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 plas ticity and size effect, relatively large burr will be produced. In order to solve the burr proble m in micromilling, ultrasonic vibration in feed direction is applied to the workpiece to complet e vibration cutting. Combined with trajectory simulation and cutting experiment, the burr forma tion 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(ƒ z ) is greater than 0.5, continuous cutting changes to intermittent cutting. Compared with TMM, UVAMM improves ch ip breaking ability, facilitates the propagation of burr crack and effectively inhibits the formatio n of burr. However, due to the influence of cutting edge radius, A /ƒ z should be set larger. Wh en the chip breaking condition is reached, the burr shape is usually tearing or flocculent. Unde r the conditions of low speed( n ), large ƒ z 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].
that burrs tend to appear on the top of the groove at lower speed(n), lower feed per tooth(ƒz) and higher cutting depth(ap). Lee et al. [8,9] found that large burrs are likely to occur when the cutting thickness is smaller than the cutting edge radius. Sun et al. [10] studied 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 ap, n and ƒz on the size of the burr, and optimized the process parameters reasonably. Mian et al. [12] found that the ratio of ƒz to the cutting edge radius is the most important factor in 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 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 process, surface defects and burrs are usually removed by postprocessing. 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 traditional micromilling(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 key for micromilling to improve the surface quality of the workpieces and lower the machining cost.
Vibration assisted machining is a machining method in which external energy is applied to the tool or workpiece during the cutting process to generate high frequency vibration. At present, 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] studied 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 mechanism was analyzed during the ultrasonic vibration assisted micromilling(UVAMM). At the same time, the high-temperature nickel-based alloys is processed with TMM and UVAMM. Combined with the chip breaking conditions, the morphology and size change of chip and burr under different cutting parameters are analyzed.

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 hmin or even greater, chips begin to form and the material recovery phenomenon still exists. However, it will not return to the original position. 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 the equation, the chip length is proportional to ƒz.
Where hmin is the minimum cutting thickness; h is the instantaneous cutting thickness; re is the cutting edge radius; θ is the rotation angle; L is the chip length; r is the tool radius.

Trajectory equation of cutting edge
Compared with TMM, the application of ultrasonic vibration changes the trajectory of the tool tip, which in turn affects the way chips are formed. Therefore, the analysis of tool tip trajectory is the basis of chip formation. Fig. 4 shows the trajectory of two adjacent tool tips in a motion cycle in TMM. The solid line and the dashed line represent the movement trajectory of the tool tip 1 and the tool tip 2, respectively.
The vibration signal 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 vƒ is the feed rate; n is the spindle speed; r is the tool radius; λ is the ratio of the vibration frequency to the spindle rotation frequency; ɷ is the ultrasonic vibration angular frequency.

Experimental materials
The micromilling cutter is the cemented carbide double-edged 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, and its chemical composition is shown in Table 2.  Cutting edge radius (μm) 5 First rear angle of base edge (°) 9 Helical angle (°) 36 Second rear angle of base edge (°) 16

Experimental equipment and arrangement
The  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 position, and the average width and height of burr were calculated according to Eq. (9) and (10).

Trajectory of cutting edge
In the machining process of UVAMM, ƒz and A are the most important parameters to change the formation mechanism of chip and burr, so it is very important to judge the relationship between ƒz 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 also be seen from Fig. 8(a-c) that when the A is 3 μm, there are the significant intersections of tool tip trajectory between tool tip1 and tool tip2, which indicates that the chip breaking is the result of the combined effect of the A and ƒz. It can be seen from the tool tip trajectory in Fig. 8(d-f) that when the A increases from 0 to 3 μm, the tool tip trajectory begins to intersect, and the chips may or have been broken. As the A increases from 3 to 6 μm, there are the significant intersections of tool tip trajectory between tool tip1 and tool tip2, 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/ƒz>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.

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.  between Tool tip1 and tool tip2 on the condition of the amplitude of 6 and 9um 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 time of ultrasonic vibration per unit distance decreases, which lead to decline the vibration effect of the cutting process. ped chips or serrated chips are easily generated with a small ƒz. However, when ƒz is more th an 5 μm/z, long and spiral chips are produced. The reason is that when ƒz is small, the mater ial 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 cutt ing thickness, reduces the influence of size effect, and makes chip formation and breaking easi er. It can be seen from the figure that the number of C-type chips increases after applying vib Chip length(μm) n(×1000r/min) ration. When A is 3 μm and ƒz 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 co mpletely transformed into C-type chip. The larger the A is, the more broken the chip is. Fig. 12 shows the variation of chip size of TMM and UVAMM under different ƒz. When the ultrasonic vibration is not applied, the chip size increases with the increase of ƒz. When ƒz 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 increase in TMM when ƒz is larger than 6 μm. It is also found that the trend of chip size is similar when A is 6 and 9um, which indicate that the larger A is beneficial for the chip-broken. When ƒz 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 ƒz 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 ƒz 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 taken 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.  Chip length(μm) n. When the A is 3 μm, the variation trend of burr size of UVAMM is the same as that of TMM. 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 reason is that the application of ultrasonic vibration can increase the chip breaking frequency and reduce the chip size, thus reducing the burr size. into larger strip burr. The reason is that with the increase of ƒz, the volume of material remov ed 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 chi p 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.  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. With the increase of A, the damage effect of the cutting tool on the material is more severe, and the cracks at the edge of the burr continue to expand, thus forming more broken burr, as shown in Fig. 17(b) and Fig. 17(c). Chen et al. [34] found that UVAMM is beneficial to the generation of cracks and the formation of chips in the cutting area. This conclusion can explain the formation of burr morphology mentioned above.

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 ƒz 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 an relate relationship between chip and burr. Under the conditions of low n, large ƒz and large amplitude, the burr suppression is more obvious.
(3) When ultrasonic vibration is applied, denser textures and longer cracks appear on the burr surface. With the increase of amplitude, the burr suppression effect is more obvious.