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Keyhole Gas Tungsten Arc Welding: a New Process Variant Brian Laurence Jarvis University of Wollongong

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Keyhole Gas Tungsten Arc Welding: a New Process Variant Brian Laurence Jarvis University of Wollongong University of Wollongong Research Online University of Wollongong Thesis Collection University of Wollongong Thesis Collections 2001 Keyhole gas tungsten arc welding: a new process variant Brian Laurence Jarvis University of Wollongong Recommended Citation Jarvis, Brian Laurence, Keyhole gas tungsten arc welding: a new process variant, Doctor of Philosophy thesis, Faculty of Engineering, University of Wollongong, 2001. http://ro.uow.edu.au/theses/1833 Research Online is the open access institutional repository for the University of Wollongong. For further information contact the UOW Library: [email protected] Keyhole Gas Tungsten Arc Welding: a new process variant. tSS!1' This photograph is an end-on view of keyhole GTAW on 8mm wall-thickness stainless steel pipe. Certification I, Brian Laurence (Laurie) Jarvis, declare that this thesis, submitted in fulfilment of the requirements for the award of Doctor of Philosophy, in the Department of Mechanical Engineering, University of Wollongong, is wholly my own work unless otherwise referenced or acknowledged. The document has not been submitted for qualifications at any other academic institution. Brian Laurence Jarvis 15m July 2001. Keyhole Gas Tungsten Arc Welding: a new process variant By Brian Laurence Jarvis B.Sc. (Hons) Flinders University, 1975 Thesis Submitted in fulfilment of the requirements for the degree of Doctor of Philosophy in Mechanical Engineering, Faculty of Engineering, University of Wollongong June 2001. Wollongong, New South Wales 1 Dedication To my Mother and Father Acknowledgements I wish to thank my adviser and supervisor, Professor Michael West, for his support and direction during this investigation. Special thanks are also due to my co-supervisor, colleague and friend, Dr Nasir Ahmed for laying the foundations for this work, and for his continued encouragement and support. Particular thanks are also extended to Ken Barton, who has played such a major role in the practical development of this process. Thanks are also extended to my many colleagues who have contributed in numerous ways. This work has been made possible through the support and generosity of the CSIRO and the CRC for Welded Structures (formerly the CRC for Metals Welding and Joining) iii Table of Contents ACKNOWLEDGEMENTS HI TABLE OF CONTENTS IV TABLE OF TABLES X TABLE OF FIGURES XII NOMENCLATURE XVITI ABSTRACT 1 1. THESIS 2 1.1. OBJECTIVES 2 /././. Introduction 2 1.1.2. Thesis 2 1.1.3. Implications 3 1.2. JUSTIFICATION STRATEGY 4 1.2.1. Justification criteria 4 1.2.2. Outline 4 1.2.3. Detailed research intentions 4 2. LITERATURE REVIEW - DEEP PENETRATION WELDING 7 2.1. INTRODUCTION 7 2.1.1. Power density infusion welding 7 2.1.2. Fluid flow in conventional arc welding. 9 2.2. KEYHOLE (DEEP PENETRATION) WELDING PROCESSES 11 2.2.1. Electron Beam 11 2.2.2. Plasma Arc 13 2.2.3. Laser Beam 13 2.3. BEAM-TO-WORK-PIECE COUPLING 15 2.3.1. Electron beam coupling 15 2.3.2. Laser beam coupling 16 2.3.3. Laser-matter interactions 19 2.4. KEYHOLE MODELS 21 iv 2.4.1. Conduction Models 22 2.4.2. Fundamental models 24 2.4.3. Dynamic behaviour 26 2.5. PLASMA ARC KEYHOLES 29 2.5.1. Process characteristics 29 2.5.2. Variable polarity plasma arc welding 30 2.5.3. Enhanced plasma arcs 31 2.5.4. Fluid flow in VPPA weldments 32 3. A PRACTICAL APPRAISAL OF KEYHOLE GTAW 34 3.1. INTRODUCTION 34 3.1.1. Background 34 3.1.2. Keyhole mode GTAW 37 3.2. EQUIPMENT 40 3.3. PROCESS PARAMETERS 42 3.3.1. Introduction 42 3.3.2. Threshold current 43 3.3.3. Experimental schedule 44 3.3.4. Travel speed 45 3.3.5. Voltage 45 3.3.6. Shielding gas 46 3.3.7. Electrode geometry 47 3.3.8. Combined effects of gas and electrode geometry: 5.6mm SAF 2205 48 3.3.9. Material 48 3.3.10. Other variables: wire feed, flow-rate and the influence of cross flow 49 3.4. PROCESS PERFORMANCE AND OPERATING WINDOWS 50 3.4.1. Joint qualification 50 3.4.2. Operational windows 56 3.4.3. Application to pipe 58 3.4.4. Process extensions 61 3.4.5. Competitiveness 62 4. THE ROLE OF SURFACE TENSION IN KEYHOLE BEHAVIOUR. 65 4.1. SURFACE TENSION IN RELATION TO KEYHOLE FAILURE 65 v 4.1.1. 2-D verses 3-D geometries 65 4.1.2. Aspects of surface tension 69 4.1.3. Keyhole failure in thick plate 71 4.1.4. Travel speed 76 4.1.5. A first rule for keyhole stability. 77 4.1.6. Control limitations 78 4.1.7. Keyhole failure in thin plate 82 4.1.8. A secondrule for keyhole stability. 85 4.1.9. Failure characteristics 87 4.2. DISPLACEMENT OF METAL IN THE GTAW KEYHOLE 89 4.2.1. Deficit 89 4.2.2. Experimental method and results 91 4.2.3. Forces required maintaining a deficit 93 4.2.4. Discussion of displacement results 95 4.2.5. Characteristics of the transition 100 4.3. MATHEMATICAL CONSIDERATIONS FOR WELD POOL SURFACES 105 4.3.1. Introduction 105 4.3.2. The equation for weld pool surfaces 105 4.3.3. Equation for axi-symmetric weld pools 108 4.3.4. Solutions for axi-symmetric weld pools Ill 4.3.5. Numerical verification 115 4.3.6. Moving weld pools 120 4.3.7. The effects of flow on the surfaces 121 4.3.8. Numerical simulation of the process 124 4.4. KEYHOLES FROM A GEOMETRIC PERSPECTIVE 124 4.4.1. Surface types 124 4.4.2. Transitions and hysteresis 126 4.4.3. Hysteresis in the M-K transition 127 4.4.4. More surfaces and porosity. 128 4.4.5. Undercut. 129 5. DISPLACEMENT FORCES IN GTAW 131 5.1. A MATHEMATICAL MODEL FOR ARC FORCES 131 5.1.1. Arc force fundamentals 131 vi 5.1.2. An historical perspective 133 5.1.3. General formulation of a model of arc force 135 5.1.4. The Converti model. 136 5.1.5. Limitations of the Converti model 137 5.1.6. Force due to radial stress 138 5.1.7. Force due to geometric divergence of current. 141 5.1.8. The Converti model as a special case of (5.7) 143 5.1.9. Current redistribution due to surface depression at the anode 144 5.1.10. Estimation of arc force values 145 5.2. ARC PRESSURE 149 5.2.1. A change of variables 149 5.2.2. The pinch pressure 150 5.2.3. Pressure due to geometric divergence of current. 151 5.2.4. Derivation of arc force from arc pressure 154 5.2.5. Arc pressure in the Converti model 156 5.2.6. Pressure distrib ution for currents displaced from the arc axis 159 5.2.7. Pressure distribution for axially peaked current distributions 161 5.2.8. Pressure at the arc core 164 5.3. EXPERIMENTAL INVESTIGATION OF CATHODIC INFLUENCES 168 5.3.1. Introduction 168 5.3.2. In-situ weighing method 168 5.3.3. Welding current 174 5.3.4. Independence from anodic influence 175 5.3.5. Electrode tip angle 777 5.3.6. Relationship of the FAF and arc voltage to tip angle 178 5.3.7. Electrode diameter 184 5.3.8. Results in terms of emission current density 185 5.3.9. Shielding gas composition 190 5.3.10. Electrode elevation 191 6. THE ROLE OF THE ARC IN KEYHOLE GTAW 194 6.1. REVIEW 194 6.1.1. Conduction through an arc 194 6.1.2. Sheath regions 196 vii 6.1.3. Arc column 198 6.1.4. The anode region 200 6.1.5. Influence of the cathode tip on arc properties 203 6.1.6. Gas composition 206 6.1.7. Numerical simulations 208 6.1.8. Alternative models 210 6.1.9. Arc characteristics and keyhole welding 211 6.2. CATHODE-KEYHOLE RELATIONSHIPS 212 6.2.1. The effect of electrode geometry on the fused area 212 6.2.2. The effect of electrode geometry on threshold current 214 6.2.3. The effect of plate thickness on threshold current 218 6.3. ARC - KEYHOLE RELATIONSHIPS 221 6.3.1. Minimum throat size 221 6.3.2. Influence of the shielding gas composition 225 6.3.3. The effect arc length on threshold current 227 7. CONDUCTIVE HEAT FLOW 229 7.1. ANALYTIC MODELS OF CONDUCTIVE HEAT FLOW 229 7.1.1. Introduction 229 7.1.2. Quasi-stationary temperature fields 229 7.1.3. Melting efficiency in 2-D (thin plate or keyhole) 232 7.1.4. Melting efficiency in 3-D (thickplate) 234 7.1.5. The heat function 237 7.1.6. Recent investigations 239 7.1.7. Dimensional analysis and its implications for keyhole shape 241 7.2. HEAT DISTRIBUTIONS IN THE HIGH-SPEED WELDING REGIME 243 7.2.1. Approximations for high welding speeds 243 7.2.2. Instantaneous application of heat at a point. 245 7.2.3. Re-calculation of melting efficiencies 248 7.2.4. Energy partitioning 250 7.2.5. The onset of the high-speed regime 252 7.2.6. Implications for heat flow from keyholes 254 7.3. MODELLING THE THERMAL FIELD ASSOCIATED WITH KEYHOLES 254 7.3.1. Solution by separation of variables 254 viii 7.3.2. Boundary conditions 255 7.3.4. Special cases 259 7.3.5. The effect of heat distribution on root face pool shape 260 8. CONCLUSIONS 264 8.1.
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