Mechanism analysis and experimental study of serrated chip formation based on ABAQUS

Zhen-zhong HE

（Chengdu Industrial Vocational Technical College，Chengdu 610000，China）

Abstract：The formation of serrated chip will lead to vibration ofmachine tool，which will reduce the cutting performance of the tool and themachining quality of the workpiece.Therefore，it is necessary to analyze the formation mechanism of serrated chip，rationally optimize the cutting parameters and reduce the formation probability of serrated chip.In this paper，ABAQUS software was used to simulate themachining process of titanium alloy and the formationmechanism of serrated chip was analyzed.The influence of cutting parameters on the degree of serrated was discussed by simulation and experiment under different cutting conditions.The results show thatwith the increase of cutting speed and feed rate，the sawdust serration degree increases gradually.With the increase of cutting angle，the sawdust serration degree decreases gradually.The research results can be used to improve themachining quality and design the process parameters.

Key words：Chip，Sawtooth degree，ABAQUS simulation，Titanium alloy

1 Introduction

Titanium alloy and other plastic materials usually form serrated chips in high-speed cutting，which makes the cutting force produce high-frequency fluctuations，aggravate the vibration of themachine tool system，reduce the tool life，and also reduce the surface quality and processing efficiency［1-2］.In the process of titanium alloy cutting，the evolution process of serrated chip is very short.At present，themechanism of serrated chip formation is still controversial.Themain research results are divided into adiabatic shear theory and periodic brittle fracture theory.

The adiabatic shear theory holds that the thermal softening effect caused by plastic deformation is greater than that caused by hardening effect during the deformation of thematerial.That is to say，the serrated chip is caused by plastic deformation under high strain，high strain rate and high temperature.

According to the periodic fracture theory，the reason of serrated chip is that the stress is greater than the tensile strength in the process of deformation，and the crack propagates from the free surface of the chip to the cutting edge，so as to produce periodic fracture.In order to analyze the causes and influencing factors of serrated chip formation and improve the machining quality and cutting efficiency of titanium alloy，this paper used ABAQUS software to simulate the machining process of titanium alloy.The formation mechanism of serrated chip during titanium alloy cuttingwas analyzed by using Johnson-Cook materialmodel.Simulation and experimental study were carried out on the serrated degree of the chip under the cutting condi-tions of different tool rake angle，cutting speed and feed rate.It provides theoretical basis for selecting cutting parameters reasonably，improving machining surface quality and tool life.

2 Form ing m echanism and geom etric characterization of serrated chip

2.1 Mechanism of serrated chip formation

In the process of cutting，when the cutting speed increases，the chip morphology usually changes from strip chip to serrated chip.At this time，the traditional shear slip theory cannot be used to explain the formation mechanism of serrated chip under high-speed cutting condition.Therefore，the formationmechanism of serrated chip is analyzed by using thermal shear theory.In cutting process，with the increase of cutting speed，the strain rate of the material in the first deformation zone increases approximately linearly［3-4］.When the cutting speed reaches a certain critical value，due to the cutting speed is toofast and the thermal conductivity of thematerial is poor，the heat will be concentrated in a very narrow rangewithout enough time to pass out.Asmostof the heat is retained in the chip，thermal softening effect occurs in the first deformation zone.Under the action of the tool rake angle，shear instability occurs in the adiabatic shear zone，resulting in the generation of serrated chip.The principle is shown in Fig.1.

Fig.1 Fo rm ation p rocess of serrated chip

2.2 Geometric characterization of chipmorphology

2.2.1 Chip thickness compression ratio

The ratio of chip thickness to cutting depth is used to express the degree of chip deformation，also known as chip thickness compression ratio［5-6］：

Where：hchis chip thickness；hDis cutting depth；φis shear angle；γ0is tool rake angle.

Generally，the larger the ratio，the greater the chip thickness and deformation.The compression ratio of chip thickness directly reflects the deformation degree of chip and is used tomeasure the deformation of the chip，as shown in Fig.2.

Fig.2 Schem atic diagram of chip deformation degree

2.2.2 Degree of serration

In order to study the serrated degree of chip，the geometric characterization model of serrated degree is established［7］：

Where：Gsis the serrated degree of the chip；h1is distance from the bottom of the chip to the top of the serrated；h2is the distance from the bottom of the chip to the root of the serrated.

3 Estab lishm ent and analysis of finite elem entm odel

3.1 Simulation model

Through the study of chip forming mechanism，it can be known that cutting speed，feed rate and tool rake angle are important factors affecting the serrated degree of chip.For further analysis，the cutting simulation model of titanium alloy workpiece was established by ABAQUS simulation software.It was assumed that the tool was treated as rigid body without deformation，and the change ofmetallographic structure caused by temperature changewas ignored［8-9］.The tool rake angle was 0°，the flank angle was 10°and the radius of tool tip arc was 0.003mm.In order to speed up the simulation convergence speed，the local refinementmethod was adopted tomesh the workpiece，and the sparsemesh processingwas carried out for the workpiece base part，as shown in Fig 3.The mesh type was quadrilateral element，and the element type was coupling analysis of temperature field and displacement field.

Fig.3 Cutting simu Iation m ode I

3.2 Establish of the constitutivemodel

The constitutive equation of titanium alloy material needs to describe the basic properties of thematerial，and can accurately reflect the high temperature and large strain rate of thematerial in the process of cutting.Therefore，the constitutive equation of Ti-6Al-4V titanium alloy was established by Johnson-Cook model.The J-Cmodel can reduce the amountof simulation calculation and speed up the calculation，and its basic expression is［10］：

Where：σis the flow stress of workpiecematerial；ε is equivalent plastic strain；˙εEquivalent plastic strain rate；˙ε0is the equivalent reference strain rate；Tris room temperature；Tmis themelting point temperature of thematerial；A，B，C，n，mrepresents the initial yield strength，strain hardening modulus，strain rate hardening coefficient，strain hardening coefficient and thermal softening coefficient respectively.

In the process of finite element simulation，the J-C constitutivemodel parameters of titanium alloy workpiece are shown in Table 1［11］：

Tab Ie 1 JC Constitu tive m ode Iparam eters

The chemical composition of Ti-6Al-4V titanium alloy is shown in Table 2.

Tab Ie 2 Chem ica Icom position of Ti-6AI-4V titanium a IIoy

3.3 Analysis of simulation results

Firstly，the influence of cutting speed on serrated chip was analyzed.The cutting speed is 200 mmin，300 m/min，400 m/min respectively，and the rake angle of the tool is 5°.The cutting thickness is 0.5 mm，and the cutting width is 6 mm.The results are shown in Fig 4.Then，the influence of tool rake angle on serrated chip was analyzed.The tool rake angle is 10°and 5°to 10°respectively，and the cutting speed is 300 m/min.The other conditions remain unchanged.The simulation results are shown in Fig 5.

Fig.4 Effect of cutting speed on chip

Fig.5 In fIuence of too I rake ang Ie on chip

The simulation results show that there are serrated chip in the process of high-speed cutting.With the increase of cutting speed or the decrease of tool rake angle，the serrated degree of chip increases continuously.This is because the larger cutting speed and the smaller tool rake anglewill lead to the increase of chip strain rate.Due to the increase of chip deformation and the sharp increase of cutting temperature，the phenomenon of concentrated shear slip is produced，and then the serrated chip is generated.

4 Experim ental tests

Cutting experiment of Ti-6Al-4V titanium alloy was carried outwith CK6180 CNC lathe.The main motor power of themachine tool is11 kW，and themaximum speed of the spindle is 1 000 r/min.The cutting tool used is PCD turning tool，which the arc of tip is 55°.The rake angle and flank angle are 0°and 10°respectively.Themain experimental equipment is shown in Fig.6.The workpiece size isφ60×300 mm，and the material has high strength and toughness.Serrated chips are easily produced in high-speed machining.The influence of different cutting parameters on the sawtooth degree of chip is analyzed by testing under differentworking conditions.

Fig.6 Experim enta Iequipm ent

4.1 Influence of cutting speed on serrated degree

The cutting experiments was carried out under different cutting speeds and compared with the simulation results.The cutting speed is100m/min，200m/min，300 m/min，400 m/min respectively.The tool rake angle is 5°and the feed rate is 0.2 r/mm.Other parameters are unchanged.The test results are shown in Fig.7.

Fig.7 In fIuence of cutting speed on serrated degree

The results show that the serrated degree of chip increaseswith the increase of cutting speed.The error between the simulation and experimental results is less than 10%，which verifies the correctness and reliability of the finite elementmodel.

4.2 Influence of feed rate on serrated degree

Cutting experiment was carried out under different feed rate conditions，and the influence of feed rate on serrated degree was analyzed.Three groups of experimentswere carried out with cutting speed of 200 m/min 300 m/min and 400 m/min respectively.During the experiment，the feed rate increased continuously，which was 0.1 mm/r，0.15 mm/r，0.2 mm/r and 0.25 mm/r respectively.The tool rake angle was 5°.The experiment results are shown in Fig 8.

Fig.8 In fIuence of feed rate on serrated degree

The results show that，under the same conditions，with the increase of feed rate，the degree of serrated cutting increases.When the cutting speed is 400m/min，the degree of serrated is the largest.This is because the increase of cutting speed，the adiabatic shear phenomenon is gradually fierce，resulting in the decrease of the contact length between the serrations and the more obvious degree of serration.If the cutting speed continues to increase，the adjacent serrated will be separated.

4.3 Influence of tool rake angle on serrated degree

The cutting experiment was carried out under the condition of different tool rake angle，and the influence of tool rake angle on serrated degree was analyzed.Three groups of experiments were carried out with cutting speed of 200 m/min 300 m/min and 400 m/min respectively.In the process of experiment，the rake angle of the tool was adjusted to-10°，-5°，0°，5°and 10°respectively，and the feed ratewas0.2 mm/r.The experiment results are shown in Fig.9.

It can be seen that at the same cutting speed，the serrated degree of chip decreases with the increase of tool rake angle in the range of the process condition.

Fig.9 In fIuence of too Irake ang Ie on saw tooth degree

5 Conclusions

As a typical hard tomachinematerial，titanium alloy is easy to produce serrated chip during cutting.Serrated chip will cause high frequency vibration of cutting force，which will affect themachining accuracy and tool life，and will have a serious negative impact on the processing efficiency and production efficiency.Therefore，reducing the generation probability of serrated chip is of great significance to improve the cutting performance.

（1）The cutting simulation of titanium alloywas carried out by using ABAQUS software to simulate the formation mechanism of serrated chip.The simulation and experimental research were carried out under different cutting conditions to discuss the influence of cutting parameters on the serrated degree of titanium alloy.

（2）The results indicate that with the increase of cutting speed and feed rate，aswell as the decrease of tool rake angle，the serrated degree of chip increases continuously.This is because the change of the above parameterswill lead to the increase of chip strain rate，the adiabatic shear phenomenon becomes more and more severe，and the cutting temperature rises sharply，so that the chip deformation is intensified.As a result，the phenomenon of concentrated shear slip is generated，resulting in serrated chip.The cutting speed has the greatest influence on the degree of serration，which leads to the decrease of the contact length between the serrations，and the degree of serration is more obvious.If the cutting speed continues to increase，the adjacent serrationswill be separated.According to the research results，reasonable matching and optimization of cutting parameters can effectively reduce the probability of serrated chips.Therefore，this paper has a certain guiding role in improving the machining quality of workpiece and designing process parameters.