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AWS - C5.5/C5.5M:2003

Recommended practices for gas tungsten arc welding

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Organization: AWS
Publication Date: 1 January 2003
Status: inactive
Page Count: 130
ICS Code (Welding processes): 25.160.10
ISBN (print): 0-87171-715-8
scope:

1.1 Scope. This document presents recommended practices for the gas tungsten arc welding (GTAW) process.1 Its purpose is to provide a fundamental explanation of the process, describe basic practices and concepts, and outline some advanced methods and applications of GTAW. These should enable welding personnel to determine the best applications of this process and evaluate its use compared with other joining processes. The section covering principles of operation will help the reader understand how the process works, the generaltypes of equipment needed, and the advantages and limitations of the gas tungsten arc welding process. The basic concepts and practices include both general and specific recommendations and technical data for equipment, consumables, procedures, variables, applications, and safety considerations. This standard makes use of U.S. Customary Units. Approximate mathematical equivalents in the International System of Units (SI) are provided for comparison in brackets [ ] or in appropriate columns in tables and figures. 1.2 Introduction to the Gas Tungsten Arc Welding (GTAW) Process. Welding as an occupation and a career is a very "special" and rewarding choice to pursue. It is one of the most interesting manufacturing disciplines as it involves both art and science. This is illustrated by manual gas tungsten arc welding (GTAW) because a person's manual dexterity, hand-eye coordination, and self discipline in combination with the correct welding procedure( s) are paramount to its success. The "art" portion is most evident when an individual welder expresses theirunique signature to the manually applied welds. Exam- ples of this would be certain welded metal sculpturesand/or a "perfectly" welded part or assembly. The "science" end of the spectrum would include recent developments such as fully automated robotic welding cells that could include through-the-torch vision that allows real-time viewing of the weld as well as real-time weld joint tracking. Also, weld parameter data acquisition and feedback control are routinely accomplished in real-time. 1.3 History. Although arc welding was first developed in the 1880s, its commercial use in the United States did not commence until the first decade of the 1900s. The years of the First World War brought the initial large-scale commercial use of arc welding, when shielded metal arcwelding (SMAW) began to replace riveting as the means of joining in the manufacture of ships. During the 1920s, H. M. Hobart and P. K. Devers performed preliminary work on using inert gases to shield the carbon or metallic electrode's welding arc and molten weld pool. In 1926 they applied for patents2 on the use of an electric welding arc in which an inert gas was independently supplied around the arc, thus replacing flux as the shielding method. Other investigators experimented with both helium and argon as shielding gases, but because of the high costs associated with these inert gases, very little commercial use was made of them at that time. By the onset of the Second World War, shielded metal arc welding had become the dominant welding process. However, there was a need within the aircraft industry for welds made with better shielding than that provided by SMAW when joining reactive metals such as aluminum and magnesium. Also, in the aircraft industry there was a need to develop an acceptable welding process to replace riveting for joining of thin gage materials. These needs led to the first commercial development of gas tungsten arc welding equipment. In 1941, R. Meredith and V. H. Pavlecka developed the first practical electrode holders (torches) using a nonconsumable electrode made from tungsten. These first torches were simply shielded metal arc welding electrode holders that had been modified to provide the shielding gas flow. A 1/8 in. [3 mm] diameter tungsten electrode was held in a copper tube through which the inert helium gas flowed to protect the electrode, weld pool, and adjacent heated areas of the workpiece. Helium was elected to provide the necessary shield because,at the time, it was the only readily available inert gas. Tungsten inert gas torches and accessories typical of that period are shown in Figures 1 and 2.3 A patent was issued for this process in 1942.4 This arc welding process was initially named "Heliarc®,"5, 6 welding because helium was used as the shielding gas. It has also been called nonconsumable electrode welding, tungsten inert gas (TIG) welding, wolfram inert gas (WIG) welding7 and tungsten-arc welding. However, the proper AWS terminology for this process is gas tungsten arc welding (GTAW), because shielding gas mixtures containing inert gases other than helium, or gases which are not inert, are sometimes used. Using a tungsten electrode and direct current power source, a stable, efficient heat source (the arc) was used to produce acceptable welds. The inert shielding gas provided full protection of the arc and weld pool, which was imperative in welding of aluminum and magnesium, because even a small amount of air could contaminate the weld. The process also allows for better control over the heat input, thus making it easier to weld thin materials When the GTAW process was first developed, SMAW was being performed utilizing direct current with electrode positive (DCEP or reverse polarity). The same power sources (DC generators of the rotating type) were thus used for the early gas tungsten arc welding process. Photographs of early GTAW/SMAW power sources are shown in Figures 3 and 4. One of the major benefits of DCEP was the tremendous cathodic cleaning action of the workpiece surface for aluminum and magnesium. However, overheating of the electrode and subsequent splitting, melting, and transfer of tungsten particles into the weld limited the useful current range. Since these early torches were air-cooled, they had only a 75 A current capacity. As a result, it soon became apparent that DCEP was not the best polarity to use for this process. By making the electrode negative, overheating was avoided and weld penetration was improved. Use of DC electrode negative (DCEN or straight polarity) proved acceptable for welding of stainless steels. However, because it did not provide the cleaning action like DCEP, it was not acceptable for welding of aluminum or magnesium, which have tenacious surface oxides that must be removed before acceptable welds can be produced. To obtain the cleaning action of DCEP along with the improved penetration characteristics and lower electrode heating of DCEN, alternating current (AC) welding power sources were developed. A high-frequency, high voltage current was superimposed over the basic welding current to stabilize the arc during current reversals. This method was successfully applied to GTAW of aluminum and magnesium. By the early 1950s, GTAW had gained acceptance in the welding industry. Argon was the most widely accepted shielding gas, followed by helium. However, because of the high cost of argon and helium gases, carbon dioxide and nitrogen were investigated as shielding gases. Since that time, numerous other gases and mixtures of gases have been used with this welding process to provide improved welding performance for some metals. These include argon-helium mixtures, argon hydrogen mixtures, and argon-nitrogen mixtures. As a tool for increasing deposition rates beyond that of the commonly used cold wire feed during gas tungsten arc welding, the hot wire feed method8 of filler metal addition was introduced. This allowed the high quality welds produced by the GTAW process without incurring the spatter produced by the consumable electrode processes. Over the last several decades, numerous improvements have been made to the GTAW equipment, process and controls. Welding power sources have been developed specifically for the GTAW process (see Figures 5 and 6). Some provide pulsed direct current and others produce variable polarity alternating welding current. Automatic arc starting systems, automatic arc length/ voltage controls, vision and penetration sensors, and positioning equipment are all commercially available. Computer controls, automatic sequence contr

abstract:

This document is designed to assist anyone who is associated with gas tungsten arc welding (GTAW). This includes welders, welding technicians, welding engineers, quality control personnel, welding... View More

Document History

C5.5/C5.5M:2003
January 1, 2003
Recommended practices for gas tungsten arc welding
1.1 Scope. This document presents recommended practices for the gas tungsten arc welding (GTAW) process.1 Its purpose is to provide a fundamental explanation of the process, describe basic practices...

References

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