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ON the EFFECT of ENVIRONMENTAL PRESSURE on GAS TUNGSTEN ARC WELDING PROCESS by G6khan G6ktug

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ON the EFFECT of ENVIRONMENTAL PRESSURE on GAS TUNGSTEN ARC WELDING PROCESS by G6khan G6ktug ON THE EFFECT OF ENVIRONMENTAL PRESSURE ON GAS TUNGSTEN ARC WELDING PROCESS by G6khan G6ktug B.Sc. in Civil Engineering, istanbul Technical University istanbul, 1986 S.M. in Civil Engineering, MIT Cambridge, 1990 SUBMITTED TO THE DEPARTMENT OF OCEAN ENGINEERING IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREES OF OCEAN ENGINEER and MASTER OF SCIENCE IN NAVAL ARCHITECTURE AND MARINE ENGINEERING at the MASSACHUSETTS INSTITUTE OF TECHNOLOGY The author hereby grants to MIT June 1994 permission to reproduce and to a) 1994 GSkhan Gktu distribute publicly paper and All rights res rved electronic copies of this thesis document In whcle or in part. Signature of Author Department of Ocean Engineering June, 1994 Certified by .. Koichi Masubuchi Thesis Supervisor Accepted by ' A. Douglas ma-ri-ielal for Graduate Studies ON THE EFFECT OF ENVIRONMENTAL PRESSURE ON GAS TUNGSTEN ARC WELDING PROCESS by G6khan G6ktug Submitted to the Department of Ocean Engineering in June, 1994 in partial fulfillment of the requirements for the degrees of Ocean Engineer and Master of Science in Naval Architecure and Marine Engineering. Abstract With the increesing need for methods that will assist the construction and repair of space structures, gas tungsten arc welding (GTAW) is considered to be a possible process. This thesis reviews the experimental work performed in Welding Systems Laboratory (WSL) at MIT and Takamatsu National College of Technolgy (TNCI), Japan. To study the basic relation between pressure and the arc behaviour of GTAW, a pressure vessel installed at WSL was used to simulate high pressure (hyperbaric) and low pressure (vacuum) conditions. The arc shape of the GTAW was monitored by a 35 mm camera installed outside the vessel as welding was conducted at different levels of pressure. Another series of experiments were carried out under the vacuum condition at THCI, at a later date. A hollow tungsten eloctrode was designed to use in GTAW on stainless steel (SUS304) specimens. The effect of the flow quantity of inert gas and the height of electrode on penetration, bead width, arc voltage, and transient arc discharge were studied. It was determined that a small amount of inert gas flow fed through the hollow electrode, made it possible to initiate and maintain a stable arc even under very low environmental pressure. Thesis Supervisor: Koichi Masubuchi Title: Kawasaki Professor of Ocean Engineering and Materials Science Acknowledgements I would like to express my gratitude to my advisor Prof. Koichi Masubuchi, for his invaluable guidance and financial and emotional support when I needed it the most. Special thanks to Dr. Y. Suita, Mr. Koga and Dr. A. Hirose for their involvement and help throughout this thesis. I am also grateful to Mr. S. Koga for his friendship and help in my work. Without him, I would have to work twice as much to finish. Thanks to Mr. Hijigate, Mr. Umekuni, Mr. Ichikawa, Mr. Meada, Mr. Kaito, Mr. Mukai, and Mr. Tanizawa for their friendship and the fun we shared. Deep thanks to Ms. Ipek Ilter for proof-reading the text, helping me with some of the drawings and text. Finally, I would like to thank to my friends here at MIT, ipekus, Hibiz, Ufaklik, Gupta, Dayucugum, my dear Hispanik Cillop and (mnog, for sticking with me in good and bad times. Table of Contents A b stract...........................................................................................................................2 Acknowledgements...............................................................................................3 Table of C ontents ..................................................................... .............................. 4 List of Figures................................................... ....................................................... 6 List of Tables.................................................................................................................. 8 1. Introduction ................................................... ..................................................... 9 1.1 Objectives and Scopes ............................................. ....... 10 1.2 Organization and Outline of theThesis ...................................... 11 1.3 Review of Previous Work ................................................................ 11 2. On Different Joining Processes.......................................... ................... 13 2.1 Advanced Welding Processes Pertinent to Space Applications ........................................................................................... 16 2.1.1 Electron Beam Welding .......................................... 17 2.1.2 Laser Beam Welding.........................................21 3. Gas Tungsten Arc Welding Process........................................25 3.1 Introduction ........................................................................ .................. 25 3.2 Principals of Process ............................................................................... 26 3.2.1 Types of Welding Torches ...................................... .... 28 3.2.1.1 Gas Cooled Torches ..................................... .... 28 3.2.1.2 Water-Cooled Torches .................................... .. 28 3.2.1.3 Parts of Welding Torches ..................................... 28 3.2.2 Electrodes ....................................................... 29 3.2.2.1 Electrode Classification ............................................... 29 3.2.2.2 Electrode Sizes ........................................ .... 30 3.2.2.3 Electrode Tip Configurations ................................. 30 3.2.2.4 Electrode Contamination ..................................... 31 3.2.3 Power Supply .............................. ...... ................. 32 3.2.3.1 Direct Current ........................................ .... 32 3.2.3.1.1 Pulsed Dc Welding .................................... 34 3.2.3.2 Alternating Current ..................................... ... 35 3.2.4 Shielding gases ................................................. 36 3.2.4.1 Argon............................... ................ 37 3.2.4.2 Characteristics ........................................ .... 38 3.2.4.3 Recommended Gas Flow Rates ............................. 38 4. Experim ental Study.................................................... ....................................... 40 4.1 Introduction ........................................................................ .................. 40 4.2 Study at M IT...................................................................... .................... 40 4.2.1 The Pressure Vessel............................... .... ....... 42 4.2.2 High Pressure Welding Process ............................................... 44 4.2.3 Low Pressure Welding Process ...................................... 51 4.2.4 Discussion and Results ....................................... ..... 55 4.3 Study at Takamatsu Institute of Technology, Japan...................... 57 4.3.1 Experimental Apparatus ...................................... .... 57 4.3.1.1 Hollow Tungsten Electrode.......................................58 4.3.2 Discussion and Results ........................................................... 60 4.3.2.1 Arc Phenomenon in Vacuum by GHTA...........61 4.3.2.2 Flow Quantity of Argon ............................................. 63 4.3.2.3 Melting of the Base Metal .................................... 66 5. C onclusion ..................................................................... .......................................... 72 6. References................................... ............................. ........................................... 74 List of Figures Figure 2.1 Shematic illustration of electron gun ..................................... 18 Figure 2.2 Shematic illustration of electron beam welding process ............. 18 Figure 2.3 Comparison of cross-sectional shape of penetration between arc welding and electron beam welding ............................................ 19 Figure 2.4 Schematic illustration of CO 2 laser resonator and processing equip m ent .............................................................. ................................................ 22 Figure 3.1 Gas tungsten arc welding equipment................. .... 26 Figure 3.2 Gas tungsten arc welding operation and torch assembly .............. 27 Figure 3.3 The variation of arc shape and fusion zone at different tip geom etries of electrode[3] .................................................................... ................ 31 Figure 3.4 Characteristic of current types for GTAW [3] .................................. 33 Figure 3.5 Waveform of pulsed direct current [3] ..................................... 34 Figure 3.6 Characteristic of variable square wave AC[3] ................................. 36 Figure 3.7 The relationship of voltage-current with argon and helium shiledin g[3] ................................................................................... .......................... 37 Figure 4.1 The pressure vessel .................................................... 42 Figure 4.2 Inside the pressure vessel ............................................... 43 Figure 4.3 Shape of arc under 0.1 MPa pressure......................45 Figure 4.4 Shape of arc under 0.17 MPa pressure ......................... 46 Figure 4.5 Shape of arc under 0.27 MPa pressure......................47 Figure 4.6 Shape of the arc under 0.38 MPa pressure...................................48 Figure 4.7 Shape of
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