Characteristics of a pulsed-current, vertical-up gas metal arc weld in steel
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I. INTRODUCTION
THE construction and maintenance of many steel structures like off-shore drilling rigs, submarine hulls, storage tanks, transmission towers, reactors, etc., often require the use of reliable positional welding in vertical, horizontal, and overhead positions. Positional welding is always a critical task, due to certain difficulties primarily arising from (1) transportation of molten filler metal from the electrode tip to the weld pool, and (2) retaining the weld pool in the proper position to form an appropriate weld geometry against the unfavorable force of gravity. Positional welding by the gas metal arc welding (GMAW) process can either be performed using a continuous current in the short-circuit mode of metal transfer or by employing a pulsed current, allowing a favorable nature of metal transfer and thermal behavior of the welding. In pulsed-current welding, the spray mode of metal transfer is achieved at a low mean current which, as a welding current, produces a short-circuiting/globular mode of metal transfer in the continuous-current GMAW process. The differing natures of metal transfer and thermal behavior of the depositing droplet at various pulse parameters give rise to different geometrical, metallurgical, and mechanical characteristics in the weld joint. During the pulsed-current GMAW process, the appropriate selection of pulse parameters, such as mean current (Im), peak current (Ip), base current (Ib), pulse duration (tp), and pulse frequency ( f ), provides a required droplet velocity to propel it against gravity and a control over the temperature (superheat) of the depositing droplet and the consequent fluidity of the weld pool, to achieve desired geometrical characteristics of the weld metal in position. The variation in temperature of a droplet under various conditions of pulsation has been estimated analytically with the help of a duly verified mathematical-model expression reported earlier.[1] It has also been observed that the interrelated nature of pulse parameters, P.K. GHOSH and S.R. GUPTA, Professors, are with the Department of Mechanical and Industrial Engineering, University of Roorkee, Roorkee 247 667, India. H.S. RANDHAWA, Assistant Professor, is with the Department of Mechanical Engineering, GZS College of Engineering and Technology, Bhatinda, Punjab, India. Manuscript submitted August 18, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS A
conforming to the energy-balance criterion of the power source, simultaneously varies with other pulse parameters during a change in any of them. This behavior suggests that control of weld quality is possible by establishing a correlation between weld characteristics, with a summarized influence of pulse parameters defined[2] as the factor f 5 ((Ib /Ip)f?tb), where tb is the pulse-off time, expressed as ((1/ f ) 2 tp). The potentiality of using f to analyze the basic characteristics of a pulsed-current GMA weld in flat position has been amply justified by several investigators.[2,3,4] In the present investigation, a square-butt weld d
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