Factors Effecting Electromagnetic Flat Sheet Forming Using the Uniform Pressure Coil

FACTORS EFFECTING ELECTROMAGNETIC FLAT SHEET FORMING USING THE UNIFORM PRESSURE COIL

A THESIS

Presented in Partial Fulfillment of the Requirements for

the Degree of Master of Science in the

Graduate School of The Ohio State University

Kristin E. Banik, B.S.

*****

The Ohio State University 2008

Master’s Examination Committee: Approved by Professor Glenn S. Daehn, Adviser

Professor Kathy Flores ______Adviser Graduate Program in Materials Science and Engineering

ABSTRACT

Electromagnetic forming is a possible alternative to sheet metal stamping. There are multiple limitations to the incumbent stamping methods including: complex alignment, changes to component shapes, and ductility issues, which often limits available formed geometry. Electromagnetic forming allows for the avoidance of some of these issues, but introduces a few other issues.

In this thesis, the issues with electromagnetic forming will be discussed in conjunction with the application of the uniform pressure coil. Also, the effects on properties of the electromagnetically formed samples in comparison to the traditional samples will be presented. These properties include hardness, formability and interface issues. Lastly, discussed in this paper is the implementation of the Photon

Doppler Velocimetry (PDV) system, a velocity measurement system used to determine the velocity of the workpiece and compare it to physics-based models of the process.

Dedicated to my parents.

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ACKNOWLEDGMENTS

I want to thank my adviser, Dr. Glenn Daehn for his support and guidance throughout graduate school and in running experiments and writing my thesis. I would also like to thank John Bradley, Steve Hatkevich and Allen Jones for their support of the work in our group as well as electromagnetic forming.

I thank all my group members, for their support and help and especially Geoff Taber for his aid in velocity measurement and experimental setup.

Lastly, I want to thank my family for their support through all the years. I could not have made it this far without the support of my parents, my brother and my sisters.

The research was supported in part by General Motors, American Trim and the Third

Frontier Grant from the State of Ohio.

VITA

January 2, 1984……………………………Born--Milwaukee, Wisconsin

2006………………………………………..B.S. Materials Science and Engineering University of Kentucky

2006-present………………………………..Graduate Research Associate The Ohio State University

FIELDS OF STUDY

Major Field: Materials Science and Engineering

TABLE OF CONTENTS

ACKNOWLEDGMENTS ...... IV

VITA...... V

LIST OF TABLES...... X

LIST OF FIGURES ...... XII

CHAPTER 1 ...... 1

CHAPTER 2 ...... 5

2.1 Traditional Sheet Metal Forming...... 5

2.2 High Velocity Forming...... 7 2.2.1 Explosive Forming...... 9 2.2.2 Electromagnetic Forming...... 10 2.2.3 Electrohydraulic Forming ...... 14

2.3 Comparison of Traditional Stamping to EM forming...... 15

2.4 Coils...... 18 2.4.1 Single Turn and Machined Coils ...... 18 2.4.2 Multiturn Coils...... 18 2.4.2.1 Uniform Pressure Coil ...... 19

2.5 Photon Doppler Velocimetry System ...... 22

CHAPTER 3 ...... 25

3.1 Traditional Stamping Simulation Setup/Pressure-pad Setup...... 25 3.1.1 Pressure-pad Comparison to Traditional Stamping ...... 27 vi

3.2 Electromagnetic Forming Experimental System ...... 28 3.2.1 Key Setup Components...... 29 3.3.2 Instrumentation Components...... 33 3.3.2.1 Vacuum and Current Measurement ...... 33 3.3.2.2 Velocity Measurement—Photon Doppler Velocimetry...... 34

3.3 Sample Analysis...... 37 3.3.1 Sample Preparation for Analysis ...... 37 3.3.2 Hardness Measurements ...... 38 3.3.3 Forming Depth Measurements...... 38

3.4 Experimental Variables...... 39 3.4.1 Discharge Energy...... 39 3.4.2 Coil Geometry...... 39 3.4.3 Vacuum...... 40 3.4.4 Standoff...... 42 3.4.5 Driver Material...... 42 3.4.6 Workpiece Material ...... 43

CHAPTER 4 ...... 44

4.1 Velocity Measurement...... 44 4.1.1 Summary...... 52

4.2 Velocity Estimates ...... 53 4.2.1 Summary...... 60

CHAPTER 5 ...... 61

5.1 Enclosure...... 62 5.1.1 Previous Generations ...... 62 5.1.2 New Generation ...... 63

5.2 Clamping Mechanism ...... 63 5.2.1 Previous Generations ...... 63 5.2.2 New Generation ...... 63

5.3 Coil...... 64 vii

5.3.1 Previous Generations ...... 64 5.3.2 New Generation ...... 66 5.3.3 Experimental and Theoretical Comparison ...... 69

5.3 Summary...... 72

CHAPTER 6 ...... 74

6.1 Hardness Effects ...... 74 6.1.1 Discharge Energy...... 74 6.1.2 Standoff...... 77 6.1.3 Comparison of pressure-pad to electromagnetic forming...... 79 6.1.4 Summary...... 82

6.2 Formability...... 83 6.2.1 Traditional versus Electromagnetically Formed...... 84 6.2.1.1 Ferritic Stainless Steel...... 84 6.2.1.2 316L Steel ...... 88 6.2.2 Coil Improvements...... 90 6.2.3 Discharge Energy...... 91 6.2.4 Lubricant...... 92 6.2.5 Summary...... 93

6.3 Interface Effects...... 93

6.4 Driver Elimination ...... 99

CHAPTER 7 ...... 101

7.1 Shearing ...... 102

7.2 Summary...... 107

CHAPTER 8 ...... 108

8.1 Critical Issues...... 108 8.1.1 Driver Elimination ...... 108 8.1.2 Vacuum Level...... 109 viii

8.1.3 Outer Channels...... 109 8.1.4 Interface ...... 110

8.2 Future Applications...... 110

8.3 Summary...... 111

CHAPTER 9 ...... 112

LIST OF REFERENCES...... 115

APPENDIX A...... 117

LIST OF TABLES

Table 2.1 High velocity explosive characteristic information...... 9

Table 2.2 Comparison of traditional stamping to electromagnetically...... 17

Table 4.1 Summary of PDV results...... 47

Table 4.2 Summary of energy and velocity data...... 51

Table 4.3 Summary of conditions for velocity experiments...... 51

Table 5.1 Summary of conditions for coil comparison...... 68

Table 5.2 Summary of conditions for coil form detph trials...... 72

Table 5.3 Summary of improvements on previous systems...... 73

Table 6.1 The discharge energy varied while holding other process parameters constant...... 76

Table 6.2 Summary of conditions for standoff studies...... 77

Table 6.3 Summary of conditions for forming studies...... 80

Table 6.4 Parameters for hardness study from pressure-pad and electromagnetically formed material with driver...... 81

Table 6.5 Pressure-pad formed materials with relative forming depth...... 84

Table 6.6 Parameters for electromagnetic forming formability study of pressure-pad versus electromagnetically formed...... 86

Table 6.7 Parameters for electromagnetic forming formability studies for ferritic stainless steel...... 87

Table 6.8 Percent full depth for ferritic stainless steel formability study...... 88

Table 6.9 Depth measurements for the different forming methods...... 90

Table 6.10 Parameters for the coil improvement experiments...... 91

Table 6.11 Forming depths for lubrication with BN studies...... 92

Table 6.12 Electromagnetic forming conditions used for the interface effects study...... 95

Table 7.1 Conditions and materials used for shearing experiments...... 106

LIST OF FIGURES

Figure 1.1 The punch is eliminated with electromagnetic forming a) traditional sheet metal forming, b) electromagnetic sheet metal forming with uniform pressure coil.... 2

Figure 2.1 Methods of high velocity forming...... 8

Figure 2.2 Schematic of explosive forming operations. a) sizing with a water filled die cavity, b) method for forming a flat panel c) use of detonation cord to prescribe the pressure distribution in an open forming system, d) use of detonation cord to form a cylinder...... 10

Figure 2.3 Schematic of electromagnetic forming...... 11

Figure 2.4 Schematic of electro-hydraulic forming...... 14

Figure 2.5 Cu-Be coil a) two separate outer channel and inner coil...... 20

Figure 2.6 Schematic of uniform pressure coil of current flow...... 21

Figure 2.7 Schematic of PDV system...... 24

Figure 3.1 Experimental pressure-pad setup for urethane pad forming. The urethane pad expands laterally during compression...... 26

Figure 3.2 Schematic comparison of traditional stamping to the pressure-pad forming...... 28

Figure 3.3 Schematic of the current electromagnetic forming setup...... 29

Figure 3.4 Standoff used in electromagnetic forming setup for uniform pressure coil, with different standoff distances...... 32

Figure 3.5 Small, subscale bipolar plate die and punch used in experimentation. .... 32 xii

Figure 3.6 Magneform 16 kJ Capacitor bank used for electromagnetic forming experiments...... 33

Figure 3.7 Schematic drawing of PDV and ring expansion system...... 36

Figure 3.8 Components for the Photon Doppler Velocimetry system...... 37

Figure 3.9 Paschen curve showing critical vacuum levels...... 41

Figure 4.1Schematic of PDV setup for use with a uniform pressure coil ...... 45

Figure 4.2 Exported data from PDV measurement system for 3.2 kJ...... 46

Figure 4.3 Exported velocity data for 1.6 kJ...... 49

Figure 4.4 Velocity profile for various distances...... 50

Figure 4.5 Schematic of die used for velocity measurement...... 52

Figure 4.6 Theoretical velocity estimates for 316L with copper driver assistance. ... 57

Figure 4.7 Theoretical and experimental velocity versus distance estimates for 316L with copper driver assistance...... 58

Figure 4.8 Theoretical velocity versus time estimates for 316L with copper driver assistance...... 59

Figure 5.1 Previous experimental setups for uniform pressure coil...... 62

Figure 5.2 Current experimental enclosure setup...... 64

Figure 5.3 The coil gap is the distance between the outer channels...... 65

Figure 5.4 a) Previous generation round lead connection (0.185 in diameter) with G-10 spacer ...... 66

xiii

Figure 5.5 The smaller coil gap increases the peak current and decreases the rise time...... 68

Figure 5.6 Theoretical velocity versus distance estimates for copper with a different coil gap...... 70

Figure 5.7 Theoretical velocity versus time estimates for copper with a different coil gap...... 71

Figure 5.8 Greater forming depth is seen in the lower coil gap...... 72

Figure 6.1 Microhardness (averaged over 20 points across channels) increases with increasing discharge energy...... 75

Figure 6.2 Percent full depth increases with increasing discharge energy...... 76

Figure 6.3 The effect of standoff on hardness of ferritic bipolar place...... 78

Figure 6.4 Percent full depth increases with increasing standoff...... 79

Figure 6.5 Increased hardness is seen with electromagnetically formed plates...... 80

Figure 6.6 Increased hardness is seen with 316L electromagnetically formed plates...... 82

Figure 6.7 A pressure-pad formed ferritic stainless steel sample with shear zones shown in white as a result of backlighting. Sample subjected to a load of 80,000 pounds or a pressure of about 35 MPa. The depth of fill is nominally 80%...... 85

Figure 6.8 Electromagnetically formed ferritic stainless steel bipolar plate. The fill depth was nominally 70%...... 86

Figure 6.9 a) Electromagnetically formed 316L b) Pressure-pad formed 316L...... 89

Figure 6.10 Cross sections of 316L for different forming methods to compare formability...... 90

Figure 6.11 Coil gap effect on formability...... 91 xiv

Figure 6.12 The interface between the driver sheet and the workpiece material can be critical...... 94

Figure 6.13 Spot welding was seen at the interface between the copper driver sheet and the workpiece material...... 95

Figure 6.14 The effects of different media used at the interface of 304L (illuminated from behind). In the no interface case the small lit spot correspond to where welding previously occurred and samples were separated...... 97

Figure 6.15 The cross sections of the materials with media at the interface...... 98

Figure 7.1 Burr can be seen in a traditional die shearing operation and no burr can be seen on the PEMS shearing...... 102

Figure 7.2 Schematic of shearing setup using the uniform pressure coil...... 103

Figure 7.3The small tensile samples used to shearing out parts exhibited less arcing during the shear operation...... 104

Figure 7.4 Current travel for each tensile bar length. The example on the left, l, is an acceptable situation and the example on the right, L, is unacceptable...... 105

Figure 7.5 Ferritic stainless steel sheared tensile bar...... 106

Figure A.1 Technical drawing for coil gap of 0.06 in...... 118

Figure A.2 Technical drawing for coil gap of 0.125 in...... 119

CHAPTER 1

INTRODUCTION

Improvements in forming flat sheets into products of usable shapes have evolved since the initial process development in the 1800’s. Currently, flat metal sheets are formed into commercial products using a die and punch assembly with traditional press forming equipment, however high velocity methods are now being introduced as an alternative to the traditional stamping method [1].

Traditional stamping mechanisms require the sheet to be formed between a die and punch. Force is applied to the punch from above and the sheet metal is formed into the die, as shown in Figure 1.1 a. There are several limitations to this method including: 1) punch and die tolerances, 2) die design change and die wear,

3) alignment of the punch and die, and 4) resistance to the sheet metal stamping due to high strain hardening rates or low modulus of elasticity [2, 3], lubrication practices and shearing.

High velocity forming uses several different methods for exerting force on the sheet stock including; explosive, electromagnetic and electro-hydraulic. Each only requires a one sided die, which significantly reduces several of the problems associated with the traditional stamping method. The high velocity forming method used in this experimental study incorporated a multi-turn coil known as a uniform pressure coil.

This method uses a high current over a short period of time such that the induced current forces the sheet metal into the die at high velocities of approximately 200 m/s depending on the discharge energy used. Figure 1.1 b shows the movement of the sheet in electromagnetic forming [1-4].

Force

a) b)

Figure 1.1 The punch is eliminated with electromagnetic forming a) traditional sheet metal forming, b) electromagnetic sheet metal forming with uniform pressure coil [2]

The studies discussed here include a comparison of the traditional stamping method to electromagnetic forming with the uniform pressure coil for flat sheet metal with a focus on the application of bipolar plates for fuel cells.

Several studies were performed to compare the traditional stamping to electromagnetic forming including hardness, formability and the effect of the interface. There are several variables in the electromagnetic forming method that were also analyzed including: 1) discharge energy, 2) coil geometry, 3) vacuum level, 4) standoff (the distance the sheet metal has to move), and 5) workpiece material. Shearing with the uniform pressure coil, was also evaluated. Lastly, a system using a Photon Doppler Velocimetry setup that utilized a laser and the resulting Doppler shift was utilized to calculate the velocity of the sheet as it was moving into the die.

The following chapters will discuss the applications of the uniform pressure coil and the results for each application, which include forming and shearing. The electromagnetic forming technique will be compared with to a traditional punch and die stamping method to show the benefits of implementing the uniform pressure coil, electromagnetic forming technique into an industrial environment. The application of a velocity measurement system (Photon Doppler Velocimetry) to measure the

3 velocity and acceleration of the workpiece will also be discussed along with the results surrounding its use.

The findings discussed in this thesis show the benefits of using the application of the uniform pressure coil in the electromagnetic forming technique compared to the traditional stamping method among these are increased hardening and formability.

It will also show that shearing of sheet metal can be successfully performed utilizing the uniform pressure coil. Lastly, the Photon Doppler measurement system can be used to measure velocity and acceleration of the moving sheet at speeds of 200 m/s.

CHAPTER 2

BACKGROUND

2.1 Traditional Sheet Metal Forming

Sheet metal forming has been used as a method to produce a variety of parts for approximately 200 years [1]. The present production process, sheet metal stamping, of forming sheet metal between two mating dies at room temperature was developed in order to produce a high volume of parts at a low cost. This process is utilized where large production runs, such as consumer appliances, electronics and automotive parts, need to be produced [3]. A schematic of traditional stamping process is illustrated in Figure 1.1 a [2].

The basic components in the traditional stamping method are the punch and die, a blank holder, and a press. The punch and die are the components that contain the pattern for the desired formed workpiece. The blank holder is used to hold the

5 unformed sheet metal (blank) against the die top to control the draw in. The last component is the press which applies the force to push the punch into the die [3].

Figure 1.1 a shows that the sheet metal is clamped on each side of the die. Often draw beads are placed on the outer channel of the die to aid in holding the sheet metal and insure against slipping [3]. Force is applied to the punch to shape the metal between the punch and the die to form the desired shape.

Traditional forming is often used in the automobile industry to produce containment assemblies, such as cases, exhaust systems, covers, lids, housings and doors because of the need to obtain the mechanical properties and provide lightweight components at high rates and low cost for overall improvements in fuel efficiency and design requirements [3].

There are at least four critical limitations to the traditional stamping method. One critical issue is the punch and die tolerances. The tolerance must be very precise in order to produce a dimensionally acceptable and defect free part every time. Second, the issue of design change and wear of the tooling is a problem [2]. This is a significant issue because as production operations evolve to current engineering design philosophies late stage changes to parts with traditional forming can be difficult. The change would have to be made to both the punch and the die, which increase the dimensional complexity. The alignment of the die and the punch can

also be a significant issue [2]. Without proper alignment, the punch can shear the

sheet stock or create local thin areas in the sheet limiting the ability to produce an

unacceptable part. This situation can become more critical as the complexity of the

part increases. A final important issue is formability of many alloys that cause sheet

metal stamping not be a viable forming method. The need to form less ductile

materials into complex shapes requires a different forming method.

2.2 High Velocity Forming

High velocity forming can be defined simply as moving a workpiece at a high rates

(over 200 m/sec) and transforming the associated kinetic energy into plastic

deformation as the workpiece contacts the die surface [1, 4]. Each of the methods

described below do not use the traditional, relatively low velocity die and punch

stamping method (1.7 x 10 -3 to 1.7 x 10 -1 m/sec) [5] to form but rather use a chemical or electrical force for acceleration [4]. High velocity forming only uses a one sided die to produce parts, so the issues associated with tolerances, machining and alignment as discussed earlier with traditional stamping [1, 2] are significantly reduced.

High velocity forming methods can provide improved formability, more uniform strain distribution in a single operation, and lightweight tooling equipment [1]. With

7 more uniform strain distribution, strain hardened alloys can be more readily shaped with reduced intermediate annealing operations.

As stated previously, there are several methods that can be considered for high velocity forming, among these are explosive forming, electromagnetic forming, and electrohydraulic forming [1, 4]. Figure 2.1 shows the methods available through high velocity forming [4].

Figure 2.1 Methods of high velocity forming [4].

2.2.1 Explosive Forming

Explosive forming can be described as using energy produced by the detonation of explosives to move a workpiece into a specified shape [4]. Several different explosives, including low explosives and high explosives, can be used. Low explosives include items like gunpowder, which simply burn and produce heat and a hot gas. These produce lower velocities (less than the speed of sound), whereas high explosives, shown in Table 2.1, can produce velocities up to 8000 m/s and energies as high as 1 MJ/kg [1, 4].

Deformation Engergy Pressure Explosive Velocity (m/s) (J/g) (GPa) 2,4,6-trinitroltoluene (TNT) 7010.4 780 16.536 Cyclotrimethylene trinitramine (RDX) 8382 1265 23.426 Pentaerythrite (PETN) 8290.56 1300 22.048 Smokeless powder <1 300 0.35

Table 2.1 High velocity explosive characteristic information [1, 4].

The operations by which explosive forming is performed depends on geometry of the part that is being produced. Figure 2.2 shows several examples of possible sheet forming methods by explosive forming [1, 4].

2.2.2 Electromagnetic Forming

Electromagnetic forming utilizes a short duration, high current surge through a coil.

Any conductive workpiece can be moved by pure electromagnetic forces, while non-conductive workpieces can be moved with the help of a conductive driver [1].

The electromagnetic forming system includes: a capacitor bank, a conductive coil or actuator, and a metallic workpiece (metal tube, flat metal sheet, etc.) [3, 4].

Electromagnetic forming can be incorporated in the production of flat sheet forming as illustrated schematically in Figure 2.3 [3]. Direct current is utilized to charge a capacitor bank for the circuit. When charged, a large current is transferred into the coil producing the primary field, and then a secondary or induced current is produced in the workpiece repelling it in the direction of the die. This occurs because according to Lenz’s Law, the primary current in the coil creates a magnetic field around it, thus inducing the secondary current in the workpiece [4, 6].

Figure 2.3 Schematic of electromagnetic forming [3].

Electromagnetic forming process is driven by the following coupled differential equations [1, 3, 4]:

d Q1 (L1I1 + MI 2 ) + R1I1 + = 0 Equation 2.1 dt C1

d (L I + MI ) + R I = 0 Equation 2.2 dt 2 2 1 2 2 where L1 = inductance of the capacitor bank and coil

R1 = resistance of the capacitor bank and coil

C1 = capacitance of the capacitor bank and coil

L2 = inductance of the workpiece

R2 = resistance of the workpiece

M = mutual inductance between the coil and the workpiece

Q1 = stored charge in capacitor bank

I1 = current in coil

I2 = current in workpiece

An approximation for the electromagnetic force per unit length between the coil and the workpiece can be make by using Equation 2.3 [3, 4].

F µ I I = o 1 2 Equation 2.3 l 2πd

where d = distance between the workpiece and coil

µo = magnetic permeability of free space

The needed electromagnetic forces can be easily achieved to surpass the plastic yielding of metal sheets and give the needed acceleration to reach the high velocity by adjusting the discharge energy in the capacitor bank [4].

Depending on the discharge energy and the material that is being formed, velocities can reach up to around 200 m/s, which improves formability at higher strain rates [6].

There are several disadvantages and advantages to electromagnetic forming. A

disadvantage is the equipment for electromagnetic forming which is not common and

can be very large depending on the discharge energy required for the application. A

larger capacitor bank is needed depending on size, shape and thickness of the material

being formed. Some of the advantages include the following: 1) the ability to control

the discharge energy in the capacitor bank to accommodate different shapes and

materials 2) higher formability is achievable when compared to metal stamping

3) improved strain distribution [6, 7] and repeatability from workpiece to workpiece.

2.2.3 Electrohydraulic Forming

Electrohydraulic forming can be viewed as a cross between electromagnetic forming and explosive forming because it uses the explosive forming speeds but uses a capacitor bank as the energy source, which is similar to electromagnetic forming [8,

9].

A schematic of electrohydraulic forming can be seen in Figure 2.4. As shown, the charge from the capacitor bank sends a pulse over the spark gap in the liquid. This converts the electrical energy into mechanical energy by vaporizing the liquid and moving the workpiece into the desired shape [1, 8, 9].

Figure 2.4 Schematic of electro-hydraulic forming [9].

2.3 Comparison of Traditional Stamping to EM forming

The focus of the current research and the remaining discussion will focus on the electromagnetic forming of sheet metals relative to the traditional stamping method and an experimental pressure-pad forming method.

As discussed previously, there are several advantages to the electromagnetic forming method. The advantages include adjustments to tooling, alignment, formability, late stage changes and forming fine details.

The tooling for traditional stamping requires a punch and a die. This requires careful alignment in order to ensure an acceptable workpiece is produced. Electromagnetic forming eliminates the punch and only requires a one sided die. With only a one sided die the alignment is much simpler [2, 7, 10].

Next, the potential forming depth of various materials in both methods is different. In traditional stamping the workpiece material must have good formability properties for dimensional accuracy. However, in electromagnetic forming because of the higher velocity and strain rate better forming depth with less formable materials can be achieved [2, 7, 10].

Third, often changes are made when new generations or designs are introduced. In traditional stamping, design changes are difficult because of tolerance requirements and changes must be made to both the punch and the die consistently. In electromagnetic forming, late stage design changes can be made more easily because changes only have to be made to one side of the die [2, 7, 10].

Lastly, electromagnetic forming can be imprint very fine details, including some similar to an etching effect. This cannot be easily performed with traditional stamping without large press forces and complex die and punch sets. These are summarized below in Table 2.2 [2, 7, 10]. Figure 1.1 demonstrates the differences in the setup and die locations of traditional stamping methods and electromagnetic forming methods.

EM Forming of Sheet Traditional Stamping Metals Tool Must have Die and Punch Single Sided Die Must be carefully aligned for Alignment Simple alignment punch and die Good material formability is Often better formablility Formability required for dimensional because of high velocity accuracy and strain rate Difficult because of Last Stage Easy because of one sided tolerance requirements Changes die between die and punch High force required to Very fine detail easily Surface Detail produce fine surface detail produced

Table 2.2 Comparison of traditional stamping to electromagnetically formed methods [2, 7, 10].

There are possible issues with the electromagnetic forming setup that will be addressed in this paper. Among these are the following: 1) robustness and coil lifetime, 2) interface sparking and 3) tool wear. Robustness and coil lifetime is important because with the introduction into an industrial environment the desire is for something that can be used for several thousands of cycles to lower costs. There are issues with interface problems between the driver and the workpiece that will be addressed later. Lastly, the wear of on the tool is of concern because of cost and difficulty of replicating some complex tooling.

2.4 Coils

The coil or actuator is a main component in the electromagnetic forming process.

Two common coil types are 1) single turn and machined and 2) multi-turn [1].

2.4.1 Single Turn and Machined Coils

Single turn and machined can be made very simply by cutting a slot and hole in a conductive plate. In order to improve the efficiency, materials with high conductivity are used, such as copper or aluminum with proper insulation. The coils can be machined out of thick plate of the conductive material to form flat sheets [1, 4].

2.4.2 Multiturn Coils

The simplest multiturn coil can be made by winding a conductive wire (copper, aluminum, etc.) around a non-conductive material, such as G-10 laminated phenolic composite. These multiturn coils can be used for expansion, compression or flat coils [1, 4]. However, there are several limitations to these coils [1]. One limit is the pressure the coil can withstand, which is determined by the composition of the coil material because the magnetic forces of the workpiece are equal and opposite.

Muliturn coils are typically limited to a pressure of 48 MPa. Another limitation is that it is hard to the produce the very fine windings required for the local field intensity [1].

2.4.2.1 Uniform Pressure Coil

The type of multiturn coil that was used in the experiments discussed here is a uniform pressure (UP) coil [11]. An in depth discussion of the uniform pressure coil is provided in the thesis of Manish Kamal of The Ohio State University [4]. There are two components in the uniform pressure coil, an outer channel and an inner coil, consisting of the coil windings. Figure 2.5 a shows the separate pieces of the coil and

Figure 2.5 b shows the coil together with the dielectric coating applied. The inner coil is dipped in a dielectric material that is non-conductive before potting in the outer channel. The inner coil and outer channel are then joined by filling the space with dielectric to isolate the inner coil from the outer channel as shown in Figure 2.5 b [4].

Outer Channel

Inner Coil

a) b)

Figure 2.5 Cu-Be coil a) two separate outer channel and inner coil b) both coil pieces secured with dielectric

A schematic of the current flow and workpiece movement for the uniform pressure coil is shown in Figure 2.6. The current flows from the capacitor into the primary coil (red, inside loop) and travels as noted in Figure 2.6. This induces a secondary current (blue, outside loop) in the fixed conductive outer channel through the workpiece. The secondary current in workpiece is repulsed by the opposing primary currents and forced at high speeds (approximately 200 m/s) into the die. A Rogowski probe is used to measure the current traveling into the coil [2, 10].

Charging Circuit

Capacitor

Rogowski Probe Vacuum box

Figure 2.6 Schematic of uniform pressure coil of current flow and workpiece movement [2].

Impact pressure is an important parameter when using the uniform pressure coil. The

impact velocity must be sufficient to assure plastic deformation when the sheet metal

impacts the die. Insufficient impact velocity can result in partially elastic strain such

that insufficient fill is obtained. The impact velocity, Vi, can be calculated through

integrating force on the sheet over the time of flight and the impact pressure, Pi, can be estimated for a planar elastic collision between two materials labeled 1 and 2 as below [2, 12]:

ρ1ρ 2C1C2 Pi = Vi Equation 2.4 [12] ρ1C1 + ρ 2C2 where: ρ = density for material

C = longitudinal wave speed for material

Since the uniform pressure coil offers the ability to move flat metal sheets at high velocity it allows for increased formability opportunities. The higher impact velocity often allows for a larger formability range when compared to a traditional stamping method. This will allow for less formable materials to be used in more complex designs [2].

2.5 Photon Doppler Velocimetry System

Previously, the system used for measuring velocity of a moving surface was the

Velocity Interferometer for Any Reflector System (VISAR). Although, VISAR has been used as the standard for a long period of time, the system is labor intensive because precise alignment and interpretation are needed. Because of the difficulty of implementing the VISAR system, the Photon Doppler Velocimetry system has been developed by Strand and co-workers [13] and is used in this study. The system to measure the actual velocity of the sheet as it moves in uniform pressure coil system is

Photon Doppler Velocimetry (PDV) system. The Photon Doppler Velocimetry setup was developed by Strand et al at Lawrence Livermore National Lab (LLNL). The

Photon Doppler Velocimetry system at Ohio State University utilizes all fiber optic sensors, which improves the accuracy and reproducibility [13, 14] more details of the setup will be discussed in Chapter 4.

The PDV system is centered on the principle of Doppler shifted light. Basically, the moving surface produces the Doppler shifted light, and when it is recombined with the incident light signal it produces a beat frequency. The beat frequency can be analyzed to calculate the velocity time profile to show how fast the material is moving at specific distances for different materials [14].

A schematic of the PDV components is shown in Figure 2.7. The system consists of a laser (the main component of the system), a splitter (can divide the laser output for several ports), a circulator (directs the laser to and from the probe and from the laser source to the detector), a probe (acts as a built-in reference surface), and an oscilloscope (collects data for processing and measurement) [14].

A fiber laser light source feeds light into a 3 port circulator. A probe on the circulator center port provides back-reflected reference light. Light emitted from the focuser is reflected back from a target. The target reflected light re-enters the circulator port 2, mixes with the reference signal and is emitted from port 3 into an optical detector. A digital oscilloscope captures the detector outputs [15].

Figure 2.7 Schematic of PDV system [14].

CHAPTER 3

EXPERIMENTAL SETUP FOR PRESSURE-PAD FORMING AND ELECTROMAGNETIC FORMING

In the experiments discussed here the three forming methods will be shown and discussed. The forming methods include: 1) traditional stamping (using a punch and a die), 2) pressure-pad (traditional stamping simulation with single-sided die), and

3) electromagnetic (using uniform pressure coil).

3.1 Traditional Stamping Simulation Setup/Pressure-pad Setup

In order to remove the effect of velocity but retain the essential features of a single-sided die arrangement, a urethane pad forming/pressure-pad method was used presently. The pressure-pad setup is shown in Figure 3.1 and described below.

Force

Steel Plate

Urethane Pad Sheet Metal Die/Punch

Figure 3.1 Experimental pressure-pad setup for urethane pad forming. The urethane pad expands laterally during compression.

The pressure-pad forming method used in experimentation was performed by placing the workpiece sheet metal on the prototype subscale bipolar plate forming die. A stiff urethane pad was placed onto the metal sheet followed by a thick steel plate and a force was applied from a 100 000 lb hydraulic press. When the force was applied, the urethane pad deformed laterally, spread over the tool producing uniform stress on the sheet metal, forming it into the die, and produced a workpiece. The urethane pad had starting dimensions of approximately 0.75 in wide x 2 in long x 0.5 in thick and during application of the force the dimensions changed to approximately 2 in wide by

3 in long by 0.04 in thick. The metal sheet was approximately 2.5 in x 4.5 in for a surface area of 11.25 in 2, therefore the nominal pressure on the sheet was approximately 5,000 psi.

3.1.1 Pressure-pad Comparison to Traditional Stamping

One important difference between the pressure-pad and the traditional stamping method is displayed in Figure 3.2. As shown in Figure 3.2 a, the metal sheet will only come in contact with one side of the tool at each particular spot. However, in pressure-pad experimental forming (shown in Figure 3.2 b), the sheet metal is forced to come in contact with the full surface of the die if the pressure is high enough. Full conformance with the tool surface provides a more aggressive deformation process in the pressure-pad forming method than the traditional metal stamping method, requiring higher local strains. On the other hand if the pressure is not high enough to cause full sheet-die conformation, low strains will be developed in pressure pad forming.

Die Sheet Metal PunchPunch a) Traditional Stamping

Urethane Sheet Metal Die b) Quasi-Static Forming

Figure 3.2 Schematic comparison of traditional stamping to the pressure-pad forming.

3.2 Electromagnetic Forming Experimental System

There are several components in the electromagnetic forming experimental system.

Figure 3.3 shows a schematic of the setup using the uniform pressure coil. An explanation of the components is below.

Clamping Force Deformable seal

Forming die Standoff Workpiece sheet

Cu driver sheet

Actuator / Coil Steel vacuum enclosure

Figure 3.3 Schematic of the current electromagnetic forming setup.

3.2.1 Key Setup Components

The key components in the uniform pressure coil system include:

Vacuum enclosure box —A sealed box to achieve the necessary vacuum

levels. The current vacuum enclosure box is made out of 0.5 in thick tool steel.

Uniform pressure coil —Key in delivering currents and providing force to move the workpiece into the die. The current coil used is made of copper beryllium, with 11 turns (windings), and is 4 in long x 3 in wide x 1.33 in deep, as shown

29 previously in Figure 2.5 b. The coils used in these studies were manufactured by

American Trim LLC in Lima, OH.

Driver sheet —Low conductivity materials need a copper or aluminum driver

sheet to aid in the movement of workpiece into the die. In the experiments where a

driver was needed, a copper driver sheet, Alloy 110, with a thickness of 0.005 in was

used.

Workpiece sheet —Desired material to be formed. Depending on the

application, various sheet metals and stock thickness were used.

Standoff —Required in order to provide a distance for the workpiece material to be accelerated to the required velocity. The distance of movement is important in order for the workpiece to gain velocity. The standoff is L-shaped (shown in Figure

3.4) and was made of tool steel. There were several standoff distance options, 2 mm,

3 mm, and 4 mm depending on the application.

Forming die —A variety of patterns can be used to produce desired pattern on the sheet metal. In these studies, a small scale fuel cell bipolar plate die with small channels was used and is shown in Figure 3.5.

Deformable seal —Used to seal the box to secure that sufficient vacuum can

be pulled. The deformable seal was a stiff urethane pad with dimensions of

6 in long x 5 in wide x 0.125 in thick in order to cover the entire top of the vacuum

enclosure box.

Clamping force —Allows for sealing of box as well as proper pressure

between the outer channel and driver sheet. The clamping force was provided by an

Enerpac hydraulic RC256 press. Typically, a nominal pressure of 6000 psi was used

to apply a force of approximately 24000 lb f to the steel plate to deform the seal over the vacuum enclosure box and provide appropriate contact between the workpiece or driver and the outer channel.

Capacitor bank —The capacitor bank provides the stored discharge energy

required to move the sheet metal. The capacitor bank used in these studies was a

16 kJ Magneform capacitor bank (shown in Figure 3.6). This capacitor bank has a

peak stored voltage of 8.66 kV, 8 capacitors of 53.4 µF each, and in the shorted

condition it has an internal inductance of about 100 nH, giving a rise time of about

10 µs.

Figure 3.4 Standoff used in electromagnetic forming setup for uniform pressure coil, with different standoff distances.

a) Die b) Punch

Figure 3.5 Small, subscale bipolar plate die and punch used in experimentation.

Figure 3.6 Magneform 16 kJ Capacitor bank used for electromagnetic forming experiments.

3.3.2 Instrumentation Components

The instrumentation components include those that measure the vacuum, current and velocity.

3.3.2.1 Vacuum and Current Measurement

Vacuum — Required in order to provide parts with no visual air pockets. The

pump used in the studies discussed here was a Trivac D76A, which typically 33 permitted vacuum levels of approximately 100 torr. The vacuum pressure was measured using a Varian ConVectorr gauge and read by using a Varian Sentorr

BA2C receiver. The ConVectorr gauge was connected to the vacuum enclosure box through a rubber vacuum hose. This was to ensure that no current would flow from the uniform pressure coil, through the box into the gauge therefore shorting out the gauge.

Current —Important to determine peak current and rise times. The current was measured using Recoil Rogowski coils rated at 200 kA/V. It was placed around one of the leads that is connected to the capacitor bank. It reads the current that passes from capacitor bank into the uniform pressure coil.

3.3.2.2 Velocity Measurement—Photon Doppler Velocimetry

The velocity of the moving workpiece can be found using a Photon Doppler

Velocimetry system. A schematic of the Photon Doppler Velocimetry system for ring expansion is shown in Figure 3.7 [15] and a photograph of the components is shown in Figure 3.8. A similar setup is used for the uniform pressure coil by simply replacing the ring expansion coil with a uniform pressure coil. A more in depth discussion of the Photon Doppler Velocimetry will be shown later.

Laser —The light source of the system is a high power erbium fiber laser. The system used in the these experiments was based on an NP Photonics 1550 nm,

1000 mW output power laser with a line width of <5 kHz [14, 15].

Splitters —Divides laser output to several fiber optic ports for multichannel operations or phase comparison [14, 15].

Circulators —Directional fiber optic device guides light from the laser out to the probe and reflected light from the probe to the detector. A 1 watt JDS Uniphase device was used in these velocity studies [14, 15].

Detectors —Short rise time based photodetector, with a high bandwidth comparable to the oscilloscope. The following experiments used a Newport model

818s detector [14, 15].

Probes —Collimating or focusing with built in reference partial reflection

surface. This system uses devices from Oz Optics Oscilloscope — A 1 GHz, 5 GS/s,

LeCroy WaveSurfer 104MXs with 4 channels is used in this system. This

oscilloscope has a large amount of data storage, 10 Mpts on each channel, allowing

storage for periods up to 2 ms at full speed on each of the 4 channels [14, 15].

Figure 3.7 Schematic drawing of PDV and ring expansion system [15].

Figure 3.8 Components for the Photon Doppler Velocimetry system

3.3 Sample Analysis

The hardness increase and forming depths from these studies were analyzed. The following discusses the procedure used in obtaining the measurements.

3.3.1 Sample Preparation for Analysis

After the samples were formed, workpieces were mounted in Struers Epofix cold mount in order to prevent the channels from being crushed in a hot mounting process.

After the samples were mounted, the workpieces were cut across using a Dremel to preserve the channels. The mounted samples had 2 mm removed from the surface to insure no damage to the channels occurred during cutting and then polished from a

200 grit to a 1 µm fine polish. After each polishing step the samples were ultrasonically cleaned in distilled water and dried with warm air.

3.3.2 Hardness Measurements

All hardness measurements discussed were performed using a Vickers microhardness tester with a 200 g load and a 20 sec hold time in accordance with ASTM standard, E 384-07, Standard Test Method for Microindentation Hardness of

Materials [16]. An average of 20 Vickers hardness points was utilized for each test condition.

3.3.3 Forming Depth Measurements

The forming depth was determined using a micrometer, by measuring the height of the channels and then subtracting the thickness of the metal sheet workpiece. The depth found was compared to the actual depth on the die to find a percent of full depth measurement. The samples were analyzed for tearing by lighting the samples from behind to determine where the failure occurred.

3.4 Experimental Variables

There are several variables that can be adjusted in order to optimize the formability, impact velocity and hardness. The variables include: 1) discharge energy, 2) coil geometry, 3) vacuum level, 4) standoff distance, 5) driver material/thickness, and

6) workpiece material.

3.4.1 Discharge Energy

The discharge energy is controlled by the capacitor bank. The capacitor bank used on experiments conducted at Ohio State University was a 16 kJ Maxwell Magneform capacitor bank. In these studies, the capacitor bank was used from 10% to 50% of the full potential (1.6 kJ to 8kJ), with all 8 capacitors on.

3.4.2 Coil Geometry

Several coil elements can be altered to improve the efficiency, robustness and longevity of the coil. Among these are the coil material, the geometry of the connections into the capacitor bank and the coil gap between the inner coil and the outer channel. In the following presented trials no coil geometry changes were made.

Technical drawings for coils are given in Appendix A.

3.4.3 Vacuum

The ability to pull a vacuum on the uniform pressure system is very important.

Because of the high speed and preciseness of the process, any foreign objects and air, between the coil and the die surface can cause imperfections in the surface. In order to remove the possibility of air pockets being visible in the finished part, a vacuum is pulled. If the vacuum level falls around 1 torr, under electrical potential the air can ionize under pressure forming a plasma arc in the chamber. The critical vacuum levels are shown in Figure 3.9 [17].

Figure 3.9 Paschen curve showing critical vacuum levels [17].

As discussed, the vacuum levels have an effect on the final workpiece. For the experiments discussed here, the vacuum level has been investigated at atmospheric pressure (nominally 760 torr) and at 100 torr. The vacuum was measured using a digital Varian ConVectorr gauge. At 100 torr, no significant air pockets have been seen. Ideally, a vacuum level of 1 torr would be reached, however with the current system this was not possible therefore the vacuum level was held to approximately

100 torr.

3.4.4 Standoff

The standoff distance is an important factor in the uniform pressure coil process. The current standoffs are L-shaped tool steel which can provide a moving distance of

2 mm, 3 mm, and 4 mm for the workpiece. Since the distance from the coil to the workpiece is fixed, the benefit of having a larger standoff distance is that the more distance the sheet has to move, a higher potential velocity can be achieved, whereas with lower standoff there is less potential impact velocity. When using less ductile materials, such as a ferritic stainless steel or titanium, less standoff is preferred in order to prevent shearing or tearing that could possibly result if higher impact velocity was achieved.

3.4.5 Driver Material

The driver material and thickness has not been investigated extensively. In these studies a copper driver of 0.005 in thickness or 0.01 in thickness has been used. A copper driver instead of an aluminum driver was used because the aluminum has a lower resistivity. Aluminum (1100 series) has a resistivity of around 2.7 x 10 -6 Ω-cm, while copper has a resistivity of around 1.7 x 10 -6 Ω-cm [18]. The thickness of the selected driver in these experiments was dependent on the application.

3.4.6 Workpiece Material

The workpiece material depends on the final component requirements. As stated earlier, if the workpiece has low conductivity, a driver sheet may be needed to form the workpiece. The discharge energy and standoff parameters are dependent on the material type and the related ductility.

CHAPTER 4

VELOCITY MEASUREMENT WITH UNIFORM PRESSURE APPLICATION

4.1 Velocity Measurement

The ability to measure the velocity of the moving sheet and the impact velocity is very important to predict the properties of the formed material. Currently, at The

Ohio State a Photon Doppler Velocimetry (PDV) system is used for velocity measurement. A schematic of the setup is shown in Figure 4.1.

Circulator PDV Probe Laser Clamp Force PDV

Detector

Rogowski Coil

Control Analysis Computer

Oscilloscope

Figure 4.1Schematic of PDV setup for use with a uniform pressure coil [14].

The individual components of the system were briefly discussed earlier. A more in depth explanation of the components can be found in Daehn et al [19].

This setup allows for the measurement of the impact velocity of the flat sheets in the uniform pressure coil. The Photon Doppler Velocimetry system also allows for the measure of current by the use of the Rowgoski coil. An example of the output data is shown in Figure 4.2, the conditions for the experiments are shown in Table 4.1.

Figure 4.2 Exported data from PDV measurement system for 3.2 kJ.

The data displayed above in Figure 4.2 is the beat signal, the current trace and the

velocity time profile (experimental parameters are shown in Table 4.1). Figure 4.2 b

shows the beat signal received from the entire experiment. As the period of the beat

signal (distance between peaks) becomes smaller, the sheet is increasing in velocity.

A display of an expanded beat signal is shown in Figure 4.2 a for the first 7 µs of

Figure 4.2 b. This shows the decrease in period size making it evident that the sheet 46 is moving faster. The velocity/time profile shown in Figure 4.2 c was found by analyzing the beat signal in Figure 4.2 b. Figure 4.2 d displays the current data received by the Rowgoski Coil. The peak current and rise time can be found by using the data collected from the Rowgoski coil. A summary of the data collected is shown in Table 4.1. The peak current collected was 95 kA and the rise time was 32 µs. The

rise time is the differential from the start of current rise to the peak current and

reflects the time that it takes for the sheet to move into the die. The peak velocity for

316L and the parameters are shown in Table 4.1 and occurs at 205 m/s.

Workpiece Material 316L Workpiece Thickness 0.004 in Driver Material Cu Driver Thickness 0.005 in Standoff 4 mm Discharge Energy 3.2 kJ Peak Current 95 kA Peak Velocity 205 m/s Rise Time 32 µs

Table 4.1 Summary of PDV results.

This data is used to determine the velocity at various standoffs and energies. By

assuming the trend of the velocity time profile is linear and triangular in shape, the

47 area under the curve, and hence the displacement of the sample can be found by using

Equation 4.1.

1 A = bh Equation 4.1 2 where A = distance traveled (standoff distance)

b = time

h = velocity

The energy and velocities for various standoff distances is shown in Figure 4.4 and

Table 4.2 displays a summary of the data. The conditions are shown in Table 4.3 for these experiments. As shown, the higher discharge energy results in a higher rate of acceleration and peak velocity at die impact. An example of the profile for 3.2 kJ is shown previously in Figure 4.2 and is shown for 1.6 kJ below in Figure 4.3.

Figure 4.3 Exported velocity data for 1.6 kJ.

225

200 4 kJ 175 3.2 kJ 150

125 1.6 kJ 100 Velocity (m/s)

0 0 0.5 1 1.5 2 2.5 3 Distance (mm)

Figure 4.4 Velocity profile for various distances.

Energy (kJ) Distance (mm) Velocity (m/s) 1.6 1 92 1.6 2 130 1.6 3 130 3.2 1 112 3.2 2 159 3.2 3 194 4 1 118 4 2 168 4 3 205

Table 4.2 Summary of energy and velocity data.

Workpiece Material 316L Workpiece Thickness 0.004 in Driver Material Cu Driver Thickness 0.005 in

Table 4.3 Summary of conditions for velocity experiments.

These measurements were taken on die similar to the schematic shown in Figure 4.5.

The small hole in the center is where the probe is inserted for velocity measurement.

The larger holes are used for the air to escape; if the air is not able to escape the velocity will be slowed down due to air resistance. Further experimentation is underway to measure velocity in a vacuum to completely avoid the air resistance.

D=0.3 in D=0.5 in D=0.5 in

2.5 in D=0.15 in

D=0.3 in

4 in

Figure 4.5 Schematic of die used for velocity measurement.

4.1.1 Summary

In summary, velocity measurement is possible with the uniform pressure application.

The ability to measure the velocity will aid in being able to predict the output properties of the workpiece and help in possible applications of the uniform pressure coil.

4.2 Velocity Estimates

The experimental velocity data can be compared to the theoretical values calculated by using equations from Kamal’s dissertation at The Ohio State University.

The theoretical inductance of the system (L sys ) can be calculated by using an experimental rise time and by using Equation 4.2

2 4t rise L = Equation 4.2 [4] sys π 2C where trise = rise time

C = system capacitance

The system capacitance for the capacitor bank used in these studies was 53.6 µF per a capacitor and there are eight capacitors [4].

The primary current as a function of time can be theoretically calculated using the system voltage, system capacitance, and system inductance as shown in Equation 4.3.

It includes a damping factor, ξ, and ringing frequency and its calculation is shown in

Equation 4.4 and Equation 4.5.

V C o L sys −ξω t I p (t) = e sin( ωt) Equation 4.3 [4] 1− ξ 2

where Vo = system voltage

ξ = damping factor

ω = ringing frequency

1 C ξ = Rsys Equation 4.4 [4] 2 Lsys where Rsys = resistance of system

1 ω = Equation 4.5 [4] LC

The induced current as a function of time of the uniform pressure system can be calculated using a coupling factor, f 2, and the primary current as a function of time as shown in Equation 4.6.

I i (t) = f 2 I p (t) Equation 4.6 [4] where f2 = coupling factor, approximately 0.70

n = number of turns in coil

The acceleration can be calculated using physics equations to find the magnetic pressure (P m). The magnetic pressure can be calculated with the peak currents and

the number of turns per unit length. It is shown in Equation 4.7 [1].

     n   n  Pm (t) = .0 0063 * Ii (t *)  * I p (t *)  Equation 4.6 [1]  ls   ls  where ls = length of solenoid

From physics pressure is force per unit area and is shown in Equation 4.7. The area can be defined as the area of the workpiece being moved.

F = Pm * A Equation 4.7 where F = force

A = area of sheet

After the force is calculated the acceleration can be determined by Equation 4.8.

F a = Equation 4.8 m where F = force

m = mass of sheet

Figure 4.6 displays the theoretical calculated velocities over specific distances for a

316L sheet moving with the assistance of the copper driver. It shows that as the energy increases the velocity in turn increases. Also, as the standoff distance increases the workpiece velocity increases.

600

500

4 kJ

400 3.2 kJ

2.4 kJ 300

Velocity (m/s) 1.6 kJ 200

100

0 0 0.5 1 1.5 2 2.5 3 Distance (mm)

Figure 4.6 Theoretical velocity estimates for 316L with copper driver assistance.

The comparison of the theoretical and the experimental velocities are shown in

Figure 4.7 and Figure 4.8 for 316L with copper driver.

500

450 4 kJ Theoretical

400

350 3.2 kJ Theoretical

300

250 1.6 kJ Theoretical Velocity Velocity (m/s) 200 4 kJ Experimental

150 3.2 kJ Experimental

100 1.6 kJ Experimental

0 0 0.5 1 1.5 2 2.5 3 Distance (mm)

Figure 4.7 Theoretical and experimental velocity versus distance estimates for 316L with copper driver assistance.

1000

900

800 4 kJ

700

600 3.2 kJ

500

Velocity (m/s) 400

300 1.6 kJ 200

100

0 0 5 10 15 20 25 30 35 40 Time (us)

Figure 4.8 Theoretical velocity versus time estimates for 316L with copper driver assistance.

The differences seen in the acceleration of the theoretical and the experimental can be accounted for in the air resistance of the moving sheet and that in the experimental setup the workpieces are constrained on the outer channels from further acceleration.

4.2.1 Summary

In summary the rise time, peak current and acceleration can be accurately measured using instrumentation including the Photon Doppler Velocimetry. An estimate of these can be made by applying the previously discussed equations involving inductance, capacitance and basic physics.

CHAPTER 5

NEW GENERATION SYSTEM MODIFICATION AND IMPACT ON RESULTS

The uniform pressure coil, setup, and technology have been changed to make the system more efficient, robust and the results more reproducible. These past forming systems included a G-10 composite (glass-cloth laminate with epoxy resin) vacuum enclosure, a copper coil, and clamping system which consisted of six bolts for fixing the plate [2]. The previous generation system is shown in Figure 5.1. The changes noted to the experimental setup were incorporated prior to the present trials. The current setup is shown in Figure 5.2.

Figure 5.1 Previous experimental setups for uniform pressure coil [4].

5.1 Enclosure

5.1.1 Previous Generations

The previous generation system used a composite box. Though the electrical conductivity is low, it was susceptible to vacuum leakage because the edges were not sealed. Also, each side wall of the box was a separate piece so after many experiments the walls would begin to bow and separate.

5.1.2 New Generation

The composite box was replaced with a welded steel box. The welded box was lined with 0.125 in thick G-10, a laminate composite with epoxy resin binder, to ensure that the conductive surfaces do not meet.

5.2 Clamping Mechanism

5.2.1 Previous Generations

In previous setups, the composite box was clamped using six bolts which were tightened by hand to minimize distortion in the box and seal for vacuum. This allowed for the clamping pressure to vary between experiments. As discussed previously, if the clamping pressure between the conducting sheet and the coil is not sufficient, arcing between the sheet and the coil is possible causing local welding between the outer channels of the coil and the sheet. If welding on the outer channels occurs, extensive cleaning of the channels is required.

5.2.2 New Generation

The issue of clamping pressure was resolved by replacing the bolts with a hydraulic press (shown in Figure 5.2). This allows for consistent pressure for each experiment thus reducing the possibility of arcing between the outer channels and the workpiece 63

causing welding. It also aides in pulling a vacuum because a seal can more easily be

achieved with a higher force. A digital vacuum gauge was added to accurately

measure the vacuum. Details of press and vacuum system can be found in Chapter 3.

Figure 5.2 Current experimental enclosure setup.

5.3 Coil

5.3.1 Previous Generations

Previous generations of the uniform pressure coil were made out of copper. Although copper provides high conductivity, it is very soft, therefore more susceptible to bending and deformation in both the leads and the center coil after several. The

64 bowing in the center of the coil can cause the dielectric to crack and encourage arcing within the coil.

The leads, which connect the capacitor bank to the uniform pressure coil, have also been modified. The previous leads were round (shown in Figure 5.4 a), resulting in connectivity problems with the capacitor bank. The connection to the bank is important to prevent arcing between the bank and the leads [2, 4]. The coil gap, shown in Figure 5.3, is the gap between the inner coil and the outer channel. In previous generations this gap was 0.125 in, a more in depth discussion of the importance of this gap will follow later.

Coil Gap

Figure 5.3 The coil gap is the distance between the outer channels and the inner coil.

5.3.2 New Generation

The coil material has changed from copper to copper beryllium. Although pure beryllium has shown to be hazardous to humans, copper beryllium alloys are relatively inert and poses no known hazards when handled properly. The stronger copper beryllium coil reduces the deformation in the leads and bowing in the inner coil.

The leads changed from a round lead to a flat rectangular leads as shown schematically in Figure 5.4. This allows for better connectivity between the capacitor bank and the coil, more surface area to allow current to flow, and less chance of arcing in the bank. For each lead connection, a G-10 insulating spacer was placed in between the leads to avoid arcing between the leads.

Leads Leads G-10 spacer a b

Figure 5.4 a) Previous generation round lead connection (0.185 in diameter) with G-10 spacer b) New generation flat rectangular (0.151 in x 0.5 in) lead connection with G-10 spacer

The efficiency of the coil has also been assessed. An increased efficiency can provide a higher peak current, faster rise time and ultimately a higher velocity.

One way to increase the efficiency is to reduce the coil gap. The greater efficiency occurs because by decreasing the inductance the maximum current increases and the maximum current squared is proportional to the magnetic pressure. The reduction in the coil gap has shown to increase the peak current and therefore decreasing rise time, thus moving the workpiece at a higher velocity. Figure 5.5 shows the increase in peak current and decrease in rise time. The conditions for the coil experiments are shown in Table 5.1. The previous coil had a gap of 0.125 in and the present coil has a gap of 0.062 in. The previous peak current was approximately 66 kA and a rise time of 26 µs and the present peak current is approximately 69 kA and rise time of 23 µs.

The reduction in coil gap resulted in a net improvement in efficiency of 10%. The technical drawing for each coil gap is shown in the Appendix.

20 Coil Gap 0.125 in Current (kA)

Coil Gap 0.062 in

-20

-40 0 50 100 150 200 250 300 Time (us)

Figure 5.5 The smaller coil gap increases the peak current and decreases the rise time.

Workpiece Material Cu Workpiece Thickness 0.005 in Standoff 4 mm Discharge Energy 1.6 kJ

Table 5.1 Summary of conditions for coil comparison

5.3.3 Experimental and Theoretical Comparison

The experimental data from the coils can be compared to the theoretical values calculated by using equations from Kamal’s dissertation at The Ohio State University and shown in Chapter 4. The change in rise time, peak current and acceleration can be found for each coil gap. The analysis of each coil follows is shown below [4] in

Figure 5.6. As shown in Figure 5.6 and Figure 5.7, by decreasing the coil gap the potential velocity gained also increases therefore increasing the impact velocity and providing better formability as shown in Figure 5.8.

400

350

Coil Gap 300 0.062 in

250

200 Coil Gap 0.125 in

Velocity (m/s) 150

100

0 0 0.5 1 1.5 2 2.5 3 Distance (mm)

Figure 5.6 Theoretical velocity versus distance estimates for copper with a different coil gap.

600

500

400 0.0625 in Coil Gap

300

Velocity (m/s) Velocity 0.125 in Coil Gap 200

100

0 0 5 10 15 20 25 30 35 40 Time (us)

Figure 5.7 Theoretical velocity versus time estimates for copper with a different coil gap

Cross sections of each coil gap are shown in Figure 5.8 and conditions are shown in

Table 5.4. The impact side is the side of the workpiece that can into contact with the die or struck the face of the die.

1 mm 1 mm

Impact Side Impact Side

a) Previous coil gap b) Current coil gap

Figure 5.8 Greater forming depth is seen in the lower coil gap.

Workpiece Material 304L Workpiece Thickness 0.004 in Driver Material Cu Driver Thickness 0.005 in Standoff 3 mm Discharge Energy 4 kJ

Table 5.2 Summary of conditions for coil form detph trials.

5.3 Summary

Table 5.5 displays a summary of the improvements of the previous system as compared to the current setup.

Past Present Enclosure G10 Composite Box Welded Steel Box Clamping Force 6 Hand Bolts Hydraulic Press Coil Material Brass, Copper Copper Beryllium Coil Gap 0.125 in 0.062 in Coil Leads Round Flat

Table 5.3 Summary of improvements on previous systems.

The coil gap effects the efficiency of the system, by reducing the coil gap a higher peak current and lower rise time with better forming depth can be reached at the same energy. This makes all aspects of the forming process more robust, because at lower energies and voltages there is less heating and less propensity for sparking.

CHAPTER 6

FORMING TRIALS

Three critical parameters were assessed with the formability trials. These included hardness, formability (measured by cavity fill) and interfaces effects.

6.1 Hardness Effects

The potential for high velocity with the uniform pressure coil provides an opportunity for shock hardening to occur as a result of the high shock pressures. According to

Golowin et al., a pressure up to 10 GPa can be achieved, which is similar to the pressure of laser shock processing [2]. Possible reasons for the hardness increase in the electromagnetic formed material are shock hardening and high plastic strain rates which both can increase the hardening rate.

6.1.1 Discharge Energy

A summary of test conditions is shown in Table 6.1. As indicated in Figure 6.1 the hardness increases with increasing discharge energy. With increasing discharge energy, the workpiece is moving at a higher velocity inducing a higher strain rate. 74 Also, by increasing the discharge energy the depth of forming increases as shown in

Figure 6.2.

300

290 285.46

280

270 261.87 260

250 242.11 242.13 240 Hardeness (HV)

230

220 As received Material 210

200 1.6 2.4 3.6 4.8 Energy (kJ)

Figure 6.1 Microhardness (averaged over 20 points across channels) increases with increasing discharge energy.

80%

70% 66.84%

60%

50%

40% 33.42%

30% Percent FullPercent (%) Depth

20% 13.37%

10% 3.34%

0% 1.6 2.4 3.6 4.8 Energy (kJ)

Figure 6.2 Percent full depth increases with increasing discharge energy.

Workpiece Material Ferritic Stainless Steel Workpiece Thickness 0.004 in Driver Material Cu Driver Thickness 0.005 in Standoff 3 mm Vacuum None

Table 6.1 The discharge energy varied while holding other process parameters constant.

6.1.2 Standoff

The effect of standoff was studied with regards to hardness effects. A summary of conditions used for the standoff study is shown in Table 6.2.

Workpiece Material Ferritic Stainless Steel Workpiece Thickness 0.004 in Driver Material Cu Driver Thickness 0.005 in Standoff 3 mm Discharge Energy 6 kJ Vacuum 100 torr

Table 6.2 Summary of conditions for standoff studies.

As shown in Figure 6.3, where each standoff was used (2 mm, 3 mm, and 4 mm), hardness increase with increasing standoff. It will be shown later how the velocity increases with the larger standoff, this can also be seen with the corresponding hardness increase. Figure 6.4 shows that the percent full depth can also increase with increasing the standoff distance. However, with less ductile material, such as the ferritic stainless steel used here failure will occur at the higher standoff, therefore the proper balance of standoff distance and energy is needed.

300

290 286.59 282.99 280.29 280

270

260

250

240 Hardness (HV)

230

220 As received Material 210

200 2 3 4 Standoff (mm)

Figure 6.3 The effect of standoff on hardness of ferritic bipolar place.

100%

90% 86.90%

80%

70% 66.84%

60.16% 60%

50%

40%

Percent FullPercent (%) Depth 30%

20%

10%

0% 2 3 4 Standoff (mm)

Figure 6.4 Percent full depth increases with increasing standoff.

6.1.3 Comparison of pressure-pad to electromagnetic forming

Copper being a good conductor, did not require a driver sheet. An increase in hardness is seen in the electromagnetically formed bipolar plates over a pressure-pad formed and as received material as shown in Figure 6.5 and Figure 6.6. A summary of conditions is shown in Table 6.3. The averages of the hardness measurements are shown with a solid line.

Workpiece Material Copper Workpiece Thickness 0.005 in Driver Material None Driver Thickness None Standoff 3 mm Discharge Energy 6 kJ Vacuum 100 torr

Table 6.3 Summary of conditions for forming studies.

120

110 Cu EM Formed 100 94 % Full Depth

Hardness Hardness (HV) 70 Cu Quasi-Static Formed 94 % Full Depth Cu Sheet 60

40 0 50 100 150 200 250 300 Distance (um)

Figure 6.5 Increased hardness is seen with electromagnetically formed plates.

Figure 6.5 shows the effect of hardening on a copper sheet. The peaks in the data occur where the higher strain occurs in the sample. This can be seen where there is greater thinning in the peaks of the sample. Both the pressure-pad and the electromagnetically formed copper sheets were formed to approximately 94% of the full depth of the die. Approximately, a 10% increase in average hardness was observed for electromagnetically formed samples when compared to the pressure-pad formed samples for the copper. An approximate 20% increase in average hardness was observed in materials which require a driver such as with 316L as shown in

Figure 6.6. A summary of conditions is shown in Table 6.4.

Workpiece Material 316L Workpiece Thickness 0.004 in Driver Material Cu Driver Thickness 0.005 in Standoff 3 mm Discharge Energy 4 kJ Vacuum 100 torr

Table 6.4 Parameters for hardness study from pressure-pad and electromagnetically formed material with driver.

330

310

290 316L EM Formed 270 77 % Full Depth

250

230 Hardness Hardness (HV) 210 316L Quasi-static 190 74 % Full Depth

170 316L Sheet

150 0 20 40 60 80 100 120 Distance (um)

Figure 6.6 Increased hardness is seen with 316L electromagnetically formed plates.

6.1.4 Summary

The hardness of various sheet materials increases with increasing discharge energy.

It was observed that standoff effects the hardness of the final workpiece which is related to the increase in impact velocity. Lastly, electromagnetic forming exhibited an approximately 10% increase in hardness over the pressure-pad formed bipolar plates.

6.2 Formability

The need for greater formability in a number of components has increased the interest in electromagnetic forming as a possible replacement to traditional stamping. The goal is to electromagnetically form materials that can not successfully be formed using traditional methods. The materials that received the most attention in these studies were a 316L, 304L and a ferritic stainless steel. The ferritic stainless steel is difficult to form as a result of its brittleness and mechanical properties, such as ductility and strength. Table 6.5 shows pressure pad formed materials and the relative formability for each. This was used to determine which materials to study further with electromagnetic forming.

Max Punch % Ranking (0-no Thickness Punch Material Pressure Full tearing, 5-extreme (in) Depth (pounds) Depth tearing) 201 0.0045 80000 0.0085 57 0 C276 0.0040 80000 0.008 54 0 304L 0.0040 80000 0.011 74 0 316L 0.0040 80000 0.007 47 0 316L 0.0060 80000 0.007 47 0 317L SS 0.0040 80000 0.013 87 0 904L 0.0045 80000 0.0095 64 0 Cu 0.0050 80000 0.014 94 0 Ferritic 0.0040 85000 0.011 74 3 Ta 0.0040 80000 0.013 87 2 Nb 0.0049 80000 0.0141 95 2 Zr 0.0049 80000 0.0131 88 4 Ti 0.0030 80000 0.011 74 5 Al-1145 0.0045 80000 0.0145 97 1

Table 6.5 Pressure-pad formed materials with relative forming depth.

6.2.1 Traditional versus Electromagnetically Formed

6.2.1.1 Ferritic Stainless Steel

The pressure-pad stamping setup was used to form the channels with a 0.004 in thick ferritic stainless steel. Using a back lighting practice, as shown in Figure 6.7, tearing can be observed in the channels of the formed workpiece. One primary goal of this work was to evaluate if high velocity forming can eliminate these tears. The ferritic stainless steel was subjected to an 80 000 lb load from the press during forming.

The ferritic stainless steel was also formed electromagnetically. Table 6.5 shows the conditions used for the formability experiments. As shown in Figure 6.8, the tearing that was evident in the pressure-pad forming has been eliminated and a workpiece with deeper channels free of tearing was formed electromagnetically.

Workpiece Material Ferritic Stainless Steel Workpiece Thickness 0.004 in Driver Material Cu Driver Thickness 0.005 in Standoff 3 mm Discharge Energy 3.2 kJ Vacuum 100 torr

Table 6.6 Parameters for electromagnetic forming formability study of pressure-pad versus electromagnetically formed.

Figure 6.8 Electromagnetically formed ferritic stainless steel bipolar plate. The fill depth was nominally 70%.

The studies on the ferritic stainless steel workpieces were completed with pressure pad and electromagnetic forming. In order to analyze the maximum forming depth a sample of each forming condition was formed to just before the failure limit.

The parameters for the pressure-pad forming and electromagnetic forming and the failure conditions are shown in Table 6.6 and the percent full depth are shown in

Table 6.7.

Workpiece Material Ferritic Stainless Steel Workpiece Thickness 0.004 in Driver Material Cu Driver Thickness 0.005 in Standoff 3 mm Vacuum 100 torr

Table 6.7 Parameters for electromagnetic forming formability studies for ferritic stainless steel.

Energy (kJ) Percent Full Depth 2.25 52 2.72 53 2.88 60 3.04 60 3.2 70

Table 6.8 Percent full depth for ferritic stainless steel formability study.

As shown, in Table 6.7 the forming depth increased with increasing energy. These

samples began to fail at 3.2 kJ. However, it is important to note that failure was not

occurring within the center channels, where typically failure occurred in the

pressure-pad formed samples. This is significant because failure in the outer region

of the die could possibly be avoided without significantly altering the die. Also, in

using the uniform pressure actuator, it is expected that at the center of the die, the

sheet will strike first and the sheet will be parallel to the nominal face of the die.

When moving towards the clamped edges, the sheet will strike at an increasing angle

and there may be increasing amounts of normal elongation of the sheet prior to strike.

Both these effects can reduce the strains to failure in these regions.

6.2.1.2 316L Steel

Traditionally stamped, pressure-pad formed and electromagnetically formed samples of 316L were also compared for formability. Figure 6.9 shows a top view of the

88 samples, whereas Figure 6.10 shows a cross sectional view of the samples. The electromagnetically formed depth was close to the depth of the traditionally formed samples. The percent full depth of the electromagnetic formed sample is deeper than the pressure-pad formed sample, however the traditionally formed sample was deeper than the electromagnetically formed sample as shown in Table 6.8.

a) b)

Figure 6.9 a) Electromagnetically formed 316L b) Pressure-pad formed 316L c) Traditionally stamped 316L 89

a) Electromagnetically formed b) Pressure-pad formed

c) Traditional formed

Figure 6.10 Cross sections of 316L for different forming methods to compare formability.

Measured Depth (in) Percent Full Depth EM formed 0.011 74% Pressure-pad formed 0.0045 64% Traditional formed 0.02 80%

Table 6.9 Depth measurements for the different forming methods.

6.2.2 Coil Improvements

As shown in Figure 6.11, an improvement in depth of forming can be seen with a reduction in coil gap. The coils used for both samples were copper beryllium with the only planned difference being the coil gap. A summary of conditions for the coil improvement study is shown in Table 6.9.

1 mm 1 mm

Impact Side Impact Side

a) Coil gap of 0.125 in b) Coil gap of 0.062 in 53 % full depth 84 % full depth

Figure 6.11 Coil gap effect on formability.

Workpiece Material 304L Workpiece Thickness 0.004 in Driver Material Cu Driver Thickness 0.005 in Standoff 3 mm Discharge Energy 4 kJ Vacuum 100 torr

Table 6.10 Parameters for the coil improvement experiments.

6.2.3 Discharge Energy

As the discharge energy increased the depth of forming typically increased. The key to achieving the desired formability is finding the proper energy in order to form the material to the desired depth without exceeding the critical strain rate and tearing the material.

6.2.4 Lubricant

A boron nitride (BN) aerosol spray lubricant from ZYP Coatings used to attempt improve formability by reducing the friction between the workpiece and the die allowing the workpiece to slide more easily over the die. The lubricant was applied to the workpiece on the die impact side. The goal was to provide a surface that could more easily across the die surface. Also this can improve the strain distribution, allowing greater part depths at similar values of peak strain. The conditions for the lubricant study were the same shown in Table 6.6 on the ferritic stainless steel. The forming depths of the lubricated studies as compared to the non-lubricated plates are shown in Table 6.10. As shown, the BN lubricant does not have much effect on the forming depth until failure. As before, the workpieces failed at the 3.2 kJ condition and the failure occurred out of the center of the channels.

Percent Full Depth-- Percent Full Depth-- Energy (kJ) unlubricated lubricated with BN 2.25 52 47 2.72 53 53 2.88 60 60 3.04 60 60 3.2 70 80

Table 6.11 Forming depths for lubrication with BN studies.

6.2.5 Summary

In summary, there are several factors that can be adjusted in order to achieve the desired formability properties. Among these are the material, coil gap, the discharge energy applied and lubrication. Also, if the impact velocity is high enough, the forming depth for electromagnetic forming method can match that of the traditional stamping method. It is shown here that with less ductile materials a greater depth of forming can be achieved than with the pressure-pad forming without failure by utilizing the correct combination of standoff and discharge energy.

6.3 Interface Effects

The importance of the interface and the effects it has on the workpiece is a critical issue under study. There are several interface regions within the electromagnetic forming system where spot welding or sticking can occur. Among these are 1) the outer channels and the driver sheet, 2) the interface between the driver sheet and the workpiece (shown in Figure 6.12) and 3) the interface between the die and the workpiece.

The sticking between the outer channel and the driver sheet has been previously investigated. This can be avoided by applying enough pressure to the outer channels and cleaning the outer channels with an abrasive paper and then with ethyl alcohol. 93

The pressure is necessary, as discussed previously, to ensure proper contact between the outer channels and the driver sheet. With appropriate care, it seems this welding between the return channel and driver can be eliminated.

The second interface is between the driver sheet and the workpiece as shown in

Figure 6.12.

Interface surfaces

Figure 6.12 The interface between the driver sheet and the workpiece material can be critical.

Sticking or spot welding can occur on the interface surface, as shown in Figure 6.13.

A summary of conditions is shown in Table 6.11.

Workpiece Material 304L Workpiece Thickness 0.004 in Driver Material Cu Driver Thickness 0.005 in Standoff 3 mm Discharge Energy 4 kJ Vacuum 100 torr

Table 6.12 Electromagnetic forming conditions used for the interface effects study.

Figure 6.13 Spot welding was seen at the interface between the copper driver sheet and the workpiece material.

The spot welding is highlighted in Figure 6.13 with blue circles. Spot welding was observed with the new coil (with the smaller gap) and the higher efficiency. The spot welding was very strong and in some cases the driver and the workpiece could not be separated from each other. This was observed in not only the 304L, but also with titanium, aluminum, and copper. In order to try and understand the cause of the spot welding all samples were used with a copper driver of 0.005 in thickness, including the copper and aluminum.

In an attempt to resolve the spot welding problem, various media was placed at the interface to prevent sticking, by physically separating the driver and the workpiece.

The media examined at the interfaces were a thin layer of vacuum grease, carbon paper and white paper each at separate points. The results, as shown in Figure 6.14, were unexpected. With the addition of media at the interface extensive shearing of the workpiece appears. However, shearing did not appear in the driver in any cases.

For the experimental media, only the 0.004 in 304L was used.

No interface material Carbon Paper

Paper Vacuum Grease

The sample with no interface material shows signs of tearing however this occurred when the sample was being removed from the driver sheet where spot welding had occurred. By placing media at the interface spot welding as eliminated but it caused extensive shearing to occur. The cross sections of the samples were examined to see where the shearing was occurring, as shown in Figure 6.15, and the shearing was highlighted with red circles.

1 mm Impact Side Impact Side 1 mm

No interface material Carbon Paper

1 mm Impact Side 1 mm Impact Side

Paper Vacuum Grease

Figure 6.15 The cross sections of the materials with media at the interface.

It is shown that the shearing occurred at the peaks away from the impact side of the die. The impact side can be defined as the side of the workpiece that came directly in contact with the die.

This issue has been resolved by ensuring the interface between the driver and workpiece is very clean. Previously, the driver and the workpiece were not cleaned using a detailed practice. With a higher efficiency coil, cleaning of the driver and workpiece surfaces pose a larger problem. By cleaning the interfaces with ethyl alcohol or acetone the problem can be eliminated.

The last interface to discuss is between the die and the workpiece. There has been no

evidence of sticking on the channels between the workpiece and the die, except at

higher energies. By adjusting the other parameters, such as standoff distances and

energy this can be avoided.

In summary, the issue of the interface has been found to have a larger impact on the

final workpiece than initially expected. The interface between the outer channels and

the driver can be controlled by cleaning the outer channels. More investigation into

the effect of the interface between the driver sheet and the workpiece and what is

occurring needs to be done. However, it was shown that the cleanliness of the

interface plays an important role in the outcome of the workpiece primarily in the

form of spot welding when using a more efficient coil. Lastly, the sticking on the

interface between the die and workpiece can be controlled by altering the standoff

and discharge energy.

6.4 Driver Elimination

The driver material is needed in order to move non-conductive materials with the uniform pressure coil as discussed previously. Several trials were performed using the ferritic stainless steel to eliminate the driver sheet. Higher energies must be used to move the material. This introduces new issues such as sticking to the die as discussed previously. The main issue is that the die material was more conductive 99 than the ferritic stainless steel. This caused the current to jump from the ferritic workpiece sheet to the die causing spot welding between the sheet and the die and damaging the die. By changing the die material to a less conductive material this problem may be eliminated.

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CHAPTER 7

SHEARING WITH UNIFORM PRESSURE COIL

In the past, shearing with electromagnetic forming has proven to work very well without the burr formation which is often seen with conventional punching if the clearance is to large, as shown in Figure 7.1. Termed Punchless Electromagnetic

Shearing (PEMS), a path coil provides the electromagnetic force to drive the material up and shear it off. Punchless Electromagnetic Shearing can be used with both the path coil and the uniform pressure system. Continuing research is being performed in this area at The Ohio State University by Scott Golowin and Glenn Daehn to further investigate the possibilities and applications of PEMS [19].

101

Figure 7.1 Burr can be seen in a traditional die shearing operation and no burr can be seen on the PEMS shearing [19].

7.1 Shearing

The ability to shear samples using the uniform pressure coil has also been investigated and successfully performed. The setup for a shearing application, illustrated in Figure 7.2, is similar to the forming setup. Shearing occurs readily in these materials and therefore no standoff is required, however a spacer on the outer channels is needed to provide the contact pressure needed with the driver sheet. In addition, a vacuum is not needed since an open die face is utilized and no shape is being generated.

102

The shearing application, by making tensile samples, using PEMS will allow tensile samples to be easily made out of sheet metal. This allows for other shearing applications using the uniform pressure coil system.

Clamping Force

Steel Block Workpiece Shearing sheet Components Cu driver sheet

Actuator / Coil

Figure 7.2 Schematic of shearing setup using the uniform pressure coil.

Tooling to produce a tensile bar shape was utilized for the shearing trials as shown in

Figure 7.3. The initial experiments included a long tensile bar. The current takes the path of least resistance, which resulted in arcing along the sheared edge. The current path is shown in Figure 7.4. With the large specimen, when the workpiece began to

103 shear, the current traveled around the sheared edge instead of through flat sheet. This problem was subsequently controlled by using the smaller tensile configuration to allow for a shorter current path and less arcing.

Figure 7.3The small tensile samples used to shearing out parts exhibited less arcing during the shear operation.

104

Current Direction

l L

Figure 7.4 Current travel for each tensile bar length. The example on the left, l, is an acceptable situation and the example on the right, L, is unacceptable.

The materials used for shearing trials included copper, aluminum, and ferritic stainless steel sheet. A table of materials, sheet thicknesses and the resulting discharge energy required is shown in Table 7.1. A thicker driver sheet was required with the ferritic stainless steel to prevent shearing from occurring in the driver sheet.

As the driver sheet begins to shear, the current follows the path of the sheared driver sheet. When the thicker driver sheet was used, a higher discharge energy was needed to move the ferritic stainless steel sheet.

105

Material Thickness (in) DriverThickness (in)Energy (kJ) Cu 0.005 ------1.28 Al-1145 O 0.005 ------0.96 Ferritic Stainless Steel 0.004 Cu 0.01 2.4

Table 7.1 Conditions and materials used for shearing experiments.

Presented in Figure 7.5 is an example of a ferritic stainless steel sheared tensile sample. There was no evident burring in the sheared samples as would be seen in a traditional press shearing method. This same result was previously reported in other electromagnetic shearing experiments (PEMS) when using a path coil.

Figure 7.5 Ferritic stainless steel sheared tensile bar.

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7.2 Summary

In summary, it has been demonstrated that shearing is possible with the uniform pressure coil. The path of the current is very important and should be carefully addressed and investigated. As shown in Figure 7.5 if the length (L) is too large the current cannot get around the sample, therefore a thicker driver can help because it fails well after the workpiece material therefore eliminating the arcing possibility.

One challenge is that it may be necessary to produce specialized tooling for shearing large components. However, the preliminary success has provided promising results for future application.

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CHAPTER 8

CRITICAL CHALLENGES AND FUTURE APPLICATIONS

There are several issues preventing electromagnetic forming technology from being implemented into an industrial setting.

8.1 Critical Issues

8.1.1 Driver Elimination

As previously discussed, a driver is used for less conductive materials. The driver is necessary, to provide energy to the less conducting material. However after it is used, the driver is essentially scrap and can not be used without further processing. This cost is generally not acceptable. By implementing a recycling/reuse system for the driver sheet the use of the driver could be more amenable to an industrial environment.

108

In order to eliminate the possibility that current can travel through the die, the die needs to be manufactured out of a low conductivity material. This can be done by use of ceramic or laminated dies. One way to eliminate the current travelling through a metallic die is by electrically floating the die or ensuring that it is electrically grounded.

8.1.2 Vacuum Level

The vacuum level has to be addressed before implementation into an industrial environment. As discussed previously, it is very important to ensure that the vacuum level does not fall in the area of the Paschen curve where ionization can occur. This could be avoided with the implementation of shut off valves and careful monitoring hardware, however this would add cost to a manufacturing process.

8.1.3 Outer Channels

There are several challenges involving the outer channels including pressure and cleanliness. The pressure on the outer channels needs be controlled carefully to ensure good contact is made between the driver sheet and the outer channels. This is to avoid arcing and spot welding on the outer channels. This surface must be cleaned occasionally to avoid potential arcing between the outer channels and the driver sheet.

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8.1.4 Interface

The avoidance of spot welding between the driver sheet and the workpiece must be addressed. Currently, the issues have been eliminated by ensuring the cleanliness of the interface surfaces. However, this could be a challenge in an industrial environment. Documented practices or a less arc sensitive tooling material maybe required to overcome this concern.

8.2 Future Applications

As previously discussed the uniform pressure coil can have several applications. The current focus is on utilizing this technique in flat sheet forming which provides desirable qualities in the formed part. These applications include forming and shearing as discussed previously but other applications are also being examined. The key is to achieve a better formability depth than can be found using traditional methods.

These processes are currently being introduced into an industrial environment with the help of General Motors, American Trim in Lima, Ohio and The Ohio State

University through the State of Ohio and the Third Frontier Grant. The program goal is to produce full scale bipolar plates. Under this grant a pilot plant is being constructed to implement experimental small scale studies which could be applied to a full scale industrial environment at American Trim in Lima, Ohio. The construction of the industrial line has begun and the problems are being addressed. With the help 110 of American Trim and General Motors this technology should be feasible for application in industry [20].

8.3 Summary

In summary, the challenges that need to be addressed are driver elimination, control of the vacuum level, continuous and uniform pressure on the outer channels of the coil and the interface issues of the driver and the workpiece. These issues are being investigated currently at The Ohio State University with the help of American Trim and General Motors. The full scale pilot plant will address production issues and take the technology one step closer to implementation into the industrial world and use for future applications.

111

CHAPTER 9

CONCLUSIONS

The applications for sheet forming are quite extensive, however, the evolution of high velocity forming has been limited by material capabilities. In electromagnetic forming, the application of the uniform pressure coil provides a uniform velocity distribution which has opened a wide range of applications including the production of bipolar fuel cell plates on an industrial environment scale.

The setup improvements, including use of a hydraulic press, new coil geometry, and steel vacuum box have allowed for more accurate and reproducible experiments to occur. As a result production variables could be assessed, as well as facilitating implementation into an industrial environment.

There are several areas that were investigated including flat sheet forming, shearing, and velocity measurement. In the area of flat sheet forming, several effects were studied including hardness increases, formability increases and interface issues of an electromagnetically formed part versus a pressure-pad formed part. 112

Approximately, a 10 % percent increase in average Vickers hardness was seen in the electromagnetically formed copper workpiece over the pressure-pad formed material and an increase in of approximately 20 % increase in average Vickers hardness was seen in the electromagnetically formed 316L workpiece with driver assistance over the pressure-pad formed material. This can be accounted for due to a higher strain rate in the electromagnetically formed material. A formability increase can be seen in several materials when using the uniform pressure coil application. Lastly, the condition of the surface of the interface between the driver sheet and the workpiece material has shown to be an important factor. This interface needs to be properly cleaned and free of contaminants in order to prevent spot welding between the driver sheet and the workpiece. Cleaning with ethyl alcohol or acetone have shown to provide acceptable surfaces.

Shearing can also be accomplished with the uniform pressure coil. Shearing using the uniform pressure coil requires no standoff and depending on the material, lower discharge energies than required for typical forming applications. A thicker driver is required for less conductive materials in order to reduce the possibility of arcing during shearing of the material.

Finally, the velocity can be measured for the uniform pressure coil using a laser and a

Photon Doppler Velocimetry (PDV) setup. The impact velocity has been measured

113 for flat sheet forming up to 205 m/s for a 316L sheet with a copper driver. Through analysis of the PDV data a specific velocity distance profile can be produced to develop relationships between impact velocity and standoff distances. This provides the tools to estimate the potential outcome properties of the workpiece.

With the support of Third Frontier Grant from the State of Ohio, General Motors and

American Trim implementation of the technology and application of the forming of bipolar fuel cell plates is being translated into an industrial setting. The pilot plant at

American Trim in Lima, Ohio is the first step in showing how it can be applied on a large scale production environment.

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LIST OF REFERENCES

1. Daehn, G.S., High Velocity Metal Forming , in ASM Handbook . 2006, ASM International. p. 405-418.

2. Golowin, S., Manish Kamal, Jianhui Shang, Jake Portier, Ahmad Din, Glenn S. Daehn, John R. Bradley, Keith E. Newman, and Steve Hatkevich, Application of Uniform Pressure Actuator for Electromagnetic Processing of Sheet Metal. Journal of Materials Engineering and Performance, 2007. 16 (4): p. 455-460.

3. Shang, J., Electromagnetically Assisted Sheet Metal Stamping , in Materials Science and Engineering . 2006, The Ohio State University: Columbus. p. 224. 4. Kamal, M., A Uniform Pressure Electromagnetic Actuator for Forming Flat Sheets , in Materials Science and Engineering . 2005, The Ohio State University: Columbus. p. 261.

5. Wang, W., R. H. Wagoner, and X.-J. Wang, Measurement of friction under sheet forming conditions. Metallurgical and Materials Transactions A, 1996. 27 (12): p. 3971-3981.

6. Daehn, G.S., M. Altynova, V.S Balanethiram, G. Fenton, M. Padmanabhan, A. Tamhane, and E. Winnard, High-Velcoity Metal Forming--An Old Technology Addresses New Problems. JOM, 1995(July): p. 42-45.

7. Kamal, M., and Glenn S. Daehn, A Uniform Pressure Electromagnetic Actuator for Forming Flat Sheets. Journal of Manufacturing Science and Engineering, 2007. 129 (April): p. 369-379.

8. EngineersHandbook.com, Electrohydraulic-Sheetmetal Forming . 2006.

9. Tamhane A. A., a.M.P., G. Fenton, M. Altynova, G.S. Daehn, V. J. Vohnout, and V. S. Balanethiram, Opportunities in High-Velocity Forming of Sheet Metal , in MetalForming Magazine . 1997. 115

10. Kamal, M., V. Cheng, T. K. Sue, J. Shang and G. S. Daehn. Replication of Microfeatures by Electromagnetic Launch and Impact . in International Conference on Micromanufacturing 2006 . 2006. Urbana, IL.

11. Electromagnetic Forming of Fuel Cell Plates .

12. Johnson, W., Impact Strength of Materials . 1970, London: Edward Arnold.

13. Strand et al . in Proceedings of SPIE . 2005. Bellingham, WA.

14. Daehn, G.S., Yuan Zhang, Scott Golowin, Kristin Banik, Anupam Vivek, Jason Johnson, Geoff Taber, Gregg Fenton, Ismael Henchi, and Pierre L'Eplattenier. Coupling Experiment and Simulation in Electromagnetic Forming Using Photon Doppler Velocimetry . in 3rd International Conference on High Speed Forming . 2008. Dortmund, Germany.

15. Taber, G., Glenn Daehn, Anupam Vivek, Photonic Doppler Velocimetry Applied to High Strain Rate Electromagnetic Ring Expansion . 2008, Columbus State Community College.

16. Standard Test Method for Microindentation of Materials , in ASTM Standards . 2007, ASM International: West Conshohocken.

17. [cited 3/26/2008]; Available from: http://www.duniway.com/images/pdf/pg/Paschen-Curve.pdf.

18. MatWeb Material Property Data . 3/26/2008 [cited; Available from: http://www.matweb.com/ . 19. Daehn, G.S. Agile Sheet Metal Forming: Basic Concepts and the Role of Electromagnetic Metal Forming . in IDDRG . 2007. Gyor, Hungary.

20. Mills, B., American Trim Receives $1 Million Grant , in Lima News . 2007: Lima.

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APPENDIX A

TECHINCAL DRAWINGS OF COILS

117

Figure A..1 Technical drawing for coil gap of 0.06 in.

118

Figure A..2 Technical drawing for coil gap of 0.125 in.

119