a lateral stretching type tribometer and a high pressure tribometer. ... friction law depends on not only the frictional stress over the flattened area but also the.
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Procedia Engineering 207 (2017) 2269–2273
International Conference on the Technology of Plasticity, ICTP 2017, 17-22 September 2017, International Conference on theCambridge, TechnologyUnited of Plasticity, Kingdom ICTP 2017, 17-22 September 2017, Cambridge, United Kingdom
Experimental investigation on friction law under starved lubrication Experimental investigation friction law under starved lubrication in on metal forming in metal forming Ye Zhanga, Zhigang Wanga*, Yasuharu Yoshikawaa, Wenzheng Donga,b Ye Zhanga, Zhigang Wanga*, Yasuharu Yoshikawaa, Wenzheng Donga,b a a
Department of Mechanical and Systems Engineering, Gifu University, Gifu, Yanagido, 501-1193, Japan b SchoolofofMechanical Mechanicaland Engineering, Xiangtan University, Hunan,Gifu, Xiangtan, 411105, China Japan Department Systems Engineering, Gifu University, Yanagido, 501-1193, b School of Mechanical Engineering, Xiangtan University, Hunan, Xiangtan, 411105, China
Abstract Abstract Friction law under starved lubrication is investigated by friction tests under a wide range of the average pressure. The lubricant thicknesslaw applied the specimen surface for frictionbytest is controlled to four levels; thin, fourth,pressure. half andThe twice of the Friction undertostarved lubrication is investigated friction tests under a wide rangevery of the average lubricant surface roughness of the specimen. Experimental that the friction undervery the thin, lubricant thickness of twice very thin is thickness applied to specimen surface for results frictionshowed test is controlled to fourlaw levels; fourth, half and of the similar to that underofdry condition. When the lubricant thickness is the largefriction enough, Coulomb friction is applicable under very surface roughness specimen. Experimental results showed that law under the lubricant thicknesseven of very thin is high pressure. similar to that under dry condition. When the lubricant thickness is large enough, Coulomb friction is applicable even under very high pressure. © 2017 The Authors. Published by Elsevier Ltd. © 2017 2017 The The under Authors. Published by by Elsevier Ltd. © Authors. Published Ltd. Peer-review responsibility of Elsevier the scientific committee of the International Conference on the Technology Peer-review under responsibility of the scientific committee of the International Conference on the Technology of Plasticity. Peer-review under of Plasticity . responsibility of the scientific committee of the International Conference on the Technology Keywords: friction of Plasticity . law; lubrication; metal forming; tribolgoy Keywords: friction law; lubrication; metal forming; tribolgoy
1. Introduction 1. Introduction Friction law is a basic and important issue in the field of process tribology. Kasuga measured the flattened area on lawsurface is a basic anddeep important issue the field of and process tribology. measured the the flattened areaarea on theFriction workpiece after drawing andin compression found that the Kasuga frictional stress over flattened theaworkpiece surfacetoafter and compression andanalyzed found that frictional stress over areaa is constant related the deep kind drawing of lubricant [1]. Bay et al. thetheaverage frictional stressthebyflattened assuming is a constant related to the lubricant Bay et al.ofanalyzed thepressure average and frictional by assuming constant frictional stress overkind the of flattened area[1]. irrespective the contact gave astress relationship betweena constant frictional stress over the flattened area irrespective of the contact pressure and gave a relationship between
* Corresponding author. Tel.: +81-58-2932516 address:author. zgwang@ * E-mail Corresponding Tel.:gifu-u.ac.jp +81-58-2932516 E-mail address: zgwang@ gifu-u.ac.jp 1877-7058 © 2017 The Authors. Published by Elsevier Ltd. Peer-review©under the scientific 1877-7058 2017responsibility The Authors. of Published by Elseviercommittee Ltd. Plasticity . Peer-review under responsibility of the scientific committee Plasticity.
of the International Conference on the Technology of of the International Conference on the Technology of
1877-7058 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the scientific committee of the International Conference on the Technology of Plasticity. 10.1016/j.proeng.2017.10.993
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Ye Zhang et al. / Procedia Engineering 207 (2017) 2269–2273 Ye Zhang / Procedia Engineering 00 (2017) 000–000
the average frictional stress and average contact pressure [2]. This relationship is the foundation of the friction law generally accepted today, i.e. a constant friction coefficient at low pressure and a constant friction shear factor at high pressure. In the previous paper [3], we proposed a new friction law for dry forming based on the experimental results using a lateral stretching type tribometer and a high pressure tribometer. The outstanding feature of this new friction law is that there exists a critical pressure and Coulomb friction is applicable up to the critical pressure and an associated frictional stress takes place at the higher pressure than the critical pressure. It has been confirmed that this friction law is valid whether the whole body is deforming plastically or not for metals with or without work-hardening [4]. Although dry forming is developed actively, forming under lubricated conditions is still the mainstream. Under lubricated conditions, friction law depends on not only the frictional stress over the flattened area but also the pressure sharing of lubricant trapped in the oil pits [5]. Moreover, microscopic plastic hydrodynamic lubrication [6] takes place and that perhaps makes the friction law complicated [7]. In the present paper, the friction law under lubricated conditions is investigated experimentally over a wide range of the average pressure and lubricant thickness. 2. Experimental conditions Fig. 1 shows the principle of the developed testing method. Since the specimen is constrained by the die, the specimen bulk is not plastically deformed even under a very high pressure. The specimen is compressed between the anvil and the punch. The punch moves upwards with a constant speed of U =0.01 mm/s, and the anvil is driven to the right with a speed of V = 0.1 mm/s. During the experiment, the compressing load and the frictional force are measured.
Fig. 1. Schematic illustration of testing method.
DLC (diamond like carbon) is coated on the anvil made of cold working die steel with roughness of 0.28μmRz. The specimen is cut off from a cold rolled steel sheet SPCC. The dimensions of the specimen are 9mm in diameter and 1mm in thickness. The surface roughness of the specimen is 4.66μmRz. The flow stress to be used in the dimensionless average pressure is Yε=0.08 = 326MPa [4]. A paraffinic mineral oil P100 is used as the lubricant (viscosity at 40℃:100×10-6m2/s). The lubricant film thickness h0 is controlled to be 0.4, 1.0, 2.0 and 7.0μm by using mixtures of the lubricant and ethanol. The specimen is cleaned by acetone and then is dipped in the mixture with ethanol. After vaporization of ethanol, the lubricant with the controlled thickness is left on the specimen surface. The lubricant thickness is calculated by using the difference in weight before and after lubricant coating. In the present testing method, a pressing force between the die and anvil is added to prevent the flow of specimen material into the gap between the die and anvil. Fig.2 shows the measurement method of the frictional force between
Ye Zhang al. / Procedia Engineering 207 (2017) 2269–2273 Author name et / Procedia Engineering 00 (2017) 000–000
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the specimen and anvil. The sliding resistance is measured before/after friction test. The difference in sliding resistance during testing and after testing is used as the frictional force between the specimen and anvil.
Fig. 2. Measurement method of frictional force.
3. Experimental results Fig.3 shows the relationship between average frictional stress and average pressure under lubricant thickness of 1μm, 2μm and 7μm. When the thickness of the coated lubricant is larger than 1μm, the friction behaviour is independent of the lubricant thickness, and the friction behaviour follows Coulomb's law. This is the same as the frictional result of the lubrication coating of solid lubricants for cold forging [4].
Fig. 3. Relationship between average frictional stress and average pressure under lubricant thickness from 1μm to 7μm.
As shown in Fig.4, when the lubricant thickness is 0.4μm, frictional behavior follows Coulomb's law when the average contact pressure pa /Y is lower than 2.0, and its friction coefficient is closer to the friction coefficient under dry condition. When the average contact pressure pa /Y is in the range of 2.0 to 4.5, the average frictional stress is constant. When the average contact pressure pa /Y is above 4.5, the average frictional stress increases again.
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Zhang et al. / Procedia Engineering 207000–000 (2017) 2269–2273 Ye Ye Zhang / Procedia Engineering 00 (2017)
Fig. 4. Relationship between average frictional stress and average pressure under lubricant thickness of 0.4μm.
The specimen surface after friction test is observed by a 3D laser microscope (OLS4100 made by OLYMPUS). No scratches on the flattened surface are observed under all conditions. The specimen surface is flattened by the contact pressure and the frictional stress. Fig. 5 shows the relationship between the specimen surface roughness Rz and the average contact pressure. Rz decreases with the increase of the average contact pressure, and reaches about 1μm at the average contact pressure pa /Y = 2.0. When the average contact pressure pa /Y is higher than 2.0, Rz is constant. This means that the deep valleys on the specimen surface with depth of 1μm are filled with lubricant and the trapped lubricant shares the contact pressure, and thus the valleys are left even under very high pressure.
Fig. 5. Variations of specimen surface roughness Rz and average pressure.
Based on the observation of the specimen surface after friction test, the contact state of the friction tool and specimen under starved lubrication is considered as shown in Fig.6. At initial stage, the lubricant is extended to almost the entire surface due to its surface tension. When the average contact pressure is low, the contact between the anvil and specimen is limited to the protruding portion of the specimen surface and the lubricant at the portion is squeezed out, and thus the lubricant thickness at the protruding portion becomes very thin as shown in Fig. 6 (b). With the increase of the average contact pressure, the specimen surface is flattened and the lubricant is concentrated in the deep valley as shown in Fig.6 (c), the interface between the anvil and the specimen surface is filled with the lubricant.
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Fig. 6. Contact state at anvil/specimen interface under starved lubrication, (a) initial stage, (b) low contact pressure, (c) high contact pressure.
4. Conclusions To examine the friction law under starved lubrication, friction tests using a specimen with a surface roughness of 4.66μmRz are carried out under the lubricant thickness of 0.4~7.0μm. The main conclusions can be summarized as follows: (1) When the thickness of applied lubricant is larger than 1/4 of the specimen surface roughness Rz, the frictional behaviour follows Coulomb's law. (2) When the thickness of the applied lubricant is about 1/10 of the surface roughness of specimen, the frictional behaviour follows Coulomb's law at low average pressure and shows a constant average frictional stress at high average pressure. References [1] Y. Kasuga. Boundary lubrication under high normal pressures, Journal of JSLE 16(1970) 748-758. (in Japanese). [2] N. Bay, T. Wanheim. Real area of contact and friction stress at high pressure sliding contact. Wear 38(1976) 201-209. [3] Z.G. Wang, Y. Yoshikawa, T. Suzuki, et al. Determination of friction law in dry metal forming with DLC coated tool, CIRP Annals Manufacturing Technology. 63 (2014) 277–280. [4] Z.G. Wang, S. Komiyama, Y. Yoshikawa, et al. Evaluation of lubricants without zinc phosphate precoat in multi-stage cold forging, CIRP Annals-Manufacturing Technology 64(2015) 285-288. [5] N. Kawai, K. Dohda, M. Saito, et al. Friction behavior in the cup ironing process of aluminum sheets, Trans. of the ASME, Journal of Engineering for Industry 114(1992) 175-180. [6] Z.G. Wang, K. Kondo, T. Mori. A consideration of optimum conditions for surface smoothing based on lubricating mechanisms in ironing process. Trans. of the ASME, Journal of Engineering for Industry 117(1995) 351-356. [7] Z.G. Wang, K. Dohda, Y. Haruyama. Effects of entraining velocity of lubricant and sliding velocity on friction behavior in stainless steel sheet rolling, Wear 260(2006) 249-257.
