In the name of god, the merciful WIRE-ARC SPRAYING SYSTEM: Particle Production, Transport, and Deposition by AmirHossein Pourmousa Abkenar A thesis submitted in conformity with the requirements for the degree of Doctor of Philosophy Graduate Department of Mechanical and Industrial Engineering University of Toronto © Copyright by AmirHossein Pourmousa Abkenar 2007 ISBN: 978-0-494-39723-7 Abstract WIRE-ARC SPRAYING SYSTEM: Particle Production, Transport, and Deposition AmirHossein Pourmousa Abkenar Doctor of Philosophy Graduate Department of Mechanical and Industrial Engineering University of Toronto 2007 Protective coatings are important to metal working. Thermal spray is a rapidly growing market, and wire-arc spraying is gaining a significant share of this market because of its low operating/equipment costs and high material/energy efficiency. Although wire-arc spraying is widely used, many of its underlying processes are not yet fundamentally understood. This work examines and explains different aspects of a wire-arc system. In wire-arc spraying, two consumable wires are continuously fed into the gun. An electric arc is struck between the tips of these two wires and continuously melts their material. A cross- flow gas removes the molten material from the wire-tips and accelerates them towards a substrate, over which the detached particles form a protective coating layer. An imaging system was developed to take pictures of the arc, and determine its length and shape. Using the information extracted from such pictures, a computational fluid dynamic model of the wire-arc torch was developed to estimate the shear stresses on the wire-tips and also sizes of primary breakups from the two electrodes. ii Shortly after primary breakups, the detached particles break up into smaller particles (secondary atomization). The size and velocity of such particles were measured in-flight using a DPV-2000 system for a range of operating parameters. A technique was developed to identify and separate the size distributions of particles produced by atomization of molten metal at either the anode or cathode by assuming that both follow a log-normal distribution. (This assumption was also verified experimentally). It was shown that particles produced by the anode are almost two times larger than those originating from the cathode. Furthermore, effect of operating parameters on size distribution of anodic and cathodic particles was investigated. Experiments were also conducted to study the effect of impact velocity and substrate temperature on the properties of individual wire-arc splats and coatings. Aluminum was sprayed onto polished stainless-steel coupons maintained at temperatures ranging from 25°C to 450°C. At low substrate temperature, droplets splashed, forming irregular splats; at higher temperatures there was no splashing and splats formed circular disks. The temperature at which the transition occurred decreased with increasing impact velocity. iii To my love, Malahat … without whom this work could have never been done, or could have been done much sooner... iv Acknowledgements I would like to take this opportunity to acknowledge my supervisors and mentors, Professor Javad Mostaghimi and Professor Sanjeev Chandra, for their invaluable guidance, encouragement, and support throughout this study. I would like to express my sincere appreciations to Professor Javad Mostaghimi for his patience and understanding. His concise but insightful comments fueled me with ideas. I extend thanks to my supervisory committee members, Professor Bendzsak, Professor Sullivan, Professor Ashgriz, and Professor Bussmann for their advice and helpful suggestions. I am also grateful to Dr. Larry Pershin and Mr. Tiegang Li for their assistance in the lab, and Ms. Brenda Fung for her excellent administrative support at the office of graduate studies. In addition, I would like to thank all my colleagues at the Center for Advanced Coating Technologies for making this journey enjoyable. My special thanks goes to Ali Abedini, my lab partner, Hanif Montazeri, my numerical handyman, Rajeev Dhiman, my heat transfer expert, Hamid Salimi, my industrial advisor, Fardad Azarmi, my political rival, Hamed Samadi, my financial advisor, and many more including Liming, Libing, Michelle, Bob, Ken, Ala, Mehdi, Afsoon, Nikoo, Andre, Reza, and many more friends in the department. I am grateful to my family, especially my parents for their never ending love and support. I would like to extend my special thanks to my father-in-law who was not only my teacher, but also my mentor in difficult times. To my wife, Malahat: Without you, your energetic essence, enthusiastic support, and unconditional kindness, I could have never completed this work. I love you and I gladly dedicate this thesis to you. v Table of Contents ABSTRACT................................................................................................................................................................II ACKNOWLEDGEMENTS.......................................................................................................................................V CHAPTER 1 INTRODUCTION..............................................................................................................................1 1.1 BACKGROUND, MOTIVATION, AND LITERATURE SURVEY...............................................................................1 1.1.1 Thermal spray.......................................................................................................................................1 1.1.2 Twin-Wire-Arc Spray............................................................................................................................5 1.1.2.1 Description of the Wire-Arc spraying process..................................................................................5 1.1.2.2 Operating parameters........................................................................................................................8 1.1.3 Brief Literature Review......................................................................................................................10 1.1.3.1 Previous Work on Droplet Production and Transport.....................................................................10 1.1.3.2 Previous Work on Bimodal Size Distribution of In-flight Particles................................................12 1.1.3.3 Previous Work on Particle Deposition............................................................................................13 1.2 STATEMENT OF OBJECTIVES..........................................................................................................................14 1.3 SCOPE OF THE PRESENT WORK.......................................................................................................................15 1.4 OUTLINE OF THE THESIS.................................................................................................................................16 CHAPTER 2 EXPERIMENTAL APPARATUS AND PROCEDURES.............................................................17 2.1 COATING AND PROCESS DIAGNOSTICS...........................................................................................................17 2.1.1 Coating Characterization...................................................................................................................17 2.1.2 Process Characterization...................................................................................................................18 2.2 VALUARC 200 SPRAYING SYSTEM AND ITS CHARACTERISTICS.....................................................................26 2.2.1 Volume-Flow-Rate..............................................................................................................................32 2.2.2 Arc Current.........................................................................................................................................33 vi CHAPTER 3 PARTICLE BREAKUP: THERMAL SPRAY GUN....................................................................38 3.1 EXPERIMENTAL STUDIES...............................................................................................................................38 3.1.1 Imaging system...................................................................................................................................39 3.1.2 Current and Voltage Fluctuations......................................................................................................45 3.2 NUMERICAL STUDIES.....................................................................................................................................49 3.2.1 Flow dynamics of the nozzle geometry...............................................................................................49 3.2.2 Simplified Arc Solution.......................................................................................................................61 3.2.3 Arc Heating in a cross flow................................................................................................................66 3.3 SIMPLIFIED BREAKUP MODEL........................................................................................................................72 CHAPTER 4 PARTICLE TRANSPORT: IN-FLIGHT PARTICLES...............................................................75 4.1 BACKGROUND................................................................................................................................................75 4.2 SPATIAL CHARACTERISTICS OF THE SPRAY...................................................................................................77 4.3 BIMODAL PARTICLE SIZE DISTRIBUTION AND SEPARATION TECHNIQUE.......................................................82 4.3.1 Size Distribution of Anodic and Cathodic Particles...........................................................................84 4.3.2 Separation Technique.........................................................................................................................86 4.3.3 Error Estimation.................................................................................................................................87 4.3.4 Effect of Varying Wire-Arc Parameters.............................................................................................92 4.4 AXIAL VARIATION OF PARTICLE PROPERTIES.................................................................................................96 4.4.1 Drag Force and Force Balance Relation...........................................................................................97 4.4.2 Heat Transfer and Exothermic Oxidation of Particles.......................................................................97 CHAPTER 5 PARTICLE DEPOSITION: SPLAT AND COATING FORMATION.....................................102 5.1 EFFECT OF SUBSTRATE TEMPERATURE ON SPLAT FORMATION....................................................................102 5.1.1 Experimental Procedure...................................................................................................................104 5.2 SPLAT MORPHOLOGY..................................................................................................................................108 vii 5.3 MODEL FOR TRANSITION TEMPERATURE.....................................................................................................112 5.4 COATING PROPERTIES..................................................................................................................................118 CHAPTER 6 CLOSURE.......................................................................................................................................122 6.1 CONCLUSIONS..............................................................................................................................................122 6.2 RECOMMENDATIONS FOR FUTURE WORK.....................................................................................................124 REFERENCE..........................................................................................................................................................125 APPENDIX A: METAL PROPERTIES...............................................................................................................132 APPENDIX B: TRANSPORT PROPERTIES OF AIR.......................................................................................133 viii List of Figures Figure 1.1 Basic principles underlying the thermal spray processes: Production, Transport, and Deposition of molten particles.......................................................................................3 Figure 1.2 Schematics of wire-arc spraying system and its major components.................................5 Figure 2.1 (a) Picture of DPV-2000 scanning unit alongside the wire-arc spraying gun detecting the in-flight particles (b) The computer system containing the DPV-2000 operating system and CPS-2000 modules; the two are connected to the scanning unit via fiber-optic cables....................................................................................................19 Figure 2.2 Schematic diagram of the DPV’s optical sensing head and its field of view [ 9].............20 Figure 2.3 (a) A schematic diagram showing the signal sensed by the DPV-2000 sensing head when a particle passes through its field of view. (b) Picture of the two slits in P4590170 photo mask...........................................................................................................20 Figure 2.4 Wire-arc sprayed stainless-steel particles are approximately spherical.........................20 Figure 2.5 Size distribution of particles was measured using two additional methods (optical picture measurements, and PSA measurements) to calibrate DPV’s particle size measurements.......................................................................................................................23 Figure 2.6 Pyrometer’s calibrated reference curve. High voltages on λ = 780 nm and λ = 850 nm photomultipliers were 700 V and 1000 V, respectively...............................................24 Figure 2.7 Surface profile of polished AISI 304L stainless steel substrate obtained using a Surface Profiling Microscope (Wyko Optical Profilometer, Veeco Instruments Inc., Woodbury, NY). Surface roughness is 7.90 nm.................................................................25 Figure 2.8 Picture of ValuArc 200 Twin Wire Arc spray system and the spray gun manufactured by Sulzer-Metco. Picture is adapted from [ 63].........................................27 Figure 2.9 Picture of ValuArc 200 Twin Wire Arc spray system during operation.........................28 Figure 2.10 Schematics of ValuArc 200 Twin-Wire-Arc Spraying System and its components. Picture is adapted from [ 2, 72].............................................................................................28 Figure 2.11 Exploded rear view of the ValuArc 200 twin-wire-arc gun and its components. The gun can be mounted on the handle and hand operated, or on a separate mount or robot and remotely operated. Picture is adapted from [ 72].............................................29 Figure 2.12 Exploded front view of the ValuArc 200 twin-wire-arc gun and its components. Picture is adapted from [ 72]................................................................................................30 ix