Deformation Behavior of Ultra-Thin Metal Foils in Strip Drawing Friction Test

Transcription

1 Key Engineering Materials Vol. 443 (2010) pp (2010) Trans Tech Publications, Switzerland doi: / Deformation Behavior of Ultra-Thin Metal Foils in Strip Drawing Friction Test Tetsuhide Shimizu 1,a, Ken-ichi Manabe 1,b and Ming Yang 2,c 1 Department of Mechanical Engineering, Graduate School of Science and Engineering, Tokyo Metropolitan University, 1-1 Minamiosawa Hachioji-shi Tokyo, Japan 2 Division of Human Mechatronics Systems, Graduate School of System Design, Tokyo Metropolitan University, Japan a simizu-tetuhide@ed.tmu.ac.jp, b manabe@tmu.ac.jp, c yang@tmu.ac.jp Keywords: Microforming; Friction; Metal foil; Strip drawing test; Surface plastic region Abstract. This study focuses on the surface deformation of thin metal foils caused by friction. To clarify the effect of the relative ratio of surface plastic region to the foil thickness on global deformation behavior, strip drawing tests for ultra thin metal foil with 20µm and 100µm thickness were conducted As a result, different surface deformation and elongation behavior under the same friction condition were observed in different thicknesses. Aided by finite element analysis of the friction test, the contribution of the deformation caused by friction to the foil elongation was investigated and the importance of the friction on material deformation in metal foil forming was demonstrated Introduction Microforming technology has been receiving much attention as one of the most economical mass production methods for microcomponents [1]. Especially, metal-foil has the great advantage to produce high-aspect three-dimensional shapes by miniaturizing the process dimensions of sheet metal forming technologies. However, due to the scale-effect, the process cannot directly be transferred to the micro-scale. One of the most important process factors in sheet metal forming is the interfacial behavior between the tool and the material. Over the last decade, basic researches of scale-effect of tribology in microforming have been performed worldwide and the low effect of lubricant in micro-scale region was reported [2,3]. Additionally it is preferred not to use a lubricant from the standpoint of releasability of tools, lubricant clogging, and the effects of meniscus and viscous forces on formability. Under dry friction conditions, friction force depends on the real contact area and the shear strength of asperity contact during Sliding tools Foil specimen sliding [4]. Thus, the predominant factor for the frictional behavior is the contact deformation behavior of surface topographies. With the miniaturization of the thickness of sheet material, the relative ratio of the plastic region in surface topographies caused by friction to the thickness becomes larger and surface shear deformation might have great influence on the global deformation behavior of the material. However, almost all of the previous studies are focusing on the influence of friction on the forming load, and the effect on material deformation has not been well studied. This study focuses on the surface deformation of thin metal foils caused by friction. The aim of this study is to clarify the effect of the relative ratio of surface plastic region to the foil thickness on global deformation behavior. In order to evaluate both friction behavior and material deformation behavior P F T 2F=2 2F=2µ P F P P:Pressure force T:Tensile force F:Friction force µ:friction coefficient Fig.1 Schematic illustration of strip drawing friction test 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 the publisher: Trans Tech Publications Ltd, Switzerland, (ID: /05/10,15:25:17)

2 Key Engineering Materials Vol under the most simple stress state, a strip drawing test (Fig.1) was adopted as a friction test for micro-sheet metal forming. A new high precision strip drawing tester was developed for ultra-thin metal foil with 20µm to 100µm thickness. Tensile force, surface roughness, surface deformation, and elongation of the foil specimen were evaluated under different normal force conditions for each thickness of the foil. In order to investigate the stress-strain state during the friction test, a finite element (FE) model of the foil strip drawing test was analyzed as well. By means of the experimental and FEM result, difference in the deformation behavior of thin-foils with the different thickness was demonstrated. The effect of the surface plastic region on the difference in material deformation behavior with the different thickness was investigated. Foil Strip Drawing Test Materials. The material used for the strip drawing test is phosphor bronze (JIS: C5191-H), of 100µm and 20µm nominal thickness. Fig.2 shows flow curves and microstructure of the phosphor bronze foil for each thickness. A tensile test of the thin foils was conducted according to the DIN H6 30 standard referred to [3]. The tensile strength for the 20µm foil is larger than 100µm foils, while the elongation is much higher for 100µm specimen. Initial surface of the foils have rolling traces with surface roughness of 0.008µmRz for 20µm foil and 0.008µmRz for100µm foil. The specimens for the friction test were produced by wire-electrical discharge machining to 2mm width and 150mm length. Experimental Setup. A new high precision strip drawing tester was developed for ultra-thin metal foils. Appearance of the friction tester is shown in Fig.3. In order to measure the normal force on the samples, a quartz sensor (Kistler; Quartz Load Washers 9001A) was mounted behind the compression tool. The strip drawing assembly was set up on the universal testing machine (Minebea; TG500NB). The machine was used to pull the foil strips upwards and tensile force was measured by 500N load cell, which was mounted on tensile machine. To achieve the high normal pressures of sheet metal forming including ironing with more than Norminal Stress [MPa] DIN H Norminal Strain [-] Fig.2 Nominal stress-strain curves of C5191-H of 20µm and 100µm thickness foils Fig.3 Appearance of foil strip drawing tester 100MPa, without any fracture of the foil specimen, a circular tool geometry with radius of 0.95mm was chosen. The surface roughness of the tools was quite smooth with 0.008µm Ra and 0.1µm Rz. The Tools were made of sintered WC-Co hard alloy (JIS: V20 tungsten-carbide-cobalt alloy). Experimental Procedure. The strip drawing friction tests were carried out for both 20 µm and 100µm thickness foils under dry friction conditions. Before setting the specimen on the tester, the tool set was defatted by acetone for every cycle of the test. To improve the testing accuracy and centering of the specimen, the strip specimens were fixed with paper frames as shown in Fig.4. Defined normal

3 112 Advances in Materials Processing IX forces ranging from 10N to 60N were set and measured simultaneously with the resulting tensile force. After loading the normal force, the strip was pulled upwards with speed of 60mm/min. The sliding length was set to 60 mm displacement of the crosshead of the universal testing machine. In order to evaluate the global elongation of the strip after testing, 4 points of gauge marks were indented on the foil surface with a Vickers hardness tester. The distance between the gauge marks was defined as gauge length and it was measured before and after the test. Additionally, for the evaluation of the surface deformation behavior, the variations of the surface roughness were measured by using a confocal laser scanning microscope. The indented positions and region for surface observation are indicated in Fig.4. Region for surface observation indentations Strip width: 2mm Specimen surface 60mm Sliding distance 20mm Indent gauge length (a) Indent position and (b) Specimen holder surface observation region and gauge length Fig.4 Specimen preparation 110mm umerical Modeling FE Model of Strip Drawing Test. In order to understand the stress-strain state during the friction test, a numerical analysis of the strip drawing test for both 20µm and 100µm foils was modeled. Simulation was carried out with an explicit dynamic finite element code, LS-DYNA ver.971. A schematic illustration of the model is shown in Fig.5. For each thickness, a 2000µm(2mm) sliding length was picked out as modeling region. To model the three dimensional deformation behavior and to output the contact data, a hexahedral 8 node mesh was used and 1 element was meshed to the width direction, assuming a plain strain state for the friction test. The mesh size was 2µm x 2 µm x 2 µm for both 20µm and 100µm foils. The foil material was assumed as isotropic elasto-plastic body and the material properties were modeled using the flow-curves 20µm model 100µm model 2000 µm Sliding distance 20µm model 100µm model 100µm 20µm Fig.5 FE model of strip drawing friction test for 20µm and 100µm foils 20µm Foil 100µm Foil Tool Unit system mg-µm-msec system Material model Elastic-plastic body Elastic-plastic body Rigid body Mass density [mg/µm -3 ] 8.83E E E-06 Young's modelus [MPa] Yield stress [MPa] Poisson's ratio Friction coefficient µs=0.16, µd=0.14 µs=0.16, µd= obtained by the tensile tests of the foils (Fig.2). The compression tools were assumed as rigid bodies. For the contact conditions, the friction force was calculated by using node constraint method and assumed by Coulomb s law. The friction coefficients between blank and tools were adjusted by comparing the output data of tensile force with experimental results. The modeled mechanical properties and friction coefficients are summarized in Table1. In the simulation, the compression 2000 µm Sliding distance Table 1 Mechanical properties and simulation conditions

4 Key Engineering Materials Vol tools were loaded with a defined normal force, ranging form 10N-50N, and the velocity was applied on the upper edge of the foil material. Both the 20µm and 100µm models were compared with the experimental results, and the validity of the model was investigated at first. The strain state during the strip drawing test was analyzed and the effect of surface friction force on the whole global elongation was discussed. Results and Discussion Friction Response of the Developed Friction Test. Fig.6 (a) shows an example of friction coefficient measurement with the developed strip drawing tester. The results show the friction coefficient under about 40N normal load conditions. The friction coefficient was calculated from the tensile force and the normal load as shown in the figure. The small difference in the value of friction coefficient seems to because by the difference of surface conditions and the material properties of each foil. Although there are difficulties using ultra-thin foil specimen, relatively stable data could be obtained from the tester. Fig.6 (b) shows the surface variation before and after the test under the conditions of Fig.6 (a). Considering the variation of the maximum height roughness Rz, the surface topography deformation is much higher for 20µm thickness foil, even though the sliding condition Relative deformation ratio Ra/Ra0 [%] Friction coefficient [-] Sliding length [mm] µ F T/2F F 20: 42.8 F 100:42.3 L:60mm V:60mm/min Normal Force [ N ] Elongation λ [ % ] Initial surface Initial surface indentation L0 = 20mm Rz=1.16µm Rz=1.69µm Initial indent gauge length L Length after friction test Elongation λ= (L-L 0) /L [%] After sliding After sliding Rz=0.71µm Rz=1.60µm (a) Measurement data of friction (b) Surface variation before and after coefficient vs. sliding distance testing for the foils with different thickness Fig.6 Friction response measured by developed strip drawing friction tester Normal Force [ N ] (a) Variation of relative deformation (b) Variation of elongation of the strip ratio of the roughness Ra Fig.7 Evaluation data of surface and global deformation under different normal force condition in each foil with different thicknesses

5 114 Advances in Materials Processing IX was same. By indenting the marks on the foil surface, surface variation before and after testing was successfully measured. Evaluation of Surface and Global Deformation for Foil Strip. Fig.7 shows the variation of the relative deformation ratio of the roughness under the different normal load conditions for each thickness of the foil. The relative deformation ratio of the roughness was defined as the rate of the arithmetic mean roughness Ra change before and after the testing. As shown in the figure, the variation became larger with increasing normal load for the 20µm thickness foil, though there are not any large variances depending on the normal force condition for the 100µm thickness foil. Corresponding to the variation of the surface roughness, the global elongation of the foil strip for 20µm thickness foil was quite large as shown in Fig.7 (b). The elongation for 20µm thickness foil shows a large increase at an exponential rate with increasing normal load. From the results, even though the test for both foils with different thickness was conducted under the same conditions, a relatively large difference in global deformation behavior was indicated. Validation of the FE Model. In order to investigate the differences of deformation behavior as shown above, an FE analysis for strip drawing test was carried out. To confirm the validation of the FE model, the tensile force of the friction test under different normal force condition was compared, as shown in the Fig.8. A good linear relation between the normal force and the tensile force was observed in each thickness condition in the experiment. Validation of the friction tester can also be confirmed from the results. By adjusting the friction coefficient in the FE model, good agreement data could be obtained and succeeded to make the FE model to investigate the stress-strain state during the strip drawing test. Global Deformation caused by Friction in Strip Drawing Test. The material flow behavior in the strip drawing test can be separated to two parts of deformation behavior: one is caused by compression of the sliding tools and another is caused by friction. To separate both deformation behaviors, an analysis with a friction coefficient µ =0 was carried out. For the evaluation of elongation, a defined node distance was picked up and the variation of the distance was defined as elongation λ. The initial length of the sliding direction was set to be 500µm length and the elongated length after the sliding was calculated. By means of the analysis with Elongation caused by friction λf [ % ] Tensile Force [N] Exp. FEM FEM Exp Normal Force [N] Exp. FEM Exp. FEM Fig.8 Validation of the FE model comparing with experiment data λ f = λ-λ m λ: Elongation of whole specimen λ m : Elongation without friction Friction force [ N ] Fig.9 Relation between global deformation caused by friction and friction force no friction, the tensile force and elongation were calculated and they were defined as material flow resistance F m, and material elongation λ m, caused by the compression of the sliding tools. By

6 Key Engineering Materials Vol subtracting both values from the experimentally validated model, the elongation caused by friction, λ f, and true friction force F f were calculated. The relation between F f and λ f is shown in Fig.9. From the results, it is clearly to understand the effect of friction for thin foil is relatively higher than for thicker material. Also the elongation increases with increasing of the friction force on the surface for 20µm thickness foil. It is conceivable that the stress-strain state and relative ratio of plastic region to the thickness may have quite large differences in different thickness of the metal foils and influence the deformation caused by friction. Further investigation will be come out in the future. Conclusions In this study, a new high precision strip drawing tester for ultra-thin metal foil with 20µm to 100µm thickness was developed. Additionally, in order to investigate the stress-strain state during the friction test, an FE model of the foil strip drawing test was analyzed as well. By means of the experimental and FEM result, the difference in the deformation behavior of thin-foils with the different thickness was investigated. 1) Newly developed strip drawing tester could successfully get the stable data for the ultra-thin foil specimen. 2) From the evaluation of both surface and global deformation behavior of the strip foils, large differences in the elongation of the strip were confirmed, even though under the same friction conditions. 3) The validity of the FE model was confirmed by comparing with the experimental data of linear relation between normal force and tensile force. 4) From the investigation of the FE analysis, the large influence of the friction on the global deformation behavior was indicated. The above results revealed that the importance of friction not only to material flow resistance on the tools, but also to material deformation it self. Especially for the ultra-thin metal foil, the friction has a more significant role in controlling the material deformation for high precision microforming. The relation between surface shear deformation and global deformation behavior of metal foil will be investigated in the future. Achkowledgement This research was partially supported by The Ministry of Education, Culture, Sports, Science and Technology (MEXT), Grant-in-Aid for JSPS fellow No.10502, The authors also would like to express their gratefully acknowledge to the support from Mr. Kuniyoshi ITO, Microfabrication Lab.Llc., for the friction tester, and from Mr.Kihei TSUTSUI, LANCEMORE Co., for the numerical modeling. References [1] U.Engel, R.Eckstein: Journal of Materials Processing Technology, Vol (2002), pp [2] U. Engel: Wear, Vol.260 (2006), pp [3] F. Vollertsen, Z. Hu: Annals of the CIRP, Vol 55, Issue 1, pp , 2006 [4] M. Nosonovsky, B. Bhushan: Journal of Tribology, Vol. 127, No.1 (2005), pp