Meng-yang QIN , Ya-jun LIU, Lan-ying XU, Yong-shun LUO

1College of Mechatronic Engineering, Guangdong Polytechnic Normal University, Guangzhou 510635, China;2School of Mechanical and Automotive Engineering, South China University of Technology, Guangzhou 510640, China

Experimental study of chip formation and cutting force during

machining Zr41.2Ti13.8Cu12.5Ni10.0Be22.5bulk metallic glass*

Meng-yang QIN?1, Ya-jun LIU2, Lan-ying XU1, Yong-shun LUO1

1CollegeofMechatronicEngineering,GuangdongPolytechnicNormalUniversity,Guangzhou510635,China;2SchoolofMechanicalandAutomotiveEngineering,SouthChinaUniversityofTechnology,Guangzhou510640,China

In order to understand cutting mechanism of amorphous alloy, chip formation and cutting force of Zr41.2Ti13.8Cu12.5Ni10.0Be22.5bulk metallic were investigated during machining process with different cutting depth by using SEM X-ray diffraction and cutting force measurement system. It is interesting that very good plasticity occurs during machining Zr-based BMG as compared to complete brittleness in tension. The chip morphology is unique and shows presence of plastic shear bands. The main cutting forceFzgets increased with the increase of cutting depth, however the values ofFxandFyare almost constant at the same time.

Metallic glass, Cutting, Chip formation, Cutting force

1.Introduction

The terminology “Metallic Glasses” or “Amorphous Metals” refers to a class of materials that exhibit a metastable amorphous atomic arrangement. Metallic glasses can be formed by a process of very rapid quenching of melt that “freezes” the microstructure and does not allow for the establishment of the classically observed crystalline structure. As a kind of Bulk metallic glass (BMG) alloys, Zr41.2Ti13.8Cu12.5Ni10.0Be22.5(Vit1) has structure with no long-range atomic order. A variety of rapid solidification techniques are used to produce BMG[1]. These materials can exhibit unique mechanical, magnetic, and corrosion properties and Vit1 is a kind of material with high hardness, high strength and almost no tension plasticity.

In the past, the requirement of rapid quenching has limited the site of metallic glass specimens and has hampered the potential of these solids for structural applications. However, recent advances in the casting of such solids have made it possible for the first time to produce large enough samples which are suitable for mechanical testing and applications. Although casting is the most commonly used method for mass-producing BMG components, machining can be an important process for the manufacture of BMG parts with stringent dimensional accuracy and surface roughness requirements. The workpiece material in machining is subject to high temperature and strain-rate deformation conditions, for example, strain rates up to 105s-1and heating rates over 105k/s can occur during chip formation[2]. Machining is therefore a simple method to investigate response of BMG under extreme deformation conditions.

There are some papers about chip formation, cutting force and tool wear in lathe-turning of the Zr52.5Ti5Cu17.9Ni14.6Al10. They paid attention to the cutting speed effect[3-4]. Cutting depth is another important parameter which affects on machining process of brittle materials mostly[5]. A concept called critical penetration depth (CPD) was described by Blackley and Scattergood for diamond turning of brittle materials[6]. After that, a lot of papers investigated CPD effect of precision machining by using different brittle materials[7-8]. But less work investigated the cutting depth effects on the machining process of bulk metallic materials. Chip formation during machining process is closed to material removal rate, cutting force, machined surface finish and tool wear[9].

In this study, the relationship between the cutting depth and chip formation was investigated. The experimental setup is first introduced. The chip morphology and cutting conditions are comprehensively studied. Cutting forces of BMG turning are analyzed. This study could be helpful to understand cutting mechanism of amorphous alloy.

2.Experimental procedure

The machining tests were conducted on a CA6140 lathe using a M10 WC-Co tool with a 0.5 mm tip radius and a 10orake angle. All tests were conducted without coolant, i.e., dry cutting. Turning tests were performed at 0.1 mm feed/rev, 0.4 m/s and six cutting depths, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6 mm. Samples were 10 mm diameter as-cast Vit1 BMG rods.

Machining chips were collected for SEM analysis. A Kistler piezoelectric dynamometer (type 9253A) and a charge amplifier were applied to measure cutting forces, and test data was transmitted to a computer with a data acquisition board.

3.Results and discussion

3.1.Chip morphology

Figure 1 shows that machining BMG produced a continuum chip type. Chip curl is another feature of the BMG chip. The two features indicate a large plastic deformation during machining process of Vit1 BMG. This is an interesting phenomenon which is different with machining other brittle materials.

Figure 1.The chip of Vit1BMG (cutting depth: 0.1～0.6 mm)

To understand chip morphology, the SEM Micrographs of BMG chips machined at 0.4 m/s with three different cutting depths are shown in Figure 2. Each chip sample has three levels of magnification to reveal the details of chip morphology. The serrated chip formation with the shape edge and shear localization is observed. The serrated BMG chips were produced during cutting processing and less localized serration formation occurs with the increase of cutting depth. Therefore, it suggests that the chip-removal process has less plastic shear deformation with the increase of cutting depth.

In order to assess the relative thermal stability and glass forming ability among the chip samples of different cutting depth, theTgandTx, which are derived from the DSC curves as shown in Figure 3, were compared. Higher the transition temperature means the thermal stability of the metallic glass is higher. The stability of the chips of BMG has little difference under the range of cutting depth from 0.1 mm to 0.6 mm. Glass transition temperatureTg, crystallization temperatureTx1andTx2are marked.

3.2.Cutting forces

The average force during the first three seconds when the tool contacts with the workpiece is used to represent the BMG. Cutting forcesFx,FyandFzwith different cutting depth are shown in Figure 4. The main cutting forceFzgets increased with the increase of cutting depth. However,FxandFyare almost constant with the increase of cutting depth.

Figure 3.DSC curves of the chip samples formed under different cutting depth.

Figure 4.Average cutting force VS cutting depth

During machining process, the force required to produce deformation on the shear plane is transmitted from the tool face to shear plane through chip. It consists of two parts of forces: shear forceFsand chip inertia forceFm, which causes the change of momentum when metal of cutting lay slips along the shear plane (as shown in Figure 5).

Figure 5. Cutting force model for machining process

During conventional machining operations on brittle materials, most of the material is removed by brittle fracture and it enables higher removal rates. Figure 6(b) shows the various stages of indentation. The material below the indenter is initially subjected to elastic deformation. As indentation continues, the material below is subjected to high hydrostatic pressure and hence an inelastic/plastic deformation zone could be produced.

Figure 6. Cutting model for Zr-based BMG

But it should consider the machining process of Zr-based BMG material by using the proposed system approach as shown in Figure 6 (a), where a cutting tool with a positive rake angle is adopted. The stress ahead the cutting edge will increase with the increase of cutting depth, so does the main cutting forceFz. When this stress reaches a particular limit, a crack forms in front of the cutting edge and no further increase in the applied load leads to the development of the crack. A part of the separated workpiece material located above the crack now serves as a cantilever. When the applied force reaches a particular limit, the fracture of workpiece material takes place at the cantilever support, as shown in Figure 6; there is no bending stress in the machining zone. As a result, final failure occurs due to pure compression of a fragment of the layer being removed. Therefore,FxandFyhave little change with the increase of cutting depth.

4.Conclusion

1) Machining of Zr41.2Ti13.8Cu12.5Ni10.0Be22.5BMG produces ductile chip removal which is the same as machining crystalline metals and it is consistent with the reported high fracture toughness.

2) Machining BMG occurs with a continuum chip. The chip morphology shows pronounced shear lamella separated by regions of shear localization. As the cutting depth increases, the SEM of chip suggests that the chip-removal process has less plastic shear deformation.

3) The main cutting forceFzgets increased with the increase of cutting depth. However, bothFxandFyhave little change with the increase of cutting depth.

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Zr41.2Ti13.8Cu12.5Ni10.0Be22.5大塊非晶合金的切屑和切削力試驗*

覃孟揚?1， 劉亞俊2， 徐蘭英1， 羅永順1

1.廣東技術(shù)師范學院 機電學院，廣州 510635；2.華南理工大學 機械與汽車工程學院，廣州 510640

為了解非晶合金的切削機理，對大塊非晶合金Zr41.2Ti13.8Cu12.5Ni10.0Be22.5進行了不同切削深度的切削實驗，然后用掃描電鏡、X光衍射儀和測力系統(tǒng)對切屑形態(tài)和切削力進行觀察和測量。實驗結(jié)果表明：Zr基非晶合金在受拉的時候，比全脆性材料有更好的塑性表現(xiàn)，其切屑形態(tài)獨特且具有塑性剪切帶特征；主切削力Fz隨切削深度的增加而增長，但Fx和Fy則幾乎沒有變化。

非晶合金；切削加工；切屑形態(tài)；切削力

TG136+.4

2014-02-25

10.3969/j.issn.1001-3881.2014.12.010

*Project supported by National Natural Science Foundation of China (No.51375101)

? Meng-yang QIN, PhD. E-mail: lp37213721@126.com