The milling performances of TiC-based cermet tools with TiN nanopowders addition against normalized medium carbon steel AISI1045

Wear 258 (2005) 1688–1695

The milling performances of TiC-based cermet tools with TiN nanopowders addition against normalized medium carbon steel AISI1045 Ning Liua,∗ , Chengliang Hana , Haidong Yangb , Yudong Xua , Min Shia , Sheng Chaoa , Feng Xieb b

a Department of Materials Science and Engineering, Hefei University of Technology, Hefei 230009, PR China Department of Mechanical and Automobile Engineering, Hefei University of Technology, Hefei 230009, PR China

Received 15 April 2004; received in revised form 3 November 2004; accepted 29 November 2004 Available online 12 January 2005

Abstract The two milling tools obtained from nearly full dense TiC-based cermets with TiN nanopowders addition, with limited percentage of monophase nickel and biphase nickel–cobalt metal binders, were tested in face milling operations, in the dry cutting of a medium carbon steel (AISI1045). Microstructure and mechanical properties were studied. Wear mechanisms (mainly diffusion and oxidation) were investigated in detail and compared each other in order to better understand key aspects due to thermal wear mechanisms. Comparing tool A with B, under the adopted cutting conditions, the tool A has a better resistance to oxidation deformation in machining medium carbon steel, but lower impact breakage property on the same conditions. © 2004 Elsevier B.V. All rights reserved. Keywords: Wear resistance; Mechanical properties; TiN nanopowders; TiC-based cermets; Milling tools

1. Introduction In the field of cutting tools, TiC-based cermets represents common and important cutting tool inserts in the field. At the moment, the most important applications of TiC cermets regard metal cutting and finishing operations [1–4]. The demand for such tool inserts continues to grow and is now second to that of WC–Co. This is due to the excellent wear resistance and chemical stability at high temperatures. TiN nanopowders mixed with different metal carbides (e.g. TiC, WC, Mo2 C) and /or binders (e.g. Co, Ni or a mixture of them) are employed to produce TiC-based cermets with TiN nanopowders addition. The binder forms a matrix where ceramic particles are dispersed [1–3]. In this kind of materials, high hardness and wear resistance, provided by ceramic ∗

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particles, are combined with toughness and thermal shock resistance, provided by binder. Cermets, compared to cemented carbides, have a better resistance to oxidation, and, during metal machining, built up edge formation and craterisation, due to the better chemical stability and high temperature hardness [5–9]. Because of these characteristics, cermets can be employed in cutting operations at higher cutting speed than cemented carbides and give better results in terms of surface roughness of the workpiece. However, cermets present some disadvantages, such as low toughness compared with that of cemented carbides [10]. Three decades have passed since nitrogen-containing cermets have been widely used in industry. Up till now, many efforts have been made to improve the comprehensive properties of cermets and cermet cutters [11–13]. Previous studies on the performance of TiC-based cermets cutting tools [4,14] showed that cermets with limited binder contents, and high hardness behave as promising tools for

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machining steel at high speed. The most important application of TiC-based cermets for cutting tools was finishing operations in working steels [15–17]. But in some applications, the limitations due to heavy interrupted cutting (e.g. milling) impede the viable cutting materials to cermets. It is well known that the main qualities required for a milling tool are excellent toughness and better wear resistance. In this work, two kinds of high toughness miller of TiCbased cermets with TiN nanopowders addition were fabricated by powder metallurgy method. And the above cermets cutting tools were tested in milling at different cutting conditions. Microstructure and wear mechanisms of the tools after milling were investigated and in order to understand the reasons of material degradation, observed during the milling tests. 2. Experimental 2.1. Materials and fabrication of the cermet indexable inserts The compositions of tool A and B cermets are listed in Table 1. Based on the previous work [18], cermets cutter containing 20 wt.% nickel shows better cutting behavior than that of cemented carbides YG8 (8 wt.%Co). Furthermore, we choose the total binder content as 24 wt.% considering that the cermet inserts should have higher toughness than that of cermet cutters. The starting powders with the average size TiC 2.56, WC 1.14, Mo 1.25, Ni 2.74, Co 2.96, C 1.05, and TiN 0.04 ␮m were mixed with WC–Co balls in ethanol for 24 h with a planetary ball mill, and then dried. The cermet indexable inserts used for the milling tests were prepared by: uniaxial pressing at 150 MPa in a hydraulic press in a simple hardened steel die to produce near-shape performs; then dewaxing, sintering at 1440 ◦ C, and finally grinding the sintered performs into the final shape with diamond wheels. The shape of the milling inserts is shown in Fig. 1. The Table 1 Chemical composition of the tested millers Millers

Tool A Tool B

Chemical composition (wt.%) TiC

WC

TiN (nm)

Mo

Ni

Co

C

35 46

15 15

10 10

15 4

24 12

0 12

1.0 1.0

Fig. 2. Schematic diagram of the position of the insert relatively to the workpiece (a) and symmetric face milling (b).

relative position of the insert to the workpiece is shown in Fig. 2 Mechanical properties, as hardness (HRA), fracture toughness (KIC ), transverse rupture strength (TRS) and density were measured as shown in Table 2. The sintered samples were examined for hardness (HRA) on the common Rockwell hardometer. The fracture toughness (KIC ) measured by the three point bending (SENB) method, was conducted by the introduction of a sharp pre-made crack the length of which is 2.5 mm and was calculated by measuring the critical rupture loading. The formula
a  a 2 3PL √ KIC = Y a Y = 1.93 − 3.07 + 14.53 2 h h 2bh  a 3  a 4  −25.11 + 25.8 h h where Y is the geometrical factor, P the critical rupture load and a is the size of the pre-crack. The transverse rupture strength testing was carried out on a Shimadzu MTS801-23 universal machine (Japan) using a constant strain rate mode. The microstructure features and wear morphology were studied with scanning electron microscopy (SEM) model LEO1530VP (Germany) in back scattered and electron secondary mode, respectively. Table 2 Mechanical properties of the tested millers Millers

Fig. 1. Shape and typical dimensions of newly developed TiC-based cermet milling inserts with TiN nanopowders addtion.

Tool A Tool B

Mechanical properties HRA

KIC (MPa.m1/2 )

TRS (M Pa)

D (g/cm3 )

90 89

15 19

1435 1602

7.1 6.9

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2.2. Milling test conditions and methods The milling tests were carried out on a 36 kw mill. To reduce the experiment periods, the tool holder carried only one insert. The workpiece material was a normalized medium carbon steel, namely AISI1045 heat treated with a hardness of HB 190 ± 5. The workpiece size was 400 mm × 100 mm × 100 mm with the length, width and height, respectively. The milling cutter was a 125 mm diameter MCV (six inserts, corner angle 75◦ , orthogonal rake angle 2◦ , axial rake angle 7◦ , radial rake angle 0◦ ). The test matrix is shown in Table 3. No lubricant was used (dry milling). The sizes of workpiece material made of medium carbon steel AISI1045 for milling impact tests are 400 mm × 10 mm × 100 mm with the length, width and height, respectively. The milling impact tests were also conducted on the previous mill with only one insert on the tool holder. When the width size of the workpiece material be reduced from 100 mm in previous common milling tests to 10 mm of this impact milling tests, the flank and crate wear behavior will become not important, but the impact of workpiece material on the insert will play a vital role to some extent. By using this simple testing method, we can evaluate the milling impact breakage resistance of insert. Milling tests were performed by using one insert for each type, Two cermet inserts (A and B) were compared. After cutting one curve under the fixed conditions, the edge of the insert at the same position on the miller was changed. All the inserts were placed on the tool holder, in order to compensate for the possible machine tool errors. After milling tests, the wear assessment criterion was the measurements of maximal flank wear (VBmax ) with usual standardized procedures by an optical microscope. Concerning the two cermet A and B inserts, the microstructures of the flank and crater were Table 3 Milling test matrix Cutting speed vc (m/min) Feed per tooth fz (mm/z) Axial cutting depth ap (mm) Radial cutting depth ae (mm)

294 0.16 0.50 100

373 0.20 0.80 100

463 0.25 1.00 100

further studied by SEM coupled with a EDS micro analyzer, respectively, in order to reveal morphological and chemical modification induced by the contact at high speed milling between work piece and tools and to define the dominant wear mechanisms.

3. Results and discussion 3.1. Microstructures of the test materials The materials of the tool A and B present a typical core/rim structure (Fig. 3), where the core is constituted by the phase TiC, whereas the rim by the (Ti, W, Mo) C solid solution. These features arise during liquid phase sintering from the complete dissolution of WC, and the partial dissolution of TiC in the liquid melt. The rim precipitates onto the cores during sintering. In addition, from Fig. 3 (a) and (b), we can found the grains of cermet tool B are larger than that of tool A, The average grain sizes for tool A and B are 2 and 5 ␮m, respectively, and carbides in tool A are well distributed. It is denoted that from previous work [12] some TiN nanoparticles distributed at the grain boundaries. It will contribute to the improvement of the fracture toughness of the material. 3.2. Milling efficiency The milling efficiency of the experimental tool A during milling tests against medium carbon steel, in comparison to the tool B, was evaluated by considering the wear of the flank (VBmax ) in finishing operations. The wear curves are shown in Fig. 4. From Fig. 4, we can see that the wear process of two kinds of the millers exhibits three moments markedly, namely primary wear phases (within 5 min), normal wear moment (5–15 min) and sharp wear moment (beyond 15 min). At the same time, it can be concluded that the experimental tool A possesses better performances in an identical cutting conditions and at higher cutting speeds. In order to deeply understand the cutting performances of the newly developed inserts A and B, milling tests were carried out changing cutting speed (vc ) and feed per tooth (fz ) and cutting depth (ap ), respectively. The flank wears ver-

Fig. 3. SEM images showing the microstructures of the milling inserts (a) tool A and (b) tool B.

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Fig. 4. Wear curves for cermet tool A and B under the milling condition of vc = 294 m/min, fz = 0.16 mm/tooth and ap = 1.0 mm.

sus time curves relative to the two tested tools with different milling parameters (such as vc , fz and ap and so on) after the achievement of a critical value of VBmax , are shown in Figs. 5–7, respectively. First of all, the effect of milling speed vc on the VB value was studied. From Fig. 5, we can draw the following result that the flank wear value increases rapidly with the increase of milling speed for the two kinds of cermet millers, and tool A behaves a small quantity of wear of VB compared to that of tool B at the same milling condition. The reason can be explained that the wearable properties of the tools stand in the breach when milling at high speed [19], and the tool A has higher hardness than the tool B. In succession, the effects of fz on wear resistant properties of two new cermet millers were researched. The testing results shown in Fig. 6 are revealed that the parameter of fz has a little influence on the flank wear of the two millers. In the end, the relation between the cutting deep ap of the two kinds of cermet millers and flank wear VB was studied and the results were illustrated in Fig. 7. It can be found that the wear VB of new cermet miller increases sharply with the increase of ap at the same cutting speed due to the increasing force of milling which leads to larger vibration and mechanical impact on the tools. And compared tool A with tool B, the increase of ap brings about little effect on the wear VB. It can be reduced to the higher fracture toughness of cermet tool B. 3.3. The milling impact breakage tests In previous work [18], the cermets cutter containing 20 wt.% nickel shows higher wear resistance and cutting life (about two times) than that of the cemented carbides YT15 (15 wt.%TiC, 79 wt.%WC, 6 wt.%Co) which widely used as cutter and insert in common cutting and milling for medium carbon steel in China, so we just choose the cemented carbides as the insert to compare our newly fabricated cermets

Fig. 5. Effect of vc on wear resistant properties of tool A and B after milling testing.

in order to make sure whether the cermets inserts are more competitive than above cemented carbides inserts, because the milling impact breakage tests have great importance in milling except other common milling tests. The milling impact breakage tests were conducted. The results were shown in Fig. 8. It was shown that the flank wear VB increase with the increase of milling impact numbers N. Tool B has the best milling impact breakage resistance, this is because the tool B contained 12% Co in which case improved the toughness. The cemented carbide YT15 has the lowest value due to the lower content of binder in which case decreased the toughness. Under the milling condition of Fig. 8(b), all inserts shown lower impact breakage resistances compared with that of Fig. 8(a), in which case has the lower impact force. The critical value of VBmax used for flank wear is 1.0 mm for common milling tests. It should be mentioned that the above tests including milling tests and milling impact breakage tests are not meant

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Fig. 6. Effect of fz on wear resistant properties of tool A and B after milling testing.

Fig. 7. Effect of ap on wear resistant properties of tool A and B after milling testing.

for determining tool life since these is just based on the results of single insert. Further, milling tests with all inserts mounted on milling holder will be conducted on in later work.

The crack dimension results are not dangerous until it grows up to a critical size that triggers a rapid break down and failure of the cutting edge. For tool A, chipping is very limited and the crater size is lower than that of the tool B, where also chipping occurred at the flank edge. At the same time, we can found that the flank wear of tool A is very homogenous. The analysis of tool wear morphology showed that tool B has a limited thermal shock resistance, as a pattern of closely spaced cracks orthogonal to the cutting edge is clearly visible on the flank (see Fig. 10). The thermal cracks that appear along the cutting edge are responsible for tool chipping at longer cutting times, although average flank wear may be very low even after tool failure in spite of tool B with low hardness (89HRA). The composition with Co binder phase (tool B) would experience sever chipping after a few minutes of interrupted cutting.

3.4. Characterization of the cermets tools after milling 3.4.1. Morphology of the flank and crater wears In order to describe the complex microstructures of the flank and crater worn, SEM–EDS analysis was employed. In the two tested tools, flank and crater wear occurred during the milling operations on the tool flank and tool face, respectively (Figs. 9 and 10). The total worn area of the insert, which increases for longer cutting time, varied for the different test tools. Fig. 9 shows an example of the changes in flank and crater wear patterns of the tool A after milling against carbon steel. From the feathers in Fig. 9, it can be noted that the insert A has a flank wear lower than the insert B at the same conditions as shown in Fig. 10 and exhibits improved performances at high speed in finishing of normalized medium carbon steel AISI1045. Thermally induced cracks on the cutting edge, due to the thermal cycling, are present in the two tested cermets.

3.5. Wear mechanisms In the contact portion between work piece and insert, abrasion and thermal cracking due to thermal fatigue can be considered concurring for wear. Moreover, SEM–EDS re-

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Fig. 10. SEM micrograph of the grooving flank wear and thermal cracks of tool B, after milling for 25 min under the condition of vc = 294 m/min, fz = 0.20 mm/tooth, and ap = 1.0 mm.

Fig. 8. The relationship between milling impact numbers and wear.

vealed a transfer of matter from the work piece to the worn tool. Due to relative movement of tool and workpiece, steel flakes and their oxides are subjected to shear and compressive stresses, enhancing fatigue wear and, consequently, micro cracking in the tool. Such a transferred mass reached and

Fig. 9. SEM micrograph of the flank and crater wear from tool A, after milling for 25 min under the condition of vc = 294 m/min, fz = 0.20 mm/tooth, and ap = 1.0 mm.

oxidized. Its adhesion to the surface tool caused the peeling that in milling gave rise to wear of the tool. In addition to the adhesion–peeling and microstructure, inter-diffusion between the rubbings surfaces is also responsible for wear [20,21]. Thermally related wear mechanisms degrading the insert material in the zone (Fig. 9) are related to the starting composition and are the consequence of oxidation phenomena occurring at the high temperature developed during milling. Such temperatures enhance not only oxidation, but also reaction with elements coming from the workpiece or the bulk of tool. From Fig. 11, we found plenty of oxygen and iron elements in the flank wear zone. The detailed results are shown in Fig. 12 and Table 4, respectively. It was reported that oxidation induces the formation of gaseous N2 and CO/CO2 , liquid MoO3 , vaporing at 1073 K and solid WO2 , subliming at 1073 K. The formation of such liquid or volatile compounds may explain the decrease of disappearance of elements like Mo and W from the oxidized areas [1–3]. Therefore, the strong surface damage in Fig. 7 can be ascribed mainly to oxidation. Oxidation of TiC-based cermets depends on the quantity of metal binder that, at high temperature, moves from the bulk towards the surface, providing accesses for oxygen that can penetrate the bulk and oxidize it. Further, heat accumulation

Fig. 11. SEM micrograph of worn flank from the tool A after milling for 25 min under the condition of vc = 294 m/min, fz = 0.20 mm/tooth, and ap = 1.0 mm, its points spectrum of the EDS correspondently shown in Fig.12.

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sion and fatigue wear, the most critical phenomena are diffusive and oxidative processes. Diffusion wears and oxidations are strongly dependent on the thermal conductivity of the tool, particularly the amount of the binder phase of TiC-based cermets considered. Acknowledgements

Fig. 12. EDS spectrum acquired in the point 89 of tool A. Table 4 EDS results of the tested tool A Element

Weight (%)

Atomic (%)

CK OK Ti K Fe K Mo L WM

0.09 42.48 13.38 28.87 1.18 14.00

0.22 74.84 7.87 14.57 0.35 2.15

Totals

100.00

100.00

of the tool depends on thermal conductivity, which is mainly related to C/N ratio and the binder phase in the TiC-based cermets [15]. In this respect, the newly developed tool A with a Ni-binder has higher hardness and a better favouring oxidation than tool B with a Ni–Co mixture binder. These features support the best performance in machining steel at high-speed milling, compared to tool B.

4. Conclusion On the basis of the experimental results presented and discussed in the present paper, the following conclusions can be drawn: Two newly developed TiC-based cermet tool A and B with TiN nanopowders addition, characterized by different starting composition were tested in machining applications in order to evaluate their potential in milling operations. Preliminary milling tests in milling medium carbon steel AISI1045 indicated that the TiC–WC–Mo–Ni cermets with TiN nanopowders addition (tool A) exhibits a better resistance to deformation at high milling speed than the TiC–WC–Mo–Ni–Co cermets with TiN nanopowders addition (tool B), but lower impact milling breakage properties. This is due to the presence of the Co binder instead of Ni improves the toughness of materials, but decreases the wear and the resistance to deformation. The failure mode of tool A and B of newly developed TiC-based cermet with TiN nanopowders addition is in the form of attrition wear when the cutting speed (vc ) is equal to or less than 373 m/min, and is in the form of breakage when the cutting speed is beyond 373 m/min. The wear mechanisms of the tools employed in milling tests were investigated by SEM–EDS analysis. Besides abra-

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