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TiCN mixed ceramic tools in turning hardened alloy steel
Wear 274–275 (2012) 442–451
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Tool life and wear mechanism of coated and uncoated Al2 O3 /TiCN mixed ceramic tools in turning hardened alloy steel K. Aslantas a,∗ , I˙ . Ucun b , A. C¸icek c a
Afyon Kocatepe University, Faculty of Techonolgy, Department of Mechanical Engineering, Afyonkarahisar, Turkey Afyon Kocatepe University, Technical Education Faculty, Department of Mechanical Education, Afyokarahisar, Turkey c Düzce University, Faculty of Technology, Department of Manufacturing Engineering, Düzce, Turkey b
a r t i c l e
i n f o
Article history: Received 22 July 2011 Received in revised form 23 November 2011 Accepted 28 November 2011 Available online 13 December 2011 Keywords: Cutting tool Steel Engineering ceramic Hard materials PVD coatings
a b s t r a c t The focus of this paper is the continuous turning of hardened AISI 52100 (∼63HRc) using coated and uncoated ceramic Al2 O3 –TiCN mixed inserts, which are cheaper than cubic boron nitride (CBN) or polycrystalline cubic boron nitride (PCBN). The machinability of hardened steel was evaluated by measurements of tool wear, tool life, and surface finish of the workpiece. Wear mechanisms and patterns of ceramic inserts in hard turning of hardened AISI 52100 are discussed. According to the results obtained, fracture and chipping type damages occur more frequently in uncoated tools, whereas crater wear is the more common type of damage in TiN coated tools. Most important result obtained from the study is that TiN coating and crater wear affect chip flow direction. In uncoated ceramic tool, the crater formation results in decrease of chip up-curl radius. Besides, uncoated cutting tool results in an increase in the temperature at the tool chip interface. This causes a thermal bi-metallic effect between the upper and lower sides of the chip that forces the chip to curl a smaller radius. Chips accumulate in front of the tool and stick to the workpiece depending on the length of the cutting time. This causes the surface quality to deteriorate. TiN coating not only ensures that the cutting tool is tougher, but also ensures that the surface quality is maintained during cutting processes. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Hard turning is a turning operation which is applied on highresistance alloy steels (45 < HRC < 65) to obtain surface roughness values that are close to those obtained in grinding (Ra ∼ 0.1 m). The workpiece materials involved include various hardened alloy steels, tool steels, case-hardened steels, superalloys, nitrided irons and hard-chrome-coated steels, and heat-treated powder metallurgical parts [1]. Although this production method is a new subject, there are quite a few studies by several researchers in the literature. These studies mostly concern the turning of AISI 52100 bearing steel, H11–H13 hot work tool steel, and AISI 4130–4340 low alloy steel using CBN, ceramic, and coated carbide tools. The most common problem encountered in the hard turning process is tool wear. Therefore, ceramic, PCBN, and CBN cutting tools are generally used for hard turning processes [2–5]. The use of alumina based ceramic tools in hard machining is an attractive alternative to grinding in order to reduce processing costs, improve material properties, and for the environmental benefits [6]. Advances in ceramic processing technology have resulted in
∗ Corresponding author. Tel.: +90 272 228 13 11; fax: +90 272 228 13 19. E-mail address: [email protected] (K. Aslantas). 0043-1648/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2011.11.010
a new generation of high performance ceramic cutting tools that exhibit properties such as fracture strength, toughness, thermal shock resistance, hardness, and wear resistance. Therefore ceramic tools are used in the machining of various types of steels and hard materials. There are different classifications of tool wear in the metal cutting process such as abrasion, adhesion, fatigue, diffusion, and chemical wear [7]. In hard turning, not only tool geometry and cutting conditions but also the cutting tool type and composition and hardness of the workpiece materials are important factors influencing wear mechanisms. Cutting type (continuous or intermittent turning) is also an important factor affecting tool wear behavior. Results on intermittent turning using a cemented carbide cutting tool showed that the wear type that generally occurred was flank wear, and the wear mechanisms were abrasion, adhesion, and oxidation [8]. The most prominent mechanisms of tool wear in typical hard turning applications have been found to be abrasion, adhesion, and diffusion [9]. Numerous studies have been carried out to describe the tool wear and wear mechanisms in the hard turning process. These studies can be categorized into four groups: (a) workpiece material type; (b) cutting tool type; (c) cutting edge geometry; (d) wear type and mechanisms. An extensive studies were performed by [10–14] to investigate the tool wear mechanisms of CBN cutting tools in turning the
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following hardened steels: AISI D2 cold work steel, AISI H11 hot work steel, 35 NiCrMo16 hot work steel, and 100 Cr6 bearing steel (AISI 52100), treated at 54 HRC. A different study was carried out to investigate the effect of workpiece material hardness on cutting forces using the finite element modeling technique [15]. Generally, in the hard turning process, steel and alloy steel materials have been used as workpiece materials. However, Aslantas et al. [16,17] investigated the wear behavior of CBN and ceramic cutting tools in turning austempered ductile iron (56HRc). Another important factor in the hard turning process is tool geometry or edge preparation. The edge preparation of the cutting tool (round or chamfered edge) affects the cutting forces, cutting temperature, and tool wear. Grzesik [18] performed an extensive study characterizing the surface roughness generated during hard turning with conventional and wiper ceramic tools at a variable feed rate. A similar study was also reported by Grzesik and Wanat [19]. Their study investigated the surface roughness produced in the turning of hardened low-chromium alloy steel using mixed alumina–titanium carbon ceramic cutting tools equipped with both conventional and wiper inserts. Their results also demonstrated that wiper inserts working with a double feed rate showed similar wear behavior. In another study, the effect of tool nose radius on finish turning of hardened AISI 52100 steels was investigated by Chou and Song [20]. Their results showed that a large tool nose radius gave a finer surface finish, but the wear behavior of a large tool nose radius was similar to that of a small nose radius tool. The alumina-based ceramic cutting tools are subjected to not only flank wear but also to crater wear and notch wear, especially where machining hard and tough materials [21]. In the literature, many studies have reported on tool wear behavior and tool life in the machining of hardened steel. The most observed wear mechanisms are abrasion and diffusion. Flank wear and crater wear are the most commonly encountered wear types due to diffusion and abrasion. Xiao [22] has observed that zirconia toughened alumina ceramic cutting tools and TiC mixed alumina ceramic tools are more suitable for machining hardened steel than other ceramic tools because of their superior flank wear resistance. Kumar et al. studied the machinability of hardened steel (EN24) using alumina based ceramic cutting tools [23]. Two types of ceramic cutting tools, namely a Ti[C,N] mixed alumina ceramic cutting tool and a zirconia toughened alumina ceramic cutting tool, were used. Their results showed that the mixed alumina ceramic cutting tool was more affected by adhesive wear. Besides, diffusion wear was higher in the mixed ceramic cutting tool, but the zirconia toughened alumina ceramic tool was not affected by diffusion wear. Hong [24] conducted tool wear studies on various ceramic tools and observed that oxide and mixed ceramic tools are more suitable for machining hardened steel than other ceramic tools. Chou et al. [25] investigated the performance and wear behavior of different CBN tools in turning of hardened AISI 52100 steel. The results indicated that low CBN content tools consistently performed better than those with high CBN content. The flank wear rates were proportional to cutting speed and high CBN tools exhibited accelerated thermal wear associated with high cutting temperatures. Arsecularatne et al. [26] studied the wear mechanisms of cutting tools. They used tungsten carbide (WC), CBN, and Polycrystalline Diamond (PCD) as cutting tool materials. Their results showed that the most likely dominant tool wear mechanism was diffusion for the WC tools and chemical wear for the CBN tools. However, Huang and Dawson [27] reported that adhesion was found to be the dominant wear mechanism when turning hardened AISI 52100 using the low CBN content insert. Literature review has indicated that considerable amounts of research effort have been devoted to the study of tool life and wear mechanism in hard turning process. So far, the performance of coated mixed ceramic tools has not been completely studied in the hard turning process. It is obviously known that coating can
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improve wear resistance and increase tool life. Therefore, this paper focuses on the performance of ceramic cutting tools coated with TiN in terms of tool life and wear resistance. 2. Materials and methods Turning tests using hardened AISI 52100 steel as the workpiece material were carried out on a Johnford T35 lathe with 10 kW spindle power and a maximum spindle speed of 6500 rpm. Bars of 50 mm diameter and 180 mm cutting length (see Fig. 1) were turned with coated and uncoated mixed ceramic cutting tools. The single point cutting tests were carried out at feed rates of 0.07, 0.11, and 0.14 mm/rev, cutting speeds of 100, 150, 200, 250, and 300 m/min, and constant depth of cut of 0.5. In turning of AISI 52100, a mixed Al2 O3 –TiCN ceramic insert containing 77% Al2 O3 , 21% TiCN, and 2% other material was used. TiN coated (TNMA 160408T IN420) and uncoated (TNMA 160408T IN23) ceramic tools were produced by Iscar. Inserts have – 6◦ rake angle, 6◦ clearance angle and 0.8 mm nose. Cutting tests were carried out according to ISO Standard No. 3685-1993 (E). The two most commonly encountered types of damage in the hard turning process are flank wear and crater wear. As the chip depth produced especially in finish turning processes is very low, wear that occurs on the tool mostly occurs in the nose area (Fig. 2). The fact that the chip section is very small and the workpiece is hard causes high cutting tensions on the cutting area. In addition, the flank geometry of the tool causes a negative chip angle and increases passive cutting forces, thus making crater wear in the rake face inevitable. In this study, tool lives are obtained by taking into consideration these two damage types that occur on the tool after the cutting process. As the type of damage that is common especially in TiN coated tools is crater type, the length KL and width KB (Fig. 2) of the crater are used to determine the tool life. 3. Experimental results 3.1. Macroscopic observation Fig. 2 integrally illustrates the images of the rake face of the TiN coated ceramic tool for different cutting times and its final state after 3.78 min. As shown in Fig. 3, crater formation occurs as early as after 0.22 min of the cutting process. The length and width of craters also increase with increased cutting times. However, it can be observed in Fig. 3 that such an increase is lower than the speed of crater formation during the first 0.22 min. High cutting speeds and material hardness cause the cutting temperature to increase. This can be observed when the cutting process is started. It is considered that the crater size increases due to the abrasive mechanism that occurs with the increased cutting speed. The high temperature that occurs on the cutting area causes the tool hot hardness to decrease. This speeds up the material loss from the tool surface. As seen in Fig. 3, the flank surface of the tool is also affected by high temperature. Local burn occurs on the tool due to the flow direction of the high-temperature chips that are produced during the turning process. 3.2. Microscopic observation It is known that the coating used on the cutting tool significantly affects the tool abrasion progress. In this study, we attempted to show the damage mechanisms that occur in cutting tools used in experimental tests conducted under similar cutting conditions. In this regard, the changes in the abrasion profiles of TiN coated and uncoated ceramic cutting tools according to the cutting time are given in Figs. 4 and 5. Crater wear on the TiN coated cutting tool
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Fig. 1. Experimental setup and dimension of workpiece material (units are in mm).
Fig. 2. Schematic view of crater wear and flank wear at tool nose (KL : length of crater, KB : crater width, VB : flank wear width, b: flank wear length).
can be clearly observed. As cutting time increases, both the length and the width of the crater also increase. In addition, flank/nose wear occurrence on the tool tip radius is also at the minimum level. As seen in Fig. 4, a minimum level of chip sticking (Built-UpEdge) (BUE) also occurred on the cutter edge. Crater formation can also be clearly observed in the cutting process carried out with the uncoated cutting tool (Fig. 5). However, small sized chipping occurs on the nose radius of the tool with increased cutting time. Two main factors affect the formation of chipping-type damage. One of these is tool vibration and the other is shock loads. In this study, even the slightest vibration occurring on the tool holder caused a
fracture of the ceramic tool. No chipping is observed on the TiN coated cutting tool that is subjected to the cutting process under the same conditions. This is because the coating applied on the ceramic tool provides toughness for the cutting tool. Therefore, a coated cutting tool is not affected by the vibrations and possible shock loads. A common point of Figs. 4 and 5 is that flank/nose wear did not occur or occurred at very small levels on both cutting tools. It is observed that chipping damage is minimal in the uncoated ceramic cutting tool at low feed rates (f ≤ 0.07 mm/rev). However, when cutting time increases, small fractures occur on the nose
Fig. 3. Optical image of the worn rake face for different cutting time (CT) at cutting speed for Vc = 250 m/min, feed rate of f = 0.07 mm/rev. and depth of cut of ap = 0.5 mm.
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Fig. 4. Vc = 150 m/min, f = 0.11 mm/rev crater wear formation in TiN coated mixed ceramic cutting tool (CL: cutting length).
Fig. 5. Vc = 150 m/min, f = 0.11 mm/rev crater wear formation in uncoated mixed ceramic cutting tool (CL: cutting length).
radius of the cutter (Fig. 6a). In addition, we can say that the nose wear on the nose area of the cutting tool also increases (Fig. 6b). BUE occurrence on the uncoated ceramic tool is local and occurs in very small sizes. However, BUE occurrence on the TiN coated cutting tool was present along the whole edge that performed the cutting process (see Fig. 4). Al2 O3 based cutting tools are basically ceramic and differ from metal materials. In addition, Al2 O3 has a more stable composition and is harder to involve in a reaction with AISI 52100 material. On the other hand, nitride (N) in TiN coating is more unstable and reacts at high temperatures. Therefore, the workpiece sticks to the tool due to the effect of high temperature caused by chipping. As a result of the SEM analyses carried out, it is found that BUE formation may cause debonding in coating material (Fig. 7). EDX analysis performed along the line in the SEM image given in Fig. 7 clearly shows that coating material has separated from the substrate material. Point “1” specified in the SEM image is the point where EDX data started to be obtained. As seen on the large graph in Fig. 7, the iron (Fe) content at this point is significantly dominant. As we get closer to point 2, iron is replaced by Ti. Because we know
that Fe comes from the workpiece, it can be said that the abrasive wear mechanism at this point of the cutting tool is effective with adhesive wear. In the small graph given in Fig. 6, detected elements are specified by the shaded line. The element Al comes from the cutting tool material. However, it is interesting that the amount is less compared to Ti and Fe. As a result of the abrasive wear mechanism, Fe on the workpiece was smeared on Ti in the cutting tool. In Fig. 8, an EDX result for an uncoated cutting ceramic cutter tool is given. In Fig. 8, the most dominant element after Al is Ti. The reason for this is that the ceramic cutting tool used is alumina based ceramic containing TiCN. BUE occurrence is also observed at the tip radius of this cutting tool. Elements dominant in the crater area are Al, Ti, and O. A very small amount of Fe can also be detected in the crater area. As we get closer to the tool tip radius, we can observe that Fe content increases. As mentioned before, such an increase is caused by the chips sticking to the cutting edge. The existence of Fe in the crater area is an indication that the workpiece sticks to the tool surface as a result of the adhesive wear mechanism. However, despite the C content, the high Al content shows that the ceramic tool does not
Fig. 6. Vc = 150 m/min, f = 0.07 mm/rev crater wear formation in uncoated mixed ceramic cutting tool (CL: cutting length).
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Fig. 7. EDX Line analysis of TiN coated mixed ceramic cutting tool.
Fig. 8. EDX Line analysis of uncoated mixed ceramic cutting tool.
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Fig. 9. Variation of average surface roughness with cutting time for coated and uncoated cutting tool.
react chemically with the workpiece despite the high temperature and compressive stress that occur during the cutting process. In addition, the C element in the crater area also follows a fixed content until the cutting edge radius. Here, the existence of C is caused by TiC. However, the increase in C towards the cutting edge is due to the high C content in 52100 material.
3.3. Variation of surface roughness As is known, the surface quality obtained in the hard turning process is close to that obtained by grinding. Generally, the average surface roughness value, Ra, obtained in the turning process is within the range 0.1–0.9 m. As tool wear directly affects surface quality, the obtained surface quality may also be considered as a tool failure criterion. Therefore changes in average surface roughness values depending on cutting time for different cutting speeds and for both coated and uncoated ceramic cutting tools are given in Fig. 9. In addition, the wear/damage status occurring on both tools until completion of the experimental tests is given in the graph. If we take into consideration the surface roughness for tool life, the value of Ra reaches 0.5 m in the coated ceramic tool, after 8 min of the cutting process. However, this period is approximately eight times less in the uncoated ceramic tool. It is observed that crater wear is prevalent on the coated tool following 10 min cutting. In addition, as a result of the abrasive wear mechanism, flank wear also occurred on the nose area. Following a 3-min cutting process, two different fracture areas occurred on the uncoated cutting tool. In particular, chipping-type fractures (see Fig. 6) that occurred on the nose area during the initial cutting periods caused large scale fractures on the chipped surface of the cutting tool. Therefore the surface roughness values of the workpiece increased rapidly. Comparing both ceramic inserts, it can be noticed that the TiN coated ceramic provided a better surface finish, probably due to its superior wear resistance, which maintained the integrity of the cutting edge for a longer period.
3.4. Effect of wear on chip flow direction Depending on the increasing cutting time, wear or chipping that occurs on both tools affects chip flow direction. Especially in coated ceramic tools, it is observed that chip flow direction changes as crater depth and length increase. When cutting is started, the chip is removed from the cutting area without contacting the workpiece. Depending on the crater depth and length, the chip is removed from the cutting area by contacting the workpiece (Fig. 10a). In the uncoated ceramic tool, chip formation and chip flow occur differently. Although a tape chip form is produced during the initial cutting time, the chip gathers in front of the tool as a result of the rapid damage that occurs on the cutting tool (Fig. 10b). As it becomes harder to remove the chip from the cutting area, the surface quality of the workpiece is negatively affected. As the temperature in the cutting area increases significantly, it is observed that the chip sticks to the processed surface of the workpiece over time. This sticking initially occurs at certain intervals (Fig. 11a). However, with the increase in chipping/fracture type damage that occurs on the tool, chip sticking occurs throughout the surface of the workpiece (Fig. 11b). 4. Tool life Tool life generally depends on workpiece material, cutting parameters (especially cutting speed), and cutting fluid. Tool wear is one of the fundamental elements that is inversely proportional to tool life and determines tool life. Apart from tool wear, tool fracture or excessive chipping and surface roughness also affect tool life. The tool rejection criteria for hard turning are employed flank wear, crater wear, notch wear, and excessive chipping or fracture. In this study, the tool life (maximum allowable machining time) is calculated from the crater wear. As flank wear or nose wear occurred on a very small scale, they are not taken into consideration in calculating the tool life in this study. As it is not possible to measure crater depth sensitively, KL = 0.65/KB = 0.1 mm is taken as the tool failure criteria and plotted against the cutting speed for coated and uncoated ceramic cutting tools in Figs. 12 and 13. Tool life values are also given
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Fig. 10. Chip formation during turning of hardened AISI 52100 steel, (a) TiN coated mixed ceramic tool, (b) uncoated mixed ceramic tool.
Fig. 11. (a) Long chip adhered to the workpiece and (b) chip welds to the workpiece surface during cutting with uncoated insert.
Fig. 12. Variation of tool life of coated and uncoated mixed ceramic inserts for f = 0.05 mm/rev.
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Fig. 13. Variation of tool life of TiN coated mixed ceramic inserts for different feed rates.
Table 1 Tool life values for coated and uncoated mixed ceramic tool. Cutting speed (m/min)
100 200 300
Tool life (min) Coated mixed ceramic
Uncoated mixed ceramic
43.03 3.2 0.47
13.34 1.73 0.18
Table 2 Tool life values of coated mixed ceramic for different cutting parameters. Tool life for coated mixed ceramic (min) Feed rate (mm/rev) Cutting speed (m/min) 100 200 300
0.07
0.11
0.14
22.99 3.33 2.93
8.89 1.39 1.32
3.4 0.42 0.18
in Tables 1 and 2. As seen in Fig. 12, both coated and uncoated cutting tools have approximately the same grades. The ratio of Tcoated /Tuncoated is 2.6 for a cutting speed of 300 m/min whereas it is about 3.2 for a cutting speed of 100 m/min. According to Fig. 12, it appears that the TiN coating has tripled the life of the cutting tool. Tool life values obtained for different feed rates for coated ceramic cutting tools are given in Fig. 13. Tool life values obtained for f = 0.07 and 0.11 mm/rev are similar to each other. While the ratio of T300 /T200 is 1.13 for feed rate f = 0.07, it is 1.05 for feed rate f = 0.11. On the other hand, the ratio of T300 /T200 is determined to be 2.3 for feed rate f = 0.14. When minimum and maximum feed values are evaluated, the ratio of T0.7 /T0.14 is approximately 6.8 for V = 100 m/min. This means that a two-fold increase in feed rate has caused a 6.8 fold decrease in tool life. 5. Discussion The most important result of this study is that TiN coating significantly increases the wear resistance of ceramic tools. In addition, it is observed that chipping-type damages occurred less in coated tools or did not occur at all. The reason for that is that TiN
coating made the ceramic tool gains a certain degree of toughness. This toughness caused a decrease in chipping type damage in the cutting tool, in particular. In cutting tests carried out under the same conditions, chipping damage in uncoated cutting tools increased with cutting time. This is also observed in the results of experiments on surface roughness. Chipping that increases over time causes catastrophic damage and fractures in the tool. As a result, the surface quality of the processed part deteriorates within a very short period of time. If evaluation is performed in terms of surface roughness, the roughness value obtained with a coated tool is better compared to that obtained with an uncoated tool. Negative rake angle is the basic factor causing the thrust force to exceed the cutting force in hard turning process. Where tool wear occurs, forces due to wear take place around the tool worn flank face of the cutting edge ABD as shown in Fig. 14a [15]. Our results imply that the hardened workpiece has an abrasive effect on the cutting tool material. High temperature and high stress value at the cutting zone cause diffusion and abrasion on the tool surface. Therefore, the cutting tool geometry deteriorates and the surface roughness values of the workpiece increase rapidly. Besides the crater wear formation on the rake face results in changing the chip flow direction. The chip up-curl radius decreases with increasing crater depth. The chip gathers in front of the tool as a result of the crater that occurs on the cutting tool (Fig. 14b). The chip temperature closes to melting point due to its small deformed chip thickness and welds to the workpiece surface. High temperature and chemical reactions that occur during the hard turning process contribute to crater wear in the cutting tool. Crater wear is the dominant wear type in both uncoated and coated cutting tools. However, crater formation in uncoated ceramic tools occurs within a shorter period of time and fractures occur on the tool tip before the crater becomes deeper. Therefore, cutting tool geometry deteriorates. Damage that occurs on the tool tip also changes the chip shape and flow direction. The thermal conductivity value of Al2 O3 material decreases with increases in temperature [28]. Lower thermal conductivity results in an increase in the temperature in the lower side of the chip. This causes a thermal bi-metallic effect between the upper and lower sides of the chip that forces
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Fig. 14. (a) Cutting geometric model under typical hard turning conditions, (b) typical chip curling pattern in the case of formation crater wear on rake face.
Fig. 15. Ti1 >Ti2 tool–chip interface temperature, qt1 < qt2 heat flow to the cutting tool, ru1 < ru1 chip up-curl radius.
the chip to curl a smaller radius [29,30] (Fig. 14). The basic ingredient of the mixed ceramic cutting tool is Al2 O3 . Al2 O3 material results in an increase in heat flow to the chip and workpiece due to its thermal conductivity properties. As a result of this a large part of the heat which occurs on the cutting area is transferred to the chip (Fig. 15). Therefore, the temperature at the tool–chip interface increases. With a small section area and a high temperature, the chip cannot be removed from the cutting area and sticks to the workpiece (see Fig. 11b). In coated mixed ceramic tool, the thermal conductivity value of TiN coating material increases with increases in temperature [28]. Therefore, the heat flow to the cutting tool increases and the temperature at the tool–chip interface decreases. The temperature difference between the upper and lower sides of the chip decreases and the chip up-curl radius increases. Chip sticking is observed in the TiN coated cutting tool. The minimum level of chip sticking (BUE formation) occurred with the uncoated tool, whereas BUE formation was at its maximum in the TiN coated cutting tool. A main reason for BUE formation might be the chemical inclination of TiN coating material with the workpiece material. The more stable structure of Al2 O3 allowed the minimum amount of BUE formation to occur.
investigated. The wear type that is dominant in both coated and uncoated cutting tools is crater wear. However, craters that occur in uncoated cutting tools are transformed into chipping or fractures within a short period of time. In addition, the minimum level of BUE formation occurred in the uncoated cutting tool. On the other hand, the coated cutting tool is more inclined to BUE formation. TiN approximately tripled both the wear resistance and the tool life. When evaluated in terms of surface roughness, the TiN coated cutting tool has a cutting distance approximately eight times longer. Besides TiN coating made the ceramic tool gain a certain degree of toughness. This toughness caused a decrease in chipping type damage in the cutting tool. The type of wear or damage that occurs on the tool also changes the direction of chip flow. Uncoated mixed ceramic tool having lower thermal conductivity results in an increase in the temperature at the tool chip interface. This causes a thermal bi-metallic effect between the upper and lower sides of the chip that forces the chip to curl a smaller radius. Besides the crater wear formation on the rake face results in changing the chip flow direction. The chip up-curl radius decreases with increasing crater depth. The chip gathers in front of the tool as a result of the crater that occurs on the cutting tool.
6. Conclusion
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Tool life and wear mechanism of coated and uncoated Al2 O3 /TiCN mixed ceramic tools in turning hardened alloy steel K. Aslantas a,∗ , I˙ . Ucun b , A. C¸icek c a
Afyon Kocatepe University, Faculty of Techonolgy, Department of Mechanical Engineering, Afyonkarahisar, Turkey Afyon Kocatepe University, Technical Education Faculty, Department of Mechanical Education, Afyokarahisar, Turkey c Düzce University, Faculty of Technology, Department of Manufacturing Engineering, Düzce, Turkey b
a r t i c l e
i n f o
Article history: Received 22 July 2011 Received in revised form 23 November 2011 Accepted 28 November 2011 Available online 13 December 2011 Keywords: Cutting tool Steel Engineering ceramic Hard materials PVD coatings
a b s t r a c t The focus of this paper is the continuous turning of hardened AISI 52100 (∼63HRc) using coated and uncoated ceramic Al2 O3 –TiCN mixed inserts, which are cheaper than cubic boron nitride (CBN) or polycrystalline cubic boron nitride (PCBN). The machinability of hardened steel was evaluated by measurements of tool wear, tool life, and surface finish of the workpiece. Wear mechanisms and patterns of ceramic inserts in hard turning of hardened AISI 52100 are discussed. According to the results obtained, fracture and chipping type damages occur more frequently in uncoated tools, whereas crater wear is the more common type of damage in TiN coated tools. Most important result obtained from the study is that TiN coating and crater wear affect chip flow direction. In uncoated ceramic tool, the crater formation results in decrease of chip up-curl radius. Besides, uncoated cutting tool results in an increase in the temperature at the tool chip interface. This causes a thermal bi-metallic effect between the upper and lower sides of the chip that forces the chip to curl a smaller radius. Chips accumulate in front of the tool and stick to the workpiece depending on the length of the cutting time. This causes the surface quality to deteriorate. TiN coating not only ensures that the cutting tool is tougher, but also ensures that the surface quality is maintained during cutting processes. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Hard turning is a turning operation which is applied on highresistance alloy steels (45 < HRC < 65) to obtain surface roughness values that are close to those obtained in grinding (Ra ∼ 0.1 m). The workpiece materials involved include various hardened alloy steels, tool steels, case-hardened steels, superalloys, nitrided irons and hard-chrome-coated steels, and heat-treated powder metallurgical parts [1]. Although this production method is a new subject, there are quite a few studies by several researchers in the literature. These studies mostly concern the turning of AISI 52100 bearing steel, H11–H13 hot work tool steel, and AISI 4130–4340 low alloy steel using CBN, ceramic, and coated carbide tools. The most common problem encountered in the hard turning process is tool wear. Therefore, ceramic, PCBN, and CBN cutting tools are generally used for hard turning processes [2–5]. The use of alumina based ceramic tools in hard machining is an attractive alternative to grinding in order to reduce processing costs, improve material properties, and for the environmental benefits [6]. Advances in ceramic processing technology have resulted in
∗ Corresponding author. Tel.: +90 272 228 13 11; fax: +90 272 228 13 19. E-mail address: [email protected] (K. Aslantas). 0043-1648/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2011.11.010
a new generation of high performance ceramic cutting tools that exhibit properties such as fracture strength, toughness, thermal shock resistance, hardness, and wear resistance. Therefore ceramic tools are used in the machining of various types of steels and hard materials. There are different classifications of tool wear in the metal cutting process such as abrasion, adhesion, fatigue, diffusion, and chemical wear [7]. In hard turning, not only tool geometry and cutting conditions but also the cutting tool type and composition and hardness of the workpiece materials are important factors influencing wear mechanisms. Cutting type (continuous or intermittent turning) is also an important factor affecting tool wear behavior. Results on intermittent turning using a cemented carbide cutting tool showed that the wear type that generally occurred was flank wear, and the wear mechanisms were abrasion, adhesion, and oxidation [8]. The most prominent mechanisms of tool wear in typical hard turning applications have been found to be abrasion, adhesion, and diffusion [9]. Numerous studies have been carried out to describe the tool wear and wear mechanisms in the hard turning process. These studies can be categorized into four groups: (a) workpiece material type; (b) cutting tool type; (c) cutting edge geometry; (d) wear type and mechanisms. An extensive studies were performed by [10–14] to investigate the tool wear mechanisms of CBN cutting tools in turning the
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following hardened steels: AISI D2 cold work steel, AISI H11 hot work steel, 35 NiCrMo16 hot work steel, and 100 Cr6 bearing steel (AISI 52100), treated at 54 HRC. A different study was carried out to investigate the effect of workpiece material hardness on cutting forces using the finite element modeling technique [15]. Generally, in the hard turning process, steel and alloy steel materials have been used as workpiece materials. However, Aslantas et al. [16,17] investigated the wear behavior of CBN and ceramic cutting tools in turning austempered ductile iron (56HRc). Another important factor in the hard turning process is tool geometry or edge preparation. The edge preparation of the cutting tool (round or chamfered edge) affects the cutting forces, cutting temperature, and tool wear. Grzesik [18] performed an extensive study characterizing the surface roughness generated during hard turning with conventional and wiper ceramic tools at a variable feed rate. A similar study was also reported by Grzesik and Wanat [19]. Their study investigated the surface roughness produced in the turning of hardened low-chromium alloy steel using mixed alumina–titanium carbon ceramic cutting tools equipped with both conventional and wiper inserts. Their results also demonstrated that wiper inserts working with a double feed rate showed similar wear behavior. In another study, the effect of tool nose radius on finish turning of hardened AISI 52100 steels was investigated by Chou and Song [20]. Their results showed that a large tool nose radius gave a finer surface finish, but the wear behavior of a large tool nose radius was similar to that of a small nose radius tool. The alumina-based ceramic cutting tools are subjected to not only flank wear but also to crater wear and notch wear, especially where machining hard and tough materials [21]. In the literature, many studies have reported on tool wear behavior and tool life in the machining of hardened steel. The most observed wear mechanisms are abrasion and diffusion. Flank wear and crater wear are the most commonly encountered wear types due to diffusion and abrasion. Xiao [22] has observed that zirconia toughened alumina ceramic cutting tools and TiC mixed alumina ceramic tools are more suitable for machining hardened steel than other ceramic tools because of their superior flank wear resistance. Kumar et al. studied the machinability of hardened steel (EN24) using alumina based ceramic cutting tools [23]. Two types of ceramic cutting tools, namely a Ti[C,N] mixed alumina ceramic cutting tool and a zirconia toughened alumina ceramic cutting tool, were used. Their results showed that the mixed alumina ceramic cutting tool was more affected by adhesive wear. Besides, diffusion wear was higher in the mixed ceramic cutting tool, but the zirconia toughened alumina ceramic tool was not affected by diffusion wear. Hong [24] conducted tool wear studies on various ceramic tools and observed that oxide and mixed ceramic tools are more suitable for machining hardened steel than other ceramic tools. Chou et al. [25] investigated the performance and wear behavior of different CBN tools in turning of hardened AISI 52100 steel. The results indicated that low CBN content tools consistently performed better than those with high CBN content. The flank wear rates were proportional to cutting speed and high CBN tools exhibited accelerated thermal wear associated with high cutting temperatures. Arsecularatne et al. [26] studied the wear mechanisms of cutting tools. They used tungsten carbide (WC), CBN, and Polycrystalline Diamond (PCD) as cutting tool materials. Their results showed that the most likely dominant tool wear mechanism was diffusion for the WC tools and chemical wear for the CBN tools. However, Huang and Dawson [27] reported that adhesion was found to be the dominant wear mechanism when turning hardened AISI 52100 using the low CBN content insert. Literature review has indicated that considerable amounts of research effort have been devoted to the study of tool life and wear mechanism in hard turning process. So far, the performance of coated mixed ceramic tools has not been completely studied in the hard turning process. It is obviously known that coating can
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improve wear resistance and increase tool life. Therefore, this paper focuses on the performance of ceramic cutting tools coated with TiN in terms of tool life and wear resistance. 2. Materials and methods Turning tests using hardened AISI 52100 steel as the workpiece material were carried out on a Johnford T35 lathe with 10 kW spindle power and a maximum spindle speed of 6500 rpm. Bars of 50 mm diameter and 180 mm cutting length (see Fig. 1) were turned with coated and uncoated mixed ceramic cutting tools. The single point cutting tests were carried out at feed rates of 0.07, 0.11, and 0.14 mm/rev, cutting speeds of 100, 150, 200, 250, and 300 m/min, and constant depth of cut of 0.5. In turning of AISI 52100, a mixed Al2 O3 –TiCN ceramic insert containing 77% Al2 O3 , 21% TiCN, and 2% other material was used. TiN coated (TNMA 160408T IN420) and uncoated (TNMA 160408T IN23) ceramic tools were produced by Iscar. Inserts have – 6◦ rake angle, 6◦ clearance angle and 0.8 mm nose. Cutting tests were carried out according to ISO Standard No. 3685-1993 (E). The two most commonly encountered types of damage in the hard turning process are flank wear and crater wear. As the chip depth produced especially in finish turning processes is very low, wear that occurs on the tool mostly occurs in the nose area (Fig. 2). The fact that the chip section is very small and the workpiece is hard causes high cutting tensions on the cutting area. In addition, the flank geometry of the tool causes a negative chip angle and increases passive cutting forces, thus making crater wear in the rake face inevitable. In this study, tool lives are obtained by taking into consideration these two damage types that occur on the tool after the cutting process. As the type of damage that is common especially in TiN coated tools is crater type, the length KL and width KB (Fig. 2) of the crater are used to determine the tool life. 3. Experimental results 3.1. Macroscopic observation Fig. 2 integrally illustrates the images of the rake face of the TiN coated ceramic tool for different cutting times and its final state after 3.78 min. As shown in Fig. 3, crater formation occurs as early as after 0.22 min of the cutting process. The length and width of craters also increase with increased cutting times. However, it can be observed in Fig. 3 that such an increase is lower than the speed of crater formation during the first 0.22 min. High cutting speeds and material hardness cause the cutting temperature to increase. This can be observed when the cutting process is started. It is considered that the crater size increases due to the abrasive mechanism that occurs with the increased cutting speed. The high temperature that occurs on the cutting area causes the tool hot hardness to decrease. This speeds up the material loss from the tool surface. As seen in Fig. 3, the flank surface of the tool is also affected by high temperature. Local burn occurs on the tool due to the flow direction of the high-temperature chips that are produced during the turning process. 3.2. Microscopic observation It is known that the coating used on the cutting tool significantly affects the tool abrasion progress. In this study, we attempted to show the damage mechanisms that occur in cutting tools used in experimental tests conducted under similar cutting conditions. In this regard, the changes in the abrasion profiles of TiN coated and uncoated ceramic cutting tools according to the cutting time are given in Figs. 4 and 5. Crater wear on the TiN coated cutting tool
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Fig. 1. Experimental setup and dimension of workpiece material (units are in mm).
Fig. 2. Schematic view of crater wear and flank wear at tool nose (KL : length of crater, KB : crater width, VB : flank wear width, b: flank wear length).
can be clearly observed. As cutting time increases, both the length and the width of the crater also increase. In addition, flank/nose wear occurrence on the tool tip radius is also at the minimum level. As seen in Fig. 4, a minimum level of chip sticking (Built-UpEdge) (BUE) also occurred on the cutter edge. Crater formation can also be clearly observed in the cutting process carried out with the uncoated cutting tool (Fig. 5). However, small sized chipping occurs on the nose radius of the tool with increased cutting time. Two main factors affect the formation of chipping-type damage. One of these is tool vibration and the other is shock loads. In this study, even the slightest vibration occurring on the tool holder caused a
fracture of the ceramic tool. No chipping is observed on the TiN coated cutting tool that is subjected to the cutting process under the same conditions. This is because the coating applied on the ceramic tool provides toughness for the cutting tool. Therefore, a coated cutting tool is not affected by the vibrations and possible shock loads. A common point of Figs. 4 and 5 is that flank/nose wear did not occur or occurred at very small levels on both cutting tools. It is observed that chipping damage is minimal in the uncoated ceramic cutting tool at low feed rates (f ≤ 0.07 mm/rev). However, when cutting time increases, small fractures occur on the nose
Fig. 3. Optical image of the worn rake face for different cutting time (CT) at cutting speed for Vc = 250 m/min, feed rate of f = 0.07 mm/rev. and depth of cut of ap = 0.5 mm.
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Fig. 4. Vc = 150 m/min, f = 0.11 mm/rev crater wear formation in TiN coated mixed ceramic cutting tool (CL: cutting length).
Fig. 5. Vc = 150 m/min, f = 0.11 mm/rev crater wear formation in uncoated mixed ceramic cutting tool (CL: cutting length).
radius of the cutter (Fig. 6a). In addition, we can say that the nose wear on the nose area of the cutting tool also increases (Fig. 6b). BUE occurrence on the uncoated ceramic tool is local and occurs in very small sizes. However, BUE occurrence on the TiN coated cutting tool was present along the whole edge that performed the cutting process (see Fig. 4). Al2 O3 based cutting tools are basically ceramic and differ from metal materials. In addition, Al2 O3 has a more stable composition and is harder to involve in a reaction with AISI 52100 material. On the other hand, nitride (N) in TiN coating is more unstable and reacts at high temperatures. Therefore, the workpiece sticks to the tool due to the effect of high temperature caused by chipping. As a result of the SEM analyses carried out, it is found that BUE formation may cause debonding in coating material (Fig. 7). EDX analysis performed along the line in the SEM image given in Fig. 7 clearly shows that coating material has separated from the substrate material. Point “1” specified in the SEM image is the point where EDX data started to be obtained. As seen on the large graph in Fig. 7, the iron (Fe) content at this point is significantly dominant. As we get closer to point 2, iron is replaced by Ti. Because we know
that Fe comes from the workpiece, it can be said that the abrasive wear mechanism at this point of the cutting tool is effective with adhesive wear. In the small graph given in Fig. 6, detected elements are specified by the shaded line. The element Al comes from the cutting tool material. However, it is interesting that the amount is less compared to Ti and Fe. As a result of the abrasive wear mechanism, Fe on the workpiece was smeared on Ti in the cutting tool. In Fig. 8, an EDX result for an uncoated cutting ceramic cutter tool is given. In Fig. 8, the most dominant element after Al is Ti. The reason for this is that the ceramic cutting tool used is alumina based ceramic containing TiCN. BUE occurrence is also observed at the tip radius of this cutting tool. Elements dominant in the crater area are Al, Ti, and O. A very small amount of Fe can also be detected in the crater area. As we get closer to the tool tip radius, we can observe that Fe content increases. As mentioned before, such an increase is caused by the chips sticking to the cutting edge. The existence of Fe in the crater area is an indication that the workpiece sticks to the tool surface as a result of the adhesive wear mechanism. However, despite the C content, the high Al content shows that the ceramic tool does not
Fig. 6. Vc = 150 m/min, f = 0.07 mm/rev crater wear formation in uncoated mixed ceramic cutting tool (CL: cutting length).
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Fig. 7. EDX Line analysis of TiN coated mixed ceramic cutting tool.
Fig. 8. EDX Line analysis of uncoated mixed ceramic cutting tool.
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Fig. 9. Variation of average surface roughness with cutting time for coated and uncoated cutting tool.
react chemically with the workpiece despite the high temperature and compressive stress that occur during the cutting process. In addition, the C element in the crater area also follows a fixed content until the cutting edge radius. Here, the existence of C is caused by TiC. However, the increase in C towards the cutting edge is due to the high C content in 52100 material.
3.3. Variation of surface roughness As is known, the surface quality obtained in the hard turning process is close to that obtained by grinding. Generally, the average surface roughness value, Ra, obtained in the turning process is within the range 0.1–0.9 m. As tool wear directly affects surface quality, the obtained surface quality may also be considered as a tool failure criterion. Therefore changes in average surface roughness values depending on cutting time for different cutting speeds and for both coated and uncoated ceramic cutting tools are given in Fig. 9. In addition, the wear/damage status occurring on both tools until completion of the experimental tests is given in the graph. If we take into consideration the surface roughness for tool life, the value of Ra reaches 0.5 m in the coated ceramic tool, after 8 min of the cutting process. However, this period is approximately eight times less in the uncoated ceramic tool. It is observed that crater wear is prevalent on the coated tool following 10 min cutting. In addition, as a result of the abrasive wear mechanism, flank wear also occurred on the nose area. Following a 3-min cutting process, two different fracture areas occurred on the uncoated cutting tool. In particular, chipping-type fractures (see Fig. 6) that occurred on the nose area during the initial cutting periods caused large scale fractures on the chipped surface of the cutting tool. Therefore the surface roughness values of the workpiece increased rapidly. Comparing both ceramic inserts, it can be noticed that the TiN coated ceramic provided a better surface finish, probably due to its superior wear resistance, which maintained the integrity of the cutting edge for a longer period.
3.4. Effect of wear on chip flow direction Depending on the increasing cutting time, wear or chipping that occurs on both tools affects chip flow direction. Especially in coated ceramic tools, it is observed that chip flow direction changes as crater depth and length increase. When cutting is started, the chip is removed from the cutting area without contacting the workpiece. Depending on the crater depth and length, the chip is removed from the cutting area by contacting the workpiece (Fig. 10a). In the uncoated ceramic tool, chip formation and chip flow occur differently. Although a tape chip form is produced during the initial cutting time, the chip gathers in front of the tool as a result of the rapid damage that occurs on the cutting tool (Fig. 10b). As it becomes harder to remove the chip from the cutting area, the surface quality of the workpiece is negatively affected. As the temperature in the cutting area increases significantly, it is observed that the chip sticks to the processed surface of the workpiece over time. This sticking initially occurs at certain intervals (Fig. 11a). However, with the increase in chipping/fracture type damage that occurs on the tool, chip sticking occurs throughout the surface of the workpiece (Fig. 11b). 4. Tool life Tool life generally depends on workpiece material, cutting parameters (especially cutting speed), and cutting fluid. Tool wear is one of the fundamental elements that is inversely proportional to tool life and determines tool life. Apart from tool wear, tool fracture or excessive chipping and surface roughness also affect tool life. The tool rejection criteria for hard turning are employed flank wear, crater wear, notch wear, and excessive chipping or fracture. In this study, the tool life (maximum allowable machining time) is calculated from the crater wear. As flank wear or nose wear occurred on a very small scale, they are not taken into consideration in calculating the tool life in this study. As it is not possible to measure crater depth sensitively, KL = 0.65/KB = 0.1 mm is taken as the tool failure criteria and plotted against the cutting speed for coated and uncoated ceramic cutting tools in Figs. 12 and 13. Tool life values are also given
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Fig. 10. Chip formation during turning of hardened AISI 52100 steel, (a) TiN coated mixed ceramic tool, (b) uncoated mixed ceramic tool.
Fig. 11. (a) Long chip adhered to the workpiece and (b) chip welds to the workpiece surface during cutting with uncoated insert.
Fig. 12. Variation of tool life of coated and uncoated mixed ceramic inserts for f = 0.05 mm/rev.
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Fig. 13. Variation of tool life of TiN coated mixed ceramic inserts for different feed rates.
Table 1 Tool life values for coated and uncoated mixed ceramic tool. Cutting speed (m/min)
100 200 300
Tool life (min) Coated mixed ceramic
Uncoated mixed ceramic
43.03 3.2 0.47
13.34 1.73 0.18
Table 2 Tool life values of coated mixed ceramic for different cutting parameters. Tool life for coated mixed ceramic (min) Feed rate (mm/rev) Cutting speed (m/min) 100 200 300
0.07
0.11
0.14
22.99 3.33 2.93
8.89 1.39 1.32
3.4 0.42 0.18
in Tables 1 and 2. As seen in Fig. 12, both coated and uncoated cutting tools have approximately the same grades. The ratio of Tcoated /Tuncoated is 2.6 for a cutting speed of 300 m/min whereas it is about 3.2 for a cutting speed of 100 m/min. According to Fig. 12, it appears that the TiN coating has tripled the life of the cutting tool. Tool life values obtained for different feed rates for coated ceramic cutting tools are given in Fig. 13. Tool life values obtained for f = 0.07 and 0.11 mm/rev are similar to each other. While the ratio of T300 /T200 is 1.13 for feed rate f = 0.07, it is 1.05 for feed rate f = 0.11. On the other hand, the ratio of T300 /T200 is determined to be 2.3 for feed rate f = 0.14. When minimum and maximum feed values are evaluated, the ratio of T0.7 /T0.14 is approximately 6.8 for V = 100 m/min. This means that a two-fold increase in feed rate has caused a 6.8 fold decrease in tool life. 5. Discussion The most important result of this study is that TiN coating significantly increases the wear resistance of ceramic tools. In addition, it is observed that chipping-type damages occurred less in coated tools or did not occur at all. The reason for that is that TiN
coating made the ceramic tool gains a certain degree of toughness. This toughness caused a decrease in chipping type damage in the cutting tool, in particular. In cutting tests carried out under the same conditions, chipping damage in uncoated cutting tools increased with cutting time. This is also observed in the results of experiments on surface roughness. Chipping that increases over time causes catastrophic damage and fractures in the tool. As a result, the surface quality of the processed part deteriorates within a very short period of time. If evaluation is performed in terms of surface roughness, the roughness value obtained with a coated tool is better compared to that obtained with an uncoated tool. Negative rake angle is the basic factor causing the thrust force to exceed the cutting force in hard turning process. Where tool wear occurs, forces due to wear take place around the tool worn flank face of the cutting edge ABD as shown in Fig. 14a [15]. Our results imply that the hardened workpiece has an abrasive effect on the cutting tool material. High temperature and high stress value at the cutting zone cause diffusion and abrasion on the tool surface. Therefore, the cutting tool geometry deteriorates and the surface roughness values of the workpiece increase rapidly. Besides the crater wear formation on the rake face results in changing the chip flow direction. The chip up-curl radius decreases with increasing crater depth. The chip gathers in front of the tool as a result of the crater that occurs on the cutting tool (Fig. 14b). The chip temperature closes to melting point due to its small deformed chip thickness and welds to the workpiece surface. High temperature and chemical reactions that occur during the hard turning process contribute to crater wear in the cutting tool. Crater wear is the dominant wear type in both uncoated and coated cutting tools. However, crater formation in uncoated ceramic tools occurs within a shorter period of time and fractures occur on the tool tip before the crater becomes deeper. Therefore, cutting tool geometry deteriorates. Damage that occurs on the tool tip also changes the chip shape and flow direction. The thermal conductivity value of Al2 O3 material decreases with increases in temperature [28]. Lower thermal conductivity results in an increase in the temperature in the lower side of the chip. This causes a thermal bi-metallic effect between the upper and lower sides of the chip that forces
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Fig. 14. (a) Cutting geometric model under typical hard turning conditions, (b) typical chip curling pattern in the case of formation crater wear on rake face.
Fig. 15. Ti1 >Ti2 tool–chip interface temperature, qt1 < qt2 heat flow to the cutting tool, ru1 < ru1 chip up-curl radius.
the chip to curl a smaller radius [29,30] (Fig. 14). The basic ingredient of the mixed ceramic cutting tool is Al2 O3 . Al2 O3 material results in an increase in heat flow to the chip and workpiece due to its thermal conductivity properties. As a result of this a large part of the heat which occurs on the cutting area is transferred to the chip (Fig. 15). Therefore, the temperature at the tool–chip interface increases. With a small section area and a high temperature, the chip cannot be removed from the cutting area and sticks to the workpiece (see Fig. 11b). In coated mixed ceramic tool, the thermal conductivity value of TiN coating material increases with increases in temperature [28]. Therefore, the heat flow to the cutting tool increases and the temperature at the tool–chip interface decreases. The temperature difference between the upper and lower sides of the chip decreases and the chip up-curl radius increases. Chip sticking is observed in the TiN coated cutting tool. The minimum level of chip sticking (BUE formation) occurred with the uncoated tool, whereas BUE formation was at its maximum in the TiN coated cutting tool. A main reason for BUE formation might be the chemical inclination of TiN coating material with the workpiece material. The more stable structure of Al2 O3 allowed the minimum amount of BUE formation to occur.
investigated. The wear type that is dominant in both coated and uncoated cutting tools is crater wear. However, craters that occur in uncoated cutting tools are transformed into chipping or fractures within a short period of time. In addition, the minimum level of BUE formation occurred in the uncoated cutting tool. On the other hand, the coated cutting tool is more inclined to BUE formation. TiN approximately tripled both the wear resistance and the tool life. When evaluated in terms of surface roughness, the TiN coated cutting tool has a cutting distance approximately eight times longer. Besides TiN coating made the ceramic tool gain a certain degree of toughness. This toughness caused a decrease in chipping type damage in the cutting tool. The type of wear or damage that occurs on the tool also changes the direction of chip flow. Uncoated mixed ceramic tool having lower thermal conductivity results in an increase in the temperature at the tool chip interface. This causes a thermal bi-metallic effect between the upper and lower sides of the chip that forces the chip to curl a smaller radius. Besides the crater wear formation on the rake face results in changing the chip flow direction. The chip up-curl radius decreases with increasing crater depth. The chip gathers in front of the tool as a result of the crater that occurs on the cutting tool.
6. Conclusion
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