Experimental investigations into effect of milling parameters on milling force and burr micromorphology for Zr-based bulk metallic glass by using groove milling

Zr-based bulk metallic glass is an amorphous alloy. In order to study the burr problem in the groove milling process of Zr-based bulk metallic glass, the milling force in the process is studied experimentally. Then the effect of milling parameters on the burr of Zr-based bulk metallic glass was studied, and the relationship between milling force and burr was analyzed. In order to further study the milling burr of amorphous alloy, under the same test parameters, the single crystal alloy material (nickel base single crystal superalloy DD5) and the traditional polycrystalline metal material (304 stainless steel) were selected for comparative test and analysis. The results show that Zr-based bulk metallic glass has good groove milling machinability. The test results provide some experimental basis for groove milling of Zr-based bulk metallic glass.


Introduction
Nowadays, many industries such as electronics, optics, aerospace, medicine, and biotechnology demand microproducts and components. The miniaturization of components has some advantages, such as portability and reduction in space, power, energy consumption, materials, and costs. To meet this demand, it is mandatory to develop an appropriate manufacturing process. This manufacturing process must be used not only for a variety of workpiece materials but also for manufacturing of three-dimensional (3D) features. Laser machining, focused ion beam (FIB) machining, electrochemical machining, and electrodischarge machining (EDM) are used to manufacture micro parts; however, these non-traditional fabrication methods have some drawbacks due to the using of limited workpiece material (such as silicon), poor productivity, and high cost. Due to these negative aspects of non-traditional fabrication methods, micromechanical machining (or micromachining) such as micromilling, microdrilling, microturning, and microgrinding has gained increasing interest in recent years. Micromechanical machining is one of the production methods for microcomponents, which is the mechanical removal of workpiece materials using miniature cutting tools.
Materials are the foundation and core of technological innovation. The invention and application of new materials have always led the global technological innovation, promoted the transformation and upgrading of high-tech manufacturing industry, and gave birth to many new industries. Amorphous alloys, high entropy alloys, and metal matrix ceramic composites are new metal alloy materials developed in recent years. More and more scholars in the field of mechanical manufacturing begin to study the processing mechanism  of these new materials and the preparation technology of parts with various machining methods [1][2][3]. Chen et al. [4] used the low-speed cutting method to conduct experimental research on Zr-based bulk metallic glass, and obtained the relevant processing characteristics of Zr-based bulk metallic glass under the condition of lowspeed machining. Ding and Wang et al. [5] used highspeed machining method to study the light emission phenomenon of zirconium-based bulk metallic glass, and conducted in-depth research on the mechanism of light emission and its control. Zhao et al. [6] established the cutting analysis model of metallic glass based on Mohr Coulomb criterion. Maroju et al. [7] used the high-speed milling method to conduct an experimental study on the surface microstructure of zirconium based bulk metallic glass after processing. Chau et al. [8] study the formation of twin serrated chips in small shear bands of bulk metallic glass by using precision micromachining. Wu et al. [9] studied the mechanical nanocutting of metallic glass by means of atomic simulation. Xiong et al. [10] used the single point diamond turning method to carry out experimental research on the machinability of bulk metallic glass and the generation mechanism of surface Memphis. Dhale et al. [11] conducted an experimental study on the chip formation mechanism and surface morphology of zirconium-based bulk metallic glass in orthogonal cutting. The reason why Zr-based bulk metallic glass is important is that it has both theoretical and practical significance. It not only enriches the theories of fracture, strengthening, and deformation in metalology and metal physics, but also enriches the theories of phonons, electrons, and dislocations in solid physics. It has great potential to become a new generation of structural materials.
There are several definitions for burrs, but they all describe the same phenomenon. Burrs are undesired but mainly unavoidable. A burr is a material accumulation, which is created on the surface during the manufacturing  of a workpiece. It extends over the intended and actual workpiece surface and has a slightly higher volume in comparison with the workpiece [12]. Burrs are uncut material remaining on the workpiece after being machined. Burrs occur at the workpiece surface in cutting as well as in shearing operations at the workpiece edges. Further, laser machining can lead to burrs as well. This essay focuses on burrs of machining and shearing processes.
Burrs are of great industrial relevance as they interfere with the workpiece performance and functionality. Ideally, workpieces would be free of burrs, but, as this is often not the case, burrs can only be reduced either by changing the machining parameters, tool path, or tool. Alternatively, the burrs will have to be removed by time consuming and expensive deburring processes.
Burrs are an economically very important issue in many machining operations. Therefore, a lot of research has been done in the last 50 years to understand, control, and minimize burr formation. The first published works on burrs cover burr formation in punching. Pekelharing [13] was the first to describe burr formation mechanisms in metal cutting processes. He described the chip formation process and closely interlinked burr formation to chip formation. First fundamental work describing an analytical model of burr formation, which enables to predict burr properties, was done by Gillespie et al. [14] published in 1976. After a basic understanding of the burr formation mechanisms was gained, research activities turned to the topic of deburring. In 2009, CIRP published a keynote paper on burrs by Aurich et al. [15]. The paper gives a review of the topic of burrs in machining operations.

Experimental materials
The test material Zr-based bulk metallic glass used in the test and its material structure are shown in Fig. 1a. The

Experimental equipment and tools
The groove milling for Zr-based BMG is conducted by using the ultra-precision lathe, as shown in Fig. 2. The milling groove contour and milling groove burr for Zr-based bulk metallic glass were observed by Ultra Plus-field emission scanning electron microscope ( Fig. 3) and VHX-1000E microscope system (Fig. 4). In the experiment, 4-edge milling cutter was used, and the helix angle is 45°.

Methods and parameters
The single factor test method was adopted in the test. The three factors are the height of milling groove, cutting speed, and workpiece feed speed. The test methods and parameters are shown in Table 1.

Milling force on milling cutter tool
The force depends on the amount of material which has to be removed by the teeth. Again the first analysis is being done in a plane (Fig. 5b). The effective direction of motion of the tip of the cutting edge is tangential to the trochoid, and the effective cutting depth is perpendicular to the effective direction of motion. The effective cutting depth can be calculated for each point by finding the intersection point between the line along the effective cutting depth and the previous trochoid, as shown in Fig. 5.
The extension of the 2D examination to the 3D milling cutter is shown in Fig. 6. Due to the helix angle, each point of the cutting edge is at a different rotational position on the tool and on a different height. So for an analytical calculation, the cutter has to be divided into height increments, and for each increment, the effective cutting depth and thus the resulting force increment has to be calculated.
Milling force and milling temperature will directly affect the size and morphology of milling groove burr. This paper will analyze the milling groove burr from milling force.
Firstly, the effect of milling groove height on milling force was investigated. It can be seen from Fig. 7 that with the continuous increase of milling groove height, the milling forces in three directions are increasing, the milling forces in X direction and Y direction are obviously increasing, and the milling forces in Z direction do not change significantly. This is because when the cutting speed and workpiece feed speed remain unchanged, the contact area between the cutting edge of the milling cutter and the metallic glass material increases, resulting in the milling force increasing with the increase of the milling groove height in the X direction and Y direction. In the Z direction, the milling force is not directly related to  the milling groove height, so the milling force remains basically unchanged. The milling force in Z direction is produced by the contact between the end face of the milling cutter and the bottom of the milling groove, which is not directly related to the generation and morphology of burrs. Figure 8 is an experimental study on milling force of single crystal metal material (Ni-based single crystal superalloy) and traditional polycrystalline metal material (304 stainless steel) under the same milling conditions. It can be seen from Fig. 8 that the milling force of Ni-based single crystal superalloy does not change significantly with the change of milling groove height, and it is stable between 9 and 12N. With the increase of milling groove height, the milling force of 304 stainless steel increases, and the change trend is similar to that of Zr-based metallic glass.
Secondly, the effect of workpiece feed speed on milling force was experimentally investigated. It can be seen from Fig. 9 that with the increase of workpiece feed speed, the milling force in X direction and Y direction first increases, then decreases and then increases. Figure 10 shows the effect of workpiece feed rate on milling force of Ni-based single crystal superalloy and 304 stainless steel. The experimental results show that the effect of workpiece feed speed on the milling force of Ni-based single crystal superalloy, and 304 stainless steel is not obvious. With the continuous increase of workpiece feed It can be seen that the milling force of Zr-based bulk metallic glass is sensitive to the workpiece feed speed, showing sinusoidal periodic changes in the whole range of the parameters of the experiments.
Finally, the effect of cutting speed on milling force was experimentally investigated. It can be seen from Fig. 11 that the milling force increases first and then maintains a stable trend with the increase of cutting speed. Figure 12 shows the effect of spindle speed (cutting speed) on the milling force of Ni-based single crystal superalloy and 304 stainless steel. Under the same experimental conditions, the milling force of the two materials does not change much with the increase of cutting speed. The milling forces in X direction and Y direction are very close. It can be seen that the milling force of Zr-based bulk metallic glass is more sensitive to the change of cutting speed than that of single crystal Ni-based superalloy and polycrystalline stainless steel.
From the above milling force test results, it can be seen that within the range of test parameters in this paper, the milling force of Zr-based bulk metallic glass is more sensitive to the changes of milling groove height, workpiece feed speed and cutting speed than that of single crystal materials (Ni-based single crystal superalloy) and polycrystalline metal materials (304 stainless steel). This shows that the change of test parameters will lead to the change of cutting temperature, which will directly lead to the material properties of Zr-based bulk metallic glass. Because amorphous alloys are very sensitive to temperature changes, the increase of temperature may lead to the gradual transformation of the brittleness of amorphous alloys into ductile materials, which will lead to the plastic rheology of amorphous alloys, and finally lead to the increase of milling force. The increase of force will also affect the material properties of amorphous alloys, because the effect of temperature and force on amorphous alloys is equivalent. Under the combined action of temperature and force, the machinability of Zr-based bulk metallic glass (amorphous alloy) will change, which will have a certain impact on the burr of milling groove. Gillespie [16] introduces four types of burrs: Poisson burr, rollover burr, tear burr, and cutoff burr, as shown in Fig. 13. The ISO 13715 standard defines the edges of workpieces as sharp, free of burrs, rounded, chamfered, or with burr (Fig. 14). Nevertheless, many companies use an in-house classification, as an overall accepted burr classification is lacking [15]. A lot of research focuses on understanding burr formation and revealing the parameters influencing burr formation. Pekelharing is the first to publish results on the investigation of burr formation. He interlinks burr formation to chip formation, as the burr formation depends on the chip formation mechanism [13]. Gillespie states six physical processes which form burrs. The processes firstly lateral flow, secondly bending of material, and thirdly tearing of chips from the workpiece result in plastic deformation of workpiece material. Finally, the redeposition of material occurs in recasting processes. The fifth process regards the incomplete cutoff of material. The last process treats burrs produced in molding or shaping processes, when the material flows into cracks [16]. Hashimura et al. have issued a schema of burr formation [17]. In his model, the burr formation mechanism is influenced by cutting conditions, tool and workpiece geometry, as well as by the mechanical properties of the workpiece material. Beier [12], and Thilow et al. [18] did a lot of work on burr formation. They all observed that a burr occurs at the tool entry or exit if the workpiece material evades the cutting process.

Basic theory of burr in groove milling
To gain a better understanding of the burr formation process and to be able to predict burr formation, the finite  [19], the state of research and future developments in modeling and simulation of burr formation are highlighted. Figure 15 is the test result of the influence of slot milling height on the morphology of milling slot burr. It can be seen from Fig. 15 that the height of the milling groove is the smallest, and the burrs on the side wall of the milling groove are the least. With the increase of milling groove height, the burr increases gradually, the burr morphology becomes more complex, and the burr volume left on the side wall of milling groove becomes larger and larger. It can be seen from Fig. 15 that the contact force between the milling cutter and the workpiece gradually increases with the increase of the height of the milling groove. The increase of milling force will lead to the increase of milling temperature. The increase of temperature and force will have an impact on the machining of block metallic glass, which is the reason for the gradual increase of milling groove burr with the increase of milling groove height.  Figure 16 is the test result of the effect of workpiece feed speed on the burr morphology for milling groove. It can be seen from Fig. 16 that with the increase of workpiece feed speed, the milling burr of Zr-based bulk metallic glass first increases and then decreases. When the workpiece feed speed is 30 μm/s, the burr of milling groove starts to increase slightly, and at 50-60 μm, the burr of milling groove is reduced. This corresponds to the previous milling force test. This shows that the milling force will affect the burr size of Zr-based bulk metallic glass. Under the combined action of force and temperature, the plastic rheology of amorphous metallic glass will occur, which will affect its material processability. Figure 17 is the test result of the effect of spindle speed (cutting speed) on the burr morphology for milling groove. The experimental results show that the change of cutting speed has no obvious effect on the burr of Zr-based bulk metallic glass milling groove within the range of test parameters. This result is basically consistent with the previous milling force test results.

Macro-and micromorphology of burr in milling groove
Zr-based bulk metallic glass is an amorphous alloy. The effects of temperature and force on its plastic flow are equivalent. We know that it is difficult to control the milling force in the machining process. Therefore, reducing the machining temperature has become the most feasible and effective machining method. In order to further study the milling burr of Zr-based bulk metallic glass, coolant was added in the milling process to observe the effect of temperature reduction on the burr. Figures 18, 19, and 20 are the experimental results of the effect of coolant on the milling burr of Zr-based bulk metallic glass. It can be seen from the test results that the number of burrs in the milled groove machined with coolant is significantly reduced, and the quality of the milled groove contour is significantly improved. The addition of coolant can properly reduce the temperature in the milling area, effectively reduce the occurrence of plastic rheology, and prevent the excessive softening of Zr-based bulk metallic glass. At the same time, the coolant plays a certain role in washing the burrs of the milling groove. Figure 18 shows the effect of groove milling height on groove burr, Fig. 19 shows the effect of workpiece feed speed on groove burr, and Fig. 20 shows the effect of cutting speed on groove burr. The effect of the test parameters on the burr of Zr-based bulk metallic glass is basically consistent with that of dry milling.
In order to better study the milling burr of amorphous alloy, the milling experiments of Ni-based single crystal superalloy DD5 with the same test parameters were carried out in this paper. Figures 21, 22, and 23 are the test results of the effect of milling groove height, workpiece feed speed, and cutting speed on its burr respectively. It can be seen from the test results that the milling burr of Ni-based single crystal superalloy DD5 is more and larger than that of Zr-based bulk metallic glass under the same test parameters. The burr is obvious on the milling groove contour. It can be seen from Fig. 21 that the effect of milling groove height on burr is not obvious, and the burr is in curly sheet shape. Figure 22 shows the effect of workpiece feed speed on the burr of Ni-based single crystal superalloy DD5 during milling. With the increase of workpiece feed speed, the burr of curl flake increases gradually. Figure 23 shows the effect of cutting speed on the milling burr of Ni-based single crystal superalloy DD5. It can be seen from the figure that with the increase of cutting speed, the curly flake burr gradually increases and changes obviously. The milling burr of Zr-based bulk metallic glass is more strip and broken flake burr.
Finally, groove milling tests with the same test parameters were carried out on 304 stainless steel, a traditional polycrystalline metal material. The test results are shown in Fig. 24. It can be seen from the figure that the quality of groove milling is not very high when the same test parameters are used for groove milling. Although the number of burrs on the milling groove contour is small, the shape and dimension accuracy of the milling groove should be relatively low from the perspective of the contour quality of the whole milling groove. A small amount of burrs after machining are in narrow strip shape.

Conclusions
(1) Within the range of test parameters in this paper, the changes of milling height and workpiece feed speed have a certain impact on the milling force and milling burr of Zr-based bulk metallic glass, and the change trend is obvious. The change of spindle speed (cutting speed) has no obvious effect on the milling force and milling groove burr. If Zr-based bulk metallic glass is to be deeply milled, and at the same time, the generation of milling groove burrs should be minimized, the workpiece feed speed can be selected at 50 μm/s-60 μm/s, Select a smaller machining depth for each tool walk and carry out multiple tool walks, and select an appropriate cutting speed at the same time. (2) When dry groove milling is applied to Zr-based bulk metallic glass, the milling force has a certain effect on the burr. The change of force will also affect the change of processing temperature. Under the combined action of temperature and force, the milling burr of Zr-based bulk metallic glass will be affected. In order to control the processing temperature, coolant is introduced into the slot milling process. The experimental results show that properly reducing the milling temperature is beneficial to reduce the burr of milling groove of Zr-based bulk metallic glass.

Fig. 23
Effect of cutting speed on burr for single crystal nickel base superalloy DD5 (h = 300 μm; f = 40 μm/s) a n = 2 × 10 4 r/min, b n = 3 × 10 4 r/ min, c n = 5 × 10 4 r/min, d n = 6 × 10 4 r/min (3) From the point of view of groove milling burr, compared with Ni-based single crystal superalloy DD5 and traditional polycrystalline metal material 304 stainless steel, Zr-based bulk metallic glass seems to have better groove milling contour quality under the same milling parameters, and the morphology types of burr are different. Most of the burrs produced during groove milling of Zr-based bulk metallic glass are thin strip burrs, the type of burr is basically Poisson burr and rollover burr. While most of the milling burrs of Ni-based single crystal superalloy DD5 and 304 stainless steel are curly flake burrs, the type of burr is basically rollover burr. From the perspective of burr generation, compared with traditional metal materials (304 stainless steel) and single crystal alloy materials (nickel based single crystal superalloy DD5), zirconium-based bulk metal glass has good slot milling machinability.

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