ABSTRACT
KARKHANIS, NIKHIL SUDHAKAR. An Integrated System Approach to Lubricant Development in Cold Forging. (Under the direction of Dr. Gracious Ngaile.)
Lubrication plays a very important role in the metal forming industry. Almost every metal forming process requires some form of lubrication or the other in order to obtain the desired component. Presently, various kinds of lubricants are being used in industry for different processes based on the lubricant properties and the process requirements.
One of the primary objectives of this study is to establish a lubricant development methodology which is based on a thorough understanding of the lubrication mechanisms involved and the metal forming process itself.
Evaluating the performance of a lubricant is an essential part of the development process. Various standard tribological tests employed in industry were used in this study. This involved both experimental and finite element analysis of the tests. Certain critical tribological parameters were identified for evaluating the performance of the lubricants and these were recorded for further analysis.
Another important objective of this study is to develop a simulation databank which would contain the finite element simulations for the major forging components used in industry. The creation of this databank is an essential part of the proposed integrated approach to lubricant development. This databank will enable lubricant developers to optimize their product for particular forging applications.
Accordingly, the appropriate tribological tests were selected for lubricant evaluation. Initially, a set of eleven lubricant formulations were taken for testing, each with a slightly different chemical formulation. In order to narrow down the suitable lubricant formulation, a low severity friction test such as the ring test was performed. Once the better performing formulations were identified, a more severe friction test was employed to narrow down on the best possible lubricant formulation. During the entire process, the lubricant developer modified the chemical formulations slightly based on the test results. The result of this was improved performance in the lubricants.
An Integrated System Approach to Lubricant Development in Cold Forging
by
Nikhil Sudhakar Karkhanis
A thesis submitted to the Graduate Faculty of North Carolina State University
In partial fulfillment of the requirements for the Degree of
Master of Science
Mechanical Engineering Raleigh, North Carolina
2008
APPROVED BY:
Dr. Jeffrey Eischen Dr. Jay Tu
Dr. Gracious Ngaile.
ii BIOGRAPHY
The author was born on the 7th of February, 1982 in the city of Pune, India. He completed his initial schooling in the city of Mumbai and later moved to Pune to continue his education. During his school years he developed an immense liking for astronomy and physics. By the seventh grade he had his own telescope and would spend many nights observing the wonders of the cosmos. His interest in physics developed his understanding of mechanics and played an important role in his decision to study mechanical engineering. He started his program in mechanical engineering at the University of Pune, in June 2000. He obtained his Bachelors degree in June 2004. After graduation, he took up a job with a reputed engineering company as a junior mechanical engineer. During the course of his work he often felt the need for a better understanding of issues in the field of mechanical engineering. As a result he decided to pursue further education in this field. He joined the Masters program in mechanical engineering at NCSU in the fall of 2005. After graduation, he intends to work in industry and further develop his engineering as well as management skills.
iii
ACKNOWLEDGEMENTS
I would like to thank Dr. Gracious Ngaile for being such a wonderful advisor and guide during my studies here at NCSU. He has always motivated me to do better and helped me out with nay problems I had. I would like to thank him for being patient with me, especially at times when things weren’t going very smoothly for me.
I greatly appreciate the help and advice given by my committee members Dr. Tu and Dr. Eischen.
I would also like to thank my lab‐mates Joey, Cristina and Chen for their support and advice during my research work. I also appreciate the help from people indirectly related to this project.
iv
TABLE OF CONTENTS
LIST OF FIGURES ... vii
LIST OF TABLES ... x
CHAPTER 1: INTRODUCTION 1.1 Motivation ... 1
1.2 Objectives and Scope ... 1
CHAPTER 2: INTRODUCTION TO MECHANICS AND TRIBOLOGY IN METAL FORMING 2.1 Interface Friction in Metal Forming ... 2
2.2 Forming Loads in various Forging Processes ... 3
2.2.1 Direct Compression in Plane Strain ... 3
2.2.2 Axisymmetric Compression ... 6
2.2.3 Rolling ... 8
2.3 Lubrication in Metal Forming... 11
2.3.1 Lubrication Mechanisms in Metal Forming ... 12
2.3.2 Desired Lubricant Properties ... 12
2.3.3 Lubricant Classification ... 13
2.3.4 Lubricant Additives ... 15
2.3.5 Lubricant Removal ... 16
2.3.6 Waste Minimization ... 17
CHAPTER 3: PROPOSED INTEGRATED SYSTEM APPROACH TO LUBRICANT DEVELOPMENT 3.1 Traditional Approach to Lubricant Development ... 19
3.1.1 Disadvantages of Present Techniques ... 21
3.2 Proposed Integrated System Approach to Lubricant Development ... 22
3.2.1 Methodology ... 22
3.2.2 Guideline for the New Forging Component Developer ... 25
3.2.3 Guideline for the Lubricant Developer ... 26
3.3 Advantages of New Approach ... 27
CHAPTER 4: FINITE ELEMENT AND EXPERIMENTAL DETERMINATION OF FRICTION 4.1 Need for Testing in Lubricant Development ... 28
4.2 Study of Tribo‐Mechanical Variables ... 28
4.2.1 Explanation of Variables ... 28
4.2.2 Interpretation of Simulation Results ... 29
4.2.3 Estimation of Local Surface Expansion ... 29
4.3 Experimental Determination of Friction ... 32
v
4.3.2 Double Cup Extrusion Test ... 35
4.3.3 Spike Test ... 37
4.3.4 Ball Penetration Test ... 38
4. 4 Finite Element Analysis of Friction Tests ... 40
4.4.1 Ring Test ... 40
4.4.2 Spike Test ... 44
4.4.3 Double Cup Extrusion Test ... 48
4.4.4 Ball Penetration Test ... 52
4.5 Selection of Appropriate Friction Test ... 56
4.6 Tribo‐ mechanical Variables and Lubricant Chemistry ... 58
CHAPTER 5: FORGING SIMULATION DATABANK 5.1 Purpose of a Simulation Database ... 59
5.2 Classification of Forged Components ... 60
5.2.1 Components made by Extrusion Process ... 60
5.2.2 Components made by Heading Process ... 62
5.2.3 Components made by Open/Closed Die Process ... 63
5.3 Simulation and Analysis of Forged Components ... 64
5.3.1 Extrusion Component ... 64
5.3.2 Summary of FEA Results ... 75
5.3.3 Heading /Upsetting Component ... 76
5.3.4 Summary of FEA Results ... 85
5.3.5 Open/Closed Die Forging ... 86
5.3.6 Summary of FEA Results ... 90
5.4 Extending Results to Similar Geometries ... 91
5.5 Geometric Factors in Extrusion Components ... 91
5.4.1 Backward Cup Extrusion ... 96
5.6 Geometric Factors in Heading Components ... 96
5.7 Effect of Strain Hardening Coefficient on the Surface Expansion ... 99
5.8 Interpolation/ Extrapolation of Results for Different Geometries ... 99
5.9 Simulation Database Structure ... 100
5.9.1 Main Part Table ... 100
5.9.2 SQL Queries and Functional Tables ... 101
5.9.3 User Interface ... 102
5.9.4 Editing the Database ... 103
5.10 Database Application Example ... 104
CHAPTER 6: CASE STUDY FOR DEVELOPMENT OF ENVIRONMENT FRIENDLY LUBRICANT 6.1 Introduction ... 110
vi
6.3 FEA Analysis of Tube Drawing ... 111
6.4 Lubricant Formulation ... 114
6.4.1 Use of Simulation Database ... 114
6.5 Testing of Lubricant Samples ... 119
6.6 Test Procedures and Parameters ... 119
6.7 Ring Test ... 119
6.7.1 Test Setup ... 120
6.7.2 Test Observation and Results ... 121
6.8 Ball Penetration Test ... 125
CHAPTER 7: CONCLUSIONS AND FUTURE WORK. 7.1 Conclusions ... 127
7.2 Future Work ... 128
REFERENCES ... 129
APPENDICES ... 131
Appendix I – Simulation Database ... 132
Appendix II – Simulation Results for Variation in Geometry and Friction ... 132
Appendix IIA – Geometric Factors in Extrusion ... 132
Appendix IIB – Geometric Factors in Heading ... 134
Appendix III –Extraction of Surface Variables in DEFORM ... 140
vii
LIST OF FIGURES
Figure 1 Variation of Friction with Normal Pressure ... 3
Figure 2 Compression in Plane Strain ... 3
Figure 3 Friction hill in plane strain compression ... 5
Figure 4 Axisymmetric Compression ... 6
Figure 5 Deformation Zone in Rolling ... 8
Figure 6 Stresses acting on Slab Element ... 9
Figure 7 The Stribeck Curve ... 11
Figure 8 Lubrication Mechanisms in Metal Working ... 12
Figure 9 Chemical Structure of the Solvents. ... 17
Figure 10 Flowchart of Traditional Lubricant Development Process ... 20
Figure 11 Schematic of the Integrated Approach to Lubricant Development ... 21
Figure 12 Design Sequence for a New Forging Component ... 23
Figure 13 Tetrahedral elements on Object Surface ... 31
Figure 14 Triangular Face on Surface ... 32
Figure 15 Calculation of Surface Expansion ... 33
Figure 16 Principle of Ring Compression Test ... 34
Figure 17 FEA Calibration Curves ... 35
Figure 18 Schematic for DCE Test ... 35
Figure 19 FEA Calibration Curves ... 36
Figure 20 Principle of the Spike Test ... 37
Figure 21 Schematic of the Ball Penetration Test... 38
Figure 22 Variation of Severity of Deformation in Ball Penetration Test ... 39
Figure 23 Forming Load Variation at Varying Reduction ... 39
Figure 24 Surface Expansion in Ring Test ... 40
Figure 25 Normal Pressure in Ring Test ... 41
Figure 26 Interface Temperature in Ring Test ... 42
Figure 27 Surface Shear Stress in Ring Test ... 42
Figure 28 Interface Pressure on Die... 43
Figure 29 Surface Expansion in Spike Test ... 44
Figure 30 Normal Pressure in Spike Test ... 45
Figure 31 Interface Temperature on Workpiece ... 45
Figure 32 Shear Stress on Surface ... 46
Figure 33 Interface Pressure on Die... 46
Figure 34 Surface Expansion in DCE ... 48
Figure 35 Normal Pressure in DCE ... 49
Figure 36 Interface Temperature in Workpiece ... 49
Figure 37 Shear Stress on Surface of Workpiece ... 50
viii
Figure 38 Interface Pressure on Die... 50
Figure 39 Surface Expansion in Ball Penetration Test ... 52
Figure 40 Normal Pressure in Ball Penetration Test ... 53
Figure 41 Interface Temperature ... 53
Figure 42 Shear Stress on Surface of Workpiece ... 54
Figure 43 Interface Pressure on Die... 54
Figure 44 Application of Simulation Database ... 59
Figure 45 Component Classification ... 60
Figure 46 Typical Components Made by Extrusion ... 61
Figure 47 Typical Components Made by Heading ... 62
Figure 48 Typical Components Made by Open/Closed Die Forging ... 63
Figure 49 Process Sequence for Pinion Gear Shaft ... 64
Figure 50 Surface Expansion in 1st and 2nd stage of Pinion Gear Shaft ... 65
Figure 51 Surface Expansion in 3rd stage of Pinion Gear Shaft ... 66
Figure 52 Effective Strain in 1st and 2nd stage of Pinion Gear Shaft ... 67
Figure 53 Effective Strain in 3rd stage of Pinion Gear Shaft ... 68
Figure 54 Normal Pressure in 1st and 2nd stage of Pinion Gear Shaft ... 69
Figure 55 Normal Pressure in 3rd stage of Pinion Gear Shaft ... 70
Figure 56 Interface Temperature Variation in 1st and 2nd stage of Pinion Shaft ... 71
Figure 57 Interface Temperature Variation in 3rd stage of Pinion Shaft ... 72
Figure 58 Surface Shear Stress Variation in 1st and 2nd stage of Pinion Shaft ... 73
Figure 59 Surface Shear Stress Variation in 3rd stage of Pinion Shaft ... 74
Figure 60 Process Sequence for Philips Head Screw ... 76
Figure 61 Surface Expansion in 1st and 2nd stage of Philips Head Screw ... 77
Figure 62 Surface Expansion in 3rd stage of Philips Head Screw ... 78
Figure 63 Effective Strain in 1st and 2nd stage of Philips Head Screw ... 79
Figure 64 Effective Strain in 3rd stage of Philips Head Screw ... 80
Figure 65 Normal Pressure in 1st and 2nd stage of Philips Head Screw ... 81
Figure 66 Normal Pressure in 3rd stage of Philips Head Screw ... 82
Figure 67 Interface Temperature in 1st and 3nd stage of Philips Head Screw ... 83
Figure 68 Surface Shear Stress in 1st and 3rd stage of Philips Head Screw ... 84
Figure 69 Forging of Ball Bearing by Upsetting Cylindrical Billet ... 86
Figure 70 Surface Expansion in Ball Bearing Forging ... 87
Figure 71 Effective Strain and Normal Pressure in Ball Bearing Forging ... 88
Figure 72 Interface Temperature and Surface Shear Stress in Ball Bearing Forging ... 89
Figure 73 Geometric Parameters in Extrusion ... 91
Figure 74 Surface Expansion Vs Extrusion Ratio ... 92
Figure 75 Surface Expansion Vs Die Half Angle (ER = 2) ... 92
Figure 76 Surface Expansion Vs Die Half Angle (ER = 3) ... 93
ix
Figure 77 Surface Shear Stress Vs Die Half Angle ... 93
Figure 78 Interface Pressure Vs Die Half Angle ... 94
Figure 79 Interface Temperature Vs Die Half Angle ... 94
Figure 80 Surface Expansion Vs Die Radius (ER = 2) ... 95
Figure 81 Surface Expansion Vs Die Radius (ER = 3) ... 95
Figure 82 Surface Expansion Vs BCE Ratio ... 96
Figure 83 Surface Expansion Vs Height Reduction ... 97
Figure 84 Interface Temperature Vs Height Reduction ... 97
Figure 85 Interface Pressure Vs Height Reduction ... 98
Figure 86 Surface Shear Stress Vs Height Reduction ... 98
Figure 87 Effect of Strain Hardening on Surface Expansion ... 99
Figure 88 Data Flow in Access ... 100
Figure 89 Main Table containing Part Data ... 101
Figure 90 User Interface for Simulation Database ... 102
Figure 91 Spiral Bevel Gears ... 104
Figure 92 Bevel Gear Data from Database (Interface Temperature) ... 106
Figure 93 Bevel Gear Data from Database (Surface Expansion) ... 107
Figure 94 Bevel Gear Data from Database (Normal Pressure) ... 108
Figure 95 Tube Drawing Process ... 110
Figure 96 Surface Expansion in Tube Drawing ... 111
Figure 97 Effective Strain and Normal Surface Pressure in Tube Extrusion ... 112
Figure 98 Surface Shear Stress in Tube Extrusion ... 113
Figure 99 Basic Composition of the Polymer Lubricants ... 115
Figure 100 Pinion Shaft Data from Database (Interface Temperature) ... 116
Figure 101 Pinion Shaft Data from Database (Surface Expansion) ... 117
Figure 102 Lubricant‐ Metal Interaction ... 118
Figure 103 Friction Test Setup ... 120
Figure 104 Ring Test Sample ... 121
Figure 105 High Temperature Results for Ring Tests ... 121
Figure 106 Results for Lubricant Drying for Ring Tests ... 122
Figure 107 Results for Enhanced Drying of Lubricant... 123
Figure 108 Results for Lubricant Dilution ... 123
Figure 109 Results for Lubricant Dilution with Enhanced Drying ... 124
Figure 110 Results for Lubricant Solid Content Variation ... 124
Figure 111 Punch Load for Lub 23 and 39 ... 126
x
LIST OF TABLES
Table 1 Results for Ring Test ... 43
Table 2 Results for Spike Test ... 47
Table 3 Results for BCE Test ... 51
Table 4 Results for Ball Penetration Test ... 55
Table 5 Test Selection for Surface Expansion ... 56
Table 6 Test Selection for Normal Pressure ... 56
Table 7 Test Selection for Interface Temperature ... 57
Table 8 Test Selection for Surface Shear Stress ... 57
Table 9 Relation between Tribo variables and Lubricant Property ... 58
Table 10 Summary of Results for Extrusion Component ... 75
Table 11 Summary of Results for Heading Component... 85
Table 12 Summary of Results for Ball Bearing Forging ... 90
Table 13 Summary of Results for Tube Drawing ... 114
CHAPTER 1 INTRODUCTION
1.1Motivation
Lubrication plays a very important role in the metal forming industry. Almost every metal forming process requires some form of lubrication or the other in order to obtain the desired component. Presently, various kinds of lubricants are being used in industry for different processes based on the lubricant properties and the process requirements. Traditionally, selection of lubricants for a given process has been more of a trial‐error process wherein the correct lubricant would be selected after testing several different lubricants and gradually narrowing down to a few.
The loss of time as well as resources associated with such a lubricant development process is no longer economically viable in today’s scenario. Thus, there is a need to develop a more scientific method of lubricant development which can be applied to industrial problems.
1.2Objectives and Scope
• One of the primary objectives of this study is to establish a lubricant development methodology which is based on a thorough understanding of the lubrication mechanisms involved and the metal forming process itself.
• Analysis of various process parameters will be done using Finite Element Method and the necessary parameters for lubricant development will be extracted. Similar analysis will be done for the various tribological tests used in lubricant development.
• Based on this study it is intended to compile a database which contains the lubrication development requirements for several typical forged components in industry.
• The scope of this study is limited to typical components in the cold forging industry.
CHAPTER 2
INTRODUCTION TO TRIBOLOGY IN METAL FORMING
Metal forming involves the plastic flow of metal without fracture. The property of ductility of metals is used to obtain different shapes and sizes of mechanical components. Various metal forming processes such as forging, rolling, extrusion etc. exist in industry. However, each process involves contact between the deforming object (die) and the deformed metal (workpiece). Friction and wear arise due to such contact and they are studied under tribology.
2.1 Interface Friction in Metal Forming
There are two basic laws used to quantify the friction that is present at the interface of die and workpiece. The first one is the Coulomb Law which essentially gives a linear relation between the normal pressure ( n) and the shear stress at the interface (τ).
σn (2.1)
This law however, is limited in its use since the shear stress cannot exceed the shear strength (k) of the material. So under certain loading conditions, the value for actually starts dropping as the normal pressure increases and . Several metal forming processes experience very high normal pressures and in such cases the Coulomb Law would not be applicable.
The second law is the Shear Friction Law which defines the interface friction in terms of a friction factor (f) or shear factor (m) [Shey, 1983].
√3 (2.2)
where, is the flow stress of the material.
So, m = 0 for no friction and m = 1 for ‘sticking’ condition (where shearing occurs before sliding). Figure 1 below shows the variation of the frictional stress ( ) with the increasing normal pressure ( .
2.2 Forming Loads in Various Forging Processes [Hosford, Caddell, 1983]
The forming loads have been derived in terms of the interface friction for some basic metal forming processes. Some of the common processes are described below.
2.2.1 Direct Compression in Plane Strain
Consider a slab being compressed between two flat dies as shown in figure 2 below.
Normal Pressure ( )
Friction
Stress
(
)
Figure 1 Variation of Friction with Normal Pressure [Bay, 1994]
Figure 2 Compression in Plane Strain
This is considered as a plane strain case since the width of the slab is much greater than its other two dimensions. Consider the forces acting on a piece of thickness ‘dx’ inside the slab. ‘P’ is the normal pressure exerted by the die on the workpiece and is the frictional stress acting at the interface. Force balance in ‘x’ direction gives:
2 0
2 (2.3)
For plane strain 2 and (this follows from the Tresca criterion where σ1 – σ3 = 2k where, k is the shear strength of the material.
Assumption: the effect of frictional stress is minimal and does not affect the principal stress directions. Hence , are considered the principal stresses).
in equation (2.3), 2 (2.4)
2
ln 2
Boundary conditions: at x = 0, = 0 and P = 2k.
ln 2
ln 2
2
2 (2.5)
It should be noted that the above expression is valid only for for for sticking friction,
so equation 2.4 becomes,
2
again integrating the above expression,
2 Applying boundary conditions: at x = 0, = 0 and P = 2k
2
2 1 (2.6)
Figure 3 below shows the variation of normal pressure required to deform slabs of different dimensions (x/h) and different friction conditions.
Figure 3 Friction Hill in Plane Strain Compression [Hosford, 1983]
It can be observed that the forming pressure increases from the edge of the slab to the center. From the lubrication perspective this is important because the lubricant will be subjected to highest pressures at the centre of the slab and thus there is a greater chance of lubricant failure in this region. One of the primary aims in this study is to estimate the friction and the above result for forming pressure is enables the process developer to relate the forming loads with the experimental or calculated values of friction.
2.2.2 Axisymmetric Compression
Figure 4 Axisymmetric Compression
Consider the case of axisymmetric compression, the forming load is derived below. Figure 4 shows a section (dr) of a ring element subjected to normal pressure ‘P’ by the flat die. σr and σθ are the radial and tangential components of stresses.
Force balance in the radial direction gives:
2 0 (2.7)
Neglecting higher order term, the above expression reduces to
2 0 (2.8)
now, for axisymmetric flow, so (this follows from the flow rule in plasticity which is similar to stress‐strain relations in elasticity).
Also, according to the Tresca cirterion σ1 – σ3 = 2k. Again and are considered as principal stresses neglecting the effects of frictional stress on the principal directions.
so, 2 where
so, and equation 2.8 becomes,
2 (2.9)
Integrating,
2
ln 2
Applying boundary conditions P = Y when r = R
ln 2
ln 2
2
(2.10)
It should be noted that the above expression is valid only for for
for sticking friction, . Substituting this in equation 2.9 we get the folowing
2
Integrating,
2
2
Applying boundary conditions P = Y when r = R
2
It can be observed that the forming pressure increases from the edge of the disc to the center. From the lubrication perspective this is important because the lubricant will be subjected to highest pressures at the centre of the slab and thus there is a greater chance of lubricant failure in this region.
2.2.3 Rolling [Rees, 2006]
Consider the case of a metal slab of thickness ‘h1’ being rolled to a final thickness of ‘h2’. In order to analyze the rolling pressure required a strip of metal of thickness ‘dx’ is considered. ‘R’ and ‘θ’ are the radius and angle subtended by the slab portion of the slab with the center of the roller. Therefore, strip of thickness ‘dx’ subtends angle ‘dθ’ with the center of the roller.
The force balance for this strip can be written as:
2 2
2 2 (2.12)
Note: for a strip ‘dx’ on the entry side of the neutral plane, the direction of the frictional force is reversed. So a force balance for both strips can be expressed in a combined form.
2 2 (2.13)
Since sticking friction condition prevails in rolling operation, we can substitute, in equation 2.11.
2 2 (2.14)
Simplifying assumption: assume rolls are replaced by flat plates i.e. θ is small. Hence we can write , 0 and h = h2 at the exit side. Equation 2.12 becomes
2
P μP
σx+ d σx σx
1
dx
μP P
Integrating,
2
2
Applying boundary conditions: at x = 0, 0 we get C1 = 0 2
2.15
At the neutral plane, x = x0,
2
Using the Tresca yield criterion with P and σx considered as the principal stresses and neglecting the effect of frictional stress on the principal stress directions, P ‐ σx = 2k
2 2
2 1 for 0 (2.16)
From the expression above it can seen that the forming pressure increases as we approach the neutral plane. Thus, this area is critical from the lubrication point of view. The above expressions are useful in predicting the forming loads in rolling. Quite often in rolling, sticking friction condition is prevalent so the forming pressure depends on the shear strength and geometry of the slab.
2.3 Factors Influencing Friction in Metal Forming
There are three main factors that affect the frictional conditions during metal forming operations. These factors are:
• Process parameters: these include interface pressure and temperature between die and workpiece, local surface strains on the formed component, sliding velocity, process duration etc.
workpiece, their respective material properties and geometry.
• Lubricant properties: these include the viscosity, chemical composition, film thickness of the lubricant.
2.3 Lubrication in Metal Forming
Proper selection of a lubricant depends on understanding the lubricating regime (i.e., film, mixed, boundary), established conventions of classifications, and an ability to interpret and apply the producer’s product data specifications to the equipment. Without this background, it is impossible to make an informed selection or substitution.
2.3.1 Lubrication Mechanisms in Metal Forming [Altan, Ngaile, Shen, 2005]
In industrial metalworking, there are four basic modes by which lubricants decrease friction.
Stribeck number (η) N/P
Figure 7 The Stribeck Curve [Campbell, 1982]
1. Hydrodynamic Lubrication: Simply stated, this means that a reduction in friction is
achieved by maintaining a constant fluid film between two solid surfaces.
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• Reduce Wear – The die should not have and wear on its surfaces. • Adapt to Conditions – Work in the changing conditions during forming.
• Material Compatibility – The lubricant cannot damage the die of the workpiece • Rapid Response – The forming process is fast requiring quick adaptation
• Durability – Lubricant film needs to endure several operations
• Controlled Finish – The lubricant must produce an acceptable finish on the part • Cooling – At high speeds the need for cooling is very important to reduce wear A good lubricant should also posses the following chemical properties:
• Stability – Fluid concentrate must store for six months without clouding or separating • Oxidative Stability – Oxidation of additives needs to be controlled especially during
heating and cooling cycles
• Emulsion Stability – Emulsion needs to be balanced in alkalinity, acidity, and HLB
• Hard‐Water Stability – Salt levels increase in baths as time goes on making hard‐water stability a must
• Mixability of Fluid Concentrate – Must allow fast and complete mixing • Foam – Air does not lubricate therefore foam should be controlled
• Residue – The lubricant should not leave a sticky or hard to clean residue on parts or die • Corrosion Inhibition – Since the use of water is common corrosion resistance is a must • Lubricity – Effective use of boundary lubricants and extreme pressure additives.
2.3.3 Lubricant Classification [Peterson, 1995]
In industrial metalworking there are four major classifications of lubricants; mineral oils, emulsions, semi synthetics and synthetics. There are various other classifications of lubricants such as waxes, greases, dry film lubricants, etc. that will not be discussed here since they are not considered to be in the realm of metalworking lubricants.
• Mineral Oils
hydrocarbon base) and naphthenic (ring structured hydrocarbon base).
Naphthenic oils have the advantage of having a much higher solubility for many types of additives. These additives are often necessary to accomplish the boundary lubrication mentioned previously as well as impart other desirable characteristics to the lubricant. It is because of this attribute that naphthenic oils are usually chosen as the base mineral oil for metalworking applications.
• Emulsions
Macro emulsions or so called “soluble oils” start as a formulated oil mixture called the concentrate which typically contains a naphthenic mineral oil base along with emulsifiers and other additives.
When used, this oil is then diluted with water to create an oil‐in‐water emulsion, typically containing five to ten percent oil. It will appear as a white, opaque solution. The relatively large size of this emulsion will generally have a lower stability and a tendency to want to separate over time. This is accelerated by the fact that metal fines and other debris accumulate in the solution providing sites for oil to coalesce. Macro emulsions have the advantage of providing good lubrication from the oil contained in the emulsion as well as additives that can be included with the oil. Additionally, the water phase is available to provide cooling.
• Semi‐Synthetics
Semi‐synthetics or micro emulsions contain emulsified oil in the range of 0.01 microns to 0.2 microns. Unlike a macro emulsion that starts as an oil with additives, the micro emulsion concentrate is already an emulsion since it contains water. Besides water, the micro emulsion will typically contain mineral oil, emulsifiers, dispersants, boundary additives, and anti‐foams. These concentrates are then diluted with water prior to use. Semi‐synthetics are generally more effective for cooling than lubricity since there is only a very small percentage of oil in the working solution. Because of the lower amount of oil and the higher amount of emulsifiers, the micro emulsion is much more stable over time than the macro emulsion. Disadvantages of this lubricant class are higher cost and difficulty in disposal.
• Synthetics
Synthetic lubricants contain no oil and typically will be made up from polyglycol, polyisobutylene or poly alpha‐olefin bases. They will appear as transparent solutions and will oftentimes contain emulsifiers, amines, dispersants and anti‐foams. These solutions also are tailored more towards cooling than lubricating. A typical application would be for high speed cutting and grinding where tool life can be extended. This is important since these solutions are much more expensive than mineral oils, demanding a return on investment. Most synthetic formulations generally are not used in severe duty applications such as deep drawing. Like semi‐synthetics, synthetics can also suffer from disposal problems. Although it would make sense that water based synthetic and semi‐synthetic lubricants would be the easiest to clean in an aqueous media, this is not always the case. If allowed to dry, the water from an oil ‐in‐ water emulsion will evaporate. The remaining lubricant will then invert to a water‐in‐oil emulsion making for a difficult to remove polymeric film. The more mineral oil in a lubricant, the better its lubricating properties. The more water contained in a lubricant, the higher it’s cooling capacity.
2.3.4 Lubricant Additives [Peterson, 1995]
There are a variety of additives that can be present in lubricants to serve many purposes. Some of the additives include corrosion inhibitors, boundary additives, extreme pressure additives, anti‐foams, emulsifiers, dispersants and viscosity index modifiers.
• Boundary Additives
These are additives that adsorb one or two molecules thick at the metal surface. They are polar in nature and provide lubrication when the fluid film wears too thin to provide hydrodynamic lubrication. Typical boundary additives are C12‐C18 saturated fatty alcohols or fatty acids. Fatty acids tend to be more effective since they can react with active oxide surfaces on the metal to form soap.
• Extreme Pressure (EP) Additives
They actually react with the metal surface to lower friction at higher pressures and temperatures. There are three well known types of EP additives; phosphorous based, chlorine based and sulfur based. Since EP additives are effective over various temperature ranges, there is usually a need for more than one in a lubricant formulation. Boundary additives and EP additives are usually found in mineral oil, emulsion type and to a lesser extent semi‐synthetic lubricants. The functions of other previously mentioned additives are generally self explanatory.
2.3.4 Lubricant Removal [Peterson, 1995]
The following will discuss the various categories of industrial cleaning and their chemistries such that a better understanding of the cleaning mechanism is possible. Certain cleaner/lubricant combinations are more compatible making the cleaning process easier and also more amenable to waste minimization.
2.3.4.1 Solvent Cleaning
This is a broad category covering anything from simple hand wiping with mineral spirits to a complex vapor degreasing system utilizing a non‐flammable chlorinated solvent. The principle is the same in any case. These solvents dissolve lubricant at the metal surface and work essentially by dilution. Most are generally non‐polar and therefore dissolve a majority of lubricants which are usually non‐polar as well. As one departs from non‐polar lubricants and moves toward more polar synthetics and semi‐synthetics, the “like dissolves like” relationship tends to be less applicable and cleaning tends to be less effective. Typical solvents are as follows:
• Paraffinic Solvents
These usually have flash points in the 120‐170° F range and are generally used in hand wiping and room temperature immersion applications.
• Aromatic Solvents
This category includes naphtha, xylenes and toluene. They are also applied via wiping and room temperature immersion.
• Terpenes
Two terpenes found in some industrial and household cleaners are shown in Figure 9. These are typically used in immersion applications, although hand wiping may also be utilized.
Figure 9 Chemical Structure of the Solvents
2.3.5 Aqueous Cleaning
Aqueous cleaning represents probably the most significant class of industrial cleaning for many reasons.
1. Environmental, safety and health concerns regarding solvents. 2. Increasing numbers of facilities utilizing it.
3. Aqueous cleaning has the most to be gained from waste minimization techniques.
Aqueous cleaners represent the most complex chemistry since they include a number of different chemicals, both organic and inorganic. They are formulated from three groups, some of which tend to overlap in function. Those groups are builders, surfactants and additives.
2.3.6 Waste Minimization
distillation which can be done on a continuous or a batch basis. Chlorinated solvents are more capable of removing polar soils than hydrocarbon solvents. Their primary disadvantage is their tendency to degrade and go “acid”. Semi‐aqueous processes tend to be most effective with mineral based, emulsion and semi‐synthetic lubricants.
The varied constituents in a synthetic lubricant could tend to make the cleaner less effective more quickly since the surfactants and emulsifiers in it could be counterproductive to the surfactants added to the semi‐aqueous formulation. Aqueous cleaners probably have the most opportunities for waste minimization since there exists more equipment for oil removal than there does for any other type of cleaning. Although mineral oils may not be as easy to remove in the cleaning process as some synthetics, they present probably the largest opportunities for waste minimization.
CHAPTER 3
PROPOSED INTEGRATED SYSTEM APPROACH TO LUBRICANT DEVELOPMENT
3.1 Traditional Approach to Lubricant Development:
The conventional method of developing lubricants in industry consists of four major activities:
• Utilizing the process designer’s experience. • Utilizing the lubricant manufacturer’s experience. • Pilot testing to evaluate the lubricant performance. • Full‐scale testing of the lubricant.
The prime considerations are film thickness and wear [Peterson, 1995]. Although film thickness can be calculated, the wear properties associated with different lubricants are more difficult to assess. Lubricants are normally tested by subjecting them to various types of physical stress. However, these tests do not completely indicate how a lubricant will perform in service. Experience has probably played a larger role than any other single criterion [Peterson, 1995]. Through a combination of testing and experience, machine manufacturers have learned which classes of lubricants will perform well in their products. It should be noted that the equipment manufacturer's recommendation should not necessarily be considered the best selection. Individual manufacturers may have different opinions based on their experience and equipment design. The concept of “best” lubricant is ambiguous because it is based on opinion. Despite this ambiguity, the manufacturer is probably in the best position to recommend a lubricant. This recommendation should be followed unless the lubricant fails to perform satisfactorily. When poor performance is evident, the manufacturer should be consulted for additional recommendations. This is especially critical if the equipment is still under warranty.
Figure 10 shows the process schematic for the traditional method of lubricant development.
Figure 10 Flowchart of Traditional Lubricant Development Process.
Process Design + Lub
Manuf.
Initial Lubricant Formulation.
Result OK? Pilot Testing.
Full Scale Testing.
Result OK?
No
No Yes
Yes
3.1.1 Disadvantages of Present Techniques
• Whenever a manufacturer contemplates a completely new product, there will very limited knowledge about the tribological factors involved. So, in such a case neither component manufacturer nor the lubricant manufacturer will able to predict the lubricant requirements accurately. This will only lead to larger number of trial and error experiments which is not the optimum way of developing a lubricant.
• Even in the case where there is some prior experience, economic factors can result in the use of non‐ optimum lubricant for a given application. For example, a lubricant manufacturer may recommend a lubricant having properties which are far superior than actually required because of a higher profit margin. Similarly, the forging company may decide to use the same lubricant for a much less demanding operation just because “it works”. The forging company may not realize the economic savings possible by the use of an optimum lubricant.
• The development time for a new lubricant using this technique will tend to be longer due to all the pilot testing involved in the initial phases of the development. This directly affects the competitiveness of the forging company in the market.
3.2 Proposed Integrated System Approach to Lubricant Development 3.2.1 Methodology:
In this study, typical forging components manufactured in industry are taken and classified into three basic categories:
a) Components produced using open/closed die forging techniques. b) Components produced using mainly extrusion techniques.
c) Components produced using mainly heading techniques.
• FEA simulations for each forging category are done and several important variables related to the tribo‐mechanics of the process are extracted from the simulation results. Such variables include the local surface strains, normal surface pressure, surface temperature etc. All these variables play a very important role in deciding the lubrication requirements for a given process. For example a region on the surface of the component being forged may show very high surface strains which may result in the failure of the lubricant film. So, a preventive measure can be using a lubricant with higher shear strength or modifying the forging process itself to reduce the surface strain. Thus a database of FEA results for all forging categories will be formed that can be directly used by lubricant developers in industry.
This is the first stage in the integrated approach as shown in the process schematic
(Fig. 11).
As shown in the schematic diagram below, each forging category will be analyzed through FEA modeling and tribo tests to determine the lubrication requirements. Figure 11 Schematic of the Integrated Approach to Lubricant Development [Ngaile, Cochran, Stark, 2006]
• Once a lubricant developer formulates an initial composition for the lubricant, this lubricant can be tested for some basic tribological performance using standardized tests. There are several basic simulative tribological tests used in industry such as the ‘ring compression test’ or ‘spike test’ to evaluate the tribological performance of a lubricant.
Computer Modeling of a Forging Process: Tribomechanical-thermal variables • Surface evolutionf(n, t, geo, etc)
• Interface pressure gradient • Surface strain gradient • Surface temp gradient • Surface stress grad • Surface pocket grad • Surface temperature
Lubricant formulation Specific forging category target characteristics Tr ib o c h e m is tr y Simulative and
process tribotests Field trial
B a si c t e st s P h ys ic a l/c h e m ica l/t h e rm a l Quantification of tribotest variables
Via FEA &
I
AI
BI
C Numerical Computer Simulation Databank Lubricant Variants Commercialization Brochures with tech data from• These tests will give the developer a fair idea as to what the lubricant composition should be. This process is shown in loop A of the process schematic.
• The next stage in the development process is to include the tribological analysis from the FEA simulations done previously. In this stage certain specific tribo tests will be targeted which closely represent the actual forgoing process. Such tests may be modifications of the basic tribological tests done earlier.
• The simulation databank will provide the developer with the required information regarding the essential variables (surface strains, temp. etc) to be tested for the forging process under consideration. This stage is shown in loop B of the process schematic. This stage will help the developer to narrow down on the chemical formulation of the lubricant.
• The final stage in this process will be to put the developed lubricant through actual field trials to evaluate its performance. Minor modifications to the lubricant formulation may be needed depending on the test results and the product will be ready for commercialization.
• Thus each lubricant formulation developed through this process will be tuned towards the specific forging process in question and there will be no requirement for a trial‐error approach before using the lubricant in industry.
3.2.2 Guideline for the New Forging Component Developer
One of the major objectives of this study is to simplify the lubricant development for a completely new forging component. Since this is an integrated development methodology, the forging component development and the lubricant development will go hand‐in‐hand. Following flowchart (Fig. 12) illustrates the suggested sequence of actions for the forging developer.
Figure 12 Design Sequence for a New Forging Component
Proposed Forging Sequence
FE Analysis
Strain/ Stress Acceptable?
Heat Treatment/ Geom Mod.
Tribo Parameters Acceptable?
Finalize Process Feedback from
Lub. No
No Yes
3.2.3 Guideline for the Lubricant Developer
Modern lubricants are a complex formulation of various chemicals each performing its own task in achieving the required lubricant performance. Almost all lubricants have a basic composition and several chemical additives. The process of selecting the appropriate chemical additives is quite an involved process by itself which is out of the scope of this study. However certain guidelines have been put forth [McClure, 2005] to make this process efficient. One of the suggested methods of additive selection is the use of statistical techniques like ‘Design of Experiment’. Following points elaborate this method
• The integrated approach suggested previously provides the lubricant developer with requirements for the basic composition of the lubricant. Now the appropriate additives must be selected.
• In the statistical ‘experiment’ the various additives are considered as ‘factors’ which are used at particular ’levels’ (varying concentration). Each experiment has an ‘output’ which is nothing but the result of the actual friction test (ring test, ball penetration etc.). • An array is created with the above parameters and the best response or output is
identified. The additives and their respective concentrations for the best output can then be identified.
The above technique is a much more scientific method of additive selection than a traditional trial‐error method.
3.3 Advantages of New Approach.
The new approach has some clear advantages over the traditional approach. Some of the major advantages are:
• Estimation of the lubricant performance requirements during the forging process development stage itself.
• Great savings in cost and time since the initial pilot testing stage and the trial‐error process is eliminated.
• The lubricant properties can be optimized for the forging process in question thus avoiding formulation of expensive lubricants which may not be required.
• Overall reduction in the time to commercialize the lubricant from the development stage.
CHAPTER 4
FINITE ELEMENT AND EXPERIMENTAL DETERMINATION OF FRICTION
4.1 Need for Testing of Lubricants
Each metalworking process has different lubricating needs based on several process parameters such as severity of deformation, process temperature, deformation forces etc. hence a lubricant having certain generic properties cannot be used for different metal working processes. Therefore lubricants have to be tested for their tribological performance in order to meet the requirements of the metal forming process in question. Also from the economic point of view, it is not feasible to use expensive lubricants exceeding the performance requirements of the process. The testing process can thus provide an answer regarding the optimum lubricant properties required for each process.
In order to identify the correct lubricant properties that are required, the tests performed on the lubricant must be representative of the actual forming process. This means that the test should be able to replicate the various tribological and tribo‐mechanical variables involved in the actual process.
So, a thorough analysis of the tests themselves is required to select the appropriate tests. The analysis of tests involves the calculation of certain tribo‐mechanical and thermal variables. Some of the variables are defined in the following section.
4.2 Study of Tribo‐Mechanical Process Variables 4.2.1 Explanation of Variables.
There are several process variables that affect the performance of a given lubricant. Some of the important variables are:
• Local Surface Expansion: This variable essentially means the strains occurring at the
• Surface Normal Pressure: The normal pressure acting on the component (and the lubricant film) can affect the integrity of the lubricant film and hence the lubrication mechanism itself. Hence it is necessary to know whether a particular lubricant film can sustain a certain level of pressure and maintain a continuous film over the component.
• Surface Normal Pressure Gradient: The knowledge of the variation of the surface
pressure can help the designer in establishing the lubrication regime i.e hydrodynamic etc., likely to be present in local areas of the workpiece. Also areas having large gradient can represent problem areas where the lubricant is likely to fail.
• Surface Strain Gradient: The variation of the surface strains helps in predicting the area
on the workpiece where the lubricant film may be subjected to high tensile forces and subsequent thinning.
• Surface Temperature Gradient: There is a temperature rise in the forged component
due to the interface friction as well as the internal plastic deformation. It is essential to find out the temperature rise at the interface of the die and component as it directly affects the property of the lubricant. This gives the designer an idea about the required thermal properties of the lubricant.
• Surface Stress Gradient: The areas of high and low friction can be identified from the
shear stress component or the surface stresses. The designer can make changes to the geometry of workpiece or die based on the variation of the surface stresses observed during the process. The surface stresses directly affect the load required to deform the component.
These are the primary variables that will be studied during the finite element as well as the experimental analyses of friction.
4.2.2 Interpretation of Simulation Results
the workpiece can be obtained easily. Also specific areas of interest can be investigated separately without going through data for the entire workpiece. This feature is also used for displaying gradients of the surface variables in critical regions of the workpiece. Results like surface expansion, normal pressure give values of only for the surface elements of the mesh whereas a result like shear stress shows values for all elements in the workpiece. In order to differentiate between surface and interior values point tracking has been used. Only critical areas have been interrogated to avoid extraction of too much data.
• Surface Expansion Results: all results for surface expansion in this thesis were obtained using a developmental code. Presently this code does not have a smoothening algorithm for displaying uniform variations of FE variables. As a result the surface expansion results show areas on the workpiece with non‐uniform values. The reader must assume a local average when using these results.
4.2.3 Estimation of Local Surface Expansion.
As described earlier, the local surface expansion that occurs during the forming process is extremely critical in the performance of the lubricant film. It is necessary to estimate the typical values of surface expansion occurring in the three categories of forging components. At present commercial FEA softwares do not have the functionality to estimate the surface expansion in the forming process. However, it is possible to calculate it using data obtained from the simulations performed in these softwares. The software used for the present study was DEFORM 3D. An algorithm to calculate the surface expansion developed at the NCSU – Advanced Manufacturing and Tribology Research Lab was used [Yang C., Report No. MD/MAE/NG‐07‐R‐1 pg 9‐23, 2007]. More details on the algorithm can be found in Appendix III.
The procedure used for calculating the surface expansion has been described in brief:
ii. T co
iii. T th D iv. U th he elements oordinates a he coordina hroughout DEFORM. Using the ne hen compare
s that form are recorded
Figure 1
ates of all su the deform w nodal coo ed to the or Figure 1 the surface d. 13 Tetrahed uch surface mation proc ordinates, th iginal areas 14 Triangula
e of the obje
dral Element
nodes are r ess. The tr
he areas of of the respe ar Face of Te
ect mesh ar
t on Object S recorded. Th racking info the surface ective triangl etrahedral E re identified Surface hese nodes ormation is triangles ar les. lement
d and their n
Figure 15 Calculation of Surface Expansion. v. The surface expansion (SE) for each element is then calculated as:
SE A A
A (4.1)
vi. The results for the surface expansion are then displayed by the GUI developed using Visual FORTRAN.
4.3 Experimental Determination of Friction
In order to evaluate the performance of a lubricant there needs to be testing method to readily mimic the conditions in different forging applications and allow measuring different tribo‐properties. There are several testing methods that test simple, medium, and severe cold‐forging applications. Some samples of tests include the simple compression, ring‐ compression, double cup extrusion, spike penetration, and ball penetration tests.
A1 A2
4.3.1 The r ring t comp the b fictio envir Estim % cha % red Ring com ring compre test consists pressed to a billet reflects
n the mor ronments the Fig mation of fric ange in inter duction in he pression tes ession test is s of the defo a percentage
s the friction re the inne
e inner diam gure 16 Prin ction factor: rnal diamete eight (ΔH): IDf st
s used to ev ormation of a e of its origi n condition er diameter meter of the nciple of Rin : er (ΔD):
= Ho
= ID
valuate the a cylinder w nal height a along the to r of the te billet increa g Compress 100 × − o f o H H 10 × − o f o ID ID D ID0 performanc with a hole d
and the chan ool/workpie est piece is ases. sion Test [So 0 00
ce of forging rilled throug nge in the in ece interface s reduced. ofuoglu, 200 H g lubricants gh its center nner diamet e. The highe In low fri
For th the p calibr obtai OD: O ID: In H: He he various fr percent redu ration curve ined for a sp Outer diame nner diamete eight. riction shear uction in the e (Figure 1 pecimen with eter er. Figure 1 r factors, the height of th 7) for speci h following g
17 FEA Calib
4.3.2 Double Cup Extrusion Test
It consists of a cylindrical billet placed inside a cavity of the same diameter with punches placed on both flat faces of the workpiece (refer Fig. 18). The top punch is then pushed down against the workpiece. The diameters of both punches are equal and less than that of the workpiece. During the forging process the friction at the interface of workpiece and die prevent the workpiece material from moving in the vertical direction. The ratio of the top and the bottom depths of the indentions made by the punches can be used to find the friction coefficient on the outer interface.
Figure 18 Schematic for DCE Test [Altan, Ngaile, Shen, 2004]
The cup height ratio, Rch, is defined by:
Rch= h1/h2 (4.4)
FE simulations are used to obtain the shear friction factor of the lubricant. Simulations are conducted with known shear friction factor values (m) and the results are plotted (Figure 19). The plot shows how the frictional condition affects the cup height ratio. It is observed that lower the friction factor, lower is the cup height ratio.
Figure 19 FEA Calibration Curves [Bunget, 2006]