Tool wear during high speed turning in situ TiCp/TiBwhybrid reinforced Ti-6Al-4V matrix composite

Ge Yingfei,Xu Jiuhua,Huan Haixiang

aSchool of Mechanical Engineering,Nanjing Institute of Technology,Nanjing 211167,China

bCollege of Mechanicalamp;Electrical Engineering,Nanjing University of Aeronauticsamp;Astronautics,Nanjing 210016,China

Tool wear during high speed turning in situ TiCp/TiBwhybrid reinforced Ti-6Al-4V matrix composite

Ge Yingfeia,*,Xu Jiuhuab,Huan Haixiangb

aSchool of Mechanical Engineering,Nanjing Institute of Technology,Nanjing 211167,China

bCollege of Mechanicalamp;Electrical Engineering,Nanjing University of Aeronauticsamp;Astronautics,Nanjing 210016,China

Chipping,adhesive wear,abrasive wear and crater wear are prevalent for both the polycrystalline diamond(PCD)and the carbide tools during high speed turning of TiCp/TiBwhybrid reinforced Ti-6Al-4V(TC4)matrix composite(TMCs).The combined effects of abrasive wear and diffusion wear caused the big crater on PCD and carbide tool rake face.Compared to the PCD,bigger size of crater was found on the carbide tool due to much higher cutting temperature and the violent chemical reaction between the Ti element in the workpiece and the WC in the tool.However,the marks of the abrasive wear looked much slighter or even could not be observed on the carbide tool especially when low levels of cutting parameters were used,which attributes to much lower hardness and smaller size of WC combined with more significant chemical degradation of carbide.When cutting TC4 using PCD tool,notch wear was the most significant wear pattern which was not found when cutting the TMCs.However,chipping,adhesive wear and crater wear were much milder when compared to the cutting of titanium matrix composite.Due to the absence of abrasive wear when cutting TC4,the generated titanium carbide on the PCD protected the tool from fast wear,which caused that the tool life for TC4 was 6–10 times longer than that for TMCs.©2016 Chinese Society of Aeronautics and Astronautics.Production and hosting by Elsevier Ltd.This is an open access article under the CC BY-NC-ND license(http://creativecommons.org/licenses/by-nc-nd/4.0/).

1.Introduction

Titanium matrix composites(TMCs)are famous for their higher specific stiffness/strength,wear/corrosion resistance,anti-fatigue performance and better high temperature properties and are recognized as one of the new materials to replace titanium alloys.1–3As an example,rocket shells,missiles shell and tail,aircraft engine sheet structure and fan,compressor,turbine blades,etc.are more and more made of TMCs.4–6However,TMCs are also notorious for their bad machinability which represents itself in excessive tool wear,poor surface finish,low productivity and high machining cost.7–12A lot of articles focused on the research of machining on titanium alloy with Polycrystalline Diamond(PCD)tools.13–17However,merely several researchers have devoted the mselves into the study of machining TMCs using PCD tools.Bejjani and Shi8investigated the turning machinability of vol10%-vol12%TiCp/TC4 composites using PCD tool under the laser assisted machining(LAM)and found that tool life increased by up to 180%but surface roughness increased by up to 15%.Ge et al.9studied the cutting force,cutting temperature,and tool life when turning vol10%(TiCp+TiBw)/TC4 composites with PCD and carbide tools.Huan et al.10have evaluated the milling machinability on vol10%(TiCp+TiBw)/TC4 composites.The optimization of cutting parameters was carried out by Aramesh et al.11to find the optimum cutting conditions for surface roughness and tool wear rate.Tool life of physical vapor deposition(PVD)coated grades was estimated by Aramesh et al.12when turning vol10%-vol12%TiCp/TC4 composites at a speed of 60 m/min.Unfortunately,tool wear and the mechanisms were seldom studied in the above researches.The present study intends to enhance the understanding of the tool wear mechanisms when high-speed turning(TiCp+TiBw)/TC4 composites using PCD and carbide tools so as to provide a theoretical basis for the engineers to select suitable processing parameters and cutting tools.

Fig.1 Met allography of in situ vol10%(TiCp+TiBw)/TC4 composite.

2.Material and methods

Titanium carbide particles and titanium boride whiskers hybrid reinforced Ti-6Al-4V(TC4)matrix composite((TiCp+TiBw)/TC4)with the reinforcement volume fraction of vol10%was used as the workpiece.The in situ((TiCp+TiBw)/TC4)composites were prepared by the consumable vacuum arc melting method and the molar ratio of the two reinforcements was 1:1.The diameter of the titanium carbide particles is 1.5–20 μm and the length of the titanium boride whiskers is 35–80 μm,as shown in Fig.1.As a comparison,the TC4 matrix was also used as the workpiece.The chemical composition of TC4 matrix is listed in Table 1.

SupowerTMPCD and Kennamet alTMcarbide cutters were used in the turning tests.Tools and their specifications used in the experiments are tabulated in Table 2.All the cutting tests were performed on the SK50P CNC lathe under wet(water-based emulsion)cutting conditions.Cutting speed range and feed rate range were 60–120 m/min and 0.05–0.08 mm/r respectively.Depth of cut was kept constant at 0.5 mm.High speed turning tests arrangements are shown in Table 3.In Table 3,tool wear appearance and tool wear mechanisms are investigated at the cutting conditions of 60–120 m/min,0.08 mm/r and 0.5 mm(trials No.1-No.7,No.9-No.11 and No.13).The cutting parameters of 120 m/min,0.05 mm/r and 0.5 mm(trials No.8 and No.12)are used to compare the tool life of PCD tools when cutting(TiCp+TiBw)/TC4 and TC4 matrix material.Trials No.1,2 and No.7,9 are used to compare the tool life of PCD and carbide tools when turning(TiCp+TiBw)/TC4 composite at the cutting speed of 60 m/min and 120 m/min,respectively.

KH-7700 three-dimensional video microscope was used to capture and measure the tool flank wear.The failure criterion was reached when the value of the tool wear VB was bigger than 0.2 mm or VBmaxwas bigger than 0.4 mm according to the ISO 8688-2 standard(1989).The tools were examined on the scanning electron microscope(SEM,Hitachi S-3400)before and after being etched for 20–30 s using a solution of HNO3(4%)+HF(2%)+H2O(94%).Energy Dispersive Spectrometer(EDS)was used for tool wear mechanisms analysis.

Kistler 9272 piezoelectric dynamometer and the associated 5019A charge amplifier were used to measure the three components cutting forces.The workpiece(TMC)-tool(PCD)natural thermocouple was used to measure the turning temperature.18The thermal induced electromotive force signals corresponding to the cutting temperatures were recorded using a HP3562A dynamic signal analyzer.The cutting temperature was calculated through calibration of the measured electromotive force using a special calibration system.19

Table 1 Main chemical composition of TC4 matrix material.

Table 2 Tools and their specifications used in experiments.

Table 3 Experiment arrangements for tool wear investigation.

Fig.2 Appearance of chipping and peeling on tool cutting edge(v=100 m/min,f=0.08 mm/r).

Fig.3 Cutting forces and raw signals(v=100 m/min,f=0.08 mm/r).

3.Results

3.1.PCD tool wear when turning vol10%(TiCp+TiBw)/TC4 composite

3.1.1.Chiping on cutting edge and peeling on rake face

When high speed turning(TiCp+TiBw)/TC4 composite,chipping is one of the main wear patterns for PCD tools,as shown in Fig.2.There are three reasons for the chipping of PCD tool:(1)the stress concentration effect on the original defects of the cutting edge(Fig.2(a));(2)the very high specific cutting force on the cutting edge where the value could reach 350–600 N/mm according to Fig.3(a);(3)the big cutting vibration of which the amplitude was 30%of the mean value of the cutting force(Fig.3(b),Fc,Ffand Fpare the main cutting force,feed force and peripheral force,respectively).Whatever the reasons are,local material on the cutting edge first reaches its ultimate strength and the crack initiates on the tool face(Fig.4)and the n the development of the crack causes the fracture of the tool material.The size of chipping is small or at a micro level on the initiate stage of tool wear(Fig.2(a))and this might develop to big damage on the cutting edge(Fig.2(c))or peeling on the rake face(Fig.2(d))with the increasing cutting time especially when high combination of cutting parameters was selected(e.g.100 m/min,0.08 mm/r,0.5 mm for the depth of cut).Generally,in this investigation,the using of higher cutting speed(120 m/min)and smaller depth of cut(0.25 mm)can effectively suppress edge chipping.

Fig.4 Micro cracks initiates on tool flank and rake face(cutting 5 min,v=80 m/min,f=0.08 mm/r).

Fig.5 Abrasive wear on flank and rake face(cutting 14 min,v=80 m/min,f=0.08 mm/r).

Fig.6 Big chipping takes place near interface of tool and workpiece outer surface(cutting 22 min,v=80 m/min,f=0.08 mm/r).

Fig.7 Adhesive wear and resultant pitting marks,crater and plunked tool material(v=120 m/min,f=0.08 mm/r).

Fig.8 EDS analysis along tool wear land(after etching and ultrasonic cleaning)(cutting 15 min,v=60 m/min,f=0.08 mm/r).

Fig.9 Cutting temperature for different tool materials and cutting speeds(VB=0.05 mm,f=0.08 mm/r).

3.1.2.Abrasive wear on flank and rake face

Fig.10 Chipping appearance of PCD when turning TC4(cutting 35 min,v=120 m/min,f=0.08 mm/r).

Abrasive wear is another main wear pattern for PCD tools when high speed cutting(TiCp+TiBw)/TC4 composite.As shown in Fig.5,severe abrasive wear presented on both of the flank and the rake face where amount of fine grooves which parallel to each other could be observed.When turning TMCs using PCD tool,the TMC is also cutting the PCD tool like a grinding wheel during which the Co bonding material and the fine diamond grains would be plunked out of the tool matrix.At the high cutting temperature and cutting force,the reinforcement in the TMC would even cause scratches on the diamond.20Under the effect of abrasive wear,when the wear land is big enough,the material big chipping will take place near the interface of the tool and the workpiece outer surface.Fig.6 shows an example of the details of this situation.

3.1.3.Adhesive wear on flank and rake face

Fig.11 Notch wear formed at the limit of cutting zone when turning TC4 using PCD(cutting 60 min,f=0.08 mm/r).

Fig.12 Adhesive wear and resultant crater when turning TC4(cutting 40 min,v=100 m/min,f=0.08 mm/r).

When high speed turning TMCs,severe adhesive wear presents on both the flank and the rake face(see Fig.7(a)and(c)).The dynamic behavior of growth and breaking off of the adhesive materials would pluck out some of the tool matrix material(the Co bonding material).Fig.7(d)shows the details of the pitting marks due to the adhesive wear on the etched rake face after cutting for a short period of time(2 min).Occasionally,massive tool material is plucked out during the course of cutting(Fig.7(e)).

It is worth mentioning that generally a striking crater is formed on the rake face after cutting for a period of time(5–10 min),which is induced by the following three main mechanisms.The first one is the severe adhesive wear.The second one is the durative abrasive wear(see Fig.5(b)).The third one is the diffusion wear and the chemical wear.As shown in Fig.8,EDS analysis shows the proof of the presence of the diffusion wear mechanism,where the Ti element of the workpiece material has diffused into the cutting tool.Fig.8 also shows that the content of the C element in the tool-chip contact zone is significantly lower than that in the unworn zone of the tool.This indicates that chemical mechanism may exist during the course of tool wear because diamond reacts with titanium at high temperature to form TiC carbides.21,22Kubanic et al.23also suggested that high temperature combined with great strain can improve the chemical reactivity of the Ti-6Al-4V(TC4)causing the degradation of the diamond and the formation of titanium carbide which is more stable than diamond.As shown in Fig.9,the cutting temperature was 300–500 °C for the PCD tool which confirms the previous hypotheses.

3.2.PCD tool wear when turning TC4

3.2.1.Chipping

When turning TC4 matrix material,chipping could also take place on the PCD tool especially when high levels of cutting parameters are used,as shown in Fig.10.However,the scale and the size of the chipping are much smaller when compared to the situation of cutting the TMC(Figs.2 and 10).

3.2.2.Notch wear

For most of the cases,an obvious notch wear can be observed when cutting TC4 matrix for a period of time,as shown in Fig.11.The scale and size of the notch depends on the cutting temperature.A high temperature combined with a low pressure at the limit of the cutting zone causes the fast graphitization of diamond which finally creates a big notch.24As show in Fig.11,the size of the notch for the cutting speed of 120 m/min is much bigger than that for the cutting speed of 80 m/min because the temperature for the cutting speed of 120 m/min is much higher.

Fig.13 Chipping,peeling and micro cracks presented on carbide tools when turning TMC (cutting30–40 s,v=120 m/min,f=0.08 mm/r).

3.2.3.Adhesive wear and crater wear

When cutting TC4,the phenomenon of material sticking on the PCD tool was also prevalent(Fig.12(a)).However,the resulting adherent wear was greatly abated and so did the crater wear compared to the cutting of TMC(Figs.7(b)and 12(b)).

3.3.Carbide tool wear when turning(TiCp+TiBw)/TC4 composite

3.3.1.Chipping and peeling

As shown in Fig.13(a)and(b),when turning TMC with carbide,chipping is prevalent and the scale and the size of the chipping are much bigger compared to the situation of cutting the TMC under the same cutting condition.This is due to the fact that the hardness and the strength of carbide is much lower than that of PCD and also the cutting forces for the carbide tool are much higher than that for the PCD tool.9Cracks can also be found near the tool wear zone(Fig.13(c)).

3.3.2.Abrasive wear

Fig.14 shows that abrasive wear marks also presents on the carbide tool when turning TMC.Surprisingly,the marks on the carbide tools are much fainter than those on the PCD tool(Figs.5 and 6).But this does not mean that the abrasive wear for the carbide tool is much slighter than that for the PCD tool(this will be discussed in Section 4).

3.3.3.Adhesive wear and crater wear

Fig.14 Abrasive wear on carbide tool when turning TMC(cutting 60 s,v=100 m/min,f=0.08 mm/r).

Fig.15 Adhesive wear and resultant crater(cutting 90 s,v=100 m/min,f=0.08 mm/r).

Fig.16 EDS analysis on wear land of rake face(cutting 20 s,v=120 m/min,f=0.08 mm/r).

As shown in Fig.15,an amount of TC4 matrix material stacks on the carbide tool when turning TMC.After being etched,a big crater is revealed.According to Corduan and Hirnbert,24this crater is caused by the chemical mechanism where the tungsten carbide from the cutting tool strongly reacts with the titanium from the workpiece to form the titanium carbide which is more stable than the tungsten carbide.In the present study,the EDS analysis(Fig.16)not only shows that the Ti element of the workpiece material has diffused into the cutting tool but also reveals that oxidization wear has taken place during turning.This attributes to the very high cutting temperature(600–700 °C,see Fig.9)when turning TMC using carbide tools.Although the depth and width of the crater are much bigger than those on the PCD tool,the wear land here is much smoother(Figs.5 and 15).

4.Discussion

4.1.Great difference of PCD tool wear when cutting TMC and TC4

As previously stated,as far as tool wear is concerned,there exists great difference between cutting the TMC and cutting the TC4 matrix when machining using PCD tools.The chipping and the crater wear when turning TMC were much more significant and severer compared to the turning of TC4 matrix.Therefore,the tool wear rate for the TMC was much higher than that for the TC4(see Fig.17).This is because the cutting temperature for the TMC is much higher than that for the TC4(Fig.9).Accordingly,the chemical wear is much stronger and more TiC are generated on the PCD tool when cutting TMC(see Section 3.1.3 for explanation).Under the abrasive effect of the reinforcement in the TMC,the TiC on the PCD tool surface are torn off shortly after the subsequent cutting and the new diamond substrate is revealed.This cyclical generation and removal of the TiC on the tool face greatly accelerated the tool wear rate.Instead,for the situation of cutting TC4,the generated TiC on the PCD tool wear land retained for a relatively long time and hence protected the cutting tool from fast wear.

4.2.Great difference of tool wear between PCD and carbide tools when cutting TMC

Fig.17 Tool wear and tool life comparison when cutting TMC and TC4 using PCD tools(v=120 m/min,f=0.05 mm/r).

Fig.18 Tool flank wear versus cutting time for PCD and carbide tools when turning TMC(f=0.08 mm/r).

As stated in Section 3.3,compared to the PCD tool,the chipping was much severer for the carbide tool especially machining at higher cutting speed.What's more,the tool wear rate for the carbide tool was also much higher than that for the PCD tool.As shown in Fig.18,the tool life of carbide tool was merely 15 s at the cutting speed of 120 m/min.Even at the cutting speed of 60 m/min,the tool life was merely 2.5 min which was 1/6–1/10 of the PCD's tool life.Even though,it looks like that the abrasive wear on the carbide tool is much slighter than that on the PCD tool(Fig.14).This phenomenon is much obvious when turning at lower cutting speed.As shown in Fig.19,it is hard to see any obvious abrasive wear marks on the rake face of carbide tool.This can be explained by the following two facts.One is the much softer and smaller tool grain(WC)for the carbide(Table 2).The other is that much severer chemical reaction took place on the tool wear land due to the much higher cutting temperature for the carbide tool(Fig.9).Therefore,the WC on the too wear land presented significant chemical degradation.When turning,the workpiece TMC was polishing the tool face like a grinding wheel and the Co material and the chemical degraded WC were removed mainly in a ductile mode except for some plucked tool matrix and pulledout WC(Fig.19).

5.Conclusions

(1)Edge chipping,peeling,adhesive wear,abrasive wear and crater wear were presented on the PCD tool when high speed turning TMC.The very high specific cutting force of 350–600 N/mm on the cutting edge is the main reason for the chipping.Accordingly,the chipping is more prevalent and severe when radical parameters are used.High cutting speed and high cutting temperature improved the diffusion of the Ti element of the TMC workpiece into the PCD tool and caused the degradation of the diamond and the formation of titanium carbide on the tool.The combined effects of abrasive wear and diffusion wear resulted in the big crater wear on the tool rake face.

(2)When cutting TC4 with PCD tool,chipping,adhesive wear,crater wear and notch wear were the main wear patterns.However,chipping,adhesive wear and crater wear were much milder when compared to the cutting of TMC.Compared with other wear patterns,the notch wear was more significant which was induced by the high temperature combined with a low pressure at the limit of the cutting zone causing the fast graphitization of diamond.Due to the absent of abrasive wear,the generated titanium carbide on the tool cutting face protected the tool from fast wear.And hence,tool life of PCD for TC4 was 6–10 times longer than that for TMC.

(3)Carbide tool presented the same wear patterns as PCD tool when high speed turning TMC.However,because of the much higher cutting temperature,the chemical reaction between the Ti element and the WC was more violent,which produced bigger crater wear on the carbide tool.Although the tool wear rate for the carbide was much higher than that for the PCD,due to the much lower hardness and smaller size of WC combined with the more significant chemical degradation of carbide,the marks of the abrasive wear on the carbide tool looked much slighter or even could not be observed especially when low levels of cutting parameters were used.In the present study,the tool life of the carbide tools was confined to 15–150 s and hence it can only be used for rough machining at lower cutting speed.

Fig.19 Tool wear appearance for carbide tool when turning under relatively low cutting speed(cutting 120 s,v=60 m/min,f=0.08 mm/r).

Acknowledgements

This study was co-supported by the National Natural Science Foundation of China(No.51275227),Nanjing Science and Technology Development Plan(201306024)of China and the Qinglan Project of Jiangsu Province(2014)of China.

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Ge Yingfeireceived the M.S.and Ph.D.degrees in mechanical engineering from Nanjing University of Aeronautical and Astronautics in 2004 and 2007 respectively,and the n became a teacher in Nanjing Institute of Technology,China.His main research interests are high performance manufacturing on difficult-to-machine materials and ultra precision machining.

Xu Jiuhuais a professor and Ph.D.supervisor at college of mechanicalamp;electrical engineering,Nanjing University of Aeronautical and Astronautics,China.He received the Ph.D.degree from the same university in 1992.His current research interests are high performance manufacturing on difficult-to-machine materials and ultra precision machining.

30 October 2015;revised 28 December 2015;accepted 31 January 2016

Available online 28 August 2016

Carbide tool;

High speed cutting;

PCD tool;

Titanium matrix composite;

Tool wear;

Turning

*Corresponding author.Tel.:+86 25 86118255.

E-mail address:yingfeige@163.com(Y.Ge).

Peer review under responsibility of Editorial Committee of CJA.