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The effect of machining on surface integrity of titanium alloy Ti–6% Al–4% V
Journal of Materials Processing Technology 166 (2005) 188–192
The effect of machining on surface integrity of titanium alloy Ti–6% Al–4% V C.H. Che-Harona,∗ , A. Jawaidb a
Department of Mechanical and Materials Engineering, Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor, Malaysia b School of Engineering, Coventry University, Coventry CV1 5FB, UK
Received 28 January 2002; received in revised form 6 July 2004; accepted 13 August 2004
Abstract This paper gives the investigation on surface integrity of rough machining of titanium alloy Ti–6% Al–4% V with uncoated carbide cutting tools. The experiments were carried out under dry cutting conditions. The cutting speeds selected in the experiment were 100, 75, 60 and 45 m min−1 . The depth of cut was kept constant at 2.0 mm. The feed rates used in the experiment were 0.35 and 0.25 mm rev−1 . Two types of insert were used in the experiments. For a range of cutting speeds, feeds, and depths of cut, measurements of surface roughness of machined surface, microhardness and work hardening backed up with scanning electron microscope were taken. The surface of titanium alloy is easily damaged during machining operations due to their poor machinability. The machined surface experienced microstructure alteration and increment in microhardness on the top white layer (≤10 m) of the machined surface. Severe microstructure alteration was observed when machining with the dull tool. In addition, surface roughness values obtained were within the limit (<6 m) stipulated by ISO for rough machining. © 2004 Elsevier B.V. All rights reserved. Keywords: Surface integrity; Titanium alloy; Carbide tool
1. Introduction Titanium alloys are extremely difficult to machine materials. The machinability of titanium and its alloys is generally considered to be poor owing to several inherent properties of materials. Titanium and titanium alloys have low thermal conductivity and high chemical reactivity with many cutting tool materials. Its low thermal conductivity increases the temperature at the cutting edge of the tool. Hence, on machining, the cutting tools wear off very rapidly due to high cutting temperature and strong adhesion between tool and workpiece material. Additionally, the low modulus of elasticity of titanium alloys and its high strength at elevated temperature further impair its machinability. Machining of titanium alloys at higher cutting speed will cause rapid chipping at the cutting edge which leads to catas∗
Corresponding author. Fax: +60 3 8259 659. E-mail address: [email protected] (C.H. Che-Haron).
0924-0136/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2004.08.012
trophic failure of the inserts [1]. A higher cutting speed also results in rapid cratering and/or plastic deformation of the cutting edge. This is due to the temperature generated which tends to be concentrated at the cutting edge closer to the nose of the inserts. The heat affected zone is very small when cutting titanium alloys. The smaller heat affected area produced is as a result of the shorter chip/tool contact length. It is mainly for this reason that the cutting speeds are limited to about 45 m min−1 when using straight grade cemented carbides (WC–Co) [2]. The rapid tool failure and chipping at the cutting edge has resulted in poor surface finish of the machined surface. It has caused not only higher surface roughness values but also higher microhardness values and severe microstructure alteration. Titanium alloys are generally used for a component, which requires the greatest reliability, and therefore the surface integrity must be maintained. According to Field and Kahles [3], when machining any component it is essential to satisfy surface integrity requirements. However, during machining
C.H. Che-Haron, A. Jawaid / Journal of Materials Processing Technology 166 (2005) 188–192
189
Table 1 Nominal chemical composition of the titanium alloys
Table 3 Properties of the cutting tools used
Work material
Insert grade (ISO)
Hardness (HNV)
Density (gm cc−1 )
Grain size (m)
CNMG 120408-883-MR4 CNMG 120408-890-MR3
1760 1753
14.95 14.92
1.0 0.68
Ti–6% Al–4% V
Chemical composition (wt.%) V
Al
N
O
H
C
Fe
4
6
0.05
0.2
0.0125
0.1
0.3
and grinding operations, the surface of titanium alloys is easily damaged because of their poor machinability. As far as the surface metallurgy of the machined component is concerned, the heat generated during cutting is a main source of damage, especially in the grinding process. Possible surface and subsurface alterations include: plastic deformation, microcracking, phase transformations and residual stress effects. Several studies on surface integrity parameters have been carried out [4–8]. When machining titanium alloys in an abusive manner an overheated white layer can be produced which results in a layer being softer or harder than the base materials [3]. The aims of this work were to investigate surface integrity effects when machining titanium alloy Ti–6% Al–4% V. The paper will explain various factors and parameters involved when machining titanium alloys with carbide tools.
carbide with 6 wt.% of cobalt as binder. The properties of the cutting tools used are shown in Table 3. 2.3. Machining tests All the machining experiments were carried out on a Cincinnati Milacron CNC lathe, Cinturn 10 CC, which was controlled by an Achramatic 850 controller. The CNC lathe has a continuously variable spindle speed. This type of lathe is particularly useful when it is required to machine bars of different diameters at the same cutting speed. Throughout experiments, the depth of cut was kept constant at 2.0 mm, and the feed rates were set at 0.25 and 0.35 mm rev−1 . The surface speed employed during the machining tests was 100, 75, 60, and 45 m min−1 . The machining experiments were carried out in a dry cutting condition. The cutting conditions used are given in Table 4.
2. Experimental procedure 2.1. Workpiece materials
3. Results and discussions
The workpiece materials used in all the experiments was a bar of an alpha-beta titanium alloy Ti–6% Al–4% V. The nominal compositions of the alloys (in wt.%) are given in Table 1 [9]. The workpiece had a microstructure, which consisted of elongated alpha phase surrounded by fine, dark etching of beta matrix. Titanium alloy Ti–6% Al–4% V is a widely used titanium alloy and offers high strength, depth hardenability and elevated temperature properties up to 400 ◦ C. The mechanical properties of tested material are shown in Table 2 [8].
The results and discussions are focused on the workpiece surface integrity aspects on the roughing operation when machining titanium alloy Ti–6% Al–4% V. The results for tool life and tool wear mechanisms for the same cutting conditions were explained in detail in previous paper [10].
2.2. Cutting tool materials Two types of carbide inserts of ISO designation CNMG 120408 were used for the machining experiments. The cutting tools used were straight tungsten carbide tools. The tools were throw-away type of rhombic shape with chipbreaker and were uncoated. Both of the inserts consisted of 94 wt.% tungsten
3.1. Surface finish and surface integrity 3.1.1. Surface finish Typical surface roughness values recorded when machining titanium alloy Ti-64 with 883 inserts at a feed rate of 0.35 mm rev−1 under dry cutting conditions is shown in Fig. 1. Slightly higher surface roughness values were recorded at a lower cutting speed. However, as the cutting speed increased, the roughness value decreased. Highest surface roughness values recorded for 883-MR4 insert Table 4 Cutting conditions for the experimental work Tool tested
Table 2 Mechanical properties of tested material Work material Ultimate tensile strength (MPa) Modulus of elasticity (×106 MPa) Hardness (HBS/10 mm/3000 kg)
883-MR4 and 890-MR3 (m min−1 )
Ti-64 827 11.3 250–300
Cutting speed V Depth of cut (mm) Feed rate (mm rev−1 ) Tool geometry
100, 75, 60, 45 2 0.25 and 0.35 Approach angle: 95◦ ; side rake angle: −6◦ ; back rake angle: −6◦ ; end relief angle: 6◦ ; side relief angle: 6◦
190
C.H. Che-Haron, A. Jawaid / Journal of Materials Processing Technology 166 (2005) 188–192
Fig. 1. Typical surface roughness when machining with 883 inserts at feed rate of 0.35 mm rev−1 .
was 3.28 m and it was recorded at the cutting speed of 60 m min−1 . Surface finish tends to become smoother toward the end of tool life. This is probably due to deformation on the flank face or adherence of the workpiece material at the tool nose. Increasing the cutting speed led to higher roughness values. However, the roughness values recorded were unstable during the intermediate cutting process. As the 883-MR4 tools wore and approached the end of their life, the roughness values recorded increased significantly, especially at cutting speeds of 45 m min−1 . The dramatic increase in the surface roughness value at cutting speed of 45 m min−1 was probably caused by rapid tool wear at the cutting edge closer to the nose and also fracture at the nose. The surface finish generated when machining titanium alloy Ti-64 with 890-MR3 tools under dry cutting conditions at feed rate of 0.25 mm rev−1 is affected by the cutting speed as shown in Fig. 2. The surface roughness values recorded for cutting speeds of 100 and 75 m min−1 tend to increase as the tools approached the end of their life. Highest surface roughness value recorded was 5.0 m and it was recorded at the end of tool life for a cutting speed of 100 m min−1 . The surface roughness value recorded for cutting speeds of 45 and 60 m min−1 were higher at the initial cutting process and reduced slightly as the tools approached the end of their life. However, further cutting has shown that the surface finish generated has improved gradually before it increased slightly as the tool failed. This is probably due to adhered material covering the cutting edge.
Fig. 2. Typical surface roughness when machining with 890 inserts at feed rate of 0.25 mm rev−1 .
Fig. 3. Microhardness value beneath the machined surface when machining with 883 insert.
3.1.2. Microhardness tests Work hardening of the deformed layer beneath the machined surface up to 0.01 mm caused higher hardness than the average hardness of the base material. However, the hardness of the subsurface at 0.02 mm below the machined surface was below the average hardness recorded for the base material. The softening effect of the material at this level was probably due to over aging of titanium alloy as a result of very high cutting temperature produced at the local surface. The low thermal conductivity of titanium alloy also caused the temperature below the machined surface to be retained. The hardness values at 0.07 mm beneath the machined surface increased drastically under all cutting conditions. Curves in Fig. 3 suggest that hardening has occurred 0.07 mm below the machined surface. The wear on the cutting edge affects the microstructure, the greatest surface hardening was found to take place when machining was carried out with worn tools. Further machining of the titanium alloy with the nearly worn tools tends to increase the hardening rate of the surface layer. Fig. 3 shows that higher values of hardness were recorded at the higher cutting speed for the same feed rate. Curves in Fig. 3 also suggest that minimal increment in hardness values were recorded when the feed rate was increased from 0.25 to 0.35 mm rev−1 at the initial cutting stage. Significant increment in the microhardness values was observed when comparing between the initial cut and the final cut. When prolonged machining was carried out with higher flank wear, the hardness of the disturbed layer of the machined surface increased significantly. The highest hardness recorded was 391 HV when machining at a cutting speed of 100 m min−1 , and feed rate of 0.35 mm rev−1 after the 883MR4 tool has failed. The highest hardness value was recorded at 0.005 mm beneath the machined surface, where the microstructures were heavily deformed. The increment in the hardness value was probably due to the work-hardening effect. However, when the microstructure was less disturbed, the increment in the hardness was small. The hardness values approached the hardness of the base material as the depth beneath the machined surface increased. At 0.32 mm beneath the machined surface, the difference in hardness was
C.H. Che-Haron, A. Jawaid / Journal of Materials Processing Technology 166 (2005) 188–192
Fig. 4. Microstructure of machined surface after 10 s of cutting at 100 m min−1 with 883 insert.
very small, which is less than 3% at the initial cutting time. However, when machining with worn tools, the hardness approached the hardness of the base material only 0.42 mm beneath the machined surface. Similar behaviour was also observed for 890-MR3 insert.
191
Fig. 6. Microstructure of machined surface after 10 s of cutting at 45 m min−1 with 890 insert.
3.1.3. Metallurgical alterations Figs. 4 and 5 show the microstructures of the machined surface produced when machining with 883-MR4 tools under dry cutting conditions. It was found that when machining under dry conditions, a thin layer of disturbed or plastically deformed layer was formed immediately underneath the machined surface. Fig. 4 shows the smooth surface with less disturbed layer of the machined surface observed at the early stage (10 s) of cutting for the cutting speed of 100 m min−1 and feed rate of 0.25 mm rev−1 . Prolonged machining with nearly worn tools produced severe plastic deformation and a thicker disturbed layer on the machined surface as shown in Fig. 5. The microstructure was observed after machining at a cutting speed of 100 m min−1 ,
and feed rate of 0.25 mm rev−1 for 2 min, where the tool reached the end of its life. The microstructures of the machined surface produced when machining with 890-MR3 tools under dry cutting conditions are shown in Figs. 6 and 7. It was found that when machining under dry conditions, a very thin layer of disturbed or plastically deformed layer was formed immediately underneath the machined surface. Fig. 6 shows the smooth surface with less disturbed layer of plastic flow after 10 s of cutting for a cutting speed of 45 m min−1 and feed rate of 0.35 mm rev−1 . Machining with nearly worn or worn tools led to the generation of irregular surfaces, which consists of tearing and plastically deformed surfaces. Prolonged machining with nearly worn tools also produced severe plastic deformation and thicker disturbed layer on the machined surface as shown in Fig. 7. It was observed that after machining for 12 min at a cutting speed of 45 m min−1 and feed rate of 0.35 mm rev−1 the tool reached its rejection criterion.
Fig. 5. Microstructure of machined surface after 2 min of cutting at 100 m min−1 with 883 insert.
Fig. 7. Microstructure of machined surface after 12 min of cutting at 45 m min−1 with 890 insert.
192
C.H. Che-Haron, A. Jawaid / Journal of Materials Processing Technology 166 (2005) 188–192
4. Conclusions
References
The following conclusions are based on the results for turning tests with straight grade cemented carbide tools on titanium alloy Ti-64 (Ti–6% Al–4% V):
[1] Z. Wang, PhD Thesis, South Bank University, UK, 1997. [2] R. Komanduri, W.R. Reed Jr., Evaluation of carbide grades and a new cutting geometry for machining titanium alloys, Wear 92 (1983) 113–123. [3] M. Field, J. Kahles, Review of surface integrity of machined components, Ann. CIRP 20 (1971) 153–163. [4] G. Byrne, J. Barry, P. Young, Surface integrity of AlSi9 machined with PCD cutting tools, Ann. CIRP 46 (1997) 489– 492. [5] A.M. Arao, M. Wise, D. Aspinwall, Tool Life and Workpiece Surface Integrity Evalutions When Machining Hardened AISI H13 and AISI E52100 Steels with Conventional Ceramic and PCBN Tool Materials, SME Technical Paper No. MR95-159, 1998. [6] W. Field, W. Koster, Surface Integrity in Conventional Machining – Chip Removal Processes, Technical Paper No. EM68, ASTME, 1968. [7] D. Watson, M. Murphy, The effect of machining on surface integrity, Manuf. Eng. (1979) 199–204. [8] C.H. Che-Haron, Tool life and surface integrity in turning titanium alloy, J. Mater. Process. Technol. 118 (2001) 231–237. [9] M.J. Donachie Jr., Titanium and Titanium Alloys; Source Book, American Society for Metals, Metals Park, OH, 1982, pp. 3–19. [10] A. Jawaid, C.H. Che-Haron, Tool wear in machining of titanium alloy Ti–6% Al–4% V, in: Proceedings of the Advances in Materials and Processing Technologies (AMPT’97), Portugal, July 22–27, 1997, pp. 562–568.
1. Straight grade cemented carbides are suitable for use in machining titanium alloy 64. The wear resistance and cutting edge strength of insert CNMG 120408-883 are superior to insert CNMG 120408-890 (finer grain size). 2. Severe tearing and plastic deformation of the machined surface were observed when machining titanium alloy Ti64, especially after prolonged machining under dry cutting conditions. At the initial stages of cutting the plastic flow of microstructure was not detected. However, at the end of cutting (when the tool failed) severe plastic flow, tearing and deformation of the microstructure was detected. This caused the formation of a white layer of hardened material on top of the machined surface, the thickness of which was less than 0.01 mm. 3. The top layer of the machined surface experience work hardening process, hence the hardness is higher than the average hardness of the workpiece materials. However, the material beneath the top layer is softer as a result of over-aging of the materials.
The effect of machining on surface integrity of titanium alloy Ti–6% Al–4% V C.H. Che-Harona,∗ , A. Jawaidb a
Department of Mechanical and Materials Engineering, Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor, Malaysia b School of Engineering, Coventry University, Coventry CV1 5FB, UK
Received 28 January 2002; received in revised form 6 July 2004; accepted 13 August 2004
Abstract This paper gives the investigation on surface integrity of rough machining of titanium alloy Ti–6% Al–4% V with uncoated carbide cutting tools. The experiments were carried out under dry cutting conditions. The cutting speeds selected in the experiment were 100, 75, 60 and 45 m min−1 . The depth of cut was kept constant at 2.0 mm. The feed rates used in the experiment were 0.35 and 0.25 mm rev−1 . Two types of insert were used in the experiments. For a range of cutting speeds, feeds, and depths of cut, measurements of surface roughness of machined surface, microhardness and work hardening backed up with scanning electron microscope were taken. The surface of titanium alloy is easily damaged during machining operations due to their poor machinability. The machined surface experienced microstructure alteration and increment in microhardness on the top white layer (≤10 m) of the machined surface. Severe microstructure alteration was observed when machining with the dull tool. In addition, surface roughness values obtained were within the limit (<6 m) stipulated by ISO for rough machining. © 2004 Elsevier B.V. All rights reserved. Keywords: Surface integrity; Titanium alloy; Carbide tool
1. Introduction Titanium alloys are extremely difficult to machine materials. The machinability of titanium and its alloys is generally considered to be poor owing to several inherent properties of materials. Titanium and titanium alloys have low thermal conductivity and high chemical reactivity with many cutting tool materials. Its low thermal conductivity increases the temperature at the cutting edge of the tool. Hence, on machining, the cutting tools wear off very rapidly due to high cutting temperature and strong adhesion between tool and workpiece material. Additionally, the low modulus of elasticity of titanium alloys and its high strength at elevated temperature further impair its machinability. Machining of titanium alloys at higher cutting speed will cause rapid chipping at the cutting edge which leads to catas∗
Corresponding author. Fax: +60 3 8259 659. E-mail address: [email protected] (C.H. Che-Haron).
0924-0136/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2004.08.012
trophic failure of the inserts [1]. A higher cutting speed also results in rapid cratering and/or plastic deformation of the cutting edge. This is due to the temperature generated which tends to be concentrated at the cutting edge closer to the nose of the inserts. The heat affected zone is very small when cutting titanium alloys. The smaller heat affected area produced is as a result of the shorter chip/tool contact length. It is mainly for this reason that the cutting speeds are limited to about 45 m min−1 when using straight grade cemented carbides (WC–Co) [2]. The rapid tool failure and chipping at the cutting edge has resulted in poor surface finish of the machined surface. It has caused not only higher surface roughness values but also higher microhardness values and severe microstructure alteration. Titanium alloys are generally used for a component, which requires the greatest reliability, and therefore the surface integrity must be maintained. According to Field and Kahles [3], when machining any component it is essential to satisfy surface integrity requirements. However, during machining
C.H. Che-Haron, A. Jawaid / Journal of Materials Processing Technology 166 (2005) 188–192
189
Table 1 Nominal chemical composition of the titanium alloys
Table 3 Properties of the cutting tools used
Work material
Insert grade (ISO)
Hardness (HNV)
Density (gm cc−1 )
Grain size (m)
CNMG 120408-883-MR4 CNMG 120408-890-MR3
1760 1753
14.95 14.92
1.0 0.68
Ti–6% Al–4% V
Chemical composition (wt.%) V
Al
N
O
H
C
Fe
4
6
0.05
0.2
0.0125
0.1
0.3
and grinding operations, the surface of titanium alloys is easily damaged because of their poor machinability. As far as the surface metallurgy of the machined component is concerned, the heat generated during cutting is a main source of damage, especially in the grinding process. Possible surface and subsurface alterations include: plastic deformation, microcracking, phase transformations and residual stress effects. Several studies on surface integrity parameters have been carried out [4–8]. When machining titanium alloys in an abusive manner an overheated white layer can be produced which results in a layer being softer or harder than the base materials [3]. The aims of this work were to investigate surface integrity effects when machining titanium alloy Ti–6% Al–4% V. The paper will explain various factors and parameters involved when machining titanium alloys with carbide tools.
carbide with 6 wt.% of cobalt as binder. The properties of the cutting tools used are shown in Table 3. 2.3. Machining tests All the machining experiments were carried out on a Cincinnati Milacron CNC lathe, Cinturn 10 CC, which was controlled by an Achramatic 850 controller. The CNC lathe has a continuously variable spindle speed. This type of lathe is particularly useful when it is required to machine bars of different diameters at the same cutting speed. Throughout experiments, the depth of cut was kept constant at 2.0 mm, and the feed rates were set at 0.25 and 0.35 mm rev−1 . The surface speed employed during the machining tests was 100, 75, 60, and 45 m min−1 . The machining experiments were carried out in a dry cutting condition. The cutting conditions used are given in Table 4.
2. Experimental procedure 2.1. Workpiece materials
3. Results and discussions
The workpiece materials used in all the experiments was a bar of an alpha-beta titanium alloy Ti–6% Al–4% V. The nominal compositions of the alloys (in wt.%) are given in Table 1 [9]. The workpiece had a microstructure, which consisted of elongated alpha phase surrounded by fine, dark etching of beta matrix. Titanium alloy Ti–6% Al–4% V is a widely used titanium alloy and offers high strength, depth hardenability and elevated temperature properties up to 400 ◦ C. The mechanical properties of tested material are shown in Table 2 [8].
The results and discussions are focused on the workpiece surface integrity aspects on the roughing operation when machining titanium alloy Ti–6% Al–4% V. The results for tool life and tool wear mechanisms for the same cutting conditions were explained in detail in previous paper [10].
2.2. Cutting tool materials Two types of carbide inserts of ISO designation CNMG 120408 were used for the machining experiments. The cutting tools used were straight tungsten carbide tools. The tools were throw-away type of rhombic shape with chipbreaker and were uncoated. Both of the inserts consisted of 94 wt.% tungsten
3.1. Surface finish and surface integrity 3.1.1. Surface finish Typical surface roughness values recorded when machining titanium alloy Ti-64 with 883 inserts at a feed rate of 0.35 mm rev−1 under dry cutting conditions is shown in Fig. 1. Slightly higher surface roughness values were recorded at a lower cutting speed. However, as the cutting speed increased, the roughness value decreased. Highest surface roughness values recorded for 883-MR4 insert Table 4 Cutting conditions for the experimental work Tool tested
Table 2 Mechanical properties of tested material Work material Ultimate tensile strength (MPa) Modulus of elasticity (×106 MPa) Hardness (HBS/10 mm/3000 kg)
883-MR4 and 890-MR3 (m min−1 )
Ti-64 827 11.3 250–300
Cutting speed V Depth of cut (mm) Feed rate (mm rev−1 ) Tool geometry
100, 75, 60, 45 2 0.25 and 0.35 Approach angle: 95◦ ; side rake angle: −6◦ ; back rake angle: −6◦ ; end relief angle: 6◦ ; side relief angle: 6◦
190
C.H. Che-Haron, A. Jawaid / Journal of Materials Processing Technology 166 (2005) 188–192
Fig. 1. Typical surface roughness when machining with 883 inserts at feed rate of 0.35 mm rev−1 .
was 3.28 m and it was recorded at the cutting speed of 60 m min−1 . Surface finish tends to become smoother toward the end of tool life. This is probably due to deformation on the flank face or adherence of the workpiece material at the tool nose. Increasing the cutting speed led to higher roughness values. However, the roughness values recorded were unstable during the intermediate cutting process. As the 883-MR4 tools wore and approached the end of their life, the roughness values recorded increased significantly, especially at cutting speeds of 45 m min−1 . The dramatic increase in the surface roughness value at cutting speed of 45 m min−1 was probably caused by rapid tool wear at the cutting edge closer to the nose and also fracture at the nose. The surface finish generated when machining titanium alloy Ti-64 with 890-MR3 tools under dry cutting conditions at feed rate of 0.25 mm rev−1 is affected by the cutting speed as shown in Fig. 2. The surface roughness values recorded for cutting speeds of 100 and 75 m min−1 tend to increase as the tools approached the end of their life. Highest surface roughness value recorded was 5.0 m and it was recorded at the end of tool life for a cutting speed of 100 m min−1 . The surface roughness value recorded for cutting speeds of 45 and 60 m min−1 were higher at the initial cutting process and reduced slightly as the tools approached the end of their life. However, further cutting has shown that the surface finish generated has improved gradually before it increased slightly as the tool failed. This is probably due to adhered material covering the cutting edge.
Fig. 2. Typical surface roughness when machining with 890 inserts at feed rate of 0.25 mm rev−1 .
Fig. 3. Microhardness value beneath the machined surface when machining with 883 insert.
3.1.2. Microhardness tests Work hardening of the deformed layer beneath the machined surface up to 0.01 mm caused higher hardness than the average hardness of the base material. However, the hardness of the subsurface at 0.02 mm below the machined surface was below the average hardness recorded for the base material. The softening effect of the material at this level was probably due to over aging of titanium alloy as a result of very high cutting temperature produced at the local surface. The low thermal conductivity of titanium alloy also caused the temperature below the machined surface to be retained. The hardness values at 0.07 mm beneath the machined surface increased drastically under all cutting conditions. Curves in Fig. 3 suggest that hardening has occurred 0.07 mm below the machined surface. The wear on the cutting edge affects the microstructure, the greatest surface hardening was found to take place when machining was carried out with worn tools. Further machining of the titanium alloy with the nearly worn tools tends to increase the hardening rate of the surface layer. Fig. 3 shows that higher values of hardness were recorded at the higher cutting speed for the same feed rate. Curves in Fig. 3 also suggest that minimal increment in hardness values were recorded when the feed rate was increased from 0.25 to 0.35 mm rev−1 at the initial cutting stage. Significant increment in the microhardness values was observed when comparing between the initial cut and the final cut. When prolonged machining was carried out with higher flank wear, the hardness of the disturbed layer of the machined surface increased significantly. The highest hardness recorded was 391 HV when machining at a cutting speed of 100 m min−1 , and feed rate of 0.35 mm rev−1 after the 883MR4 tool has failed. The highest hardness value was recorded at 0.005 mm beneath the machined surface, where the microstructures were heavily deformed. The increment in the hardness value was probably due to the work-hardening effect. However, when the microstructure was less disturbed, the increment in the hardness was small. The hardness values approached the hardness of the base material as the depth beneath the machined surface increased. At 0.32 mm beneath the machined surface, the difference in hardness was
C.H. Che-Haron, A. Jawaid / Journal of Materials Processing Technology 166 (2005) 188–192
Fig. 4. Microstructure of machined surface after 10 s of cutting at 100 m min−1 with 883 insert.
very small, which is less than 3% at the initial cutting time. However, when machining with worn tools, the hardness approached the hardness of the base material only 0.42 mm beneath the machined surface. Similar behaviour was also observed for 890-MR3 insert.
191
Fig. 6. Microstructure of machined surface after 10 s of cutting at 45 m min−1 with 890 insert.
3.1.3. Metallurgical alterations Figs. 4 and 5 show the microstructures of the machined surface produced when machining with 883-MR4 tools under dry cutting conditions. It was found that when machining under dry conditions, a thin layer of disturbed or plastically deformed layer was formed immediately underneath the machined surface. Fig. 4 shows the smooth surface with less disturbed layer of the machined surface observed at the early stage (10 s) of cutting for the cutting speed of 100 m min−1 and feed rate of 0.25 mm rev−1 . Prolonged machining with nearly worn tools produced severe plastic deformation and a thicker disturbed layer on the machined surface as shown in Fig. 5. The microstructure was observed after machining at a cutting speed of 100 m min−1 ,
and feed rate of 0.25 mm rev−1 for 2 min, where the tool reached the end of its life. The microstructures of the machined surface produced when machining with 890-MR3 tools under dry cutting conditions are shown in Figs. 6 and 7. It was found that when machining under dry conditions, a very thin layer of disturbed or plastically deformed layer was formed immediately underneath the machined surface. Fig. 6 shows the smooth surface with less disturbed layer of plastic flow after 10 s of cutting for a cutting speed of 45 m min−1 and feed rate of 0.35 mm rev−1 . Machining with nearly worn or worn tools led to the generation of irregular surfaces, which consists of tearing and plastically deformed surfaces. Prolonged machining with nearly worn tools also produced severe plastic deformation and thicker disturbed layer on the machined surface as shown in Fig. 7. It was observed that after machining for 12 min at a cutting speed of 45 m min−1 and feed rate of 0.35 mm rev−1 the tool reached its rejection criterion.
Fig. 5. Microstructure of machined surface after 2 min of cutting at 100 m min−1 with 883 insert.
Fig. 7. Microstructure of machined surface after 12 min of cutting at 45 m min−1 with 890 insert.
192
C.H. Che-Haron, A. Jawaid / Journal of Materials Processing Technology 166 (2005) 188–192
4. Conclusions
References
The following conclusions are based on the results for turning tests with straight grade cemented carbide tools on titanium alloy Ti-64 (Ti–6% Al–4% V):
[1] Z. Wang, PhD Thesis, South Bank University, UK, 1997. [2] R. Komanduri, W.R. Reed Jr., Evaluation of carbide grades and a new cutting geometry for machining titanium alloys, Wear 92 (1983) 113–123. [3] M. Field, J. Kahles, Review of surface integrity of machined components, Ann. CIRP 20 (1971) 153–163. [4] G. Byrne, J. Barry, P. Young, Surface integrity of AlSi9 machined with PCD cutting tools, Ann. CIRP 46 (1997) 489– 492. [5] A.M. Arao, M. Wise, D. Aspinwall, Tool Life and Workpiece Surface Integrity Evalutions When Machining Hardened AISI H13 and AISI E52100 Steels with Conventional Ceramic and PCBN Tool Materials, SME Technical Paper No. MR95-159, 1998. [6] W. Field, W. Koster, Surface Integrity in Conventional Machining – Chip Removal Processes, Technical Paper No. EM68, ASTME, 1968. [7] D. Watson, M. Murphy, The effect of machining on surface integrity, Manuf. Eng. (1979) 199–204. [8] C.H. Che-Haron, Tool life and surface integrity in turning titanium alloy, J. Mater. Process. Technol. 118 (2001) 231–237. [9] M.J. Donachie Jr., Titanium and Titanium Alloys; Source Book, American Society for Metals, Metals Park, OH, 1982, pp. 3–19. [10] A. Jawaid, C.H. Che-Haron, Tool wear in machining of titanium alloy Ti–6% Al–4% V, in: Proceedings of the Advances in Materials and Processing Technologies (AMPT’97), Portugal, July 22–27, 1997, pp. 562–568.
1. Straight grade cemented carbides are suitable for use in machining titanium alloy 64. The wear resistance and cutting edge strength of insert CNMG 120408-883 are superior to insert CNMG 120408-890 (finer grain size). 2. Severe tearing and plastic deformation of the machined surface were observed when machining titanium alloy Ti64, especially after prolonged machining under dry cutting conditions. At the initial stages of cutting the plastic flow of microstructure was not detected. However, at the end of cutting (when the tool failed) severe plastic flow, tearing and deformation of the microstructure was detected. This caused the formation of a white layer of hardened material on top of the machined surface, the thickness of which was less than 0.01 mm. 3. The top layer of the machined surface experience work hardening process, hence the hardness is higher than the average hardness of the workpiece materials. However, the material beneath the top layer is softer as a result of over-aging of the materials.