Experimental investigation on hard milling of high strength steel using PVD-AlTiN coated cemented carbide tool

    Experimental investigation on hard milling of high strength steel using PVD-AlTiN coated cemented carbide tool Qinglong An, Changying Wang, Jinyang Xu, Pulin Liu, Ming Chen PII: DOI: Reference:

S0263-4368(13)00240-0 doi: 10.1016/j.ijrmhm.2013.11.007 RMHM 3706

To appear in:

International Journal of Refractory Metals and Hard Materials

Received date: Accepted date:

13 September 2013 15 November 2013

Please cite this article as: An Qinglong, Wang Changying, Xu Jinyang, Liu Pulin, Chen Ming, Experimental investigation on hard milling of high strength steel using PVD-AlTiN coated cemented carbide tool, International Journal of Refractory Metals and Hard Materials (2013), doi: 10.1016/j.ijrmhm.2013.11.007

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Experimental investigation on hard milling of high strength steel using PVD-AlTiN coated cemented carbide tool

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Qinglong An1*, Changying Wang1, Jinyang Xu1, Pulin Liu2, Ming Chen1 1

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School of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai, 200240, P. R. China 2

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Shanghai Aerospace Precision Machinery Research Institute, Shanghai, 200240, P. R. China *E-mail addresses: [email protected]

Abstract: High strength steel 30Cr3SiNiMoVA (30Cr3) is usually used to manufacture the key parts in

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aviation industry owing to its outstanding mechanical properties. However, 30Cr3 has poor machinability due to its high strength and high hardness. Hard milling is an efficiency way to machine high strength

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steels. This paper investigated hard milling of 30Cr3 using a PVD-AlTiN coated cemented carbide tool with regard to cutting forces, surface roughness, chip formation and tool wear, respectively. The

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experimental results indicated that the increase of cutting speed from 70 to 110 m/min led to direct

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reduction of cutting forces and improvement of surface finish, while both feed rate and depth of cut had negative effect on surface finish. The occurrence of oxidation on chip surfaces under high cutting

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temperature makes the chips show different colors which are strongly influenced by cutting speed, and sawtooth chip was observed with the occurrence of the thermo-plastic instability within the primary shear zone. Micro chipping and coating peeling were confirmed to be the primary tool failure modes. Serious

30Cr3.

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abrasion wear and adhesive wear with some oxidative wear were the main wear mode in hard milling of

Keywords: high strength steel, hard milling, AlTiN, surface roughness, chip formation, tool wear

1 Introduction Demand for high strength steel 30Cr3SiNiMoVA (30Cr3) is growing in the aerospace and aviation industry due to its superior mechanical properties, such as high strength, strong corrosion resistance, high hardness, etc. High strength steel contains different amounts of various alloying elements such as Si, Mo, Ni, etc. Those alloying elements provide solid solution strengthening, resulting in the formation of numerous martensites with high strength and high hardness. 30Cr3 can achieve an ultimate tensile strength of 1800MPa and a high hardness of HRC 50-55 after an appropriate quenching process.

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Although 30Cr3 has excellent mechanical properties, it is a typical difficult-to-cut material. The poor machinability of 30Cr3 can be summarized as follows. (1) Serious tool wear: 30Cr not only has high

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tensile strength, but also has good toughness. During the machining process, the contact length of the

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tool-chip interface is small and the stress and heat are concentrated in the cutting area which easily lead to

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crater wear and flank wear and in turn tool life reduces. (2) High cutting forces: Ultra-high shear strength leads to the direct rise of cutting forces and high level tool vibration in the machining process. Under the same cutting condition, the main cutting force is 120%~150% higher than that for 45 steel [1, 2]. (3) High

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cutting temperature: The thermal conductivity of AISI 1045 steel is 50.2 W(m∙k)-1 , while the thermal conductivity of 30Cr3 is only 29.3 W(m∙k)-1 [3]. Low heat conduction of 30Cr3 may lead to a high cutting

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temperature in the cutting zone during the machining process and accelerate tool wear [4]. Traditionally, grinding processes are always the final finishing operations of hardened steels. In recent

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years, hard cutting operations have been considered an attractive alternative to traditional finish grinding

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operations [5, 6]. Firstly, hard cutting can achieve the same surface quality as grinding processes when appropriate cutting parameters are employed [7]. Secondly, hard cutting operations have become a

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profitable alternative instead of grinding for hardened steel, because it can reduce manufacturing costs, improve production efficiency and eliminate the environmental impact of coolant [8, 9]. However, the reliability of hard cutting processes is often unpredictable [10]. The main factors affecting the reliability of

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hard machining are surface integrity and tool wear. Surface roughness is one of the important indicators of surface integrity and cutting forces are the indispensable variables to monitor the hard cutting process. Numerous research works have been done with regard to surface roughness and cutting forces in hard machining. Paulo Davim et al.[11] investigated the machinability of cold work tool steel with ceramic tools using statistical techniques. The results showed that it is possible to obtain a surface roughness (Ra<0.8m) with appropriate cutting parameters and the primary factors affecting the surface roughness were tool wear and feed rate. Ebrahimi et al.[12] studied the machinability in hard turning of microalloyed and quenched-tempered steels at different cutting conditions. It was found that each of the materials showed high cutting forces at low cutting speeds ascribed to the low temperature and formation of the build-up edges on the contact zone. According to some other studies [13, 14], better surface quality and lower cutting forces could be obtained with higher

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cutting speeds, but machining at too high of a speed may accelerate tool wear. In the machining process, tool wear is an important factor directly affecting the surface quality of the

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machined parts and an important parameter in evaluating the performance of the cutting tools [15]. The

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exploring of tool wear mechanisms in hard machining is beneficial to understand the metal removal,

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revealing important friction phenomena occurring during the cutting process, and determining optimal cutting conditions [7, 16]. Conventional cutting tools usually suffer rapid wear rates because of high temperature and strong adhesive wear between the tool and the work material during the machining

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process of hardened steels [17]. In order to tackle these problems, coated tools are applied to reduce tool wear and improve cutting condition [9, 18]. It is reported that PVD ((Ti, Al)N-TiN coatings outperform

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CVD Ti(C, N)-Al2O3 coatings at the same cutting condition for machining high strength steel 30CrMnSiNi2A [19]. Halil et al.[20] investigated the influence of three different coatings (nanolayer

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AlTiN/TiN, commercial TiN/TiAlN and multilayer nanocomposite TiAlSiN/TiSiN/TiAlN) on the cutting

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forces and surface roughness during face milling of AISI D2 cold work tool steel. It was found that the coating has no significant effect on cutting forces and surface roughness. Chinchanikar et al. [21] carried

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out experiments of hard turning of hardened AISI 4340 steel with coated carbide cemented inserts. They observed that PVD coated inserts could achieve better surface roughness, indicated by the polished and bright back surface of the chip, compared to the rough surface showing some parallel strips with CVD

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coated inserts. Paulo Davim et al.[11] found that the tool wear is highly influenced by the cutting velocity (54.7%) followed by cutting time (13.4%) in hard turning of cold work tool steel with ceramic tools. This present work concerns the fundamental cutting characteristics of hard milling of high strength steel 30Cr3 with a PVD-AlTiN coated tool. The cutting force, surface roughness, chip morphology and tool wear were experimentally investigated. The objective of the present paper is to explore the hard milling mechanisms of high strength steels.

2 Material and methods 2.1 Workpiece material A 30Cr3 block with the dimensions of 100mm×100mm×50mm was used in the milling experiments. Through improving the chemical composition, applying advanced smelting and heat treatment processes, 30Cr3 has outstanding mechanical properties. The chemical composition of 30Cr3 is shown in Table 1.

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The content of alloying elements Cr, Ni, Mn and Mo is beneficial for the improvement of the strength and hardness of 30Cr3. Fig. 1 shows the metallographic microstructure of 30Cr3 after a heat treatment of

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15min normalizing at 930℃, 15min oil quenching at 910℃ and 2h tempering at 250℃. As shown in Fig.

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1, various needle-shaped martensites were observed in the material matrix. The presence of these

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martensites makes the strength and hardness of 30Cr3 higher than most other materials. In addition, these martensites were surrounded by several small black particles which were carbide materials obtained after quenching process. These carbide materials usually have small grain size effect which could improve the

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mechanical properties of 30Cr3. Table 2 presents the comparison of room temperature mechanical properties of 30Cr3 after oil quenching at 900℃, compared with Inconel718 and Ti-6Al-4V [22]. It is

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evident that 30Cr3 has the highest tensile strength and hardness. On the contrary, these martensites with high strength and high hardness cause severe mechanical and thermal shock to the cutting tool, which

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would lead to the worst machinability compared with Inconel718 and Ti-6Al-4V.

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2.2 Cutting tool

Coated cemented carbide end mill type JABRO–JS522 from Seco Tools was used in the experiments.

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The tool edges of the end mill experienced special strengthening treatment which can greatly meet the cutting performance requirements of high-speed hard milling of high strength steels. Geometry parameters of the end mill used in the experiments were listed in Table 3. The tool features a rigid 0.9-degree tapered

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neck design which can reduce tool deflection in the milling operation, reduce milling vibration and improve surface finish. It also features a wear-resistant polished physical vapor deposition (PVD) aluminium titanium nitride (AlTiN) with Al content of 60 at.% and WC-8 wt.% Co carbide substrate. The AlTiN coating has the same structure as titanium aluminum nitride (TiAlN) coating and both of them belong to a kind of multi-layer coated materials composed of TiN, Al2O3 and TiCN materials. The only difference between them is that AlTiN coatings have an Al content higher than 50 at.%, while, TiAlN coatings contain less than 50% Al. Al content is one of the key parameters which influence the critical properties of a coating. The optimal Al content is 60-70 at.% [16]. The higher Al content will change the crystal structure and lattice distortion of the coatings and the increase of the aluminum element will enhance the performance of high hardness and high oxidation resistance under high temperature conditions. In AlTiN crystal film, Al atoms replace part of Ti atoms in TiN, which lead to lattice distortion. As we all

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know, when lattice distortion increases, on the one hand, grain boundaries increase, and on the other hand, dislocation density increases, therefore, the deformation of the crystal will be more difficult resulting in an

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increase of coating hardness. When the Al content of AlTiN coating changes, the hardness of the coating

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also changes. Researches have shown that when the Al and Ti content ratio reaches 1.5, the hardness of

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AlTiN can obtain a maximum hardness of 3200HV [23].

Ascribing to its high hardness and high oxidation resistance under high temperature conditions, the JS522 end mill coated with AlTiN can obtain better machining performance, especially for high speed

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machining hardened steels, high temperature alloys and some other difficult-to-cut materials. 2.3 Experimental conditions

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Fig. 2 shows the experimental setup used in hard milling tests. The milling tests were performed on a DMU70V CNC machining center with a spindle speed range of 20~12000 r/min. Orthogonal methodology

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was adopted in the experimental design. A revised L16 (45) orthogonal array was selected. Cutting speed vc,

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feed rate f and radial depth of cut aw were selected as the factors and each of them had four levels, as illustrated in Table 4. Axial depth of cut ap remained 10mm during all of the tests and the type of end

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milling operation was down milling with no coolant, as schematically illustrated in Fig. 3. In terms of measurement device, a four-component force dynamometer Kistler 9272 was mounted under the workpiece to gather cutting force signals. The measured signals were transmitted to signal

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amplifiers Kistler 5017B, then to an A/D board and recorded on a personal computer using data acquisition software Labview 7.1, as shown in Fig. 3. Surface roughness of the machined surface was measured by a portable profilometer Mitutoyo SJ201. The values of surface roughness are the averages of six measurements at different locations of the corresponding machined surface. A Nikon photomicroscope system was used to observe the morphology of the chips. In addition, a digital microscope system Keyence VHX-600 and a Philips XL30-FEG scanning electronic microscope (SEM) equipped with an energy dispersive X-ray spectrometer (EDS) were used to study the flank wear width VB of the cutting tool.

3 Results and analysis 3.1 Cutting forces and surface roughness During the end milling operation, milling forces can be divided into three components referring to feed force Fx, radial force Fy, and axial force Fz. However, Fz usually keeps a very small value, has minor

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effects on the cutting process, and is not susceptible to the variations of cutting parameters. Therefore, the following analysis will focus on the effects of cutting parameters i.e. vc, f and aw on milling forces in terms

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of Fx and Fy. The milling forces Fx and Fy were all measured at the stable stage of the milling operation and

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the average values are calculated as the final value.

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Fig. 4 presents the main effect plots-mean values for Fx and Fy and Ra. It is observed that Fy is larger than Fx which indicates that Fy is the main cutting force. Fx and Fy decrease gradually with the increase of cutting speed vc in the interval of 70~110 m/min. This result reveals that high-speed machining is beneficial

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to reduce cutting force in hard milling process which can be ascribed to the effect of softening of workpiece material under the high temperature in the main shear deformation region [24].

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In contrast to vc, both f and aw have a negative effect on Fx and Fy. But the effect of f on the milling forces is relatively small compared with aw ascribed to the small range of feed rate. While aw has the

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dominant effect on the increase of the milling forces.

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Surface roughness of the machined surface is an important indicator of the surface quality, and correspondingly, also a key factor influenced by tool wear. Fig. 4c illustrates the effects of the cutting

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parameters (vc, f and aw) on surface roughness. Basically, vc decreases the surface roughness value especially for speeds up to 70m/min as shown in Fig. 4c. This can be attributed to the drop of cutting forces in the end milling operation that improves the

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stability of the machining system. Alternatively, an increase of f or aw deteriorates surface finish with aw noted as a determinative factor. It is well known that the generated surface comprises helicoid furrows resulting from the tool shape and the form of tool-part movements [24]. In end milling operation, surface roughness usually has a close relationship with the feed rate f and radial width of cut aw . When factors of f and aw were high, cutting forces and tool wear were exacerbated which thereby undermined the machined surface and deteriorated the surface finish. Consequently, changing f from 0.08 to 0.14mm/rev and aw from 0.1 to 0.7mm resulted in a direct rise of the surface roughness (Ra); whereas, augmenting vc from 70 to 110 m/min caused an obvious drop of the surface roughness. Comprehensively, A4B1C1 (vc=110m/min, f=0.08mm/rev and aw=0.1mm) is found to be the optimal combination of cutting parameters in theory.

3.2 Chip morphology and chip formation

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High-speed hard milling of hardened steel generates a high cutting temperature which will influence the chip formation and the chip morphology. Fig. 5 shows the progress of the chip shapes and chip colors

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versus the cutting speed with a magnification of ×20. The macro-shapes of the chips remained almost

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unchanged, i.e. they all kept the same curling pattern belonging to a spiral chip type. However, the chips

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displayed quite different colors under different cutting speeds. As the cutting speed increased, the color of the generated chips changed from original silver at 50m/min, to dark brown at 70m/min, purple at 90m/min and blue at 110m/min.

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This phenomenon was closely related to the cutting temperature. In hard milling of hardened steels, a large amount of cutting heat would be generated at the tool-chip interface. There were two sources of the

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generated heat on the tool-chip interface. One is due to the intensive friction between the chips and the tool, and the other came from the shear deformation at the back of the chips which led to a high temperature of

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the back surface of the chips. It should be highlighted that the temperature difference between the front and

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back surface of the chip directly determined the chip curling degree due to the principle of thermal expansion and contraction. According to this and the curling degree of the chips, it could be inferred that

equivalent.

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the temperature differences between the front and back surface under different cutting speed were nearly

Since the bulk cutting heat was carried away by chips and dissipated into the air, the high

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temperature of the air surrounding the surfaces of the chips favored oxidation, making the chips show different colors under different cutting conditions. The formation of oxide layers (especially iron oxide) on the chip surface directly determines the color of the chips. The potential iron oxides are Fe2O3, FeO and Fe3O4. Since the colors of Fe2O3, FeO, and Fe3O4 at the solid state were red-brown, black and black, respectively, the colors of the generated chips varied with the presence of specific iron oxides or their combinations, which depended on the degree of oxidation. With the increase of cutting speed, more cutting heat would be generated through the chip surface, making the chip suffer higher cutting temperature and severe oxidation. The high temperature (especially exceeding 1478℃) may reduce Fe2O3 to FeO and Fe3O4 [25]. Higher amounts of FeO and Fe3O4 make the chip color darker and darker. Therefore, the cutting temperature could also be estimated if the relationships between the colors of the chips for this material and the temperatures of the chips were known.

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In addition to color, the morphorlogy of the free and back surface of the chips were differrent. The combined actions of high contact pressures, frictional forces and high tempretures make the back surface

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smoother and shinier than the free surface of the chips (see Fig. 6). Although the distribution of contact

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pressure, frictional forces and temperature at the tool/chip interface vary with the cutting parameters, the

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back surfaces of the chips have very similar charateristics. Besides, sawtooth chips were also observed in this study when machining high strength steel 30Cr3. The metallographic structure of the generated chip cross section has been presented in Fig. 7. Sawtooth chips are the typical microstructure characteristic in

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hard machining [26-28]. According to Fig. 7, there are three regions can be observed, i.e., 1—plastic deformation zone, 2—adiabatic shear region, and 3—lower deformed zone. During saw-tooth chip

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formation, there were three phenomena involved. The first one was material strain hardening due to the large plastic deformation of the chip. The second was the thermal softening effect due to a high cutting

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temperature. The third one was the quenching effect of the chip by the high cutting temperature with the

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intermittent cutting operation. The quenching took place when the temperature at the tool/chip interface was above the austenization temperature for 30Cr3. Consequently, Sawtooth chips were formed with a

3.3 Tool wear

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structure that consists of untempered martensite with residual austenite.

Flank wear width VB of the JS522 end mill was measured with respect to the cutting length l, as

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shown in Fig. 8. The tool wear experiment was carried out under the cutting condition of vc=110m/min, f=0.08mm/rev, aw=0.7mm and ap=10mm. Average flank wear width VB=0.3mm was selected as the wear criterion of the end mill according to international standard-Tool life testing in milling (ISO 8688-2). As shown in Fig. 8, the AlTiN coated tool underwent the initial, normal and rapid wear stages during milling. There was a rapid tool wear rate at the initial wear stage, since the surface roughness of the opposing surfaces was high and the actual contact zone suffered high pressure and severe friction. Before l reached 30m, the cutter reached a steady wear stage, and tool flank wear was controlled below 0.135mm which can be ascribed to the effective protection and excellent wear resistance of the AlTiN coating material. When l reached over 30m, micro chipping was found to take place on tool nose and side cutting edge as shown in Fig. 8, which made the cutter quickly enter the rapid wear stage. The appearance of micro chipping demonstrated that the main cutting edges suffered high specific pressures in hard milling,

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which were the result of the interaction between machined material hardened phases and the components of the tool. In order to quantitatively analyze the functional relation between VB and l, a polynomial

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function was employed to fit the wear curve of the used tool. The fitting expression is given by:

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VB=6.05+7.19l  4.17 103 l 2  1.40 102 l 3  3.58 104 l 4

(1)

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where, VB is measured in micrometers and l is measured in meters. The Analysis of Variance (ANOVA) between the fitted value and the measured value was conducted, as illustrated in Table 5. The value of P is almost equal to 1 which indicates that the wear curve is fitted very well.

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During the tool wear experiment, the cutting forces were also measured and the corresponding force components at different cutting lengths were formed. Fig. 9 demonstrates the changes of the cutting forces

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during tool life for the end mill. As the dominant force component, Fy shows a very similar trend to the corresponding wear curve for this end mill. In addition, at the steady stage of the flank wear curve, the

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smallest force component Fz is almost unchanged which can be ascribed to the complexity of the

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machining process [16].

Table 6 lists the the percentage increase of force components and the correlation of wear to cutting

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forces. We can see that the cutting force components are highly correlated to the tool flank wear. Therefore, force components, especially the primary component Fy are recommended for monitoring the status of the tool wear.

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Fig. 10 presents the SEM images and EDS analyses of the worn tool when the cutting length reaches 30m. As shown in Fig. 10a, no build-up edge (BUE) is found on the main cutting edge of the tool. The high strength of 30Cr3 makes it exhibit brittleness to some extent, even at a high temperature. Crater wear is obvious on the tool rake face and is mainly caused by the high cutting temperature on tool-chip interface. Via the EDS analysis, the main elements of position A are W and C together with a small amount of Fe and O elements. No Ti, Al and N elements are found at point A, which indicates that the coating has peeled completely from the tool. The occurrence of coating peeling may exacerbate the friction between the chips and tool rake face and make the tool substrate lose heat protection, which thereby deteriorates the cutting condition of the second shear zone. With the absence of the coating, the strength of the cutting edges reduces greatly, thereby causing micro chipping of the tool. Besides, the appearance of Fe and O elements confirms that oxidative wear and adhesive wear has taken place at the rake face.

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As presented in Fig. 10b, many grooves are observed on the flank face, due to serious abrasive wear. This abrasive wear is mainly caused by numerous hard particles (particularly the martensite hard phase) in

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the workpiece that are able to cut into the coating material. In addition, due to severe contact and friction

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between the machined surface and the tool flank face when machining high strength steel, adhesive wear is

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particularly active on the tool flank face with the help of high stress and temperature. The large amount of Fe element at position B as shown in Fig. 10b confirms the occurrence of adhesive wear. Because the adhesive wear usually undergoes an adhesive-peeling-adhesive dynamic process, it will take away parts of

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the tool material periodically. The appearance of Ti, Al and N elements reveals that the coating has not peeling completely. Ascribed to the negative effects of micro chipping and coating peeling on the cutting

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edges, the cutter was soon deprived of the protection from coating material and failed quickly in its rapid

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wear stage.

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4 Conclusions

This research mainly concerns the machinability evaluation in hard milling of high strength steel 30Cr3 by

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using a PVD-AlTiN coated carbide tool with regard to cutting forces, surface roughness, chip morphology and tool wear, respectively. Based on the results, several conclusions can be drawn as follows. (1) The increase of cutting speed from 70 to 110 m/min led to direct reduction of cutting forces and

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improvement of surface finish. The cutting parameters combination of low feed rate and low width of cut with high cutting speed is confirmed to be beneficial for minimizing the cutting forces and surface roughness through the method of extremum difference analysis. (2) The occurrence of oxidation on chip surfaces makes the chips show different colors which are strongly influenced by cutting speed. As the cutting speed increased, the color of the generated chips changed from original silver at 50m/min, to dark brown at 70m/min, purple at 90m/min and blue at 110m/min. (3) Tool life of the coated cemented carbide end mill is approximately 40m under the cutting parameters of vc=110m/min, f=0.08mm/rev, aw=0.7mm and ap=10mm according to the wear criterion VB=0.3mm. A correlation analysis between the cutting forces and tool flank wear showed that the radial force components and feed force components 97% and 96% correlated to the tool flank wear, respectively.

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(4) The main failure modes of the PVD-coated end mill are micro chipping and coating peeling in hard milling of 30Cr3. Crater wear and micro chipping are found on the rake face, while grooves and coating

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peeling are confirmed to be the main failure modes of the flank face. The serious abrasion wear and

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adhesive wear with some oxidative wear are the main wear mode in hard milling of 30Cr3.

Acknowledgments

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This work was supported by National Natural Science Foundation of China (No. 51105253), National Basic Research Program of China (No.2011CB706804) and SAST-SJTU joint research center for

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aerospace advanced technology ( USCAST2012-16).

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2009;49:805-13.

[26] Barry J, Byrne G. The Mechanisms of Chip Formation in Machining Hardened Steels. Journal of Manufacturing Science and Engineering. 2002;124:528-35.

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[27] Guo Y, Yen DW. A FEM study on mechanisms of discontinuous chip formation in hard machining. Journal of Materials Processing Technology. 2004;155:1350-6. [28] Kishawy H, Wilcox J. Tool wear and chip formation during hard turning with self-propelled rotary tools. International Journal of Machine Tools and Manufacture. 2003;43:433-9.

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Figure Captions

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Fig. 1 Metallographic microstructure of 30Cr3 (after 15min normalizing at 930 ℃, 15min oil quenching at

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910℃ and 2h tempering at 250℃)

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Fig. 2 Experimental setup in hard milling of 30Cr3

Fig. 3 Schematic illustration of the milling operation and the cutting forces data acquisition system Fig. 4 Main effect plots-mean values for (a) force Fx, (b) radial force Fy, (c) surface roughness Ra

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Fig. 5 Chip colors under different cutting speed: (a) vc=50m/min, (b) vc=70m/min, (c) vc=90m/min and (d) vc=110m/min (f=0.08mm/rev, aw=0.7mm and ap=10mm), ×20

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Fig 6 SEM of chip morphology under the cutting condition of vc=110m/min, f=0.08mm/rev, aw=0.7mm (a) back surface (b) free surface

Fig. 7 Saw-tooth chip obtained from the milling operation under the condition of vc=110m/min, f=0.08mm/rev,

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aw=0.7mm and ap=10mm. Three regions of the saw-tooth chip: 1—plastic deformation zone of the chip

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corresponding to the secondary shear zone; 2—adiabatic shear region corresponding to the primary shear zone; 3—lower deformed zone due to saw-tooth chip formation.

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Fig. 8 Tool flank wear VB curve versus cutting length (vc=110m/min, f=0.08mm/rev, aw=0.7mm, ap=10mm) Fig. 9 The relationship between cutting force and cutting length of tool

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Fig. 10 SEM images and EDS analyses of the end mill (cutting length l=30m). (a) rake face. (b) flank face

Table Captions

Table 1 Chemical composition of high strength steel 30Cr3 (wt.%) Table 2 Mechanical properties of 30Cr3, Inconel718 and Ti-6Al-4V Table 3

Geometry parameters of end mill JS522

Table 4 Factors and levels selected for the milling experiments Table 5 ANOVA table of fitted value Table 6 Correlation of wear to cutting forces and the percentage of the force increment

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100m

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Carbide particles

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Needle-shaped martensites

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Fig. 1 Metallographic microstructure of 30Cr3 (after 15min normalizing at 930℃, 15min oil quenching at 910℃

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and 2h tempering at 250℃)

Spindle

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Coated Tool

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115

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60

Workpiece Fixture

DynamometerKistler 9272

Fig. 2 Experimental setup in hard milling of 30Cr3

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A

z

n z

x

ap

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A

Workpiece

A/D Board Force Signals

PC (Labview)

Dynamometer

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Workbench

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Signal Amplifiers

aw

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f

y

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D

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Fig. 3 Schematic illustration of the milling operation and the cutting forces data acquisition system

Fig. 4 Main effect plots-mean values for (a) feed force Fx, (b) radial force Fy and (c) surface roughness Ra

D

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IP

T

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Fig. 5 Chip colors under different cutting speed: (a) vc=50m/min, (b) vc=70m/min, (c) vc=90m/min and (d)

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vc=110m/min (f=0.08mm/rev, aw=0.7mm and ap=10mm), ×20

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a

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b

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surface (b) free surface

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Fig 6 SEM of chip morphology under the cutting condition of vc=110m/min, f=0.08mm/rev, aw=0.7mm (a) back

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TE

D

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Fig. 7 Saw-tooth chip obtained from the milling operation under the condition of vc=110m/min, f=0.08mm/rev, aw=0.7mm and ap=10mm. Three regions of the saw-tooth chip: 1—plastic deformation zone of the chip

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corresponding to the secondary shear zone; 2—adiabatic shear region corresponding to the primary shear zone; 3—lower deformed zone due to saw-tooth chip formation.

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Micro chipping

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VB=0.3mm

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VB=0.13mm

Fy

Fx

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Fz

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750 500 250

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Cutting forces (N)

1000

0

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Fig. 8 Tool flank wear VB curve versus cutting length (vc=110m/min, f=0.08mm/rev, aw=0.7mm, ap=10mm)

0

5

10 15 20 25 30 Cutting length l (m)

35

40

Fig. 9 The relationship between cutting force and cutting length of tool

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386

a

309 231

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W 154 77

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Crater wear

W

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Micro chipping

Fe

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Intensity (a. u.)

A

0 0

Rake face

0.5

1.0 1.5 2.0

2.5 3.0

3.5 4.0

4.5 5.0

Energy / keV 671

B

Fe

536 402

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Intensity (a. u.)

b

Ti

268

Al O

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134

N

0

Coating chipping

0

D

Flank face

1.0

2.0 3.0

4.0

5.0 6.0

7.0

8.0 9.0

10

Energy / keV

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Fig. 10 SEM images and EDS analyses of the end mill (cutting length l=30m). (a) rake face. (b) flank face

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Table 1 Chemical composition of high strength steel 30Cr3 (wt.%) C

Cr

Si

Ni

Mn

Mo

V

Fe

Content

0.34

3.2

1.2

1.2

0.8

0.8

0.15

Balance

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Element

Tensile strength Material

 b (MPa)

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Table 2 Mechanical properties of 30Cr3, Inconel718 and Ti-6Al-4V

Yield strength

Ductility

Hardness

 0.2 (MPa)

 (%)

(HRc)

1640

12.8

50-55

1854

Inconel718(solution treated and aged)

1350

1170

16

38~44

Ti-6Al-4V(annealed)

950

880

14

30~36

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Geometry parameters of end mill JS522

Diameter (mm)

 0 ()

()

Coating material

12

10

40

PVD-AlTiN

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Table 3

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30Cr3(normalized and oil quenched)

Cutting teeth

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Tool type

2

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JABRO–JS522

Table 4 Factors and levels selected for the milling experiments

Level

B-Feed rate f (mm/rev)

C-Radial depth of cut aw (mm)

50

0.08

0.1

70

0.10

0.3

3

90

0.12

0.5

4

110

0.14

0.7

1 2

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A-Cutting speed vc (m/min)

Factor

Table 5 ANOVA table of fitted value Source Columns

SS ~0

DOF

MS

1

~0 8071.82

Error

177580

22

Total

177580

23

F-ratio 4.05×10

SS, sum of squares; DOF, degree of freedom; MS, mean squares; P, percent contribution.

P -7

0.9995

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Table 6 Correlation of wear to cutting forces and the percentage of the force increment Fx (N)

Fy (N)

Fz (N)

Cutting length, 5m

0.041

402

662

188

Cutting length,30m

0.075

529

889

254

Percentage of force increment (%)

-

32%

34%

35%

Correlation to VB

-

0.97

0.92

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VB (mm)

0.96

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HIGHLIGHTS OF OUR RESEARCH

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1. The machinability of high strength steel 30Cr3SiNiMoVA was evaluated by using a

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PVD-AlTiN coated cemented carbide tool in hard milling.

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2. Low feed rate, low width of cut with high cutting speed are beneficial for minimizing the cutting forces and surface roughness.

3. Radial force and feed force are conformed to be highly correlated to the tool flank wear

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with the correlation of 97% and 96%, respectively.

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4. The main failure modes of the PVD-coated tool are micro chipping and coating peeling.