Evaluation of Lubricant Performance Using the Cup Drawing Test (CDT)

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1 Evaluation of Lubricant Performance Using the Cup Drawing Test (CDT) By Cliff Hoschouer 1, Jeff Jeffery 2, Frank Kenny 2, David Diaz Infante 3 and Taylan Altan 3 1 Shiloh Industries, 2 IRMCO, 3 Center for Precision Forming Lubrication plays an important role in sheet metal forming by affecting metal flow and reducing the possibility of fracture during forming. There are several methods to test the lubricant performance such as the Strip Draw Test (SDT), the Twist Compression Test (TCT) or the Cup Drawing Test (CDT). Among these, the CDT emulates quite well the pressure, velocity and temperature conditions that exist in stamping. Using a 300 ton servo press located at Hyson and the CDT tooling, designed and built by IRMCO, 24 lubricants from 4 different companies have been tested using blanks from mild steel, HSLA, Stainless Steel and AHSS. In addition, average part temperatures were measured using a FLIR infrared camera. 1 Cup Drawing Test (CDT) The schematic of CDT is shown in Figure 1. The cup is always drawn to the same depth of 80 mm. The details of this test were discussed in previous articles published in Stamping Journal [2, 3]. In evaluating the lubricant performance using the CDT, the sheet is not drawn completely and some flange remains at the end of the operation. The perimeter of the flange is measured for each specimen. The length of the flange perimeter is an indication of the lubricant performance. The smaller the perimeter, for a given Blank Holder Force (BHF), the better is the lubricant, i.e. the lower is the Coefficient of Friction (COF). This case represents a better lubrication performance. In case the flange perimeters don t show reasonable differences for different lubricants, the load vs stroke data can be used to determine the lubricant performance. The lower is the maximum forming load, the better is the lubricant. 2 Experimental and FE Simulation Results During this set of experiments, a total of 390 samples (blanks of 12 inches diameter) were used and 4 different variables were measured in order to evaluate the lubricant performance; namely the average flange perimeter of the formed cup, the average blank temperature, the average die temperature and the punch force. While some materials such as mild steels may not show a significant temperature increase in forming, others such as AHSS may reach temperatures above 200 F in high volume production. Due to

2 temperature generation from the blank, the tooling may expand thermally reducing the clearance between the punch and the die creating undesirable forming conditions. Therefore, in these cases, selecting a lubricant, that performs well at high temperatures and also has a cooling effect is very important. The temperature of the drawn cup was recorded using a FLIR infrared camera which was triggered manually as soon as the cup was visible, as shown in Figure 2. While the part surface may lose some temperature, during the time when the cup is ejected, this measurement gives a good estimation of the temperature generated during forming. The forming speed was kept constant during the experiments; however, the BHF was selected according to the blank material and thickness. In ranking the lubricants, the average temperature and the average flange perimeter were used as the main parameters. As two examples, the set of lubricants used with mild steel and AHSS blanks, are shown in Figure 3. For each material the best five lubricants were selected based on the criteria mentioned above (i.e. average flange perimeter and average temperature). Lubricants X and J consistently performed best for all the materials tested. A FE model was set up using PAM-STAMP software. The experimental parameters such as forming speed or draw depth used in the experiments were reproduced in the simulations. Using this methodology it is possible to estimate the CoF of the lubricant by matching the flange perimeter measured on the experimental samples with the corresponding values obtained thru FE simulation, as seen in Table 1. Figure 5 shows that the highest temperature due to material deformation occurs around the die corner radius, as estimated using FE simulations. On the other hand, the temperature in the cup, near the punch corner radius does not show any significant change in temperature, since the deformation in that region is small. For comparison purposes, the experiment for the lubricant E shown in Figure 3 was used in the simulation to predict temperatures, Figure 5. The FE simulation and experimental results are shown in Table 1. It is seen that the temperatures estimated using simulations are close to the experimental values for the tested case. 3 Acknowledgments The authors would like to thank Mr. Darrell Quander and Mr. Ethan McLaughlin of HYSON for hosting the research team and for sharing their equipment (servo cushion and servo press) during the lubrication tests. David Diaz Infante (diazinfantehernandez.1@.osu.edu) is a graduate research associate at the Center for Precision Forming (CPF) at The Ohio State University, 1971 Neil Ave., Room 339 Baker Systems Engineering Building, Columbus, OH Taylan Altan

3 is a professor emeritus and director of CPF, and 4 References [1] Mao, T., and Altan, T., Evaluation of Dry-Film and Wet Lubricants for Aluminum Stamping, Stamping Journal, Sept/Oct, 2015, p. 16 [2] Fallahiarezoodar, A., et al., Examining Lubricant Performance in Forming AHSS, Stamping Journal, Jan/Feb., 2015, p. 10 Table 1: Comparison of FE Simulation VS Experimental Results Parameter FE simulation (CoF=0.12) Experiment Flange 781 mm 781 mm Max Temperature 49 C (120.3 F) 50.9 C (123.5 F) Figure 1: Schematic of deep drawing tool used in this study [1]

4 Figure 2: Example of Temperatures on the drawn cup recorded with a FLIR infrared camera during experiments, after the drawn cup is ejected from the tool Mild Steel Average Perimeter mm Average Temp F Lubricant Top five lubricants (G, J, W, X and Z) Figure 3: Average Flange Perimeter and Temperature Measured in Drawn Cup (dots indicated in the figure) for Various Lubricants (Mild Steel Blanks)

5 Average Perimeter mm AHSS A AA B C D E F G H I J K L M N O P Q R V W X Y Z Lubricant Top five lubricants (AA, B, J, M and X) Average Temp F Figure 4: Average Flange Perimeter and Temperature Measured in Drawn Cups (dots indicated in the figure) for Various Lubricants (AHSS blanks) Figure 5: Temperature distributions on the cup according to FE simulations. Materials: Mild steel (270MPa UTS) 2 mm, and DP mm. Ram speed: 80mm/s. Initial temperature: 20 C.