Long Ju School of Mechanical Engineering, University of Science and Technology Beijing, 30 Xueyuan Road, Haidian, Beijing 100083, China e-mail: [email protected] 1 Tingting Mao 2 Center for Precision Forming, The Ohio State University, 339 Baker Systems, 1971 Neil Avenue, Columbus, OH 43210 e-mail: [email protected] 43 5 Julio Malpica Honda Engineering of North America, Inc., 24000 Honda Parkway, Marysville, OH 43040 e-mail: [email protected] Taylan Altan1 Center for Precision Forming, The Ohio State University, 339 Baker Systems, 1971 Neil Avenue, Columbus, OH 43210 e-mail: [email protected] 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 Evaluation of Lubricants for Stamping of Al 5182-O Aluminum Sheet Using Cup Drawing Test Lubricants are necessary to avoid adhesion, galling, and scratching in aluminum stamping processes. In this study, various lubricants, including dry lubes and wet lubes, were evaluated using cup drawing test (CDT) for stamping of Al 5182-O aluminum sheets. The effects of surface texturing, with electro-discharge texturing (EDT) and mill finish (MF), on the friction behavior were also investigated. Furthermore, the methodology to evaluate the performance of lubricants was established based on (a) maximum applicable blank holder force (BHF) and (b) draw-in length in flange or flange perimeter of formed cups. Finite element (FE) simulations were carried out to determine the coefficient of friction (CoF) at tool–workpiece interface during deep drawing under different lubrication conditions. Flow stress data of Al 5182-O material were obtained using viscous pressure bulge (VPB) and tensile tests. In this study, it was confirmed that, in forming Al 5182-O, dry film lubricants have better lubricity than wet lubricants. A better lubrication condition was found with EDT surface texture. [DOI: 10.1115/1.4030750] Keywords: aluminum stamping, lubrication, surface texturing, finite element analysis surface finishes: MF and EDT [3]. By using 2D and 3D measurements, it is reported that EDT can provide a higher closed void Lightweight materials, such as aluminum alloys, are increasarea ratio in the sheet surface to ensure a better lubricant distribuingly used in sheet metal forming of automotive parts. In forming tion and transport than MF. As a result, a lower CoF could be aluminum alloys, adhesive wear and galling, due to insufficient achieved by using EDT [9,10]. From a microscopic scale point of lubricant, can greatly affect the quality of stamped parts [1]. The view, the characterization of the asperities and valleys on the die right lubricant can reduce or even avoid failures associated with and workpiece surfaces could provide more reasonable expiations wrinkling and premature fracture, thus increasing the process winfor many tribological phenomena in sheet metal forming [11]. For dow [2]. There are mainly two types of lubricants used for alumia given tool, sheet material, and surface finish, the friction is also num sheet metal forming: dry-film lubricants and mineral oil affected by rolling direction of the sheet, sliding direction of the based lubricants. Lubricants used in stamping lines are mainly contacts [9] and the contact pressure as well as relative velocity based on mineral oils. They have good deep drawing performance and interface temperature [12]. The well-known Stribeck curve and can be used in forming complex parts, such as inner door pan- was applied to describe the dependency of friction on contact els with most metals [3]. In recent studies, dry film lubricants pressure, sliding velocity, and dynamic viscosity of the lubricant have been used in forming to avoid scratching and galling of the [13]. From the experimental study on the strip drawing of Al tools [4,5]. Regarding the evaluation of various lubricants, twist sheets, adhesive wear was found to occur as a result of local peak compression test [6], deep drawing test (including CDT [7], and stress and temperature rise [1]. strip drawing test [8]) are found to be most practical testing methIn the present study, a methodology was established to evaluate ods. Using inverse analysis and the measured data from CDT the performance of different lubricants for stamping of Al 5182-O tests, the CoF can be estimated with FE simulation. At the Center sheets. Selected lubricants were evaluated by using CDT in a of Precision Forming (CPF), it was concluded that, compared with hydraulic press. The effects of two different surface texturing, MF other lubricant testing method, the CDT can emulate the “nearand EDT, were investigated in detail. The specific objectives are production” conditions well [7]. It should be noted that the present to: (a) select the lubricants that perform best in drawing of alumitesting method applies primarily for deep drawing operations. num sheet 5182-O, (b) compare the friction behavior between MF However, in conducting FE simulations of stamping operations, surface and EDT surface, and (c) determine the CoF at the for practical reasons, only one average value of CoF is used. tool–workpiece interface under various lubrication conditions. Thus, the CoF obtained from the CDT can be used in FE analysis The obtained CoF for selected lubricants can be used as input into of stamping. FE simulations of stamping with complex geometries used in The surface finish of the sheet also affects the lubrication durautomotive production, such as hoods, decklids, and doors. ing forming. Aluminum sheet is commercially available in two 1 Introduction 1 Corresponding author. Contributed by the Manufacturing Engineering Division of ASME for publication in the JOURNAL OF MANUFACTURING SCIENCE AND ENGINEERING. Manuscript received January 14, 2015; final manuscript received May 19, 2015; published online xx xx, xxxx. Assoc. Editor: Blair E. Carlson. 2 Experimental Procedures 2.1 Sheet Materials. In the present study, the sheet materials were characterized by both standard tensile test and VPB test. The tensile tests were conducted in rolling, diagonal, and transverse Journal of Manufacturing Science and Engineering CCopyright V 2015 by ASME ID: asme3b2server Time: 19:39 I Path: D:/ASME/Support/XML_Signal_Tmp/AS-MANU150085 MONTH 2015, Vol. 00 / 000000-1 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 J_ID: MANU DOI: 10.1115/1.4030750 Date: 6-June-15 Stage: Page: 2 Total Pages: 9 PROOF COPY [MANU-15-1035] Table 1 Mechanical properties of Al 5182-O from tensile tests (conducted by Honda R&D) Fig. 1 Flow stress curves obtained from tensile test and VPB test 74 2.2 Lubricant Samples and Surface Textures. In this study, 75 14 lubricants were evaluated. All the lubricants are used as 76 received, except lubricant I, which had to be diluted with water in 77 78 the ratio of 1:3 (lubricant/water). The coating weight applied on 79 the blank was 1 g/m2 for all lubricants. The details of lubricant 80 type and application method are summarized in Table 2. The time 81 82 period, from the application of a wet lubricant until the test is 83 completed, was approximately 5 min. 84 Application of lubricants on the blank was done by using a pip85 ette and draw down bar #0. Each lubricant was spread all over the 86 87 blank surface on both sides uniformly. Three drops of the lubri88 cant were applied on each side of blank to achieve a coating 89 90 weight (1.0 g/m2 6 0.3 g/m2). Since it is difficult to manually 91 apply the dry film lubricant in the laboratory scale, some of the 92 dry film lubricants were required to be precoated by the lubricant 93 94 companies with the same amount as wet lubricants. The coating weight/amount was verified by measuring a few samples using the 95 96 laboratory balance before and after lube application. Additionally, 97 98 the applied lubricant thickness was verified by using a portable 99 tool (Microderm CMS). The coating weight measured by Micro100 derm CMS tool was around 1.24 g/m 2, which was within the range 101 102 of the coating requirement, recommended by the lubricant 103 suppliers. 104 The 5182-O aluminum sheets were available with two different surface texturing, MF and EDT. Three-dimensional view of EDT surface texture measurement, conducted by Honda Engineering, is shown in Fig. 2. In order to have better understanding about the difference in the surface topography, the roughness parameters, Ra and Rz, were compared in Table 3. It can be seen that the EDT texture provide higher deviation values and the pocket structure on the surface can definitely effect the lubricated contacts. directions, respectively. Table 1 lists the material properties obtained from the tensile tests. VPB test, developed by CPF, could provide higher strain values under biaxial state of stress [14]. Figure 1 shows the flow stress curves obtained from the two tests. It can be seen that the flow stress could be obtained at true strain of about 0.5 with the VPB test. 2.3 Principles of CDT–CDT. The cup drawing process is extensively used to evaluate the performance of lubricants. The schematic of CDT is illustrated in Fig. 3. In cup drawing, the most severe friction takes place at the flange and die corners. Sample Ultimate tensile stress (MPa) Yield stress (MPa) R-value at strain 15 (%) 291.1 291.4 279.8 279.8 285.8 284.9 128 125.8 120.9 121.7 123.7 124.1 0.69 0.72 1.08 1.1 0.79 0.88 RD1 RD2 DD1 DD2 TD1 TD2 68 69 70 71 72 73 Table 2 Lubricants and their details as provided by lube companies Lubricant code A B C D E F G Lubricant type Petroleum oil Draw down bar #0 Petroleum oil Draw down bar #0 Petroleum oil, additive blend Draw down bar #0 Petroleum oil, additive blend Draw down bar #0 Emulsified oil (semisynthetic) Draw down bar #0 Dry film Mineral oil based Lube company applies Application method Lubricant code H I Lubricant type Mineral oil based Draw down bar #0 Mineral oil based Draw down bar #0 Application method J K L Draw down bar #0 M N Water based Water based Water based Dry film Dry film Draw down bar #0 Draw down bar #0 Draw down bar #0 Lube company applies Lube company applies Fig. 2 3D measurements of surface texture (Al 5182-O, 1.5 mm): (a) EDT and (b) MF, provided by Honda Engineering 000000-2 / Vol. 00, MONTH 2015 ID: asme3b2server Time: 19:39 I Path: D:/ASME/Support/XML_Signal_Tmp/AS-MANU150085 Transactions of the ASME 105 106 107 108 J_ID: MANU DOI: 10.1115/1.4030750 Date: 6-June-15 Stage: Page: 3 Total Pages: 9 PROOF COPY [MANU-15-1035] Table 3 Measured roughness parameters in different areas on the EDT and MF surfaces Raa (lm) Area 1 Area 2 Area 3 Area 4 Area 5 109 110 111 112 113 114 Rzb (lm) EDT MF EDT MF 1.719 1.414 1.111 1.463 1.197 1.574 1.282 1.121 1.485 1.238 16.991 14.546 12.022 14.3 13.505 14.03 11.145 11.135 12.96 10.5 The following evaluation criteria are used to determine the “better” performance of the selected lubricants: (a) higher applicable BHF for the successfully drawn part and (b) shorter perimeter in flange area (the perimeter is used in evaluation of the test results because the formed flange is not exactly round and the measurement of the perimeter avoids measuring and averaging several draw-in lengths (Ld)). 2.4 Description of the Tooling and Test Procedure. The CDTs were carried out in a 160 ton hydraulic press, which has a maximum ram speed of 300 mm/s (12 in./s). The schematic of the tooling is shown in Fig. 4. A draw die attached to the upper ram a Ra is the arithmetic average of the absolute values. moves down and forms a cup sample with a stationary punch. The b Rz is average distance between the highest peak and lowest valley in each preset constant BHF is provided by the CNC hydraulic cushion sampling length. pins. During the CDT, the punch force is measured by a load cell and the die displacement is recorded by an infrared laser sensor. The lubrication condition in this flange area influences the thickness The flange perimeter is measured using a flexible tape. The cup draw test is selected because it is relatively simple to distribution in the side wall of drawn cup. It also affects the draw-in conduct and it communicates to the practical stamping engineer length, Ld. As the blank holder pressure (Pb) increases, the frichow lubrication affects the metal flow and possible cup fracture. tional stress (s) also increases based on Coulomb’s law (Eq. (1)) In this study, the draw ratio, the ratio of initial blank diameter to cup diameter, was selected to be 2.0. In order to leave enough s ¼ l Á Pb (1) flange area for measuring, the draw depth 80 mm was selected. Round blanks with 304.8 mm diameter were cut from Al 5182-O where s ¼ frictional shear stress, l ¼ coefficient of friction, and sheet. Pb ¼ blank holder pressure. Fig. 3 Schematic of the cup drawing operation (CDT) and tooling Fig. 4 Deep draw tooling used in CDT [7] Journal of Manufacturing Science and Engineering ID: asme3b2server Time: 19:39 I Path: D:/ASME/Support/XML_Signal_Tmp/AS-MANU150085 MONTH 2015, Vol. 00 / 000000-3 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 AQ1 J_ID: MANU DOI: 10.1115/1.4030750 Date: 6-June-15 Stage: Page: 4 Total Pages: 9 PROOF COPY [MANU-15-1035] Table 4 FE simulation parameters for obtaining BHF range for CDT Parameter Description Sheet material Anisotropic values (average) Flow stress curve Yield criterion Blank diameter BHF CoF Punch stroke Forming speed Al 5182-O, 1.5 mm R0 ¼ 0.71, R45 ¼ 1.09, R90 ¼ 0.84 VPB test data, as shown in Fig. 1 Yld 2000-2d 304.8 mm 16–24 ton 0.1, 0.11, 0.12 80 mm 40 mm/s Table 5 Predicted results under various BHF and CoF BHF CoF 16 18 20 22 24 0.1 0.11 0.12 Formed Formed Formed Formed Formed Fracture Formed Fracture Fracture Fracture Fracture Fracture Fracture Fracture — 139 3 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 In order to obtain a workable range of BHF for the tests, the FE analysis, using the commercial code PAM-STAMP, was carried out to simulate the CDT process. The FE model was built in commercial software PAM-STAMP. The sheet was modeled with four node quadrilateral shell elements, and the tools were assumed to be rigid. The flow stress curve of Al 5182-O was considered to be independent of strain-rate in the deformation rate used in CDT. The flow stress curve obtained from VPB test was used in the FE model. The yield criterion Yld 2000-2d, having a better description of the anisotropic behavior of Al alloys, was used in the FE model. The required eight yield parameters were characterized in Numisheet 2005 Benchmark for Al 5182-O [15]. All the simulations were conducted under constant BHF and punch speed. Table 4 summarizes the input parameters in the FE simulation. In the FE simulations, the critical thinning (maximum thinning) value of 20%, selected to predict the failure in forming of Al 5182-O is suggested by industrial experience from some of the companies that form Al alloy sheets. The cup is formed, when the Fig. 6 Flange perimeter recorded for 13 lubricant tests at 16 ton BHF (B, D, H, and N failed at 16 ton BHF). Note: The y-axis on the graph does not start from 0. The error bands show the deviation between samples. FE Simulation of the CDT Fig. 7 Flange perimeter recorded for five lubricant tests at 17 ton BHF (A, C, E, I, and J failed at 17 ton BHF). Note: The y-axis on the graph does not start from 0. The error bands show the deviation between samples. Fig. 5 Test procedure for the evaluation of lubricants 000000-4 / Vol. 00, MONTH 2015 ID: asme3b2server Time: 19:39 I Path: D:/ASME/Support/XML_Signal_Tmp/AS-MANU150085 Transactions of the ASME J_ID: MANU DOI: 10.1115/1.4030750 Date: 6-June-15 Stage: Page: 5 Total Pages: 9 PROOF COPY [MANU-15-1035] Fig. 8 Comparison between MF and EDT at (a) 16 ton BHF and (b) 17 ton BHF Table 6 Flange perimeters prediction by FE simulations BHF (ton) 16 17 CoF0.06 0.08 0.1 0.12 0.06 0.08 0.1 0.12 Flange perimeter (mm) 735 740 745 750 736 741 746 752 158 159 160 161 162 maximum thinning value is below 20%. Otherwise, the cup gets fracture. As shown in Table 5, the drawability of 5182-O using the present die set was predicted under different BHF and CoF. It was determined that 16–18 ton is a reasonable range for evaluation of the performance of various lubricants in this study. 163 4 164 165 166 167 168 4.1 Test Results. Based on the FE simulation results listed in Table 5, the CDTs were conducted under BHF 16 ton, 17 ton, and 18 ton, respectively; 17 ton was found to be the maximum useful BHF for the experiments, since all the cups drawn with all lubricants cracked when BHF was 18 ton. The consistency of Lubrication Tests, Analysis, and Results lubrication condition is determined by using three samples for each test. The test procedure for the evaluation of all lubricants is shown in Fig. 5. In the tests, the measurements of Ld could be easily affected by the concentricity deviation of the sample position on the die. Thus, the performance of the lubricants was evaluated by measuring the flange perimeter of the drawn cup using a tape. At 16 ton BHF, all lubricants listed in Table 2 were tested. Lubricants that cracked at 16 ton (B, D, H, and N) were not considered for next round of evaluation, using 17 ton BHF. As shown in Fig. 6, lubricant F shows the minimum flange perimeter among all the tested lubricants, indicating a better performance at 16 ton BHF. At 17 ton BHF, cups could only form with five lubricants, including three wet lubricants G, K, L and two dry film lubricants F, M. The results under 17 ton BHF are shown in Fig. 7. It can be concluded that, among all the wet lubricants, G, K, and L could be determined as “better performing wet lubricants.” The dry film lubricant F performed “best” at both 16 and 17 ton BHFs. 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 4.2 Effect of Surface Texturing. The friction behavior of Al 5182-O with EDT surface and MF surface are compared, as 187 188 Fig. 9 Comparison of flange perimeters obtained from simulation and experiment to predict the CoF at 16 ton BHF for Al 5182O/MF Journal of Manufacturing Science and Engineering ID: asme3b2server Time: 19:39 I Path: D:/ASME/Support/XML_Signal_Tmp/AS-MANU150085 MONTH 2015, Vol. 00 / 000000-5 J_ID: MANU DOI: 10.1115/1.4030750 Date: 6-June-15 Stage: Page: 6 Total Pages: 9 PROOF COPY [MANU-15-1035] Fig. 10 Comparison of flange perimeters obtained from simulation and experiment to predict the CoF at 17 ton BHF for Al 5182O/MF Fig. 11 Thickness comparison between experiment and simulation of drawn cup with lubricant F: (a) rolling direction and (b) transverse direction 000000-6 / Vol. 00, MONTH 2015 ID: asme3b2server Time: 19:39 I Path: D:/ASME/Support/XML_Signal_Tmp/AS-MANU150085 Transactions of the ASME J_ID: MANU DOI: 10.1115/1.4030750 Date: 6-June-15 Stage: Page: 7 Total Pages: 9 PROOF COPY [MANU-15-1035] Fig. 12 Thickness comparison between experiment and simulation drawn cup with lubricant I: (a) rolling direction and (b) transverse direction 189 190 191 192 193 194 shown in Fig. 8. Lubricants K, J, and L were first tested at 16 ton BHF. However, when BHF was increased from 16 ton to 17 ton, the cups fractured when using lubricant J. As shown in Fig. 8, the cups formed with EDT surface show smaller flange perimeters. It can be concluded that, comparing to MF, EDT provides a better lubrication condition in the current experimental procedure. 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 4.3 Prediction of CoF at Tool–Workpiece Interface. To determine the CoF, the perimeter of the flange obtained at the end of stroke in the simulation is compared to the results obtained in the experiment using PAM-STAMP. The parameters used as input to FE simulation are the same as parameters listed in Table 4, except that (1) BHFs used in the simulation are 16 ton and 17 ton and (2) the CoFs are 0.06, 0.08, 0.1, and 0.12. The predicted flange perimeters under different BHFs and CoFs are shown in Table 6. As shown in Figs. 9 and 10, the flange perimeters obtained from FE simulations and experiments are compared to determine the CoFs for different lubricants under 16 ton BHF and 17 ton BHF with MF blanks. It can be concluded that (i) the CoFs for lubricants G and K are in the range of 0.08–0.1, (ii) the CoF for lubricant F is around 0.08, and (iii) the CoF for the lubricant I is around 0.12. 4.4 Comparison of Cup Thickness Variation Obtained From Simulations and Experiments. In order to verify the simulation results (CoF), the thickness distributions in the drawn cup obtained from experiments and simulations are compared. The test results for lubricants F and I are selected to verify the results of simulations. As mentioned in Sec. 4.3, the CoF of F is around 0.08 and the CoF of I is around 0.12. The cup samples drawn with lubricants F and I were cut in rolling and transverse directions. The thickness measurements along the rolling direction and transverse direction were completed using Starrett micrometer and are shown in Figs. 11 and 12. It is seen that FE simulations provided a good match with experimental results and the maximum error is around 5%. 210 211 212 213 214 215 216 217 218 219 220 221 222 5 223 Summary and Conclusions In this study, the methodology for evaluating the performance of various commercial lubricants for forming aluminum alloy sheets was established by using CDT and FE analysis. Two surface textures were investigated and compared with different lubricants. The principal conclusions from this study are summarized as follows: Journal of Manufacturing Science and Engineering ID: asme3b2server Time: 19:39 I Path: D:/ASME/Support/XML_Signal_Tmp/AS-MANU150085 MONTH 2015, Vol. 00 / 000000-7 224 225 226 227 228 229 J_ID: MANU DOI: 10.1115/1.4030750 Date: 6-June-15 Stage: Page: 8 Total Pages: 9 PROOF COPY [MANU-15-1035] 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 (1) CDTs were utilized to evaluate various wet and dry lubricants under different BHFs for Al 5182-O sheets with EDT and MF surfaces. The effect of deformation on surface finish, as discussed in Ref. [16], was considered. The criteria used in this study were able distinguish the performance of selected lubricants. (2) One of the dry film lubricants was found to perform better than wet lubricants with the same amount of coating weight (1 g/m2). In addition, EDT surfaces, with higher roughness, could provide a better lubrication condition than MF surfaces for aluminum stamping. (3) The FE model used in PAM-STAMP yields reasonable estimate of CoFs and accurate thickness predictions when deep drawing conditions prevail. The lowest CoF 0.08 was determined for one dry film lubricant under the current experimental condition. Transactions of 246 Acknowledgment 247 248 249 250 251 252 253 254 The authors would like to extend special thanks to the member companies of the Center of Precision Forming (CPF) that funded this study. Special thanks are due to Honda North American Engineering center (Milan Jurich, Doug Staats), Honda R&D (Jim Dykeman) and Honda Engineering (Dennis O’Connor) for supporting this project. The authors also thank to the lubricant companies that provided samples and the Edison Welding Institute (Hyunok Kim) for assisting in conducting the experimental part of this study. 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