enhancing impact with high frequency, which can profit the transition from the ductile to the brittle fracture and form more broken burr. Chen et al. [37] also found that UVAMM is beneficial to the generation of cracks and the destruction of materials.

Conclusions
In this study, Inconel718 was machined by TMM and UVAMM, and the influence of ultrasonic vibration on burr was analyzed. The main findings are summarized as follows: (1) When the ratio of A to ƒ z is greater than 0.5, continuous cutting changes to intermittent cutting. At this time, the spiral chip gradually changes into C-type chip. (2) Combining trajectory simulation and cutting experiment, it is found that UVAMM can effectively reduce the chip size and burr size. By observing the change of chips and burrs, it is found that there is a related relationship between chip and burr. Under the conditions of low n, large ƒ z , and large amplitude, the burr suppression is more obvious. Data availability Not applicable.
Code availability Not applicable.

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Competing interests
The authors declare no competing interests.