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A methodology for off-line evaluation of new environmentally friendly tribo-systems for sheet metal forming
CIRP Annals - Manufacturing Technology 62 (2013) 231–234
Contents lists available at SciVerse ScienceDirect
CIRP Annals - Manufacturing Technology jou rnal homep age : ht t p: // ees .e lse vi er . com /ci r p/ def a ult . asp
A methodology for off-line evaluation of new environmentally friendly tribo-systems for sheet metal forming Ermanno Ceron, Niels Bay (1)* Department of Mechanical Engineering, Technical University of Denmark, Denmark
A R T I C L E I N F O
A B S T R A C T
Keywords: Stamping Lubrication Off-line testing
Increasing focus on environmental issues in industrial production has urged sheet stamping companies to look for new tribo-systems in order to substitute hazardous lubricants such as chlorinated paraffin oils. Production testing of new lubricants is, however, costly and makes industry reluctant towards testing alternative solutions. The present paper presents a methodology for off-line testing of new tribo-systems based on numerical modelling of production process as well as laboratory test to adjust the latter combined with testing of selected tribo-systems on a new automatic sheet-tribo-tester emulating typical sheet forming production processes. Final testing of the tribo-systems in production verifies the methodology. ß 2013 CIRP.
1. Introduction Stamping of sheet metal by deep drawing, ironing and punching in tribologically difficult materials e.g. stainless steel, high strength steel, aluminium alloys and titanium alloys requires very efficient tribo-systems often involving environmentally hazardous lubricants such as chlorinated paraffin oils to avoid lubricant film breakdown and galling, which leads to poor surface quality of formed components and eventually production stop. Fig. 1 shows an example of acceptable and unacceptable surface quality, the latter due to the occurrence of galling. During the last decade legislation in Europe [1] and Japan [2,3] has been increasingly restrictive with respect to the industrial application of such lubricants and many sheet stamping industries are struggling to find alternative, environmentally benign lubricants [4,5]. Testing of possible alternatives in the production line is, however, costly and problematic due to the required production stops and cleaning of tools before introducing the new test lubricant, as well as production stops caused by possible lubricant film breakdown and pick-up of workpiece material on polished tool surfaces, which requires dismounting of tools and re-polishing. The present paper describes in detail a methodology for off-line evaluation of new, environmentally friendly tribo-systems proposed in [6]. The methodology is here applied to an industrial production process consisting of deep drawing followed by two redrawing operations of a stainless steel component, where chlorinated paraffin oil has hitherto been used.
2. Methodology for off-line testing 2.1. Sheet tribo-test equipment The application of Bending-Under-Tension tests to simulate the tribological conditions in the die shoulder during deep drawing has * Corresponding author. 0007-8506/$ – see front matter ß 2013 CIRP. http://dx.doi.org/10.1016/j.cirp.2013.03.062
Fig. 1. Multistage deep drawn component, (a) good surface quality and (b) poor surface quality.
been utilized by a large number of authors starting with Littlewood and Wallace [7]. Later developments include Wilson et al. [8], Weinmann and Kernovsky [9], Wang et al. [10] and Vallance and Matlock [11], which give a good review on the different test variants. All of these tests are based on one or a very limited number of strokes leaving no possibilities for long term testing of slowly developing pick-up. The only test allowing this is to the best of the authors’ knowledge the one developed by Hennig and Groche [12], where back tension is ensured by a draw bead located ahead of the BUT tool. The present paper presents a more flexible solution introducing direct control with feedback, which allows the back tension force to be varied during stroke. A new, universal, automatic sheet-tribo-tester has been developed for the purpose [6]. Fig. 2 shows the equipment, which has three hydraulically powered movements controlled by a PLC which communicates with a PC provided with a dedicated LabView programme, in which all main parameters are set. The innovative feature of the machine is the possibility to run test from a coil, enabling repeated testing at similar rate as in industrial production. This makes it possible to emulate the graduate, but often slow build-up of pick-up of workpiece material on the tool surface occurring in production, which is impossible in the simple, above mentioned laboratory tests [7–11], where the time between repetitions allows the tool to
E. Ceron, N. Bay / CIRP Annals - Manufacturing Technology 62 (2013) 231–234
232
cool down [13]. The equipment is designed for the main sheet tribo-tests [14], e.g. Bending-Under-Tension (BUT) testing, DrawBead-Testing (DBT) or Strip-Reduction-Testing (SRT) with adjustable sliding length (0–250 mm), sliding speed (0–150 mm/s), cycle time (0–95 spm) and total number of strokes.
speed is 40 spm in a 250 t mechanical press. The height of the final cup is 20 mm. The analysis is focused on the third operation, which is the most critical as regards the limit of lubrication. The radius of curvature of the die in operation 3 is r = 3.5 mm. Previous experience in the company has shown that the choice of the environmentally hazardous lubricant and the tool coating was the
Fig. 3. Cup produced in progressive tool system.
only feasible tribo-system due to the severity of the production process. 4. Numerical analysis Fig. 2. DTU-MEK’s automatically controlled sheet-tribo-tester.
2.2. Procedure Starting point in the proposed procedure for off-line testing is an existing production process, with a hazardous tribo-system that has to be replaced with an environmentally friendly one. The production platform defines important tribo-parameters such as normal pressure, surface expansion, sliding length, sliding speed etc., which can be quantified by numerically simulation. After this process characterization, an appropriate simulative test is selected based on the type of deformation process. A numerical analysis of the laboratory test is performed to determine and adjust the tribo-conditions to be comparable with those in the production process. After that, a thorough planning of the simulative test is done determining factors such as number of repetitions, idle time between strokes and initial tool temperature. A new tribo-system is then selected for testing. A first campaign of screening tests is performed with triboparameters similar to those in the production to check whether the tribo-system is a promising alternative. In case of poor results one of two routes may be followed. Either a new tribo-system may be selected or, if possible, modification of the production platform/ workpiece geometry may be introduced in order to lower the tribological severity. In case of good results more comprehensive off-line testing is performed varying critical parameters such as normal pressure, sliding length, rate of testing and idle time, in order to define the working window of the tribo-system. Again, this could lead to either good or poor results and, as described above, there are two possible choices in case of poor results. If good results are achieved the next step is to test the tribo-system in production. If satisfactory results are achieved in the production test the procedure is successfully completed, and the old tribo-system may be substituted. Since it is practically impossible to reproduce the production conditions exactly, the production test may also lead to unsatisfactory results in spite of promising laboratory test results. In that case the same procedure as mentioned above is applied. 3. Case study The methodology is applied to an industrial case shown in Fig. 3 manufacturing a cup by deep drawing (1) and two redrawings (2)–(3) and a flange pressing (4) in a progressive tool system. The workpiece material is 1 mm stainless steel strip EN 1.4301, tool material is powder metallurgical tool steel UHB Vanadis 6 (62 HRC) PVD coated with TiAlN. Chlorinated paraffin oil Iloform PN226 is currently used as lubricant. The production
With selection of the production process the first step in the proposed methodology is taken. The next step is to determine the tribo-parameters in the considered, third operation by numerical analysis. In this case the most important parameter is considered to be the normal pressure at the tool/workpiece interface. The three deep drawing operations (1)–(3) are simulated with a 2D axisymmetric model using the elastic-plastic FE code LS-DYNA1. The coefficient of friction is calibrated comparing the measured punch force with the calculated one. Fig. 4 shows the normal stress in the y direction in the local Cartesian system of each element, where the local y direction on elements distributed on the outer surface of the curvature is perpendicular to the circumference. The
Fig. 4. Normal pressure at the interface workpiece/die. Operation (3).
stress displayed on the outer elements can thus be interpreted as normal pressure at the interface. The numerical analysis shows that the contact area is quite small, approximately equal to the sheet thickness, and the normal pressure is very high with peak values around 1000 MPa. The most suitable simulative test to choose for emulation of the production conditions in operation (3) is Bending-UnderTension (BUT). In this test a strip is bent 908 and drawn over a nonrotating tool pin. The equipment is earlier described by Andreasen et al. [15]. The 908 BUT test with a tool pin of same radius r = 3.5 mm as in operation (3) of the production test is also numerically analyzed in LS-DYNA. Fig. 5a shows that this leads to insufficient normal pressure even with the highest possible back tension stress, where pmax = 250 MPa. The limit is set by the yield stress of the undeformed strip. In order to reach a normal stress of same size as in the production process, a new tool geometry was developed for the BUT test. In the new tool, having the same tool radius as the former, the contact during 908 BUT testing is limited to 458, see Fig. 5b. This configuration leads to a localized peak in normal pressure around 428 of pmax = 1000 MPa due to the abrupt exit, i.e. a maximum normal pressure comparable to that in the production tool.
E. Ceron, N. Bay / CIRP Annals - Manufacturing Technology 62 (2013) 231–234
233
6. Experimental results 6.1. BUT testing The torque acting on the BUT tool pin was registered by a piezoelectric transducer, whereas, the drawing force and the back tension force were measured by strain gauge transducers. All data were aquired by a PC with LabView. As described by Andreasen et al. [15] the torque is very sensitive to friction and as such a good indication of onset and growth of pick-up. Fig. 6a shows the torque and drawing force for tribo-system 1. In this case the test was successful, since no galling appeared within 1500 strokes. The force and torque increase slightly during the first 200 strokes after which they reach a steady state. The initial increase is probably due to micro pick-up of workpiece material on the tool surface, which after running-in becomes stable.
(a) 30
10000 8000
20
6000
15 4000
10
EN 1.4301
5
2000
0 5. Experimental setup
0
Table 1 Investigated tribo-systems. Tribo-system Workpiece material 1 2 3
Tool material Lubricant
EN 1.4301 Vancron 40 DP 800 Vancron 40 EN 1.4162 (LDX 21011) Vancron 40
rhenus SU 166 A ANTICORIT PLS 100 T rhenus SU 166 A
According to the numerical analysis, the back tension should be
sb = 300 MPa for all three materials. Such a large value was, however, not applicable for EN 1.4301, being too close to the yield stress. Preliminary tests showed rapid fluctuations in the drawing force which induced a variation of the back tension that exceeded the yield stress s0 and caused fracture of the strip. The back tension was therefore decreased to 200 MPa implying lower but still fairly high normal pressure. The sliding length was set equal to the height of the cup, 20 mm. The sliding speed was set to 50 mm/s, which is less than half the one applied in production. Even though the maximum speed in the BUT test is 150 mm/s, it was decided to reduce it to 1/ 3, since problems with fluctuation in the drawing force during acceleration caused fracture of the strip. Each test was repeated 1500 times with a rate of 40 strokes/min corresponding to the production rate.
1000 stroke
torque
0 1500
drawing force
(b) 60
15000
50
torque [Nm]
Three new tribo-systems were selected to be analyzed in BUT testing as well as production testing. They are described in Table 1. Besides the existing production process forming EN 1.4301 with yield stress s0 = 320 MPa two other materials were investigated, i.e. an advanced high strength steel DP800 with yield stress s0 = 620 MPa and a lean duplex stainless steel EN 1.4162 (LDX 2101) with yield stress s0 = 550 MPa. As tool material was chosen an uncoated, powder metallurgical tool steel UHB Vancron 40 hardened and tempered to 63 HRC. The tool surface was manually polished with emery paper grade 1000 in transversal direction reaching a final roughness of Ra = 0.06 mm. The workpiece material is a strip with cross section 1 mm 30 mm fed from a coil reel during testing. The lubricant applied in tribo-systems 1 and 3 is environmentally benign, highly refined, mineral oil with EP additives, viscosity 150 mm2/s at 40 8C, from Rhenus Lub. The lubricant applied in tribo-system 2 is corrosion protective mineral oil, viscosity 100 mm2/s at 40 8C, from FUCHS. Both lubricants were applied by means of two felt rolls.
500
40
10000
30 20
5000
10
EN 1.4162
0 0
10
20 stroke
torque
force [N]
Fig. 5. (a) Normal stresses in 2D model of: (a) quarter of square BUT tool pin with 908 round edges and (b) quarter of new tool geometry with confined tool curvature.
force [N]
torque [Nm]
25
0 30
40
drawing force
Fig. 6. Torque and drawing force versus number of strokes (a) tribo-system 1 and (b) tribo-system 3.
The results for tribo-system 2 are not shown but they have the same constant trend as No. 1, although without the initial increase. Fig. 6b shows the test results for tribo-system 3 (lean duplex steel), where severe galling occurred. In this particular test the back tension was decreased to 200 MPa (as for EN 1.4301) in order to investigate, whether deep drawing could be performed at lower normal pressure. It is noticed that the torque and force increase abruptly after 30 strokes. This is due to severe galling eventually leading to fracture of the strip. The limit of lubrication for this system is thus far below production conditions. Fig. 7a–c shows the tool surface after testing the three different tribo-systems 1–3. Sliding direction of the strip is from bottom to top. The exit edge, where maximum normal pressure is reached, is indicated in each picture. As regards tribo-systems 1 and 2, almost no pick-up is noticed, while tribo-system 3 results in severe pickup, which led to the before mentioned fracture of the strip. In this test campaign only the production speed was varied in order to find the limit of lubrication as a function of this parameter. No galling occurred for any of the two tribo-systems 1 and 2, when running up to the maximum speed of 95 strokes/min. 6.2. Production testing The simulative tests showed promising results for tribosystems 1 and 2. According to the methodology, it was therefore
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furthermore carried out at increased production speed of 95 strokes/min again with all 1500 parts being approved. 7. Conclusion A methodology for off-line evaluation of tribo-systems for sheet metal forming was presented in this paper and applied to an industrial case study. The methodology follows a logical path, which combines numerical and experimental analysis to ensure realistic tribo-conditions emulating those in production. An important goal of the procedure is to increase the chance of success before production trial of more environmentally benign tribo-systems as possible substitutes to hazardous ones. The results prove that the methodology works and can give detailed information as regards the limit of lubrication of the tribo-system investigated thereby diminishing costly production trials. Acknowledgments The authors thank Grundfos A/S, Outokumpu Stainless AB, Outokumpu Stainless Research Foundation, SSAB and Uddeholm AB for their economic and technical support.
References
Fig. 7. Tool surface after testing: (a) tribo-system 1, (b) tribo-system 2 and (c) tribosystem 3.
Fig. 8. (a) Production sample, good tribo-system 1 and (b) production sample, acceptable tribo-system 2.
decided to test them in production. Corresponding to the laboratory tests 1500 cups were produced with a production speed of 40 spm. The die in step No. 3 was polished in circumferential direction obtaining the same surfaces topography as in the BUT tools. Fig. 8a and b shows the last two samples produced for tribosystem 1 and 2 respectively. They both satisfy the requirements to surface quality. As regards tribo-system 1 production testing was
[1] European Parliament, Council, 2006, REACH, EC Regulation No. 1907/2006 of the European Parliament and of the Council. [2] Ministry of Environment, Government of Japan, http://www.env.go.jp/en/ laws/policy/basic/index.html, January 2013. [3] Ministry of Environment, Government of Japan, http://www.env.go.jp/en/ laws/recycle/01.pdf, January 2013. [4] Ike H (2004) Report on Metal Forming Tribology Research in Japan with Special Focus on Environmental Aspects. in Bay N, (Ed.) Proceedings of the International Conference on Tribology in Manufacture Processing, vol. 1, 29–38. [5] Bay N, Azushima A, Groche P, Ishibashi I, Merklein M, Morishita M, Nakamura T, Schmid S, Yoshida M (2010) Environmentally Benign Tribo-systems for Metal Forming. CIRP Annals – Manufacturing Technology 59/2:760–780. [6] Ceron E, Bay N (2012) Testing and Prediction of Limits of Lubrication in Sheet Metal Forming. Proceedings of the International Deep Drawing Research Group Conference, Mumbai, 251–257. [7] Littlewood M, Wallace JF (1964) The Effect of Surface Finish and Lubrication on the Friction Variation Involved in the Sheet-metal-forming Process. Sheet Metal Industry 41:925–930. [8] Wilson WRD, Malkani HG, Saha PK (1991) Boundary Friction Measurements Using a New Sheet Metal Forming Simulator. Transactions of NAMRC/SME 19:37–42. [9] Weinmann KJ, Kernovsky SK (1996) Friction Studies in Sheet Metal Forming Based on a Unique Die Shoulder Force Transducer for Sheet Metal Forming Research. Annals of CIRP 45/1:269–272. [10] Wang W, Wagoner RH, Wang X-J (1996) Measurement of Friction under Sheet Forming Conditions. Metallurgical and Materials Transactions A 27A:3971– 3981. [11] Vallance DW, Matlock DK (1992) Application of the Bending-Under-Tension Friction Test to Coated Sheet Steels. Journal of Materials Engineering and Performance 1/5:685–693. [12] Hennig R, Groche P (2004) Investigation of Abrasive Behavior of Zinc-coated Steel Sheets with a New Laboratory Test Method. Production Engineering 11/ 2:17–22. [13] Nielsen PS, Friis KS, Bay N (2011) Testing and Modelling of New Tribo-systems for Sheet Metal Forming of Stainless Steel in Laboratory and Production. Proceedings of the IMechE Part J Journal of Engineering Tribology Special issue on Tribology Research in Scandinavia 225:1036–1047. [14] Bay N, Olsson DD, Andreasen JL (2008) Lubricant Test Methods for Sheet Metal Forming. Tribology International 41/9:844–853. [15] Andreasen JL, Olsson DD, Chodnikiewicz K, Bay N (2006) Bending Under Tension Test with Direct Friction Measurement. IMechE Part B Journal of Engineering Manufacture 220:73–80.
Contents lists available at SciVerse ScienceDirect
CIRP Annals - Manufacturing Technology jou rnal homep age : ht t p: // ees .e lse vi er . com /ci r p/ def a ult . asp
A methodology for off-line evaluation of new environmentally friendly tribo-systems for sheet metal forming Ermanno Ceron, Niels Bay (1)* Department of Mechanical Engineering, Technical University of Denmark, Denmark
A R T I C L E I N F O
A B S T R A C T
Keywords: Stamping Lubrication Off-line testing
Increasing focus on environmental issues in industrial production has urged sheet stamping companies to look for new tribo-systems in order to substitute hazardous lubricants such as chlorinated paraffin oils. Production testing of new lubricants is, however, costly and makes industry reluctant towards testing alternative solutions. The present paper presents a methodology for off-line testing of new tribo-systems based on numerical modelling of production process as well as laboratory test to adjust the latter combined with testing of selected tribo-systems on a new automatic sheet-tribo-tester emulating typical sheet forming production processes. Final testing of the tribo-systems in production verifies the methodology. ß 2013 CIRP.
1. Introduction Stamping of sheet metal by deep drawing, ironing and punching in tribologically difficult materials e.g. stainless steel, high strength steel, aluminium alloys and titanium alloys requires very efficient tribo-systems often involving environmentally hazardous lubricants such as chlorinated paraffin oils to avoid lubricant film breakdown and galling, which leads to poor surface quality of formed components and eventually production stop. Fig. 1 shows an example of acceptable and unacceptable surface quality, the latter due to the occurrence of galling. During the last decade legislation in Europe [1] and Japan [2,3] has been increasingly restrictive with respect to the industrial application of such lubricants and many sheet stamping industries are struggling to find alternative, environmentally benign lubricants [4,5]. Testing of possible alternatives in the production line is, however, costly and problematic due to the required production stops and cleaning of tools before introducing the new test lubricant, as well as production stops caused by possible lubricant film breakdown and pick-up of workpiece material on polished tool surfaces, which requires dismounting of tools and re-polishing. The present paper describes in detail a methodology for off-line evaluation of new, environmentally friendly tribo-systems proposed in [6]. The methodology is here applied to an industrial production process consisting of deep drawing followed by two redrawing operations of a stainless steel component, where chlorinated paraffin oil has hitherto been used.
2. Methodology for off-line testing 2.1. Sheet tribo-test equipment The application of Bending-Under-Tension tests to simulate the tribological conditions in the die shoulder during deep drawing has * Corresponding author. 0007-8506/$ – see front matter ß 2013 CIRP. http://dx.doi.org/10.1016/j.cirp.2013.03.062
Fig. 1. Multistage deep drawn component, (a) good surface quality and (b) poor surface quality.
been utilized by a large number of authors starting with Littlewood and Wallace [7]. Later developments include Wilson et al. [8], Weinmann and Kernovsky [9], Wang et al. [10] and Vallance and Matlock [11], which give a good review on the different test variants. All of these tests are based on one or a very limited number of strokes leaving no possibilities for long term testing of slowly developing pick-up. The only test allowing this is to the best of the authors’ knowledge the one developed by Hennig and Groche [12], where back tension is ensured by a draw bead located ahead of the BUT tool. The present paper presents a more flexible solution introducing direct control with feedback, which allows the back tension force to be varied during stroke. A new, universal, automatic sheet-tribo-tester has been developed for the purpose [6]. Fig. 2 shows the equipment, which has three hydraulically powered movements controlled by a PLC which communicates with a PC provided with a dedicated LabView programme, in which all main parameters are set. The innovative feature of the machine is the possibility to run test from a coil, enabling repeated testing at similar rate as in industrial production. This makes it possible to emulate the graduate, but often slow build-up of pick-up of workpiece material on the tool surface occurring in production, which is impossible in the simple, above mentioned laboratory tests [7–11], where the time between repetitions allows the tool to
E. Ceron, N. Bay / CIRP Annals - Manufacturing Technology 62 (2013) 231–234
232
cool down [13]. The equipment is designed for the main sheet tribo-tests [14], e.g. Bending-Under-Tension (BUT) testing, DrawBead-Testing (DBT) or Strip-Reduction-Testing (SRT) with adjustable sliding length (0–250 mm), sliding speed (0–150 mm/s), cycle time (0–95 spm) and total number of strokes.
speed is 40 spm in a 250 t mechanical press. The height of the final cup is 20 mm. The analysis is focused on the third operation, which is the most critical as regards the limit of lubrication. The radius of curvature of the die in operation 3 is r = 3.5 mm. Previous experience in the company has shown that the choice of the environmentally hazardous lubricant and the tool coating was the
Fig. 3. Cup produced in progressive tool system.
only feasible tribo-system due to the severity of the production process. 4. Numerical analysis Fig. 2. DTU-MEK’s automatically controlled sheet-tribo-tester.
2.2. Procedure Starting point in the proposed procedure for off-line testing is an existing production process, with a hazardous tribo-system that has to be replaced with an environmentally friendly one. The production platform defines important tribo-parameters such as normal pressure, surface expansion, sliding length, sliding speed etc., which can be quantified by numerically simulation. After this process characterization, an appropriate simulative test is selected based on the type of deformation process. A numerical analysis of the laboratory test is performed to determine and adjust the tribo-conditions to be comparable with those in the production process. After that, a thorough planning of the simulative test is done determining factors such as number of repetitions, idle time between strokes and initial tool temperature. A new tribo-system is then selected for testing. A first campaign of screening tests is performed with triboparameters similar to those in the production to check whether the tribo-system is a promising alternative. In case of poor results one of two routes may be followed. Either a new tribo-system may be selected or, if possible, modification of the production platform/ workpiece geometry may be introduced in order to lower the tribological severity. In case of good results more comprehensive off-line testing is performed varying critical parameters such as normal pressure, sliding length, rate of testing and idle time, in order to define the working window of the tribo-system. Again, this could lead to either good or poor results and, as described above, there are two possible choices in case of poor results. If good results are achieved the next step is to test the tribo-system in production. If satisfactory results are achieved in the production test the procedure is successfully completed, and the old tribo-system may be substituted. Since it is practically impossible to reproduce the production conditions exactly, the production test may also lead to unsatisfactory results in spite of promising laboratory test results. In that case the same procedure as mentioned above is applied. 3. Case study The methodology is applied to an industrial case shown in Fig. 3 manufacturing a cup by deep drawing (1) and two redrawings (2)–(3) and a flange pressing (4) in a progressive tool system. The workpiece material is 1 mm stainless steel strip EN 1.4301, tool material is powder metallurgical tool steel UHB Vanadis 6 (62 HRC) PVD coated with TiAlN. Chlorinated paraffin oil Iloform PN226 is currently used as lubricant. The production
With selection of the production process the first step in the proposed methodology is taken. The next step is to determine the tribo-parameters in the considered, third operation by numerical analysis. In this case the most important parameter is considered to be the normal pressure at the tool/workpiece interface. The three deep drawing operations (1)–(3) are simulated with a 2D axisymmetric model using the elastic-plastic FE code LS-DYNA1. The coefficient of friction is calibrated comparing the measured punch force with the calculated one. Fig. 4 shows the normal stress in the y direction in the local Cartesian system of each element, where the local y direction on elements distributed on the outer surface of the curvature is perpendicular to the circumference. The
Fig. 4. Normal pressure at the interface workpiece/die. Operation (3).
stress displayed on the outer elements can thus be interpreted as normal pressure at the interface. The numerical analysis shows that the contact area is quite small, approximately equal to the sheet thickness, and the normal pressure is very high with peak values around 1000 MPa. The most suitable simulative test to choose for emulation of the production conditions in operation (3) is Bending-UnderTension (BUT). In this test a strip is bent 908 and drawn over a nonrotating tool pin. The equipment is earlier described by Andreasen et al. [15]. The 908 BUT test with a tool pin of same radius r = 3.5 mm as in operation (3) of the production test is also numerically analyzed in LS-DYNA. Fig. 5a shows that this leads to insufficient normal pressure even with the highest possible back tension stress, where pmax = 250 MPa. The limit is set by the yield stress of the undeformed strip. In order to reach a normal stress of same size as in the production process, a new tool geometry was developed for the BUT test. In the new tool, having the same tool radius as the former, the contact during 908 BUT testing is limited to 458, see Fig. 5b. This configuration leads to a localized peak in normal pressure around 428 of pmax = 1000 MPa due to the abrupt exit, i.e. a maximum normal pressure comparable to that in the production tool.
E. Ceron, N. Bay / CIRP Annals - Manufacturing Technology 62 (2013) 231–234
233
6. Experimental results 6.1. BUT testing The torque acting on the BUT tool pin was registered by a piezoelectric transducer, whereas, the drawing force and the back tension force were measured by strain gauge transducers. All data were aquired by a PC with LabView. As described by Andreasen et al. [15] the torque is very sensitive to friction and as such a good indication of onset and growth of pick-up. Fig. 6a shows the torque and drawing force for tribo-system 1. In this case the test was successful, since no galling appeared within 1500 strokes. The force and torque increase slightly during the first 200 strokes after which they reach a steady state. The initial increase is probably due to micro pick-up of workpiece material on the tool surface, which after running-in becomes stable.
(a) 30
10000 8000
20
6000
15 4000
10
EN 1.4301
5
2000
0 5. Experimental setup
0
Table 1 Investigated tribo-systems. Tribo-system Workpiece material 1 2 3
Tool material Lubricant
EN 1.4301 Vancron 40 DP 800 Vancron 40 EN 1.4162 (LDX 21011) Vancron 40
rhenus SU 166 A ANTICORIT PLS 100 T rhenus SU 166 A
According to the numerical analysis, the back tension should be
sb = 300 MPa for all three materials. Such a large value was, however, not applicable for EN 1.4301, being too close to the yield stress. Preliminary tests showed rapid fluctuations in the drawing force which induced a variation of the back tension that exceeded the yield stress s0 and caused fracture of the strip. The back tension was therefore decreased to 200 MPa implying lower but still fairly high normal pressure. The sliding length was set equal to the height of the cup, 20 mm. The sliding speed was set to 50 mm/s, which is less than half the one applied in production. Even though the maximum speed in the BUT test is 150 mm/s, it was decided to reduce it to 1/ 3, since problems with fluctuation in the drawing force during acceleration caused fracture of the strip. Each test was repeated 1500 times with a rate of 40 strokes/min corresponding to the production rate.
1000 stroke
torque
0 1500
drawing force
(b) 60
15000
50
torque [Nm]
Three new tribo-systems were selected to be analyzed in BUT testing as well as production testing. They are described in Table 1. Besides the existing production process forming EN 1.4301 with yield stress s0 = 320 MPa two other materials were investigated, i.e. an advanced high strength steel DP800 with yield stress s0 = 620 MPa and a lean duplex stainless steel EN 1.4162 (LDX 2101) with yield stress s0 = 550 MPa. As tool material was chosen an uncoated, powder metallurgical tool steel UHB Vancron 40 hardened and tempered to 63 HRC. The tool surface was manually polished with emery paper grade 1000 in transversal direction reaching a final roughness of Ra = 0.06 mm. The workpiece material is a strip with cross section 1 mm 30 mm fed from a coil reel during testing. The lubricant applied in tribo-systems 1 and 3 is environmentally benign, highly refined, mineral oil with EP additives, viscosity 150 mm2/s at 40 8C, from Rhenus Lub. The lubricant applied in tribo-system 2 is corrosion protective mineral oil, viscosity 100 mm2/s at 40 8C, from FUCHS. Both lubricants were applied by means of two felt rolls.
500
40
10000
30 20
5000
10
EN 1.4162
0 0
10
20 stroke
torque
force [N]
Fig. 5. (a) Normal stresses in 2D model of: (a) quarter of square BUT tool pin with 908 round edges and (b) quarter of new tool geometry with confined tool curvature.
force [N]
torque [Nm]
25
0 30
40
drawing force
Fig. 6. Torque and drawing force versus number of strokes (a) tribo-system 1 and (b) tribo-system 3.
The results for tribo-system 2 are not shown but they have the same constant trend as No. 1, although without the initial increase. Fig. 6b shows the test results for tribo-system 3 (lean duplex steel), where severe galling occurred. In this particular test the back tension was decreased to 200 MPa (as for EN 1.4301) in order to investigate, whether deep drawing could be performed at lower normal pressure. It is noticed that the torque and force increase abruptly after 30 strokes. This is due to severe galling eventually leading to fracture of the strip. The limit of lubrication for this system is thus far below production conditions. Fig. 7a–c shows the tool surface after testing the three different tribo-systems 1–3. Sliding direction of the strip is from bottom to top. The exit edge, where maximum normal pressure is reached, is indicated in each picture. As regards tribo-systems 1 and 2, almost no pick-up is noticed, while tribo-system 3 results in severe pickup, which led to the before mentioned fracture of the strip. In this test campaign only the production speed was varied in order to find the limit of lubrication as a function of this parameter. No galling occurred for any of the two tribo-systems 1 and 2, when running up to the maximum speed of 95 strokes/min. 6.2. Production testing The simulative tests showed promising results for tribosystems 1 and 2. According to the methodology, it was therefore
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furthermore carried out at increased production speed of 95 strokes/min again with all 1500 parts being approved. 7. Conclusion A methodology for off-line evaluation of tribo-systems for sheet metal forming was presented in this paper and applied to an industrial case study. The methodology follows a logical path, which combines numerical and experimental analysis to ensure realistic tribo-conditions emulating those in production. An important goal of the procedure is to increase the chance of success before production trial of more environmentally benign tribo-systems as possible substitutes to hazardous ones. The results prove that the methodology works and can give detailed information as regards the limit of lubrication of the tribo-system investigated thereby diminishing costly production trials. Acknowledgments The authors thank Grundfos A/S, Outokumpu Stainless AB, Outokumpu Stainless Research Foundation, SSAB and Uddeholm AB for their economic and technical support.
References
Fig. 7. Tool surface after testing: (a) tribo-system 1, (b) tribo-system 2 and (c) tribosystem 3.
Fig. 8. (a) Production sample, good tribo-system 1 and (b) production sample, acceptable tribo-system 2.
decided to test them in production. Corresponding to the laboratory tests 1500 cups were produced with a production speed of 40 spm. The die in step No. 3 was polished in circumferential direction obtaining the same surfaces topography as in the BUT tools. Fig. 8a and b shows the last two samples produced for tribosystem 1 and 2 respectively. They both satisfy the requirements to surface quality. As regards tribo-system 1 production testing was
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