Journal of Materials Processing Technology 129 (2002) 222±226

Wear patterns and mechanisms of cutting tools in high-speed face milling Z.Q. Liu*, X. Ai, H. Zhang, Z.T. Wang, Y. Wan School of Mechanical Engineering, Shandong University, 73 Jingshi Road, Jinan, Shandong 250061, PR China

Abstract High-speed machining has received important interest because it leads to an increase of productivity and a better workpiece surface quality. However, at high cutting speeds, the tool wear increases dramatically due to the high temperature at the tool±workpiece interface. Tool wear impairs the surface ®nish and hence the tool life is reduced. That is why an important objective of metal cutting research has been the assessment of tool wear patterns and mechanisms. In this paper, the wear performances of PCBN tool, ceramic tool, coated carbide tool and ®ne-grained carbide tool in high-speed face milling are presented when cutting cast iron, 45# tempered carbon steel and 45# hardened carbon steel. The tool wear patterns were examined through a toolmaker's microscope. The research results show that the tool wear types differed in various matching of materials between the cutting tool and the workpiece. The dominant wear patterns observed were rake face wear, ¯ank wear, chipping, fracture and breakage. The main wear mechanisms were mechanical friction, adhesion, diffusion and chemical wear promoted by cutting forces and high cutting temperature. Hence, the important considerations of high-speed cutting tool materials are high heatresistance and wear-resistance, and chemical stability as well as resistance to the failure of coatings. The research results will be of great bene®t in the design and the selection of tool materials and in the control of tool wear in high-speed machining processes. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Cutting tool; Wear; High-speed machining; Face milling

1. Introduction A primary objective of manufacturing operations is the ef®cient production of accurate parts. In machining processes, signi®cant research has focused on ways to increase material removal rates without sacri®cing workpiece accuracy. Two basic ways to raise material removal rates are to increase the cutting speed and the chip cross-section. Increasing the chip cross-section is limited owing to de¯ection and stability constraints, so that high-speed machining becomes the major process to be exploited. The high-speed machining processes can produce more accurate parts as well as reduce the costs associated with assembly and ®xture storage by allowing several process procedures to be combined into a monolithic one. There has been a strong resurgence of interest in high-speed machining, so that it has become one of the most promising advanced manufacturing technologies in the last 20 years [1]. It can adapt to the keen competition and rapid change of the market. High-speed machining technology has already

* Corresponding author. E-mail address: [email protected] (Z.Q. Liu).

been applied in many manufacturing industries such as the aviation and aerospace industry, the automobile industry, and the mould industry, to cut steel, cast iron and its alloys, aluminum and magnesium alloys, super alloys of nickelbased, cobalt-based, ferrous-based, titanium-based, etc. and composite materials [2]. The application ®elds of high-speed machining are continuing to expand rapidly. High-speed machining, however, leads to the increase of cutting temperature and to the tool material to soften. It thereby causes tool wear and tool fracture. Hence, tool materials with high wear resistance take a crucial role in high-speed machining. Recent developments in tool materials have opened the door to the wide applications of highspeed machining. The tool material options for high-speed machining were reviewed comprehensively by Kramer [3] and Ai [4]. polycrystalline diamond (PCD), polycrystalline cubic boron nitride (PCBN), ceramics, cermets, coated and uncoated cemented carbides, and ®ne-grained cemented carbides represent a range of tool materials applicable to high-speed machining. Much research has been conducted on the temperature and wear performance of these tool materials among the cutting speed range from 200 to 1000 m/min [5±9]. Reports on the wear performance of these materials above the cutting speed 1000 m/min are still

0924-0136/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 0 1 3 6 ( 0 2 ) 0 0 6 0 5 - 2

Z.Q. Liu et al. / Journal of Materials Processing Technology 129 (2002) 222±226

lacking. Different tool wear modes and characteristics may be observed at this cutting speed range. Manufacturing engineers frequently use high-speed face milling to remove the greatest possible quantity of material from the workpiece. In this paper, the wear performances of a PCBN tool, a ceramic tool, a coated carbide tool and a ®negrained cemented carbide tool were investigated experimentally when face milling cast iron, and carbon steel at a cutting speed of 1100 m/min. 2. Experiments 2.1. Workpiece In this study, cast iron and 45# carbon steel (0.45% carbon) were selected as the workpiece materials. The 45# carbon steels were supplied in both the tempered and hardened conditions. Each workpiece was checked for its hardness. The values of the hardness measurements were 135±150 HB for cast iron, 35±40 HRC for 45# tempered steel, and 45±50 HRC for 45# hardened steel. 2.2. Cutting tools and machining conditions PCBN (50% CBN), Si3N4-based ceramic, pure Al2O3 ceramic, Al2O3-based ceramic, Al2 O3 ‡ TiC coated carbide, and ®ne-grained grain cemented carbide tool inserts were used in the milling tests. The whole inserts had an ISO designation of SPKN 150608 ED. The inserts were mounted rigidly on a face cutter body of 100 mm diameter, which was interfaced with the machine tool spindle using a standard BT 40 tool holder. The assembled tool geometry and cutting conditions are given in Table 1. 2.3. Experimental techniques All of the machining tests were performed on a vertical CNC machining center with a maximum spindle rotational speed of 10,000 rpm and a 15 kW drive motor. The CNC machining center was operated under speci®ed machining conditions described above. The machining tests were carried out dry. Except for early breakage, the tool wear patterns Table 1 Tool geometry and cutting conditions Cutting conditions

Parameters

Cutter diameter (mm) Axial rake angle

é100 58 (ceramic, PPCBN), 08 (coated carbide, carbide) 78 (ceramic, PCBN), 78 (coated carbide, carbide) 0.1 5 Single 758

Radial rake angle Feed per tooth (mm/z) Cutting width (mm) Number of tooth Approach angle

223

were observed through a toolmaker's microscope after 1 min of cutting. 3. Results and discussion Different modes of tool failure including rake face wear, ¯ank wear, micro-chipping, chipping, notch wear, breakage, and plastic deformation were observed in this study. Among these tool wear patterns, rake face wear, ¯ank face wear, chipping, and breakage were the main modes of tool wear in the ultra-high-speed face milling of both cast iron and carbon steel, whereas notch wear and plastic deformation were the main tool wear types in the high-speed machining of titanium-based alloys reported by other researchers [10,11]. These tool wear patterns in ultra-high-speed face milling suggested that the tool wear mechanisms were diffusion, attrition, and wear by chemical interaction, etc. It is common for several tool wear patterns to be found to appear simultaneously with the same tool and to have an effect on each other. 3.1. Tool wear on the rake face Generally, the end of tool life is determined by excessive wear of the tool ¯ank at conventional cutting speed. In highspeed machining, however, tool wear on the rake face predominates and therefore tool life is determined with the wear deepening until edge failure results [12]. Typical tool wear on the rake face when in the high-speed face milling of 45# tempered steel using an Al2O3-based ceramic tool and a ®ne-grained cemented carbide tool is shown in Fig. 1(a) and (b), respectively. From this ®gure, it can be seen that the tool wear on rake face in high-speed machining is different from crater wear in conventional cutting speed machining. In the conventional cutting speed range, the tool wear on the rake face occurred in the form of a pit called the crater, which was formed at some distance from the cutting edge, while the tool wear on rake face during high-speed machining was adjacent to the cutting edge. Actually, the maximum depth of tool wear on the rake face occurred on the main cutting edge. Experimental results showed that increasing the cutting speed even further led to the reduction of the wear area. However, the depth of wear area increased at the same time. This mode of tool wear is mainly due to too high cutting temperature on the rake face. Extreme high cutting speed leads to very high temperature (800±1000 8C) occurring at the vicinity of the main cutting edge where the maximum depth of tool wear on rake face occurs. The hardness of the tool materials decreases at such high cutting temperatures, which aggravates the abrasive wear on tool rake face. For a ®ne-grained cemented carbide tool, high cutting temperature also resulted in diffusion, adhesion, plastic deformation, etc. The tool±chip contact length is shorter in high-speed machining than at conventional cutting speed, which causes

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Fig. 1. Tool wear on rake face in high-speed face milling of 45# tempered steel.

Fig. 2. Flank wear in high-speed face milling.

the cutting force to be concentrated adjacent to the main cutting edge. The softer cutting edge due to high temperature under the concentrated cutting force near the cutting edge leads to deformation and de¯ection. This is one of the main factors, in particular, for ®ne-grained cemented carbide tool wear. Additionally, the intermittent milling process may cause mechanical impact and thermal shock. Mechanical and thermal shocks are non-negligible factors to in forming this kind of tool wear morphology. Thus, the combined effect of cutting force and cutting temperature is the main factor to lead to this type of tool wear on the rake face at high cutting speed. The matching of mechanical, physical and chemical properties between the tool and the workpiece materials at high temperature is a very important factor in the highspeed machining process. It was clearly observed in this study that both the wear zone and the depth of wear area on the rake face for a ceramic tool were greater than for a ®negrained cemented carbide tool. It was shown that the ceramic tool had a better performance in high-speed face milling steel through its high wear-resistance and high-temperature capability.

conventional cutting speed. The widest of the wear land in high-speed machining, however, is in the vicinity of the tool nose. This is different from conventional cutting where tool ¯ank wear occurs at the point furthest from the tool nose. This is due to the same facts as described above for tool wear on the rake face. Flank wear is also the most common pattern in high-speed cutting. In this study, ¯ank wear was observed throughout the whole of the machining tests.

3.2. Flank wear This pattern of wear produces wear lands on the side and end ¯anks of the tool on account of the abrasive action of the machined surface. In Fig. 2(a) and (b), it is noted that the wear land is not of uniform width, is the same as in

3.3. Cracking and chipping If the cutting edge appears jagged or there are cavities or depressions in the wear land, it means that chipping has occurred. Small chips breaks off from the tool cutting edge on account of mechanical impact, transient thermal stresses due to cycled heating and cooling in intermittent machining operations, chatter and excessive cratering and ¯ank wear. In ultra-high-speed milling, the cutting tool is exposed not only to rapid and high thermal as well as mechanical shocks every time it enters and exits the workpiece, but also to high cutting temperature. Traction-pressure-alternating stress therefore arises and leads to longitudinal and transverse cracks. Fig. 3(a) shows a crack that has been initiated without the loss of parent tool material when using a pure Al2O3 ceramic tool to cut cast iron. When cracks spread, two kinds of chipping mode were observed in this study: (1) strip chipping along the boundary of the major ¯ank and the minor ¯ank (Fig. 3(b)), which is caused by the mechanical

Z.Q. Liu et al. / Journal of Materials Processing Technology 129 (2002) 222±226

225

Fig. 4. Breakage of high-speed cutting tool.

observed when a using Al2 O3 ‡ TiC-coated cemented carbide tool to machine the cast iron. 3.4. Breakage

Fig. 3. Chipping and crack when high-speed cutting cast iron.

fatigue cracks on the ¯ank; and (2) shell®sh-like chipping of a large area on the tool rake face (Fig. 3(c)), which leaves a cavity on the rake face. The coating serves as a region of easy crack initiation, increasing the tendency towards fracture. The high elastic modulus of the coating causes intensi®cation of the stress in the brittle coating. Due to low resistance to failure of the coating, small chipping on the tool corner (Fig. 3(d)) was

Tool materials with low transverse rupture strength and fracture toughness are prone to fracture. The transverse rupture strength for PCBN tool in this study is only 0.57 GPa, while for cemented carbide steel this value is usually greater than 1.8 GPa. Generally, the lower-CBNcontent PCBN tool materials do not have effective crystalto-crystal bonding, which weakens the mechanical strength and impact resistance [7]. Thus, a lower-CBN-content PCBN tool is not suitable for the high-speed machining of steels with a hardness less than 45±50 HRC, and is especially unsuitable in an intermittent machining process. It can be seen from Fig. 4(a) that both the main and the minor cutting edges of the PCBN tool (50% CBN) were broken down completely at the ®rst pass cutting. Hence, it is necessary to select a higher-CBN-content PCBN tool material (more than 90% CBN) in intermittent high-speed operations such as milling. Due to Si3N4-based ceramic tool having a lower hardness and higher in®nity with steel than Al2O3-based ceramic tool material, catastrophic failure of the Si3N4-based ceramic tool (see Fig. 4(b)) was observed at the beginning of the cutting process, whilst only a normal wear pattern occurred when high machining the same hardened steel under the same cutting conditions with an Al2O3-based ceramic tool (see Fig. 2(b)).

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Tool breakage is very dangerous due to the in¯uence of centrifugal force in high rotational speed in the milling process. Security measures to prevent the launch of a broken tool should be taken during the design of high-speed machine tools.

3. Tool wear on the rake face and flank wear during highspeed machining were located adjacent to the cutting edge.

Acknowledgements 4. Conclusions Ultra-high-speed face milling experiments for cast iron and carbon steel were performed using a PCBN tool, a ceramic tool, a coated carbide tool and a ®ne-grained cemented carbide tool. The tool wear patterns at a cutting speed of 1100 m/min were observed through a toolmaker's microscope. The tool wear mechanisms in high-speed machining were analyzed. The main results obtained in this study are summarized as follows: 1. The main tool wear types differed in various matchings of materials between the cutting tool and the workpiece. The dominant wear patterns observed in this study were rake face wear, flank wear, chipping, and breakage. The optimal matching of mechanical, physical, and chemical properties between the tool and the workpiece materials is very important in high-speed machining. 2. The tool wear factors in high-speed milling operations were high cutting temperature, rapid and high thermal shock as well as mechanical impact.

This project is supported by the National Natural Science Foundation of China (through grant no. 50105012) and the State Education Committee Fund for Returned Students from Abroad (through grant no. 2000-23). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]

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