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Keywords: Cold forging; Backward extrusion; Friction coefficient; Tribology .... It is natural to consider that the friction coefficients are not a constant value on the ...

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ScienceDirect Procedia Engineering 81 (2014) 1866 – 1871

11th International Conference on Technology of Plasticity, ICTP 2014, 19-24 October 2014, Nagoya Congress Center, Nagoya, Japan

Measurement of friction coefficient by backward extrusion with rotating tool under severe forming conditions Masatoshi Sawamura*, Yasuhiro Yogo, Michiaki Kamiyama, Noritoshi Iwata Toyota Central R&D Labs., inc., 41-1, Yokomichi, Nagakute, Aichi 480-1192, Japan

Abstract In cold forging processes, friction between materials and dies strongly influences material flow, forging load and damage to the dies. Therefore, several methods to evaluate friction have been proposed. However those methods are not well simulated for the severe conditions occurring in actual forging processes. In this study, a new method to evaluate friction coefficients is proposed. The method applies backward extrusion, which enables us to create high pressure, large surface area expansion and a long sliding distance, for the measurements. The unique feature of the die set used in this method is a rotating punch at a very slow angular velocity. By a combination of measured load and torque, friction coefficients can be monitored throughout the measurement process. Friction coefficients were measured for four types of lubrications; mineral oil, cold forging oil, bonderizing and dry-in-place. With these results, the proposed method enables us to measure friction coefficients under severe forming conditions. © 2014 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license © 2014 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/3.0/). Selection and peer-review under responsibility of Nagoya University and Toyohashi University of Technology. Selection and peer-review under responsibility of the Department of Materials Science and Engineering, Nagoya University Keywords: Cold forging; Backward extrusion; Friction coefficient; Tribology

1. Introduction In cold forging processes, friction between materials and dies strongly influences material flow, forging load and damage to the dies. Therefore, reduction of friction forces is one of the important subjects to be investigated.

* Corresponding author. Tel.: +81-561-71-7566; fax: +81-561-63-6948. E-mail address: [email protected]

1877-7058 © 2014 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.org/licenses/by-nc-nd/3.0/). Selection and peer-review under responsibility of the Department of Materials Science and Engineering, Nagoya University doi:10.1016/j.proeng.2014.10.247

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Several methods to evaluate friction have been proposed by the Japan Society for Technology of Plasticity (1993). However these methods are not well simulating severe conditions occurring in actual forging processes. If friction coefficients, which are the values to represent friction conditions, can be measured under similar conditions to actual cold forging processes, that makes it possible to select a suitable lubricant for each forging process and improve calculation accuracy in computer aided engineering. Hansen and Bay (1986) proposed a measurement method for friction coefficients using a punch consisting of two parts; an inner punch and an outer ring punch. The outer ring punch rotates to measure both load and torque for calculating the friction coefficient. They showed the measured results under low pressure with aluminum. Danno et al. (1983) and Kitamura et al. (1996) applied backward extrusion to evaluate severe friction conditions with steel. They measured forming load, and then they examined performance of lubricants based on the measured load. However, they did not measure friction coefficients. The authors proposed an improved backward extrusion equipment with a rotating punch for torque measurement to adapt the existing method to severe friction conditions in Sawamura et al. (2012). In this paper, a measurement method for friction coefficients with the developed equipment and the results are shown. 2. Experiment 2.1. Experimental method In Fig. 1, the developed equipment is shown. With this equipment, it is possible to measure friction coefficients under the high pressure and long sliding distances which occur when forging steel. A billet was backward extruded with the rotating punch at a very slow rotational velocity. The rotational velocity was determined not to influence material flow. Through the backward extrusion, the punch rotated only 6º. Load cells were installed below the container to measure torque and load for calculating friction coefficients. On the contacting surface between the bottom of the billet and the container, grid patterned snags were made to suppress slippage. The punch and container can be heated up to 623 K (350 ºC) with the heaters. The equipment was placed in a servo press (3000kN). A hydraulic die cushion (500 kN), placed below the equipment was used to knock-out the billet after testing. The punch and billet dimensions are shown in Fig. 1(a) and (c). The reduction in area of the billet was 50 %. The high speed tool steel (SKH51 of 60 HRC) was applied for the punch and container. The contacting surface of the punch and container with the billet was lapped to 0.4 Rz. The material of the billet was spheroidizing annealed S10C (surface roughness: 6.3 Rz, 180 HV). The penetration depth into the billet was determined to be twice as long as the punch stroke due to the 50 % reduction in area of the billet. Four types of lubricants were applied for the measurement; additive–free paraffinic mineral oil with three viscosity grades (VG22, VG100, VG460), sulfur adding cold forging oil, dry-in-place, and bonderizing. Those lubricants were applied to the billet and no lubrication was used on both the punch and the container. The billet and the container were used at room temperature, and the range of temperatures of the punch was set between room temperature and 623 K (350 ºC).

ĭ (a)

Punch rotation(1rpm) Punch torque, M Punch load, P

(c)

ĭ

ĭ21.2

Penetration depth

Punch Ⅸ

Container

Billet

8

H

ĭ

Before

After Fig. 1. Die-set for measurements of friction coefficients. (a) specimen, (b) die set and (c) punch.

ĭ

Land 2

(b)

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Masatoshi Sawamura et al. / Procedia Engineering 81 (2014) 1866 – 1871

2.2. Calculation of friction coefficient The friction coefficient was calculated with Eq. (1) proposed by Kamiyama et al. (2012) using a combination of the measured load, P (kN), and Torque, M (N·m). Eq. (1) was obtained with calculation results using the finite element software, FORGE (Transvalor S.A.) with consideration of pressure distribution on the punch. ȝ= 0.00709(M/P)3 - 0.0121(M/P)2 + 0.0937(M/P).

(1)

3. Results

800

Lubricant : Bonderizing Punch velocity : 150mm s-1 Penetration depth : 24mm Punch temperature : 298K

600

Punch load Punch stroke Period for evaluation of friction coefficient

400

200

2

204

206

208 20: Time (s)

Fig. 2. Measured punch stroke, load and torque.

30

20

10

Punch torque

0

40

3

0 304

Punch stroke (mm)

Punch load, kN, Punch torque (N m)

In Fig. 2, measurements of the punch stroke, load and torque are shown. The punch reached the bottom dead center at 0.4 s, and was released at 1 s. Therefore, evaluation of friction coefficients was carried out while the punch was at the bottom dead center (from 0.5 to 0.9 s). The velocity of the punch was at a constant value from its initial position until 4 mm above the bottom dead center. Therefore, average velocity of the punch was defined as the velocity until 5 mm above the bottom dead center. In Fig. 3, the effect of the viscosity of the mineral oils is shown. The friction coefficients increased over time in all three viscosities of the mineral oils. (Fig. 3(a)). In Fig. 3(b), the relationship between kinematic viscosity and the averaged friction coefficients are shown. Lower kinematic viscosity shows higher friction coefficients. Photographs of the punch after testing are shown in Fig. 3(c). Severe adhesion resulting in galling resulting was observed when using lower viscosity oils (VG22 and VG100). However, when using the higher viscosity oil (VG460), only light adhesion was observed. Based on these photographs, it was confirmed that the measured friction coefficients were well influenced by the magnitude of adhesion. In Fig. 4, the influence of the penetration depth on the friction coefficients is shown. The friction coefficients of the dry-in-place and bonderizing specimens decreased with increasing penetration depth. On the contrary, friction coefficients of cold forging oil specimens initially decreased, then drastically increased for penetration depth over 40 mm. Under these conditions, severe adhesion was observed at the punch land. In Fig. 5, the influence of the punch temperatures on the friction coefficients at 44 mm of penetration depth was shown. The friction coefficients of dry-in-place specimens were higher than that of bonderizing specimens at all temperatures. The friction coefficients with dry-in-place specimens decreased from room temperature to 473 K (200 ºC), then increased over 473 K (200 ºC). On the contrary, the friction coefficients with bonderizing specimens decreased gradually with increasing temperature. It is confirmed by these results that the developed equipment enables us to calculate friction coefficients with torque and load data gathered under severe friction conditions.

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Masatoshi Sawamura et al. / Procedia Engineering 81 (2014) 1866 – 1871

(a)

0.12 Punch velocity : 120mm s-1 Penetration depth : 24mm Punch temperature : 298K

Friction coefficient ȝ

0.1

Viscosity grade of the mineral oils

VG22

0.08 0.06

VG100

0.04

VG460

0.02 0 207

208

.

209 Time (s)

20:

20;

(b)

0.12 Punch velocity : 120mm s-1 Penetration depth : 24mm Punch temperature : 298K

Friction coefficient ȝ

0.1 0.08 0.06 0.04 0.02 0 32

322 3222 Kinematic viscosity of the mineral oil (cSt)

(c)

1mm

Viscosity grade of the mineral oils

Observed area

VG22 Adhesion

Forming direction

Punch land

VG100 Adhesion

Punch land

VG460 Light adhesion

Punch land

Fig. 3. Effects of viscosity of the mineral oils. (a) Behavior of friction coefficients, (b) averaged friction coefficients and (c) photographs of the punch.

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Masatoshi Sawamura et al. / Procedia Engineering 81 (2014) 1866 – 1871 (a)

(b)

0.08

1mm

Friction coefficient ȝ

Punch velocity : 100~150mm s-1 Punch temperature : 298K

10mm

Cold forging oil

0.06

Adhesion

Punch nose

Punch land Adhesion

Dry-in-place 0.04

0.02 Forming direction

Bonderizing 0 2

42 62 Penetration depth (mm)

82

Fig. 4. Influence of penetration depth on the friction coefficients. (a) Averaged friction coefficients and (b) photographs of the punch.

0.08

Friction coefficient ȝ

Punch velocity : 150mm s-1 Penetration depth : 44mm

0.06 Dry-in-place 0.04

0.02 Bonderizing 0 472

572 672 772 Punch temperature (K)

872

Fig. 5. Influence of punch temperature on the friction coefficients at 44 mm of the penetration depth.

It is natural to consider that the friction coefficients are not a constant value on the entire surface of the punch. Here is discussed the area which the measured friction coefficients represent. In Fig. 4(b), the friction coefficient was 0.051 under the severe adhesion condition. This value is recognized as a small value. This becomes an argument to suspect the validity of the developed method. The reason why such a small value was measured was due to the contribution ratio of the areas of the punch on the friction coefficients. The effect of the length of the punch land on torque is shown in Fig. 6. As shown in this figure, the contribution of the punch land on torque, and also friction coefficients, was about 10 %. Therefore, when severe adhesion occurred at the punch land, friction coefficients were less than 0.1. This result shows that the measured friction coefficient by the proposed method represent the average value of the entire surface of the punch.

Masatoshi Sawamura et al. / Procedia Engineering 81 (2014) 1866 – 1871

Punch torque (N m)

400

300

200

Length of the punch land 6mm

Punch velocity : 150mm s-1 Penetration depth : 44mm Punch temperature : 298K

2mm

100

0 Dry-in-place

Bonderizing

Fig. 6. Effect of length of punch land on torque. .

4. Conclusions Using the developed equipment, friction coefficients were calculated with the torque and load measured during backward extrusion using steel specimens. Through the measurements, the following results were obtained. (1) With the measured torque and load, friction coefficients can be calculated. (2) The friction coefficient of dry-in-place specimens showed higher values than that of bonderizing specimens at all testing temperatures. Changes in friction coefficients with testing temperatures were different between the dry-in-place and bonderizing. (3) When severe adhesion occurred on the punch land, the measured friction coefficient was less than 0.1. This is because the contribution ratio of the punch land was much smaller than the other areas. (4) The friction coefficients which are measured with the developed method are the average value of the entire surface of the punch at the bottom dead center. References Danno, A., Abe, K., Nonoyama, F., 1983. Evaluation of Lubricating Performance of Zinc Phosphate Coatings by Cold Piercing Test, Journal of JSTP, 24-265, 213-220. Hansen, B. G., Bay, N., 1986.Two New Methods for Testing Lubricants for Cold Forging, Journal of Mechanical Working Technology, 13, 189-204. Kamiyama, M., Yogo, Y., Sawamura, M., Iwata, N., 2012. Numerical Evaluation Study for Instantaneous Friction Coefficient by a Developed Backward Extrusion Test Using a Rotating Tool, The Proceedings of the 63rd Japanese Joint Conference for the Technology of Plasticity, 249-250. Kitamura, K., Ohmori, T., Danno, A., 1996. Evaluation of Performance of Cold Forging Oils Using Backward Can Extrusion Test, Journal of JSTP, 37-429, 1083-1088. Sawamura, M., Yogo, Y., Kamiyama, M., Iwata, N., 2012. Measurement of Instantaneous Friction Coefficient Under Severe Friction Conditions by a Developed Backward Extrusion Test Using a Rotating Tool, The Proceedings of the 63rd Japanese Joint Conference for the Technology of Plasticity, 251-252. The Japan Society for Technology of Plasticity; 1993. Process Tribology - Lubrication in Metal Forming -, CORONA, 65-89.

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