EVALUATION OF TWIST COMPRESSIONS TEST (TCT) AND CUP DRAW TEST (CDT) FOR DETERMINING THE PERFORMANCE OF LUBRICANTS FOR SHEET METAL FORMING OPERATIONS

Transcription

1 EVALUATION OF TWIST COMPRESSIONS TEST (TCT) AND CUP DRAW TEST (CDT) FOR DETERMINING THE PERFORMANCE OF LUBRICANTS FOR SHEET METAL FORMING OPERATIONS THESIS Presented in Partial Fulfillment of the Requirements for the Degree of Master of Science in Graduate School of The Ohio State University By Sahika Ruzgar Peker, B.S. Graduate Program in Industrial and Systems Engineering The Ohio State University 2016 Thesis Committee: Dr. Taylan Altan, Advisor Dr. Jerald Brevick

2 Copyright by Sahika Ruzgar Peker 2016

3 ABSTRACT Lubrication is one of the process variables that affects the quality of the stamping part. With an appropriate lubrication, quality of the formed part can be improved, and scrap rate can be reduced. The objective of this study is to provide detailed information about Cup Drawing Test (CDT) and Twist Compression Test (TCT) as two common methods to evaluate lubricants performance. In this study, different types of lubricants were evaluated for different sheet materials using CDT and TCT. In CDT, the perimeters of the drawn cups were used to evaluate lubricants performance. TCT factors were used in TCT to compare the performance of the lubricants. Finite element (FE) simulations were used to predict the temperature distribution in the sheet during both CDT and TCT. According to simulation results, temperature generated at the tool-sheet interface in TCT do not reflect the conditions exists in practical stamping operations. Additionally, TCT results show larger scatter. However, easiness in the application and short process time, make TCT useful to compare the performance of various lubricant additives. ii

4 DEDICATION To my parents, Akın and Sevtap Peker, And my sister Irmak Peker iii

5 ACKNOWLEDGEMENTS I would like to express my deepest gratitude to my advisor, Dr. Taylan Altan for providing me an opportunity to be a part of Center for Precision Forming (CPF) and his exceptional guidance during my studies. I extend my sincere thanks to Dr. Jerald Brevick for serving on the committee and for his continuous support through my Master s program. I am grateful to Quaker Chemical, Roy Miller, Vincenzo Disantis and Lilly Kull for their support for this research. I also extend my thanks to Dr. Hyunok Kim for his valuable inputs to this study. Sincere thanks are extended to all the students and visiting scholars at CPF for their support during my graduate studies. In particular, I would like to mention Suraj Appachu, David Diaz Infante, Zeming Yin and Berk Aykas. My special thanks to Ali Fallahiarezoodar whose advice guided me through my graduate school work. Above all, I would like to thank my family, and friends, especially Güçhan Özbilgin, without their support, I could not have done it. iv

6 VITA B.S. Manufacturing Engineering, Istanbul Technical University, Turkey 2014 to present.....graduate Research Associate, Center for Precision Forming (CPF), The Ohio State University Columbus, Ohio, USA FIELD OF STUDY Major Field: Industrial and Systems Engineering v

7 TABLE OF CONTENTS ABSTRACT... ii DEDICATION... iii ACKNOWLEDGEMENTS... iv VITA... v TABLE OF CONTENTS... vi LIST OF TABLES... ix LIST OF FIGURES... x CHAPTER 1: INTRODUCTION Lubrication in Sheet Metal Forming Parameters Affecting the Sheet Metal Forming Process [Altan, and Tekkaya 2012] Blank Properties Tool Properties Lubrication / Friction Forming Machines and Cushions... 3 CHAPTER 2: OBJECTIVES AND METHODOLOGY Objectives Methodology... 4 CHAPTER 3: LITERATURE REVIEW Friction Models and Lubrication Mechanisms in Metal Forming Processes Friction Models in Metal Forming Primary Lubrication Mechanisms in Metal Forming... 8 vi

8 3.1.3 Secondary Lubrication Mechanisms in Metal Forming Surface Roughness of the Materials and Tool Wear Mechanisms Surface Roughness of the Materials Wear Mechanisms in Sheet Metal Forming Types of Lubricants Used in Sheet Metal Forming [Altan, and Tekkaya 2012] Test Methods to Evaluate Lubricants Draw Bead Test Strip Drawing Test Twist Compression Test Cup Drawing Test CHAPTER 4: CUP DRAWING TEST Principles of Cup Drawing Test Evaluation of the Lubrication Performance for Different Lubricants Experiments: Day One Experiments: Day Two Conclusions Process Simulation Using Finite Element Methods Simulation Results Prediction of Average Speed and Pressures during CDT, to be used to Emulate the Same Conditions at TCT Conclusions CHAPTER 5: TWIST COMPRESSION TEST Principle of Twist Compression Test Preliminary Experimental TCT Results Experimental TCT Results Evaluation of Experimental TCT Results Conclusions Process Simulation Using Finite Element Methods Conclusions vii

9 5.5 Surface Roughness Measurements CHAPTER 6: COMPARISON OF CDT AND TCT AND OVERALL CONCLUSIONS CHAPTER 7: SUMMARY REFERENCES APPENDIX A Experimental Results of Preliminary TCTs APPENDIX B Experimental Results of TCTs APPENDIX C Validation of TCT Model by using Literature [Kim 2008] viii

10 LIST OF TABLES Table 4.1: Lubricant name and symbols Table 4.2 Experimental matrix and results obtained for seven lubricants tested in day one Table 4.3 Experimental matrix and results obtained for seven lubricants tested in day two Table 4.4 Experimental matrix and results obtained for three lubricants at high blank holder force (BHF) tested in day two Table 4.5 Lubricant Rating Table 5.1 Experimental Matrix for Preliminary TCT Experiments Table 5.2 Experimental Matrix for TCT Experiments (conducted by Quaker Chemical) Table 5.3 Detailed Experimental Matrix for TCT Experiments (conducted by Quaker Chemical) Table 5.4 Inputs for TCT Simulations (COF was obtained from TCTs) ix

11 LIST OF FIGURES Figure 1.1 Factors that affect friction and lubrication in sheet metal forming [Altan, and Tekkaya 2012]... 2 Figure 3.1 Relationship between contact pressure and frictional shear stress. [Altan, and Tekkaya, 2012]... 6 Figure 3.2 Stribeck curve showing onset of various lubrication mechanisms, η, lubricant viscosity; v, sliding velocity; p, normal pressure; μ, coefficient of friction. [Kim, 2008]... 9 Figure 3.3: Schematic of Draw Bead Test [Keeler 2008] Figure 3.4: Schematic of Strip Draw Test [Hol 2016] Figure 4.1: Schematic of cup drawing Figure 4.2: Deep Drawing Tooling (used by CPF) Figure 4.3: Schematic of tooling geometry of cup drawing simulation [Kim, 2008]18 Figure 4.4: Average flange perimeter in cup drawing test when using DP590 for different lubricants. One of the specimens were fractured when using lubricant E. Lubricants C, F, and G all failed to form any samples Figure 4.5: Average flange perimeter in cup drawing test when using DP980 for different lubricants. Two specimens were fractured when using lubricant E. Lubricants C, D, F, and G all failed to form any samples Figure 4.6: Average flange perimeter in cup drawing test when using DP980 for different lubricants tasted at day two. Lubricant I failed to form any samples. Lubricant A was used on day during set-up and had approximately mm flange perimeter, but it was not recorded Figure 4.7: Average flange perimeter in cup drawing test when using DP980 to evaluate the performance of lubricant H and J at higher blank holder force. x

12 Only one test for lubricant A and 3 tests for lubricants H and J were performed Figure 4.8 Resulting geometry of the model built by DEFORM 2D Figure 4.9: DP590 Flow Stress Curve Obtained From Tensile and Bulge Tests Figure 4.10: Speed vs Stroke curve used in Simulation to represent mechanical press characteristic (15spm) Figure 4.11: Location of the Calculated Maximum and Minimum Temperatures that Occurs in the Blank (DP590 (1.4mm), 30mm/s punch speed (hydraulic press), COF=0.1) Figure 4.12: Increase in maximum temperature on the blank due to deformation and friction (DP590 (1.4mm), 30mm/s punch speed (hydraulic press)) Figure 4.13: Increase in maximum temperature in the blank due to deformation and friction for different punch speeds (DP590 (1.4mm), at 80mm stroke, 30mm/s and 60mm/s hydraulic press, 15spm mechanical press) Figure 4.14: Effect of the punch speed on increase in temperature due to deformation (DP590 (1.4mm), 30mm/s and 60mm/s hydraulic press, 15spm mechanical press) Figure 4.15: Effect of the punch speed on increase in max. temperature due to friction only (COF=0.1, DP590 (1.4mm), 30mm/s and 60mm/s hydraulic press, 15spm mechanical press) Figure 4.16: Effect of the material on temperature increase in CDT Figure 4.17: Average speed and pressure at the die corner area (DP590, t=1.4mm, 30mm/sec punch speed) Figure 5.1: Schematic of TCT [Kim, 2008] Figure 5.2: TCT Tool Insert (Dimensions are in mm) [Kim, 2008] Figure 5.3: Example COF - Time curves obtained from TCT for DP590 (t=1.4mm) with lubricant B xi

13 Figure 5.4: Example COF - Time curves obtained from TCT for DP590 (t=1.4mm), lubricant B, 10ksi, 10rpm and 1g/m 2 coating weight Figure 5.5: Example COF vs Time curve Figure 5.6: Experimental TCT Results for 10ksi Tool Pressure Figure 5.7: Experimental TCT Results for 20ksi Tool Pressure Figure 5.8: Experimental TCT Results for 30ksi Tool Pressure Figure 5.9: Experimental TCT Results for 30rpm Tool Speed Figure 5.10: Experimental TCT Results for Different Specimen Materials (10ksi Tool Pressure and 10rpm Speed) Figure 5.11: TCT Simulation Model [Kim 2008] Figure 5.12: TCT Simulation Input: DP590 Flow Stress Curve Obtained From Tensile and Bulge Tests Figure 5.13: Temperature Distribution in the Specimen (See figure 5.11) (10rpm, 10ksi, 1g/m 2, the longest run (Case A), room temperature 20⁰C) Figure 5.14: Increase in Maximum Temperature in TCT Specimens (DP590 (t=1.4mm), lubricant B, 10rpm, 10ksi) Figure 5.15: Increase in Maximum Temperature in TCT Specimens (DP590 (t=1.4mm), lubricant B, 30rpm, 30ksi) Figure 5.16: Increase in Maximum Temperature in TCT Specimens (Fully flooded, longest run) Figure 5.16: Surface Roughness Measurement Device (SURFTEST SV-3100) Figure 5.17 Comparison of Surface Roughness of materials Figure 6.1 Example of TCT Results (DP590, 20ksi, 20rpm, lubricant B, 2g/m²) xii

14 CHAPTER 1: INTRODUCTION 1.1 Lubrication in Sheet Metal Forming Lubrication plays an important role in sheet metal forming processes as it reduces the friction at the tool-workpiece interface, thus enhancing the ability to produce a good part integrity. With good lubrication, in addition to the reduction in die wear, failures associated with wrinkling, fracture, and strain concentration can be reduced or avoided. There are different lubricant evaluation methods to compare lubricant performance such as Draw Bead Test, Strip Drawing Test, Twist Compression Test (TCT), Cup Drawing Test (CDT). However, not all of them can emulate the actual stamping conditions. The cup drawing test (CDT) was successfully used for evaluation lubricant by various European manufacturers. [Pfestorf, 2002, Tolazzi et al. 2004, and Wagner et al. 2002]. Cup draw test (CDT) emulates the conditions that exist in practical deep drawing and stamping operations. It is widely used to evaluate and compare the performance of lubricants under stamping conditions. On the other hand, twist compression test (TCT) is being increasingly used by lubricant suppliers/developers because of the short processing time and convenience to evaluate and compare the performance of lubricants and additives. 1

15 1.2 Parameters Affecting the Sheet Metal Forming Process [Altan, and Tekkaya 2012] A sheet metal forming operation can be considered as a system is composed of all the input variables. Blank and tooling properties, tool-workpiece interface conditions, plastic deformation mechanics, final product characteristics, and forming environment affect this system. Figure 1.1 Factors that affect friction and lubrication in sheet metal forming [Altan, and Tekkaya 2012] 2

16 1.2.1 Blank Properties Mechanical properties of the blank such as flow stress curve of the material, anisotropy, and surface finish and coating (if any) affect the quality of the final part. Additionally, the dimensions of the initial blank will affects the final geometry Tool Properties Different types of deformation modes and forces can be observed during the forming and drawing operations. Other than the tool material, and coating, punch and die corner radii, clearance affect the sheet forming operation and part quality Lubrication / Friction Since lubricant affects the frictional forces at the tool-workpiece interface, material flow during the forming and drawing operations depends on the lubrication. Good lubrication conditions would lead to lower scrap rate and better quality Forming Machines and Cushions Forming speed might affect the quality of the formed part because it affects the temperature conditions during the forming operation. Individually programmable cushions will provide a locally controlled material flow during the sheet metal forming operation. This flexibility in the blank holder force can be adjusted according to the incoming sheet material properties and thicknesses. 3

17 CHAPTER 2: OBJECTIVES AND METHODOLOGY 2.1 Objectives The overall research objective of the proposed study is to provide general information about Cup Drawing Test (CDT) and Twist Compression Test (TCT) as methods to evaluate lubricants. The specific objectives are to: Estimate and compare temperature increase at the sheet/tool interface in both CDT and TCT. Estimate the temperatures and pressures that occur in both CDT and TCT. Determine the limitations of CDT and TCT and investigate how TCT can be used to evaluate lubricants and additives. 2.2 Methodology The following approach was used to evaluate lubrication test methods. 1. Cup Drawing Test (CDT) was conducted for the selected material and lubricant 2. Twist Compression Tests (TCT) were conducted with different lubricants and for selected test conditions 3. FE simulations were used to predict the temperatures during CDT and TCT 4

18 CHAPTER 3: LITERATURE REVIEW 3.1 Friction Models and Lubrication Mechanisms in Metal Forming Processes Friction Models in Metal Forming Coulomb s friction model and Shear friction model are two commonly used models to describe the frictional condition in the metal forming process. According to Coulomb s friction model coefficient of friction (COF) is calculated by using formula below (Equation 3.1) with following parameters: τ f = frictional shear stress μ = coefficient of friction ρ = normal pressure τ f = μ ρ (3.1) Interface pressure can reach a multiple of the yield stress of the yield stress of the material. Since, the shear stress, τ f, cannot exceed the shear strength, k, of the deformed material that is normally the workpiece Coulomb s model is not valid at high contact pressure levels. When μρ exceeds the shear strength, τ f, the coefficient of friction becomes meaningless. 5

19 The shear friction model was proposed by Orowan to avoid the limitation of Coulomb s model [Orawan, 1943]. The frictional shear stress, τ f, is proportional to the normal pressure at low pressure like Coulomb s model, but it equals to the shear strength, k, at high pressure and it is calculated by using formula below (Equation 3.2) with following parameters: f = friction factor m = shear factor k = shear strength σ = flow stress τ f = f σ =m σ = mk (0 m 1) (3.2) 3 Figure 3.1 Relationship between contact pressure and frictional shear stress. [Altan, and Tekkaya, 2012] 6

20 Wanheim and Bay [Wanheim et al. 1974, and Bay et al. 1975] proposed a modified shear friction model, to consider the effect of real contact area ratio on friction. As shown in Equation 3.3, the friction shear stress, τ f, is a function of the real contact area ratio in this model, with following parameters: f = modified frictional factor m = modified shear factor σ = flow stress a = real contact ratio = real contact area/apparent contact area (Ar/Aa) τ f = f ak = m σ 3 (3.3) Another complex model to take lubricant effect into account was proposed for boundary and mixed lubrication regimes by Bowden, and Tabor (1967), as shown in Equation 3.4, where, a = real contact ratio, τ a = average shear stress at contacting asperity peaks τ b = average shear stress (lower stress) at the lubricant pockets τ f = a τ a + (1- a) τ b (3.4) 7

21 Although this approach is the most detailed expression of the lubricant behavior, it is hard to be used in forming simulations. More models to represent lubrication conditions in sheet metal forming are proposed by different studies. [Lee et al. 2002, Wang et al. 2014, and Hol et al. 2015] Primary Lubrication Mechanisms in Metal Forming Different lubrication mechanisms may be seen in sheet metal forming, dry condition, boundary condition, mixed-film lubrication, and hydrodynamic lubrication. Dry condition means no lubrication at the mating surfaces, this lead high friction. The dry condition is often used when the frictional conditions do not have a significant effect on the part quality or when the material is formable enough to form without lubricants. The most commonly encountered lubrication condition is the boundary condition which is defined as a condition where the solid surfaces are so close that surface interaction between single or multimolecular films of lubricants and the solid asperities dominated the contact. [Bhushan, 2002] Another lubricant condition is the mixed-layer lubrication. The micro peaks of the metal surface become filled with lubricant in this case. 8

22 Hydrodynamic lubrication is observed during high-speed sheet rolling operations where hydrodynamic conditions occur at the material-tool interface. The Stribeck curve illustrates lubrication mechanisms as shown in the figure below (Figure 3.2) as a function of lubricant viscosity, η, sliding velocity, ν, and normal pressure, ρ. [Schey, 1983] Figure 3.2 Stribeck curve showing onset of various lubrication mechanisms, η, lubricant viscosity; v, sliding velocity; p, normal pressure; μ, coefficient of friction. [Kim, 2008] 9

23 3.1.3 Secondary Lubrication Mechanisms in Metal Forming Two different types of secondary lubrication mechanisms may be observed when the trapped lubricant permeates to the real contact surface; Micro-Plasto-Hydro-Static Lubrication (MPHSL) and Micro-Plasto-Hydro-Dynamic Lubrication (MPHDL). A thin film occurs between the surface asperities and the die where boundary lubrication previously dominated when lubricant escapes from the valleys of the surface. This leads reduced frictional stress. Many researchers provide experimental evidence of MPHSL and MPHDL. [Azushima et al. 1991, Bech et al. 1998, Le, and Sutcliffe, 2003]. To enhance the MPHSL and MPHDL mechanisms by optimizing textured sheet may help to reduce surface friction. [Pfestorf et al. 1998] 3.2 Surface Roughness of the Materials and Tool Wear Mechanisms Surface Roughness of the Materials Surface roughness describes the deviations in the normal vector direction of a real surface from its ideal form. If these deviations are large, that surface named rough, and if they are small surface named smooth. [Bhushan 2001] Lee et al. (2002), proposed a friction model which considers lubricant velocity and surface roughness. According to this study, the coefficient of friction becomes high for the extremely high and low surface roughness conditions. 10

24 Surface roughness is generally expressed as an average value Ra which is an arithmetic average value of the deviation of the trace above and below the center line [Bhushan 2001]. However, Ra does not differentiate between valleys and peaks. Rz which defines as the maximum height profile and Rq which measures the root mean square deviation of the profile are alternative ways of measuring of roughness Wear Mechanisms in Sheet Metal Forming Gradual material removal from contacting surfaces named wear. Wear occurs in sheet metal forming in two basic forms, adhesive and abrasive. Adhesive Wear Adhesive wear is the most critical wear type in sheet metal forming. Particles are removed from one surface attached to the other surface temporarily or permanently during adhesive wear. Adhesive wear shows itself in different forms during different forming operations such as scoring, powdering and galling. According to Kim et al. (2008), higher contact pressure and temperatures during forming of galvanized Advance High Strength Steels (AHSS) may lead to lubricant breakdown and powdering or galling. To reduce the adhesive wear, more lubricant may be applied or chemical composition of one material might be changed to lower the adhesion between sheet and tool surfaces. 11

25 Abrasive Wear When one hard surface removes/cuts material from the other surface it is called abrasive wear. In sheet metal forming abrasive wear does not occur often. Often, abrasive wear caused by adhesive wear which may generate abrasive particles. 3.3 Types of Lubricants Used in Sheet Metal Forming [Altan, and Tekkaya 2012] Stamping lubricants are either water or petroleum based, and by varying the additive content can have widely different properties and performance qualities depending upon the process. There are several parameters should be considered while selecting lubricants for sheet metal forming operations such as forming operation type, lubricant application methods, types of additives, corrosion control, cleanliness and removal methods, compatibility with pre-lubricants, the effect on additional forming operations, environmental safety, and recycling. Lubricants can be divided into following categories; oils, soluble oils, semisynthetic, synthetics, and dry-film lubricants. Petroleum-based oils are commonly used for light-duty stamping operations. Various types of additives are used to enhance the lubricant performance. Extreme pressure additives are widely used in heavy-duty metal stamping operations to prevent metal to metal contact during stamping. Lubricants with extreme temperature additives stick to the hot workpiece and create a friction reducing barrier while lubricant without extreme temperature additives may become thinner and burn. 12

26 3.4 Test Methods to Evaluate Lubricants Draw Bead Test In this test, a lubricated sheet specimen pulls through the draw bead dies and pulling forces are measuring during the test [Figure 3.3]. Velocity and blank holder force can be adjusted to emulate the actual forming conditions. Since frictional forces and pulling forces change with the changes in lubricant type or amount, a number competitive analysis can be used to evaluate the lubricant performance. Figure 3.3: Schematic of Draw Bead Test [Keeler 2008] 13

27 3.4.2 Strip Drawing Test A strip drawn between two dies during this test. Several types of dies are used. The replacement of flat dies by cylindrical ones leads to higher contact pressures. The amount of stretching with the differences in the coefficient of friction [Figure 3.4]. This test can be performed at different speeds, speed profiles, die radii or material. This test was developed to test the materials with limited formability due to the difficulty to conduct deep drawing test. Figure 3.4: Schematic of Strip Draw Test [Hol 2016] Twist Compression Test Twist Compression Test (TCT) is widely used by lubricant suppliers/developers to evaluate and compare the performance of lubricants and additives. The tool is rotated under axial force and torque is calculated during the test. [Kim 2008] 14

28 3.4.4 Cup Drawing Test Cup Drawing Test (CDT) emulates the conditions that exist in practical deep drawing and stamping operations. It is widely used to evaluate and compare the performance of lubricants under stamping conditions. In CDT, the cup is drawn to a predetermined depth at a given blankholder force. The perimeter of the drawn cup is measured as an indication of lubricant performance. The CDT allows determining the average coefficient of friction (COF) [Diaz et al. 2015]. 15

29 CHAPTER 4: CUP DRAWING TEST In this part of the study, the deep drawing test, and FE Simulations are used to evaluate the performance of various stamping lubricants and predict temperature conditions during forming. 4.1 Principles of Cup Drawing Test The cup drawing process is used extensively to evaluate stamping lubricant performance. Since the most severe friction takes place at the flange area, lubrication conditions at this region influence the thinning and failure in the side wall of the drawn cup and draw-in length. Lubricants can be compared according to their maximum applicable blank holder force without failure in the cup wall. The effectiveness of lubricants are being analyzed based on the following criteria: The maximum necessary punch force, where lower force implies better lubrication, The maximum blank holder force without fracture, where higher blank holder force shows better lubrication, Thinning distribution on the circumferential length (the better lubrication, the lesser thinning), Measurement of the draw-in length and perimeter of the flange, where the shorter perimeter and the larger draw-in length means better lubrication, Visual inspection of galling (the lower galling, the better the lubricant) 16

30 Figure 4.1: Schematic of cup drawing 4.2 Evaluation of the Lubrication Performance for Different Lubricants In evaluating the lubrication performance using cup drawing test, the sheet is not drawn completely and some flange remains. The perimeter of the flange is measured for each specimen. Lower flange perimeter shows the sheet is more drawn and less stretched which represents better lubrication performance. Cup drawing tools are used in this study and the tool geometries are shown in Figure 4.2 and 4.3. The draw depth (punch stroke) is about 80mm and the punch speed is about 30mm/sec. 17

31 Figure 4.2: Deep Drawing Tooling (used by CPF) Figure 4.3: Schematic of tooling geometry of cup drawing simulation [Kim, 2008] 18

32 Cup drawing tests for two different materials (DP590 and DP980) and ten different lubricants including dry and wet lubricants are conducted to evaluate the performance of each lubricant in terms of lubrication quality and reduction of friction in forming of advanced high strength steels (AHSS). Due to a large number of specimens and lubricants, it was not possible to complete all experiments in one day. Also, all lubricants were not available at the same time. Therefore, experiments are conducted on two separate days (one week apart). Lubricant A which was tested in day one and showed the best lubrication performance, among the rest of the other lubricants except B, was initially selected as the base for the experiments (on all days). Thus, lubricant A, was tested again on day two to be sure about the same tooling set up and results were used for comparison of the other lubricants tested in this day. Table 1 shows the lubricants name and symbol which were tested on day 1 and 2. Lubricant code Table 4.1: Lubricant name and symbols Testing Type of Lubricant day A 1 and 2 Polymer Synthetic B 1 Polymer Synthetic C 1 Mineral oil w/rust inhibitors D 1 E 1 F 1 Polymer Water based G 1 Mineral oil w/ extreme pressure additives H 2 Mineral oil w/ extreme pressure additives I 2 Dry film, higher hardness hot melt J 2 Mineral oil w/ extreme paraffinic pressure additives 19

33 In order to reduce the effect of applying different amounts of lubricant and different amounts of spread on test results, all lubricants except lubricant I (dry lubricant) were applied by one person in the same way. Lubricant I was applied to the sheet metal by the provider company. All wet lubricants were applied using a #0 draw down bar and 3 drops of lubricant per sheet side. This method has been shown in past work to give ~+/- 0.3 g/m 2 tolerance, however, the coating weights were not measured or recorded for the current study. The following is the method used for applying the wet lubricant described step by step: 1) Fiber Glass Plate was placed on top of the work bench and cleaned using acetone 2) The draw down bar (#0) was cleaned using acetone after putting on clean gloves 3) Sheet samples of the required material were then cleaned (wiped off) of debris using a paper towel 4) The sheet sample was placed on the fiberglass plate and three drops of the required lubricant were applied on one side 5) The lubricant was then spread over the entire surface of the sample using a #0 draw down bar 6) The sample was then turned over and the same was repeated for the other side 7) The sample was then placed in a cleaned cup draw tooling (Between different lubricants the die, blankholder, and punch are cleaned with acetone. Between samples of the same lubricant the tool surfaces are just wiped with paper towels.) 20

34 8) For each lubricant the gloves were changed and work bench/glass plate and draw down bar were cleaned using acetone. 9) Aforementioned steps (3 through 8) were then repeated Experiments: Day One For DP590 and DP980 sheet materials, 5 or 6 specimens are tested for each lubricant, unless three failures (fracture) in a row occur. A 650 KN blank holder force was applied for DP590 and 600 KN for DP980. After the test, if the cup was formed successfully, the perimeter of the flange is measured. For lubricants C, F, and G, none of the specimens could be formed successfully. Therefore, these three lubricants are eliminated from the study. For lubricant D, only two samples (out of 6) were formed using DP590 and none of the DP980 specimens could be formed successfully. This shows that lubricant D also cannot provide a good lubrication performance during the forming of AHSS using these testing conditions. Failure occurs in one of the six DP590 specimens when using lubricant E while two of the DP980 specimen cracked using this lubricant. As is shown in Figure 4.4 (for DP590) and Figure 4.5 (for DP980) for both materials, lubricant B provides less flange perimeter compared to other lubricants followed by lubricant A. Since cracking occurs in some specimen except when using lubricants A and B, it can be concluded that they have the best lubrication performance in forming of AHSS compare to the other five lubricants tested in the day one conditions. Table 4.2 shows the experiment matrix and test results obtained in day one 21

35 Table 4.2 Experimental matrix and results obtained for seven lubricants tested in day one. Lubricant symbol A (day one) B C D E F G Material Blank holder force (KN) Number Number of of successfully specimen tested formed cup Average of flange perimeter (mm) Notes / Comments DP Thick lube, easily spread DP Part gets hot Part Temperature DP is noticeably high with each test DP DP DP DP DP Part gets very Hot 2.Lube forms beads and doesn t spread out very well 1. Lube hard to spread out 2.Part gets very hot DP Lube hard to spread out 2.Part gets very hot DP Part gets very hot DP Lube spreads but beads up DP DP Lube spreads well DP

36 Figure 4.4: Average flange perimeter in cup drawing test when using DP590 for different lubricants. One of the specimens were fractured when using lubricant E. Lubricants C, F, and G all failed to form any samples. Figure 4.5: Average flange perimeter in cup drawing test when using DP980 for different lubricants. Two specimens were fractured when using lubricant E. Lubricants C, D, F, and G all failed to form any samples. 23

37 4.2.2 Experiments: Day Two In the second day of the experiment, lubricant A selected as the base, was tested again to compare the tool set up and testing conditions with the first day. Cup drawing test with several different blankholder forces were conducted using both materials (DP590 and DP980) to find the force that provides similar flange perimeter as day one. Due to the lack of the specimens and time, for DP980, only one sample was tested for each blank holder force. The selected force was used for the rest of the experiments. 700KN blank holder force was selected for DP590 and 600KN for DP980.For lubricant I which is the dry lubricant, contrary to what was expected, none of the specimens could form successfully. Therefore, several lower blank holder forces were tested to check the possibility of forming specimens with this lubricant. However, only for DP980 with 500KN blank holder force one of the two tested specimens can be formed successfully when using lubricant I. This shows, at least for these experimental conditions, lubricant I is not appropriate for forming AHSS. 24

38 Table 4.3 Experimental matrix and results obtained for seven lubricants tested in Lubricant symbol J H I Material Blank holder force (KN) day two Number of Number of specimens successfully tested formed cup Average of flange perimeter (mm) DP DP DP DP DP590 DP Notes/ Comments Thick and Spreads well and is ok to wipe Very Viscous and Does not spread Parts get well extremely Figure 4.6 shows the results for DP980. As mentioned before, lubricant I did not provide appropriate lubrication performance and was eliminated from the study. As shown in this figure, for DP980, lubricant H provided lower flange perimeters following with lubricant J. Based on the results, both lubricants H and J provided better lubrication conditions compared to lubricant A (which had a perimeter ~ mm during set-up, but was not recorded). The measured average perimeters for lubricant H is lower than lubricant B (results from day one), which showed the lowest average flange perimeter in day one. The average flange perimeters are almost similar when using lubricant J or B. 25

39 Figure 4.6: Average flange perimeter in cup drawing test when using DP980 for different lubricants tasted at day two. Lubricant I failed to form any samples. Lubricant A was used on day during set-up and had approximately mm flange perimeter, but it was not recorded. However, results when increasing the blank holder force to 700KN for DP590 showed that lubricant H (failed 2 of 3 tests) provides similar flange perimeter (but worse performance) to lubricant A (failed 1 of 3 tests), and lubrication performance of lubricant J (failed 2 of 3 tests) worse than these two lubricants. Extra tests were conducted using DP980 to evaluate the performance of lubricants J and H at higher blank holder force (i.e. 750KN). Table 4.4 shows the experiment matrix and Figure 4.7 shows the results of these tests. Due to the lack of specimen and time, only one test was conducted with lubricant A and three specimens were tested for each lubricant H and J. 26

40 The measured flange perimeter when using lubricant A (one test) was 719mm while the average flanges perimeter for lubricant H and J were mm and 727mm, respectively. This shows that the performance of lubricant H reduced and provides almost similar lubrication performance as lubricant A when using higher blank holder force while lubrication performance of lubricant J drops below the performance of lubricant A by increasing the blank holder force. Table 4.4 Experimental matrix and results obtained for three lubricants at high blank holder force (BHF) tested in day two. Lubricant symbol Material Blank holder force (KN) Number of specimens tested Number of successfully formed cup Average of flange perimeter (mm) A (day wo) J H DP DP DP DP DP DP

41 Figure 4.7: Average flange perimeter in cup drawing test when using DP980 to evaluate the performance of lubricant H and J at higher blank holder force. Only one test for lubricant A and 3 tests for lubricants H and J were performed Conclusions Lubricant B was seen to perform better than A, and H was slightly better than A but hard to spread out on the sheet samples. Additionally, it was observed that A produced similar flange perimeters to H (~ mm), but were not recorded on the day that H, J, and I were tested. It was noticed that part temperatures were high, especially for DP980. In large flange parts and fractured parts the temperatures were very high (hard to handle). Simulations are run to predict temperatures during the cup drawing operations (Chapter 4.1). 28

42 There was only one dry film tested (lubricant I ). Temperatures were especially high in those samples, and the temperature was concentrated in the top portion of the part (as opposed to the wall in most other lubricants). The cracks also occurred in the punch radius, as opposed to the part wall in other lubricants. I also performed very poorly, where it could only form DP590 with BHF less than 45 tons and DP980 with 50 tons. There are two hypothesis for the poor performance, 1) the lubricant lacks heat dissipation capability because it is not a wet (oil or water) lubricant, and 2) the lubricant has too high of COF in the punch radius which causes the sample to fail prematurely in the radius instead of the wall (concentrating the thinning over the punch radius). Lubricant Perimeter Rank Table 4.5 Lubricant Rating COF (calculated) Observations/Notes A 3/ (DP980) Was used as base, spread and formed well B 1/ (DP980) Spread very well, seemed to be best lubricant C - - Did not spread well, beads up, cups fractured D 5/ (DP590) Hard to spread, barely formed DP590 E 4/10 >0.1 (DP980) Hard to spread out, formed DP590, barely formed DP980 F - - Spreads, but then beads up, cups fractured G - - Spreads well, cups fractured H 2/ (DP980) Very tacky, hard to spread, cups form very well I - - Dry film, cups fractured in punch radius J 6/ (DP980) Thick, but spreads well, cups form well 29

43 4.3 Process Simulation Using Finite Element Methods For this study the model showed on Figure 4.3 was used and the resulting axisymmetric geometry built on DEFORM 2D version 11.0 can be seen in Figure 4.8. For this simulation, it was chosen that the Blank Holder, Die, and Punch would behave as rigid objects while the sheet would behave as a plastic object. Figure 4.8 Resulting geometry of the model built by DEFORM 2D D590 (t=1.4mm) is used for this simulation. Its flow curve (σ=k.ε n ) obtained from Tensile and viscous pressure bulge tests (VPB) (Strength coefficient (K) =1311MPa, Strain hardening exponent (n) = 0.27) can be seen in the Figure 4.9. Constant simulation parameters for the all simulations are as follows: Blank Holder Force = 600KN Final stroke = 83mm Blank diameter = 152.4mm E-modulus (DP590) = 180GPa 30

44 Stress (MPa) Strain Figure 4.9: DP590 Flow Stress Curve Obtained From Tensile and Bulge Tests Simulations are run for three different punch speeds (30mm/s, 60mm/s, and 15spm) and two coefficient of friction values (0.0 and 0.1) for each speed to calculate temperature increase due to friction and deformation. Mechanical press characteristics are chosen arbitrarily with 15spm angular speed and 640mm stroke (TDC to BDC) with forming from α= 40⁰; 560mm to 640mm. Ram velocity (V) shown in Figure 4.10 is calculated by using formula below (Equation 4.1) with following parameters: S= Stroke (2r) n= rotational speed α= crank angle r= crank radius = s/2 V = Sπn 60 sinα Equation (4.1) 31

45 Figure 4.10: Speed vs Stroke curve used in Simulation to represent mechanical press characteristic (15spm) Simulation Results Simulation results for COF = 0.0 are used to determine the temperature increase due to the plastic deformation only. To be able to calculate temperature increase due to friction, simulation results with COF = 0.0 and COF = 0.1 are compared (results for COF = 0.0 are subtracted from the results for COF = 0.1). To compare the simulation results, the maximum temperature that occurs on the blank during the cup drawing operation is considered as shown in Figure

46 Increase in temperature (max.) (⁰C) Punch Corner Min. Temp. 35⁰C Max. Temp. 105⁰C Die Corner Figure 4.11: Location of the Calculated Maximum and Minimum Temperatures that Occurs in the Blank (DP590 (1.4mm), 30mm/s punch speed (hydraulic press), COF=0.1) Increase in maximum temperature on the blank (near to die corner) due to deformation and friction for the punch speeds 30mm/s, 60mm/s and 15spm are shown in Figure 4.12, and Stroke (mm) 30mm/s, COF=0.1 30mm/s, COF=0.0; Temperature increase due to deformation 30mm/s; Temperature increase due to friction (Δ between two curves (COF=0.0 and 0.1) Figure 4.12: Increase in maximum temperature on the blank due to deformation and friction (DP590 (1.4mm), 30mm/s punch speed (hydraulic press)) 33

47 Increase in temperature (max.) (⁰C) COF = COF = 0.0, temperature increase due to deformation mm/s (Hydraulic press) 60mm/s (Hydraulic press) Punch Speed 15spm (Mechanical press) Temperature increase due to friction only (COF=0.1) Figure 4.13: Increase in maximum temperature in the blank due to deformation and friction for different punch speeds (DP590 (1.4mm), at 80mm stroke, 30mm/s and 60mm/s hydraulic press, 15spm mechanical press) The maximum temperature is higher at higher press speed (60mm/s vs 30mm/s) due to the shorter forming time which reduces the heat transfer. The same observation is valid for maximum temperature calculated with friction. Effect of the punch speed on the increase in temperature due to deformation can be seen in Figure 4.14 and effect of the punch speed on the increase in temperature due to friction can be seen in Figure

48 Increase in temperature (max.) (⁰C) Increase in temperature (max.) (⁰C) Stroke (mm) 60mm/s, COF=0.0; Temperature increase due to deformation (Hydraulic press) 15spm, COF=0.0; Temperature increase due to deformation (Mechanical press) 30mm/s, COF=0.0; Temperature increase due to deformation (Hydraulic press) Figure 4.14: Effect of the punch speed on increase in temperature due to deformation (DP590 (1.4mm), 30mm/s and 60mm/s hydraulic press, 15spm mechanical press) Stroke (mm) 30mm/s; Temperature increase due to friction (Hydraulic press) 60mm/s; Temperature increase due to friction (Hydraulic press) 15spm; Temperature increase due to friction (Mechanical press) Figure 4.15: Effect of the punch speed on increase in max. temperature due to friction only (COF=0.1, DP590 (1.4mm), 30mm/s and 60mm/s hydraulic press, 15spm mechanical press) 35

49 Increase in temperature (max.) (⁰C) Figure 4.16 shows the effect of material on temperature increase in forming in hydraulic press DP980 (1.5mm) DP590 (1.4mm) DP980 (1.5mm) DP590 (1.4mm) Stroke (mm) Figure 4.16: Effect of the material on temperature increase in CDT 36

50 4.3.2 Prediction of Average Speed and Pressures during CDT, to be used to Emulate the Same Conditions at TCT In general tool speed and pressures for TCT are determined according to personal experience and previous trials for a given material and lubricant. Since, speeds and pressure are changing during cup drawing, average pressure and strains at the die corner area are predicted to emulate the same conditions in TCT. In this study, average speeds and pressures are predicted for the 30mm/sec constant punch speed (hydraulic press) during forming of DP590 (t=1.4mm) which was used in CDT lubrication study previously. Average speed and pressure at the die corner area (where the temperature are maximum) are shown in Figure Figure 4.17: Average speed and pressure at the die corner area (DP590, t=1.4mm, 30mm/sec punch speed) 37

51 4.3.3 Conclusions Plastic deformation has much larger effect than friction on the increase in temperature in CDT. Most of the heat is due to plastic deformation, about 70 to 100 %. Increase in temperature due to plastic deformation for different punch speeds has a similar trend (Increase in temperature due to deformation is increasing with higher punch speed, Figure 4.14). Increase in temperature due to friction only for different punch speeds (hydraulic and mechanical presses) varies during the stroke (Figure 4.15). They may be due to numerical errors (temperatures calculated are in the range of 4-8⁰C). Since plastic deformation has a significant effect on the increase in temperature, material properties (especially the flow stress curve) affect temperature results significantly. According to simulation results, to emulate the forming conditions (speed and pressure) in CDT run for 30mm/sec punch speed, 10rpm tool speed, and 10ksi tool pressure should be used in TCT. 38

52 CHAPTER 5: TWIST COMPRESSION TEST In this study, the Twist Compression Test (TCT) and Finite Element Analysis are used to evaluate lubricant performance, investigating the effect of tool speed, tool pressure, specimen material (surface roughness) and lubricant amount on TCT results, and predicting temperatures that occur in TCT. 5.1 Principle of Twist Compression Test In TCT, a rotating annular tool is pressed against a fixed sheet specimen while the pressure and torque are being measured. Schematic of TCT is shown in Figure 5.1 and dimensions of the tooling are shown in Figure 5.2. During the test, COF is calculated by using formula below (Equation 5.1) with following parameters: T = torque (Nm) r = mean radius of the tool (mm) P = applied pressure (MPa) A = cross-sectional area of the tool (mm²) T COF = rpa (5.1) 39

53 Figure 5.1: Schematic of TCT [Kim, 2008] Figure 5.2: TCT Tool Insert (Dimensions are in mm) [Kim, 2008] 5.2 Preliminary Experimental TCT Results Preliminary TCTs have been conducted for three different specimen materials and two different lubricants, shown in Table 5.1. Each condition was repeated four times. Tool speed and pressures are selected according to the previous TCT results for Advance High Strength Steels (AHSSs) and Aluminum alloys. 40

54 According to Cup Draw Test (CDT) results, lubricant B is one of the best lubricants to form Advance High Strength Steels (AHSS), and lubricant J is the worst one among the six tested lubricants. Table 5.1 Experimental Matrix for Preliminary TCT Experiments Material Lubricant Rotational Speed Pressure DP590 Lubricant B 10rpm 45ksi (t=1.4mm) Lubricant J DP980 Lubricant B 10rpm 45ksi (t=1.5mm) Lubricant J Al5182-O Lubricant B 10rpm 5ksi (t=1.2mm) Lubricant J COF curves as function of time are calculated (Appendix A). Figure 5.3 shows an example of test results for DP590 (t=1.4mm), and lubricant B. Figure 5.3: Example COF - Time curves obtained from TCT for DP590 (t=1.4mm) with lubricant B 41

55 According to TCT results: As seen in Appendix A, the average COF for lubricant B (better lubricant, based on CDT) (0.08) is higher than average COF for lubricant J (worse lubricant, based on CDT) (0.05) for DP980 and DP590. Also, lubricant B is breaking down sooner (around 15 seconds) than lubricant J (around 25 seconds). Thus, the CDT and TCT results do not agree. Test results for Al5182-O do not show a significant difference for different lubricants (B and J). As shown in Figure 5.3, TCT results of different runs for the same conditions can be significantly different. Therefore, evaluation of lubricant by using TCT may not be accurate. 5.3 Experimental TCT Results After running preliminary TCTs, understanding the basics and the methodology to analyze the data, to be able to investigate the effect of tool speed and pressure, lubricant amount, and specimen materials, TCTs had run for 33 different test conditions according to experimental matrix below (Table 5.2) by Quaker Chemical. Each condition was repeated for at least 3 times (samples) resulting 146 total number of runs. (Table 5.3) 42

56 Table 5.2 Experimental Matrix for TCT Experiments (conducted by Quaker Materials DP590 (t=1.4mm) TWIP900 (1.2mm) CP800 (t=1.4mm) Rotational Speed (rpm) Chemical) Pressure (ksi) 10, 20, 30 10, 20, Application of lubricant 1 g/m 2 ( +/- 0.3g/m 2 ) 2 g/m 2 ( +/- 0.3g/m 2 ) fully flood 1 g/m 2, 2 g/m 2 ( +/- 0.3g/m 2 ) and fully flood 1 g/m 2, 2 g/m 2 ( +/- 0.3g/m 2 ) and fully flood COF vs time curves for all runs are calculated and can be found in Appendix B. An example curve for DP590 (t=1.4mm), and lubricant B,10ksi, 10rpm and 1g/m 2 coating weight can be seen in Figure 5.4. Figure 5.4: Example COF - Time curves obtained from TCT for DP590 (t=1.4mm), lubricant B, 10ksi, 10rpm and 1g/m 2 coating weight 43

57 Table 5.3 Detailed Experimental Matrix for TCT Experiments (conducted by Metal Substrate DP 590 TWIP 900 CP 800 Test Pressure (ksi) Tool rpm Quaker Chemical) # of Test Target Coating Weight Actual Coating Weight fully flooded fully flooded g/m g/m g/m g/m g/m2 (+/-0.3g/m2) 1.26 g/m g/m g/m g/m g/m g/m g/m g/m g/m g/m2 (+/-0.3g/m2) 2.13 g/m g/m g/m g/m g/m2 4 fully flooded fully flooded 4 1 g/m2 (+/-0.3g/m2) 0.96 g/m g/m2 (+/-0.3g/m2) 2.11 g/m2 4 fully flooded fully flooded 4 1 g/m2 (+/-0.3g/m2) 1.14 g/m2 4 2 g/m2 (+/-0.3g/m2) 2.20 g/m2 44

58 5.3.1 Evaluation of Experimental TCT Results As shown in Figure 5.5, COFb is the COF where the tool pressure reaches to desired value and stays stable. On the other hand, COFe is the COF where the lubricant has broken down. Breaking down points for each run have been determined by three researchers and then average of three were used. Figure 5.5: Example COF vs Time curve Due to the inconsistency of the TCT results, averaging the results of 4 runs provides an error. Thus, two out of four samples (the shortest and longest runs, as discussed below) are considered for each condition (pressures 10, 20, 30 ksi; speeds 10, 20, 30 rpm, lubrication 1g/m², 2g/m², fully flooded). 45

59 The duration of the tests are calculated by using the formula below (Equation 5.2) with following parameters: Time(b) = When the tool pressure reaches to desired value (sec) Time(e) = When the lubricant has broken down (sec) Run time = Time(e) Time(b) (5.2) The longest (Case A) and shortest (Case B) runs are considered for a given condition and TCT factors are calculated for Case A and B according to formula below (Equation 5.3) with following parameters (this is the common procedure to evaluate TCT results): COFb = COF where the tool pressure reaches to desired value and stays stable COFe = COF where the lubricant has broken down TCT Factor= (Time(e) Time (b)) / ((0.2 x COFb) + (0.8 x COFe)) (5.3) An average TCT factor is calculated by using formula below (Equation 5.4) for each condition where the higher TCT factor implies better lubrication Average TCT Factor = TCT Factor (Case A) + TCT Factor (Case B) 2 (5.4) TCT Factor vs lubricant amount for different speeds are plotted for each pressure values (Figure ). TCT Factor vs lubricant amount and different materials are plotted (Figure 5.10) 46

60 Average TCT Factor Average TCT Factor Figure 5.6: Experimental TCT Results for 10ksi Tool Pressure Figure 5.7: Experimental TCT Results for 20ksi Tool Pressure 47

61 Average TCT Factor Average TCT Factor Figure 5.8: Experimental TCT Results for 30ksi Tool Pressure Figure 5.9: Experimental TCT Results for 30rpm Tool Speed 48

62 Average TCT Factor Figure 5.10: Experimental TCT Results for Different Specimen Materials (10ksi Tool Pressure and 10rpm Speed) Conclusions TCT factor decreases (decreased lubricant performance) with increasing tool speed and increasing tool pressure. Increase in lubricant amount increases the TCT Factor which means better lubricant performance, TCT Factors for TWIP900 are higher than TCT Factors for DP590 and CP800 for investigated cases (10rpm and 10ksi). The difference is most probably due to the difference in surface finish or coating of the steels. 49

63 5.4 Process Simulation Using Finite Element Methods Since it is difficult to measure the temperature at the tool-specimen interface during the experiment, FE simulation of TCT was conducted to estimate the interface temperature using the commercial FEM software DEFORM 2-D version The FE model of TCT is shown in Figure To validate the accuracy of the simulation model, a simulation was run by using same conditions that used by Kim (2008). Simulation results of the new model were compared with provided experimental measurements and simulation results (Appendix C) The flow stress of the sheet material DP590 was obtained using the tensile and viscous pressure bulge test (Figure 5.12) [Gutscher et al. 2004]. COF vs time curves obtained from experiments were used as an input for TCT simulations to predict the temperature and compare with temperature results of CDT simulations. Curves obtained from the longest and shortest runs (Case A and Case B) were used to predict temperatures, by using speed and pressure values that are as close as possible to values obtained by CDT simulations (TCT values; 10ksi pressure and 10RPM speed appears to be closest to conditions estimated in CDT (the pressure and speed around the die corner area)). 50

64 Figure 5.11: TCT Simulation Model [Kim 2008] Table 5.4 Inputs for TCT Simulations (COF was obtained from TCTs) Workpiece Tool Workpiece Holder Material DP590 D2 D2 Speed Contact pressure Coefficient of friction 10rpm and 30rpm 10ksi and 30ksi f(time) Thermal Conductivity 50 (W/m⁰K) [Kim et al. 2008] Heat transfer coefficient Between workpiece and tool and workpiece holder 11 (W/m⁰K) [Kim et al. 2008] Initial temperature 20⁰C 20⁰C 20⁰C 51

65 Stress (MPa) Strain Figure 5.12: TCT Simulation Input: DP590 Flow Stress Curve Obtained From Tensile and Bulge Tests Temperatures that occur at the tool/sheet interface were estimated. An example of the heat distribution on the specimen is shown in Figure The increase in maximum temperatures that occur in the specimens are plotted as shown in Figure Figure 5.13: Temperature Distribution in the Specimen (See figure 5.11) (10rpm, 10ksi, 1g/m 2, the longest run (Case A), room temperature 20⁰C) 52

66 Increase in temperature (max.) (⁰C) g/m2, the longest run 1g/m2, the shorthest run 2g/m2, the longest run 2g/m2, the shorthest run Fully flood, the longest run Time (s) Fully flood, the shorthest run Figure 5.14: Increase in Maximum Temperature in TCT Specimens (DP590 (t=1.4mm), lubricant B, 10rpm, 10ksi) TCT curves obtained from the cases with the most severe conditions (30ksi pressure and 30RPM speed) were also used to predict the temperature. TCT results of 3 lubricant amounts were used for the simulations (1g/m², 2g/m² and fully flooded). The increase in maximum temperatures occur in the specimens plotted as shown in Figure

67 Increase in temperature (max.) (⁰C) Time (s) 1g/m2, the longest run 1g/m2, the shorthest run 2g/m2, the longest runt 2g/m2, the shorthest run Fully flood, the longest run Fully flood, the shorthest run Figure 5.15: Increase in Maximum Temperature in TCT Specimens (DP590 (t=1.4mm), lubricant B, 30rpm, 30ksi) Simulation results for investigated tool pressure and speed values (10ksi and 10rpm, and 30ksi and 30rpm) are compared to show the effect of the tool pressure and speed on temperature increase in the specimen (Figure 5.16). It seems that increasing tool speed and pressure results in an increase in temperature. 54

68 Increase in temperature (max.) (⁰C) rpm, 30psi 10rpm, 10psi Time (s) Figure 5.16: Increase in Maximum Temperature in TCT Specimens (Fully flooded, longest run) Conclusions Maximum temperature rise at the tool-sheet interface is reduced when the coating weight of the lubricant is increased from 1 and 2g/m 2 to fully flooded condition (Figure 5.14 and 5.15) For the TCT conditions that represent the similar pressure and speed as CDT experiments (run by CPF), maximum temperature rise at the tool-sheet interface is relatively small, around 5⁰C (TCT simulation results) (Figure 5.14), because TCT does not involve metal deformation. 55

69 When tool pressure and speed increase from 10ksi and 10rpm to 30ksi and 30rpm, the maximum increase in temperature at the tool-sheet interface increases from 5⁰C to 19⁰C or more (Figure 5.16). 5.5 Surface Roughness Measurements Surface roughness and lubricant viscosity have a strong interacting effect on the coefficient of friction [Ghalme et al. 2013]. Since the same lubricant (Lubricant B) was used in TCT for different materials (DP590, CP800, and TWIP900), the difference between materials due to the interactive effect of surface roughness and lubricant viscosity depends on surface roughness only. Figure 5.16: Surface Roughness Measurement Device (SURFTEST SV-3100) 56

70 Average Surface Roughness, Ra (μm) Surface roughness measurements have been done for all materials by using a stylus profiler with 2-micron diameter tip. Since rolling directions of the materials were unknown, direction names are given randomly. Measurements have been repeated for 6 times for each direction and measurement length was 30mm (Figure 5.17) Rolling Direction (assumed) Transverse Direction (assumed) 0 DP590 (1.4mm) CP800 (1.4mm) TWIP900 (1.2mm) Materials Figure 5.17 Comparison of Surface Roughness of materials The difference in average surface roughness (Ra) between two directions is not significant for all tested materials. The error range of the average surface roughness of CP800 is higher than other materials. This might be due to the scratches on the sheet surface. Average surface roughness of TWIP900 is significantly lower than other two materials which are around 0.35μm (Ra of DP590=1.35μm; Ra of CP800=1.00μm). 57

71 CHAPTER 6: COMPARISON OF CDT AND TCT AND OVERALL CONCLUSIONS 1. Adjusting the test parameters and evaluating the machine output for TCT is difficult. Figure 6.1 Example of TCT Results (DP590, 20ksi, 20rpm, lubricant B, 2g/m²) Determining the breaking down point is not straightforward since in most of the cases the breaking down point is not clear (Figure 6.1) There are significant differences between TCT runs for the same setup conditions (specimen material, tool speed, tool pressure, and lubricant), as observed from test results (Figure 6.1). 58

72 Commonly used tool pressure (45ksi) for TCT experiments is much higher than the pressures at CDT (used by CPF, around 10ksi). This can affect the lubrication performance. In cup drawing, lubricant B shows better results than J but the TCT results show lower COF for lubricant J. Lubricant J has pressure additives, this might be the one of the reasons why lubricant J performs better at TCT where the pressures are higher. 2. The effect of plastic deformation on the temperature where in the formed part in CDT, is significantly more than the effect of friction work. In TCT, there is no plastic deformation so the temperature increase is only due to the friction. Therefore, increase in temperature in TCT (for the speed and pressure values that emulate CDT conditions) is significantly lower than in CDT (around 5⁰C in TCT and around 90⁰C in CDT) (Figure 4.12 and 5.14). 3. Using CDT is a better choice than TCT to evaluate the performance of lubricants for sheet metal forming applications. 59

73 CHAPTER 7: SUMMARY According to this study, 1. Temperatures during TCT are significantly lower than temperatures during CDT even for most severe TCT conditions (around 32⁰C in TCT and around 90⁰C in CDT ), 2. Tool pressures that used in TCT are higher than pressures occurs in CDT, 3. There is an inconsistency in TCT results (COF vs time curves for the same specimen material, tool speed, tool pressure, and lubricant vary), In summary pressure and temperature conditions found in TCT do not reflect the sheet-tool interface conditions that exist in practical stamping conditions. Therefore, to evaluate the performance of various lubricants for stamping applications, TCT has limited value. However, TCT is fast and easy to conduct. Thus, it is useful in comparing the performance of various additives in lubricants. 60

74 REFERENCES [Altan, and Tekkaya 2012] Altan, T., and Tekkaya, E. (2012). Sheet Metal Forming Fundamentals and Applications, Vol. 1 Ch. 7 Friction and Lubrication, ASM International. [Azushima et al. 1991] Azushima, A., Uda, M., and Kudo, H. (1991). An Interpretation of the Speed Dependence of the Coefficient of Friction under the Micro-PHL Condition in Sheet Drawing. Annals of CIRP, 40, [Bay et al. 1975] Bay, N., Wanheim, T. and Petersen, A.S., (1975). Ra and the average effective strain of surface asperities deformed in metal forming processes. Wear, 34, [Bech et al. 1998] Bech, J., Bay, N., and Eriksen, M. (1998). A Study of Mechanism of Liquid Lubrication in Metal Forming, Annals of the CIRP, 47, [Bhushan 2001] Bhushan, B. (2001). Surface roughness analysis and measurement techniques. Modern tribology handbook, 1, [Bhushan 2002] Bhushan, B. (2002). Introduction to Tribology. John Wiley and Sons. [Bowden, and Tabor, 1967] Bowden, F.P., and Tabor, D. (1967). Friction and Lubrication, Methuen & Co. LTD, London, U.K. [Diaz et al. 2015] Diaz, D., Fallahiarezoodar, A., Appachu, S., Altan T. (2015). CPF-2.3_14_01: Evaluation of the lubrication performance and effect of press speed upon lubrication in forming AHSS, The Ohio State University. 61

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78 APPENDIX A Experimental Results of Preliminary TCTs DP 590 (1.4mm), lubricant B, 45ksi tool pressure, 10rpm tool speed, fully flooded DP 590 (1.4mm), lubricant J, 45ksi tool pressure, 10rpm tool speed, fully flooded 65

79 DP 980 (1.5mm), lubricant B, 45ksi tool pressure, 10rpm tool speed, fully flooded DP 980 (1.5mm), lubricant J, 45ksi tool pressure, 10rpm tool speed, fully flooded 66

80 Al 5182-O (1.2mm), lubricant B and J, 5ksi tool pressure, 10rpm tool speed, fully flooded 67

81 APPENDIX B Experimental Results of TCTs DP 590 (1.4mm), lubricant B, 10ksi tool pressure, 10rpm tool speed, 1g/m2 coating weight: DP 590 (1.4mm), lubricant B, 20ksi tool pressure, 10rpm tool speed, 1g/m2 coating weight: 68

82 DP 590 (1.4mm), lubricant B, 30ksi tool pressure, 10rpm tool speed, 1g/m2 coating weight: DP 590 (1.4mm), lubricant B, 10ksi tool pressure, 20rpm tool speed, 1g/m2 coating weight: 69

83 DP 590 (1.4mm), lubricant B, 20ksi tool pressure, 20rpm tool speed, 1g/m2 coating weight: DP 590 (1.4mm), lubricant B, 30ksi tool pressure, 20rpm tool speed, 1g/m2 coating weight: 70

84 DP 590 (1.4mm), lubricant B, 10ksi tool pressure, 30rpm tool speed, 1g/m2 coating weight: DP 590 (1.4mm), lubricant B, 20ksi tool pressure, 30rpm tool speed, 1g/m2 coating weight: 71

85 DP 590 (1.4mm), lubricant B, 30ksi tool pressure, 30rpm tool speed, 1g/m2 coating weight: DP 590 (1.4mm), lubricant B, 10ksi tool pressure, 10rpm tool speed, 2g/m2 coating weight: 72

86 DP 590 (1.4mm), lubricant B, 20ksi tool pressure, 10rpm tool speed, 2g/m2 coating weight: DP 590 (1.4mm), lubricant B, 30ksi tool pressure, 10rpm tool speed, 2g/m2 coating weight: 73

87 DP 590 (1.4mm), lubricant B, 10ksi tool pressure, 20rpm tool speed, 2g/m2 coating weight: DP 590 (1.4mm), lubricant B, 20ksi tool pressure, 20rpm tool speed, 2g/m2 coating weight: 74

88 DP 590 (1.4mm), lubricant B, 30ksi tool pressure, 20rpm tool speed, 2g/m2 coating weight: DP 590 (1.4mm), lubricant B, 10ksi tool pressure, 30rpm tool speed, 2g/m2 coating weight: 75

89 DP 590 (1.4mm), lubricant B, 20ksi tool pressure, 30rpm tool speed, 2g/m2 coating weight: DP 590 (1.4mm), lubricant B, 30ksi tool pressure, 30rpm tool speed, 2g/m2 coating weight: 76

90 DP 590 (1.4mm), lubricant B, 10ksi tool pressure, 10rpm tool speed, fully flooded: DP 590 (1.4mm), lubricant B, 20ksi tool pressure, 10rpm tool speed, fully flooded: 77

91 DP 590 (1.4mm), lubricant B, 30ksi tool pressure, 10rpm tool speed, fully flooded: DP 590 (1.4mm), lubricant B, 10ksi tool pressure, 20rpm tool speed, fully flooded: 78

92 DP 590 (1.4mm), lubricant B, 20ksi tool pressure, 20rpm tool speed, fully flooded: DP 590 (1.4mm), lubricant B, 30ksi tool pressure, 20rpm tool speed, fully flooded: 79

93 DP 590 (1.4mm), lubricant B, 10ksi tool pressure, 30rpm tool speed, fully flooded: DP 590 (1.4mm), lubricant B, 20ksi tool pressure, 30rpm tool speed, fully flooded: 80

94 DP 590 (1.4mm), lubricant B, 30ksi tool pressure, 30rpm tool speed, fully flooded: CP 800 (1.4mm), lubricant B, 10ksi tool pressure, 10rpm tool speed, 1g/m2 coating weight: 81

95 CP 800 (1.4mm), lubricant B, 10ksi tool pressure, 10rpm tool speed, 2g/m2 coating weight: CP 800 (1.4mm), lubricant B, 10ksi tool pressure, 10rpm tool speed, fully flooded: 82

96 TWIP 900 (1.2mm), lubricant B, 10ksi tool pressure, 10rpm tool speed, 1g/m2 coating weight: TWIP 900 (1.2mm), lubricant B, 10ksi tool pressure, 10rpm tool speed, 2g/m2 coating weight: 83

97 TWIP 900 (1.2mm), lubricant B, 10ksi tool pressure, 10rpm tool speed, fully flooded: Specimen names; we are using the following designation: Material Pressure (ksi) Speed (RPM) - Coating weight (G/m2 or F- flooded) Run # 84

98 85

99 86

100 87

101 88

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103 90

104 APPENDIX C Validation of TCT Model by using Literature [Kim 2008] The simulation was run by using same conditions with earlier work done by Kim at CPF. Simulation results are compared with experimental results to validate new simulation model. Simulation inputs are shown in following 3 figures; 91

105 Geometry of the model that used in simulations shown in following figure; 92