Investigation on the Cutting Process of Plasma Sprayed Iron Base Alloys

Size: px
Start display at page:

Download "Investigation on the Cutting Process of Plasma Sprayed Iron Base Alloys"

Transcription

1 Key Engineering Materials Online: ISSN: , Vols , pp doi: / Trans Tech Publications, Switzerland Investigation on the Cutting Process of Plasma Sprayed Iron Base Alloys Kan Ding 1,a, Hiroyuki Sasahara 2,b, Syuji Adachi 3,c and Kimio Nishimura 3,d 1 Graduate School of Bio-Applications and System Engineering, Tokyo University of Agriculture and Technology, Tokyo, Japan 2 Department of Mechanical Systems Engineering, Tokyo University of Agriculture and Technology, Tokyo, Japan 3 Nissan Motor Co. Ltd., Yokohama, Japan a 1984.kan@gmail.com, b sasahara@cc.tuat.ac.jp, c syuji-adachi@mail.nissan.co.jp, d ki-nishimura@mail.nissan.co.jp Keywords: Cutting, Plasma spray coated cylinder bore, Boring, Tool wear, Force. Abstract. In the recent years, the current technology enables only the molten iron base alloys, sprayed on the aluminum alloy engine block thus it can function as a cylinder bore. However, the machinability performance of plasma spray coated cylinder bore in boring process is poor because of severe tool wear compared with the previous cast iron cylinder bore. This paper deals with the results obtained at boring process of plasma sprayed iron base alloys coating to clarify the root cause of tool wear. Preliminary fine boring and turning experiments are conducted on the plasma sprayed cylinder bore, and tool wear, tool failure modes and cutting force were also investigated. The result shows plasma spray coated cylinder bore recorded larger cutting force than the cast iron cylinder bore. Also, this work shows that abrasive effect by the hard oxide particles on the cross-sectioned of machined layer is superior when fine boring plasma spraying iron base alloys coating. 1. Introduction As a surface treatment method to the coated metal parts, the plasma spraying technology has been increased in recent years. This is because of the demand of a high surface hardness, excellent resistance against wear, heat and corrosion, and good adiabaticity, etc. These characteristics can be provided on the metallic surface by forming the thermal spray coating to mechanical parts. In the automotive industry, the plasma spraying is widely used, especially in the cylinder bore of engine block. For instance, conventional cylinder bore is made of cast iron. Instead of the conventional cylinder bore, the current technology enables very thin cylinder bore by spraying the molten iron base alloys on the aluminum alloy engine block, and subsequently functioned as a cylinder bore. The thickness of plasma spray coated cylinder bore in comparison to the conventional cylinder bore is 0.2 mm and 2.0 mm respectively. Thus it contributed to reducing the entire engines weight. In addition, it has been reported there are several advantages by using the plasma thermal spray coatings, such as the decrease in the friction between piston ring and bore inner surface. Furthermore, it also improved in wear and abrasion resistance of cylinder bore and fuel cost of engine can be saved [1]. On the other hand, very rough surface of cylinder bore is obtained after the plasma spraying process. Therefore, surface finishing process is necessary, and normally a honing process is executed as finishing process. Moreover, time and cost can be reduced by replacing the honing process to the fine boring process. However, during the fine boring process of the iron base alloy plasma spray coated cylinder bore, the machinability performance is poor because of severe tool wear compared to the conventional cast iron cylinder bore. There are limited numbers of studies on the cutting process of plasma thermal spray coatings. Inui and others [2] have clarified the machinability of a self-fluxing alloy applied by thermal spraying to the machine parts, by evaluate the tool wear, cutting force and the finished surface when cutting with different kinds of CBN tools. Itomura and others [3] investigated machinability of the thermal All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (ID: , Pennsylvania State University, University Park, USA-13/05/16,07:33:35)

2 822 Advances in Precision Engineering sprayed materials made of 13% chromium stainless steels, eutectoid steel and aluminum bronzes. Specifically, the cutting conditions, the cutting force and roughness of the machined surface when cutting with WC-Co tool were examined. However, the research related to the iron base alloy plasma thermal spray coating for the engine cylinder bore is hardly found. Therefore, the objective of this research is to clarify the machinability of the iron base alloy thermal spray coating and the cause of the severe tool wear. The experiments on fine boring process with actual iron base alloy plasma spray coated cylinder bore were conducted. Then the effect of the content rate of CBN and binder on the tool wear was examined. In addition the cutting force during the turning process was measured and compared. 2. Fine boring experiment with iron base alloy plasma spray coated cylinder bore A finish machining of the iron base alloy plasma spray coated cylinder bore is almost executed by the honing process. So the tool wear condition, the machined surface conditions and other details during the fine boring process was still uncomprehending. Therefore, in the first experiment, a fine boring was conducted with actual iron base alloy plasma spray coated cylinder bore and the tool wear progress was measured. The experimental equipment and arrangement are shown in Fig. 1. Machining center (MAZAK-VCN-410A) was used in the experiment. The test piece was fixed on the table of the machining center with the clamping jig. The test piece (L-TP) was sectioned from an actual iron base alloy plasma spraying engine block as shown in Fig. 1 (3 bores). The diameter and height of the bore were 95 mm and 150 mm respectively. The thickness of the thermal spray coating is approximately 0.6 mm. A boring bar was used in the fine boring process. Fig.1 Fine boring experiment The K10 grade carbide tool and CBN tool which contains more than 90% with the metal system binder were used throughout the experiments. The tool shape is TCGT Moreover, the CBN tool has a chamfer (0.12 mm in width) on the edge. The experimental conditions are as follows, cutting speed ranging from 85 to 400 mm/min, feed rate was kept constant at 0.19 mm/rev and depth of cut was 0.25 mm. The water soluble type coolant was used in this experiment. For the observation and measurement of the tool wear progress, the cutting process was stopped at every prescribed time, and then the maximum width of tool flank wear was measured using the microscope. In addition, the surface roughness of the machined surface was measured along the feed direction. The total cutting length to complete one cylinder bore is approximately 130 m. The result of the flank wear of the carbide tool (cutting speed V=85 mm/min) and CBN-A tool (V=400 mm/min) are shown in Fig. 2. From this figure, it shows that the tool flank wear is remarkably growing in spite of a short cutting time in both cases. Particularly, it was impossible to continue the machining by the carbide tool. Moreover, the tool wear progress is shown in Fig. 3. As shown in this figure, the carbide tool exhibited a very high tool wear rate and the flank wear land exceeded 0.2 mm at the cutting length 90 m. Furthermore, tool wear rate of the CBN tool is faster than the common case of the cast iron cutting. In addition, three levels of cutting speed (85 mm/min, 200 mm/min, and 400 mm/min) were examined in the experiment by using CBN tool. As a result, the tool wear rate increases when cutting speed increased. This can be thought that a thermal factor influences the tool wear. As for the machined surface, the average surface roughness Ra at the bottom part of the cylinder bore by the carbide tool is almost 6 μm. This is due to the remarkably growing of flank wear rate. On the other hand, the average surface roughness when machined with CBN tool was maintained around 3 μm. Fig. 4 shows an example of the machined surface observed by scanning electron microscope (SEM) when using CBN-A tool at the cutting speed of 200 mm/min. Cutting feed marks can be

3 Key Engineering Materials Vols observed clearly on top of the machined surface. Moreover, there are some pinholes and some fractured part by the dropout of the inclusion particle on the machined surface. This is attributed to the existence of inclusion particle and pores in the thermal spray coating, and subsequently contributed to the mechanical effect on the tool wear. (a)carbide Flank wear width V B (max) mm (b)cbn-a Fig.2 The flank wear of tools Cutting length m Fig.3 Wear progress curves of carbide and CBN-A in plasma spray coated cylinder bore s fine boring process 3. Turning process of the iron base alloy thermal spray coating A cutting force measurement and the observation on the condition of cutting point during the fine boring process are difficult. However, the cutting force may influence all elements of machinability directly or indirectly. Therefore, in order to investigate the cause of the tool wear, turning experiments with iron base alloy plasma spray coated cylinder is carried out under the dry conditions. The experimental setup is shown in Fig. 5. Sprayed iron base alloy is same with the case on the cylinder bore. The thickness of the thermal spray coating is from 1 to 2 mm, the diameter and length of the test piece is 142 mm and 100 mm respectively. In addition, the cast iron bore (FC250) of 84 mm in the diameter and 130 mm in length is used as a comparison. Table 1 (a) and (b) shows the cutting tool grade and cutting conditions respectively. Fig.5 Cutting force measurement experiment Carbide V85 f0.19 D0.25 WET CBN-A V85 f0.19 D0.25 WET CBN-A V200 f0.19 D0.25 WET CBN-A V400 f0.19 D0.25 WET (a) Cutting tools Types and Content of the bonding agent CBN-A metal system (less than 10%) CBN tools CBN-B ceramics system (more than 50%) Carbide tools K-type (b) Cutting conditions Cutting speed [m/min] 85~400 Feed rate [mm/rev] 0.19 Depth of cut [mm] 0.25 Fig.4 An example of the machined surface Table 1 Cutting tools and cutting conditions 3.1 Cutting force Three components of the cutting force (main cutting force: F p, feed force: F f, thrust force: F v ) during the turning process of the thermal spray coating were measured using 3 channels piezoelectric dynamometer. Fig. 6 shows the result of cutting forces when the thermal spray coating and the cast iron were cut at the cutting speed of 85 m/min, feed rate of 0.19 mm/rev and the depth of cut of 0.25 mm. For both thermal spray coating and cast irons, the Cutting forces F p, F v, F f N p F p F v F f 0 Plasma Sprayed Coating Cast Iron Fig.6 Cutting forces of the thermal spray coating and cast iron in turning process

4 824 Advances in Precision Engineering highest cutting forces appeared at the main cutting force and the feed force was the smallest in the cutting force components. It is very clearly the three components of cutting force during the turning process of thermal spray coating are larger than the case of the cast iron, especially main cutting force is about twice or higher. 3.2 Characteristics of thermal spray coatings In this experiment, two types of thermal spray coatings were used, those are sprayed on the inner side of engines cylinder block and on the outside of the aluminum cylinder. Spray conditions and the wire material of both are the same. The test piece spraying process were made by three different manufacturers namely L-TP (engine cylinder bore), C-TP1 (aluminum cylinder), and C-TP2 (aluminum cylinder). The observation of metallic structure and the hardness of these thermal spray coatings were conducted by using confocal microscope and hardness tester respectively. Fig. 7 shows the metallic structure on the cross sections of three types of the thermal spray coatings. There are some features in the thermal spray coatings structure depending on the substrate material shape (at either periphery or inner periphery) and the difference of the plasma spraying technique. Wavy surface was observed on test piece L-TP. Also, the amount of the Fe-oxide (gray parts) of the test piece L-TP is larger. Furthermore, a lot of porosity (black parts) is included in all the thermal spray coatings. The results of hardness of the thermal spray coatings are as follows: L-TP is from 310 to 700 HV, C-TP1 is from 300 to 660 HV and C-TP2 is from 280 to 460 HV. Meanwhile, the cast iron recorded the hardness ranges from 360 to 420 HV. The hardness value of all kinds of the thermal spray coating has some range of dispersion. It is because the hardness of the oxide part shows high value, it sometimes reaches 700 HV. Therefore, the mechanical wear possibly has a large influence on the tool wear because that a great amount of hard particle exist in the sprayed layer. Moreover, thermal conductivity of the thermal spray coating layer was measured by a laser flash method. As a result, the thermal conductivity of thermal spray coating at the room temperature is 20 W/m K, and it is very low compared with a common cast iron (46 W/m K). This is attributed to the existence of a great amount of porosity and the oxide in the thermal spray coating as shown in Fig. 7. The sample of chip obtained when cutting the thermal spray coating was mounted, ground, polished and etched in order to analyze the chip segmentation. The cross section of the chip of the thermal spray coating is shown in Fig. 8. It can be observed that the sawtooth chip formation can be obtained by cutting the thermal spray coating which is the same as the chip obtained by the cutting of the low thermal conductivity titanium alloy. It is contemplated that this is because the deformation concentrates on the part that softening by partial temperature-rise during the machining [4]. Therefore, it is can be expected that the cutting temperature is very high during cutting the thermal spray coating. 3.3 Influence of the binder material of CBN tool (a) L-TP (c) C-TP2 (b) C-TP1 Fig.7 Cross sections of the thermal spray coatings Fig.8 Cutting chip of the thermal spray coating It is difficult to obtain a large crystal of CBN particle, so the CBN tool is usually sintered by using the particle of 1 to 3 μm with various binder materials. Therefore, the characteristic and the machinability of CBN tool were depending on the binder material and its content rate. For the turning experiment, two kinds CBN tools with different binder material content were used namely CBN-A

5 Key Engineering Materials Vols (contains 90% or more CBN) and CBN-B (contains 50% or more CBN). The results of tool wear progress for both tools were compared. The condition of the tools after machining was observed by using scanning electron microscope (SEM). Fig. 9 (a) shows the CBN-A when turning at the cutting speed of 85 mm/min, feed rate of 0.19 mm/rev, depth of cut of 0.25 mm and cutting length about 1400 m. Fig. 9 (b) shows the CBN-B when turning at the cutting speed of 200 mm/min, feed rate of 0.19 mm/rev, depth of cut of 0.25 mm and cutting length about 1500 m. A large amount of adhesion of test piece material was observed on flank faces of both tools. Moreover, a lot of micro chippings can be observed on the chamfer region of the CBN-B tool. This is thought that the large amount of binder material were wears off, subsequently the CBN particles were removed from the substrate. Moreover, after prolonged machining, the large amount of test piece material was adhered on the cutting edge thus contributed to the severe chipping or catastrophic failure. On the other hand, adhesion wear mechanism was insignificant for the low binder material content (10%) of CBN-A tool under the various cutting conditions. Therefore, the CBN-A tool with less contents of binder material is thought to be effective for the cutting of the thermal spray coating. However, the affinity between the binder material and the thermal spray coating is not clear yet, thus it will be necessary to still examine on the aspect of reaction, diffusion and etc. (a)cbn-a (b)cbn-b Fig.9 SEM photograph of the CBN tools after cutting process 4. Conclusions (1) The tool wear is remarkable during the fine boring process of the iron base alloy plasma spray coated cylinder bore. (2) The cutting force of the thermal spray coating is larger than that of the cast iron, especially the main cutting force. (3) The thermal spray coating contains hard particles which reach over 700 HV. It shows that the abrasion by hard particle influences the rapid tool wear. (4) Thermal conductivity of thermal spray coating measured by laser flash method is very low. A large amount of adhesion is observed on the tools. Thus it can be suggested that adhesion wear is due to the chemical reaction between the tool and the plasma spraying coated surface. Acknowledgment The authors wish to acknowledge the support for this research by Sumitomo Electric Hardmetal Corp. References [1] G. Barbezat: Surf. & Coat. Technol. Vol. 200 (2005), p.1990 [2] Y. Inui, T. Hayami and T. Ikuta: J. Japan Soc. Prec. Eng. Vol. 56(7) (1990), p.1229 [3] S. Itomura, W. Shikawa, J. Yamaguchi, G. Ueno and T. Fukushima: Bull Fac. Eng. Uni. Ryukyus Vol.49 (1995), p.1 [4] M.C. Shaw and A. Vyas: CIRP Annals-Manuf. Technol. Vol. 47 (1) (1998), p.77

6 Advances in Precision Engineering / Investigation on the Cutting Process of Plasma Sprayed Iron Base Alloys /