Scholarly article on topic 'High Speed Machining of Inconel 718 Focusing on Wear Behaviors of PCBN Cutting Tool'

High Speed Machining of Inconel 718 Focusing on Wear Behaviors of PCBN Cutting Tool Academic research paper on "Materials engineering"

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{"High speed machining" / "Inconel 718" / CBN / Tribology / Wear}

Abstract of research paper on Materials engineering, author of scientific article — Haruki Tanaka, Tatsuya Sugihara, Toshiyuki Enomoto

Abstract Recently, PCBN cutting tools have received a great deal of attention as a material for cutting tools to achieve high performance machining of Inconel 718. In this study, turning and orthogonal cutting experiments on Inconel 718 employing two types of PCBN cutting tool were conducted in order to evaluate the tool life and identify the wear mechanisms, at a wide range of cutting speeds. The results of the experiments indicated that the tool life and wear behavior of the PCBN cutting tools significantly depend on the cutting speed and material structures, and a PCBN tool, which has a low CBN content with TiN-based ceramic binder, shows excellent wear resistance in high speed machining of Inconel 718.

Academic research paper on topic "High Speed Machining of Inconel 718 Focusing on Wear Behaviors of PCBN Cutting Tool"

Available online at www.sciencedirect.com

ScienceDirect

Procedia CIRP 46 (2016) 545 - 548

www.elsevier.com/looate/procedia

7th HPC 2016 - CIRP Conference on High Performance Cutting

High speed machining of Inconel 718 focusing on wear behaviors of PCBN cutting tool

Haruki TANAKAa, Tatsuya SUGIHARAa*, Toshiyuki ENOMOTOa

a Department of Mechanical Engineering, Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan

* Corresponding author. Tel.: +81-6-6879-7287; fax: +81-6-6879-7287. E-mail address: t-sugihara@mech.eng.osaka-u.ac.jp

Abstract

Recently, PCBN cutting tools have received a great deal of attention as a material for cutting tools to achieve high performance machining of Inconel 718. In this study, turning and orthogonal cutting experiments on Inconel 718 employing two types of PCBN cutting tool were conducted in order to evaluate the tool life and identify the wear mechanisms, at a wide range of cutting speeds. The results of the experiments indicated that the tool life and wear behavior of the PCBN cutting tools significantly depend on the cutting speed and material structures, and a PCBN tool, which has a low CBN content with TiN-based ceramic binder, shows excellent wear resistance in high speed machining of Inconel 718.

© 2016 The Authors.PublishedbyElsevier B.V This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of the International Scientific Committee of 7th HPC 2016 in the person of the Conference Chair Prof. Matthias Putz

Keywords: High speed machining; Inconel 718; CBN; Tribology; Wear;

1. Introduction

Inconel 718 is one of the most important alloys among the nickel and nickel-based alloys due to its excellent properties under very high temperature conditions, and widely used as a material for aviation, turbines and nuclear power plant applications. However, Inconel 718 is classified as a "difficult-to-cut material" because of its peculiar characteristics such as low thermal conductivity, high tendency to work hardening and high affinity for tool materials [1-4]. Recently, polycrystalline cubic boron nitride (PCBN) has received a great deal of attention as a material for cutting tools to achieve high performance machining of Inconel 718, and Uhlmann and Ederer reported its performances and potential in high speed turning of Inconel 718 up to 1250 m/min [5]. On the other hand, although it is known that the performances of PCBN tools are greatly affected by their material structures including CBN grain size, CBN content and type of binder material [2, 6], suitable material structures of the PCBN cutting tool for high performance machining of Inconel 718 are still under

discussion. For example, while tool manufacturers generally recommended a PCBN tool with a high CBN content [1, 7], Costes et al. presented that a PCBN tool with low CBN content below 60% showed a better wear resistance [6].

In this study, in order to obtain a guideline for suitable material structures of a PCBN cutting tool especially in high speed machining of Inconel 718, cutting experiments with two different PCBN cutting tools at a wide range of cutting speeds (20 m/min - 300 m/min) were conducted and wear behaviors of each tool were investigated. Through a series of experiments, the detailed relationship between the cutting performances and the material structures of the PCBN tools were discussed.

2. Experimental details

2.1. Experimental setup

The workpiece material was solution treated and aged Inconel 718 with a hardness of 40 HRc. The chemical composition is shown in Table 1.

2212-8271 © 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.Org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of the International Scientific Committee of 7th HPC 2016 in the person of the Conference Chair

Prof. Matthias Putz

doi:10.1016/j.procir.2016.03.120

Firstly, turning experiments with two types of PCBN cutting tool (Sumitomo Electric Industries Ltd., 2NU-CNGA120412-LF), as stated in the next section, were conducted until cutting length reached 300 m to confirm the tool life of each tool. Then, orthogonal cutting experiments were carried out to investigate the detailed wear behaviors of the tools. In the latter experiments, Inconel 718 pipe with a thickness of 2 mm and outer diameter of 100 mm was used as a workpiece, as shown in Fig. 1. Table 2 lists the cutting conditions. All experiments were conducted with three different cutting speeds (20, 100, 300 m/min), and cutting fluid (NEOS Co., Ltd., Finecut 2500) was supplied.

2.2. PCBN cutting tools

In this paper, two types of PCBN cutting tools with different material structure were prepared. Fig. 2 shows the micro material structures of each PCBN cutting tool, and Table 3 lists the contents and physical properties. PCBN-A (Sumitomo Electric Industries, Ltd., BN7000) is a high CBN content tool with cobalt binder, and PCBN-B, which is a newly developed PCBN tool produced by Sumitomo Electric Industries, Ltd., has finer CBN grains with TiN-based ceramic binder. As shown in Table 3, PCBN-A has superior mechanical properties including higher hardness and transverse strength. On the other hand, although thermal conductivity of PCBN-A is higher than that of PCBN-B, PCBN-B is expected to show excellent heat resisting properties because the ceramic binder phase has much better chemical stability with respect to nickel compared with CBN grains [1, 5].

After orthogonal cutting experiments, cross sections of the worn tools fabricated by focused ion beam milling [8] (Hitachi High-Technologies Corp., IM4000), as shown in Fig. 3, were observed by using SEM (Hitachi High-Technologies Corp., TM-3000), in order to investigate the detailed wear behaviors of each cutting tool.

Table 1. Chemical composition of Inconel 718 (in % of mass).

Table 2. Cutting conditions.

Workpiece

Ni Cr Nb Mo Ti Al

53.98 18.11 5.44 3.00 1.01 0.53

Fig. 1. Experimental set-up of orthogonal turning test

CBN grain Binder CBN grain Binder

Cutting tool Tool geometry PCBN cutting tool Rake angle Flank angle 20 m/min (a) BN7000 (b) Developed PCBN 5° 6°

Cutting speed 100 m/min 300 m/min

Cutting length Feed rate Turning Orthogonal cutting 0.10 mm/rev 300 m 25 m

Cutting fluid Soluble type (US A2) Supply rate Fine cut 2500, Neos 6.1 L/min

Table 3. Contents and physical properties of prepared CBN cutting tool.

Type of tool PCBN-A (BN7000) PCBN-B (Developed CBN)

CBN content (vol%) 85~95 55~65

Grain size (^m) 2~3 1~2

Vickers hardness: HV (GPa) 41~44 30~32

Fracture toughness : KIC (MPa • m1/2) 9~11 5~6

Three-point bending strength: a (GPa) Thermal conductivity (W/mK) Binder material 2.0~2.2 75-85 Co-based alloy 1.5~1.6 35-55 TiN-based ceramics

"Observed'

Fig. 3. Method for observing cross section of worn cutting tool

nanmaJHafiWriHIil ■ 1

Catastrophic ^^^tc-ol failure ■L KU

(a) PCBN-A (BN7000) (b) PCBN-B (Developed CBN) Fig. 2. Micro material structures of PCBN cutting tools

Fig. 4. SEM images of tool rake face after 300 m cutting

3. Results and discussion

3.1. Results of turning experiments

Fig. 4 shows the SEM images of the tool rake face after 300 m cutting at each cutting speed. As shown in Fig. 4 (e), catastrophic tool failure occurred in PCBN-B at the cutting speed of 100 m/min. Furthermore, Fig. 5 shows the relationship between the cutting speed and depth of crater wear measured by using a two-dimensional stylus type profile instrument (Kosaka Lab. Ltd., SE-3500K). In this figure, the result of PCBN-B at the cutting speed of 100 m/min is treated as an exceptional case when the depth of the crater wear exceeded 50 ^m.

These figures clearly indicate that the PCBN tool life significantly depends on the cutting speed and material structures of the PCBN tools. In the case of PCBN-A, better wear resistance

Inconel 718

Fig. 5. Relationship between cutting speed and depth of crater wear

was obtained at the cutting speed of 100 m/min, which is substantially equal to the recommended cutting speed for the tool, and the curved course of the tool life graph is in good agreement with the previous studies [3]. On the other hand, PCBN-B showed different trends compared with PCBN-A, and Fig. 5 indicates that the wear amount of PCBN-B was drastically decreased when increasing the cutting speed to 300 m/min.

3.2. Cutting speed of 20 m/min

Fig. 6 shows the SEM images of the tool rake face after orthogonal cutting experiments at the cutting speed of 20 m/min and Fig. 7 shows the results of cross section observation of the CBN cutting tools which were sliced along the line indicated as A-A' and B-B' in Fig. 6, respectively.

As shown in Fig. 6, severe adhesion of Inconel 718 occurred on the tool rake face and tool substrate materials were partially exposed in both cutting tools. Furthermore, Fig. 7 indicates that the bottom of the crater wear after machining had large asperities, and some cracks that penetrate into the tool substrate material can be observed as can be seen from Fig. 7 (a). These

results suggest the tool substrate material was flaked when the adhesion was removed from the tool rake face, resulting in the exacerbation of the crater wear, in low speed machining of Inconel 718. Under such cutting conditions, sever crater wear occurs in both PCBN-A and PCBN-B as indicated in Fig. 5, although the scale and frequency of the adhesion and its removal were different.

3.3. Cutting speed of 100 m/min

Figs. 8 and 9 show the SEM images of the tool rake face and results of cross section observations after orthogonal cutting experiments at the cutting speed of 100 m/min. As shown in these figures, in contrast to the results at the low cutting speed, the thick adhesion layer on the tool rake face disappeared at this cutting speed, suggesting that the cutting temperature at the tool-chip interface was elevated over recrystallization temperature of the workpiece material at high cutting speed. In addition, as shown in Fig. 9 (a) and (b), the surface of the worn rake face became smooth, indicating that CBN grains themselves were worn. As our previous study indicated [9], this result means that the diffusion due to the high cutting temperature causes the crater wear at this cutting speed.

It is well known that thermal wear including diffusion, chemical reaction and oxidation tends to be dominant in high speed machining, whereas mechanical wear, such as adhesion and its flaking or abrasive wear are the most significant types of wear at lower cutting speed [10]. Thus, the above mentioned results show that the dominant factor for exacerbating the crater wear changes from mechanical factor, such as adhesion and its removal, to thermal load due to high cutting temperature when increasing the cutting speed from 20 m/min to 100 m/min in the machining of Inconel 718.

300 Mm

Adhesion layer

--■Cutting edge

(a) PCBN-A (BN7000) (b) PCBN-B (Developed CBN)

Fig. 6. SEM images of rake face after machining at cutting speed of 20 m/min

(a) PCBN-A (BN7000) (b) PCBN-B (Developed PCBN)

Fig. 8. SEM images of rake face after machining at cutting speed of 100 m/min

(a) PCBN-A (BN7000)

(a) PCBN-A (BN7000)

(b) PCBN-B (Developed PCBN) Fig. 7. Cross section observations after machining at cutting speed of 20 m/min

(b) PCBN-B (Developed PCBN) Fig. 9. Cross section observations after machining at cutting speed of 100 m/min

On the other hand, as shown in Fig. 8 (b) and Fig. 9 (b), cutting edge chipping was observed only in PCBN-B. This is because a large mechanical load is still applied to the cutting edge at the cutting speed of 100 m/min, and the poor mechanical properties such as lower transverse strength of PCBN-B lead to the cutting edge chipping. Eventually, catastrophic tool failure occurred, as shown in Fig. 4 (e).

3.4. Cutting speed of300 m/min

Figs. 10 and 11 show the SEM images of the tool rake face, results of cross section observations and sectional profiles of each cutting tool after orthogonal cutting experiments at the cutting speed of 300 m/min. As shown in these figures, the surface of the worn rake face of both PCBN-A and PCBN-B became smooth, in the same way as the results at the cutting speed of 100 m/min, although the wear rate was higher. On the other hand, no cutting edge chipping was observed even in PCBN-B. These results suggest that the dominant factor for exacerbating crater wear completely changed from the mechanical wear to thermal wear when increasing the cutting speed to 300 m/ min. In addition, it has been pointed out that the TRS of a PCBN with a low CBN content and ceramic binder phase increases as temperature increases [11], and this can be another reason for the better chipping resistance of PCBN-B.

Under such conditions, as shown in Fig. 5, PCBN-B, which has a low CBN content and TiN-based ceramic binder phase, shows excellent crater wear resistance compared with PCBN-A. From these results, it can be said that PCBN-B has the potential to be a major candidate to achieve high speed machining of Inconel 718.

(a) PCBN-A (BN7000) (b) PCBN-B (Developed PCBN)

Fig. 10. SEM images of rake face after machining at cutting speed of 300 m/min

E' , __Raku ,ace E

■ v !v v " -..

' lOum ■ ■

Flank, face - ..A-", ■ :: 50

(a) PCBN-A (BN7000)

F .. Cutting edge Рак

\ * ': ЧРЙ«r 10 um

Flank face'- - ■ • ■— M

(b) PCBN-B (Developed PCBN) Fig. 11. Cross section observations after machining at cutting speed of 300 m/min

4. Conclusion

In this study, turning and orthogonal cutting experiments on Inconel 718 employing two types of PCBN cutting tool were conducted in order to achieve high speed machining of Inconel 718 with the following findings:

(1) The results of turning experiments showed that the PCBN tool life significantly depends on the cutting speed and material structures of the PCBN tools. While the PCBN tool with a high CBN content (PCBN-A) shows the same tendency as observed in the conventional studies, PCBN-B indicates excellent wear resistance especially at cutting speed over 300 m/min.

(2) At low cutting speed, crater wear is exacerbated by the cycle of adhesions of workpiece material and their removals, and sever crater wear occurred regardless of the types of cutting tools.

(3) When increasing cutting speed to 100 m/min, diffusion due to high cutting temperature is a dominant factor for promoting crater wear. On the other hand, cutting edge chipping occurred in PCBN-B which is inferior in mechanical properties, leading to catastrophic tool failure.

(4) At the cutting speed of 300 m/min, the dominant factor for exacerbating crater wear completely changes from mechanical wear to thermal wear, and PCBN-B with a low CBN content and TiN-based ceramic binder phase shows excellent crater wear resistance due to its superior heat resistant property and chemical stability.

Acknowledgements

We thank everyone at Sumitomo Electric Industries Ltd., and Neos Co., Ltd., for their invaluable assistance and advice. This work was supported by JSPS KAKENHI Grant Number 24860040.

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