Experimental study of chip formation and cutting force during machining Zr41．2 Ti13．8 Cu12．5 Ni10．0 Be22．5 bulk metallic glass*

Meng-yang QIN，Ya-jun LIU，Lan-ying XU，Yong-shun LUO College of Mechatronic Engineering，Guangdong Polytechnic Normal University，Guangzhou 50635，China;School of Mechanical and Automotive Engineering，South China University of Technology，Guangzhou 50640，China

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 strainrate 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 BMGturning 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 BMGchips 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．

Figure 2．SEM micrographs of Vit1 BMG chips machined at cutting speed 0．4 m/s

In order to assess the relative thermal stability and glass forming ability among the chip samples of different cutting depth，the Tgand Tx，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 temperature Tg，crystallization temperature Tx1and Tx2are 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 forces Fx，Fyand Fzwith different cutting depth are shown in Figure 4．The main cutting force Fzgets increased with the increase of cutting depth．However，Fxand Fyare 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 VScutting 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 force Fsand chip inertia force Fm，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 force Fz．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，Fxand Fyhave 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 force Fzgets increased with the increase of cutting depth．However，both Fxand Fyhave little change with the increase of cutting depth．

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