Welding-arc:different Processes for many uses.SOLUTIONS with Effective, Powerful Advice
Welding-arc, tig, mig, tig torches, miller/lincoln welding machines, welding cables, orbital welding, wire feeders, welding gun, smaw, submerged-arc welding, stick welders, plasma arc welding, gmaw, gtaw, covered electrode, solidified slag, underwater welding, flux cored arc welding, stud welding, welding links, welding tips, improving welding results, joining questions needing answers: these are some of the items developed in this Site for the benefit of interested readers. What is in here for me? Condensed information, practical know-how on Welding-arc, quick, time saving reference. A reminder of old catastrophic failures. Order in the large variety of processes. Recommended Practices. Description of less common or less used processes. In a previous page on WELDING PROCESSES, a short review was offered grouping processes according to some general characteristic. Here we are going to enter into some more detail for the class of FUSION WELDING processes known and described as Welding-arc processes. UPDATED PAGE
The heat source... Welding-arc processes as a group are identified by the fact that the heat source for melting metal is due to an Arc Discharge of electrical energy, obtained and directed into the workpiece through electrodes specifically designed to supply power in a useful form to get the welding job done. An Article on the physical characteristics of the Electric Arc was published in our Practical Welding Letter No. 12, issue of August 2004. To read the Article click on PWL#012. To receive at no cost all the issues of PWL as they are published, please click on Subscription. The origins of Welding-arc processes can be found towards the end of the nineteenth century when trials were first attempted to master the newly discovered form of energy. The impressive technological advances developed during the twentieth century produced a mature series of variations on the theme of Welding-arc, which effectively adapted the principles to ever changing and more exacting requirements. Welding-arc history was not without setbacks: the most famous example is the occurrence of spectacular failures which plagued eight all-welded ships of the Liberty class, built during World War II, which suddenly broke in two as a consequence of exposure to high seas under particularly cold weather. The lessons learned during the thorough investigations that followed to clarify the underlying causes, were essential to the progress of knowledge in this particular metallurgical matter. It was found that it was not Welding-arc as such to blame: rather it was an unknown before characteristic of certain steels to become brittle at low temperatures together with a design that did not anticipate the need to include "crack arrestors" to avoid the catastrophic spread of cracks through the whole structure. For a short overview of these events, the interested reader is referred to an article that can be read by clicking here. Stick welding... Omitting historical Welding-arc developments which have at present no practical interest we shall describe first the Shielded Metal-Arc Welding (SMAW) which is the most prominent manual Welding-arc process currently employed in a very extensive range of applications due to its wide versatility. Here the electric Welding-arc is struck between a covered electrode (stick) and the workpiece, connected to opposite polarities of an electric power supply. Alternating current can be used with simple transformers. Direct current is preferred for certain properties or materials.
Electrode positive (reverse polarity) in Welding-arc has a hotter electrode, with larger diameter for the same current.
Electrode negative (straight polarity) concentrates heat on the workpiece. Filler metal is provided by melting the tip of the consumable electrode by the Welding-arc, while the molten surroundings of the workpiece are protected from air contamination by the gaseous products of decomposition of the fluxing cover. During cooling and solidifying of the molten metal, the weld bead remains covered by a layer of solidified slag, which further protects the cooling joint. Shielded Metal-Arc Welding is performed using Welding-arc equipment that is quite compact and portable, with long welding cables between the power supply and the actual welding spot. Welds can be performed, by an experienced welder, in all positions, even overhead. By controlling the current employed, both thin and thick welds can be done, within the working range of the equipment. Most of the industrial materials, except those with low melting point, can be welded by Welding-arc processes, using electrodes of matching composition, and sometimes using special procedures involving pre- and post-heating. An Article on Understanding AWS Classification of shielded Filler Metal Electrodes was published in issue 1 of Practical Welding Letter for September 2003. To see the article click on PWL#01. The economy of the Shielded Metal process is occasionally at loss when compared with other Welding-arc processes capable of continuous metal deposition, because of the necessity of interrupting the work to discard the remaining butt-end and to clamp a new electrode in the holder. Time is also needed to remove the solidified slag between passes. In any case most Welding-arc welders work only for a specified duty cycle at every Current level, to prevent overheating. But in general, except when the work can be mechanized for long stretches of uniform welds, this remains the process of choice for industrial structures, bridges, ships, where there is no alternative to expert manual work. High productivity... SUBMERGED ARC WELDING is a successful mechanized semi-automatic process, where the electric Welding-arc is struck between a bare electrode wire unreeling from a spool and the prepared joint, under the cover of a granulated flux, continuously provided just ahead of the advancing weld front. To be economic the process must be applied to long joints, with a certain minimum thickness, with special equipment and trained operators. The Welding-arc is not visible and must be regulated by controls that do not depend upon its appearance. The equipment carriage, which provides linear motion and wire feeding besides supplying a sliding electrical contact to the electrode, moves steadily over the joint with positive means of following the joint centerline, which is not in the direct sight of the operator. Alternatively the welding head may be stationary, and the joint may roll uniformly under it, i. e. for circular welds. The solidified slag remaining on the weld must be removed as usual by manual mechanical means, and for reasons of economy the excess of unmelted granulated flux must be collected, by vacuum or by other devices, for reuse. This Welding-arc process is quite successful for well prepared joints of long stretches, or for circular joints of large cylindrical bodies. The weld is always performed in flat position and there is no spatter because of the action of the flux. For multipass welds there is the inconvenience of the necessity of complete removal of solidified slag from the previous beads, before the deposition of the following one. An Article on Filler metals for Submerged Arc Welding was published In Section 4 in Issue 29 of Practical Welding Letter for January 2006. To read the article click on PWL#029. Note: This process has nothing to do and must not be confused with Underwater Welding, which is a manual operation conducted by divers in the sea for repair or construction of structures under water. Another one... FLUX-CORED ARC WELDING is another Welding-arc process where the heat for melting is provided by an electric arc discharged between the workpiece and a special hollow wire consumable electrode containing in its interior a suitable flux for providing shielding of molten metal from air by the protective gases evolved during combustion. For improved protection of steel work the shielding may be augmented by an auxiliary continuous stream of carbon dioxide (CO2) gas. This development addresses the concerns of increasing productivity by the continuous supply of the consumable electrode wire from a spool. This Welding-arc method is successfully applied in a wide variety of instances either as a semi automatic or as a full automatic process in competition to other ones. The main drawback is the usual need to dispose of the solid slag remaining on the weld bead and possibly of the thicker fumes. Mig for you... GAS METAL-ARC WELDING (GMAW) called also MIG (for Metal Inert Gas) is a Welding-arc development related to the preceding one which depends solely on the supply of an external shielding inert gas, either carbon dioxide or helium or argon, while the continuous consumable electrode solid wire is provided from a spool. Heat for melting comes from the electric Welding-arc between electrode and workpiece. This process overcomes the difficulties, confronted by other ones, of various position welding, the limited length of stick electrodes, and the need to dispose of the solidified slag of flux shielded methods. The finer electrode wire used permits higher Welding-arc current density and higher deposition rate. Therefore productivity is high and the process has economic advantages in well defined applications. However equipment tends to be more complex and less portable, and also more expensive. The special electrode holder with wire feed and gas supply may be too cumbersome to use for hard to reach places where the use of stick welding (SMAW) is more fit. See a detailed page on Mig Welding Tips by clicking on this underlined title. See: Recommended Practices for Shielding Gases for Welding and Plasma Arc Cutting Tip!: Most important is the external condition of the wire, usually copper plated for increased electrical conductivity and corrosion prevention. Welding should never be attempted with filler wire in poor surface condition. In GMAW different types of metal transfer from the filler wire to the molten pool occur, depending on the current density and type and on the protective gas. At low Welding-arc current density, the transfer occurs in short circuit mode, so called because the elongated drop bridges the gap between electrode and work and the arc is momentarily extinguished. The molten metal bridge is broken by the electric pinch force. Then the arc is renewed and a new cycle begins with a certain frequency depending on the electrical circuit. At a certain current density, which varies with material and filler size, there is no more shorting of the arc, the shape of the metal transfer becomes that of large globular drops which fall by gravity, and therefore can be used only for flat or horizontal position. At even higher current density the filler metal transfer takes the shape of a spray of very fine droplets forcefully projected from the electrode tip.
An article titled MIG/MAG Welding which covers important
considerations on GMAW was published in the November 2004 of
Practical Welding Letter Issue No. 15, in Section 11 - Contributions. A series of four articles on Selecting GMAW Parameters was published in the following issues of Practical Welding Letter. To read them click on:
PWL#023
Another spray type, called pulsed-arc transfer, occurs with equipment provided with the facility of modulating current at regular intervals: this, because of its lower heat input, is suitable for welding thinner sections than with conventional spray type, and with less distortion. With direct current reverse polarity (electrode positive) the strong squeezing Welding-arc force produces an action tending to pinch the molten metal and to project the drops across the arc unto the work. This contributes to a stable arc and to uniform metal transfer. Also heat input is greatest at the work side, inherent cleaning action is active, penetration is greater and weld deposits are of higher quality. With straight polarity (electrode negative) the greatest heat is developed at the electrode tip, there is no more pinch effect and metal transfer becomes of large globular type. The arc is now instable, with high burn-off rates and high spatter. Gas type has much influence. The use of alternating Welding-arc current extinguishes the arc every half cycle, so that it becomes inherently unstable. Open circuit voltage has to be higher in argon, to permit arc restriking. Helium atmosphere is not suitable. Penetration is somewhere between that of both polarities. An Article on Process Extensions of GMAW was published in our Practical Welding Letter issue 30 for February 2006. To see the article click on PWL#030.
And Tig for all... GAS TUNGSTEN-ARC WELDING (GTAW) often called TIG (for Tungsten Inert Gas), is a process where the electric Welding-arc takes place between the workpiece and a nonconsumable electrode. Shielding gas (usually argon, less commonly helium) is provided through a special tig torch. The process works in all positions either as a manual or as a semi-automatic operation. It is most useful when no filler metal is required, especially for thin materials. Some special means of initiating the Welding-arc are required when the electrode is still cold, possibly by using a high frequency spark between work and electrode. See a detailed page on Tig Welding Tips by clicking on this underlined title. Recommended Practices for Gas Tungsten Arc Welding
Specification for Tungsten and Tungsten Alloy Electrodes for Arc Welding and Cutting Tip!: At the end of a stretch of weld, while the arc is interrupted, the gas flow must be maintained for a while to protect the hot metal until it cools off. Most often the work needs to be well prepared and fixtured to avoid movement and deformation. Frequently an auxiliary argon flow must be supplied from the back side. A specialized automated version, called orbital welding, used for welding tube ends to tube-sheets for boilers, or for circumferential welds around tubes, provides the controlled speed and the guided circular motion of torch and filler as needed. The need of filler metal, if at all required, depends on the thickness, the shape and the desired characteristics of the weld. When filler is added, a rod is manually fed from the side through the arc until its tip melts and is added to the molten pool. For automatic welding the filler wire may be added continuously through a wire feeder at constant speed, while the tig torch moves relative to the workpiece. Better yet... PLASMA-ARC WELDING (PAW) is a Welding-arc process where the heat for melting is obtained by an electric arc struck between a nonconsumable tungsten electrode and a constricting orifice in the torch itself. The name Plasma is that of a gas ionized at elevated temperature by its passage through an electric arc. The Welding-arc in question is called a nontransferred arc, established between the electrode and the inside of the orifice body. The heat for melting is input to the workpiece only by the very hot ionized gas in plasma state. Concentration of energy is increased, permitting a narrower weld to be obtained as compared to that of GTAW (Tig), although a special welding technique (keyhole) may be required. The workpiece may be part of the electrical circuit (transferred arc plasma) or may not be part of it (nontransferred arc). In this case the arc voltage is independent from the distance between electrode and workpiece, the arc is more stable and tungsten contamination is eliminated. See a detailed page on Plasma Welding Tips by clicking on this underlined title. See: However equipment, spares and consumables expenditures are higher, and the welder must be specially trained to use this method successfully. Welds may be made with or without the addition of filler metal. Very thin sheets have been welded successfully. Weld of higher quality than with other processes can be obtained, frequently for the problematic root pass, while successive passes are done with more economical processes. An Article on Plasma Arc Welding was published in the Feb. 2005 Issue No. 18 of Practical Welding Letter. To see the article click on PWL#018. Although with certain specific jobs the selections can sometimes overlap, as a general indication one can establish the following rules: Aluminum and magnesium are preferably welded by Alternating Current (GTAW) but Direct Current Reverse Polarity (electrode positive), which gives good penetration and cathodic cleaning action, is used with GMAW either with Welding-arc constant current (CC) or with Constant Voltage (CV) power supplies. SMAW and SAW are unsuited for these alloys. Steels can use all types of current with subtle differences: For SMAW, one can use DC or AC: it depends on the type of covering of the electrode. Low hydrogen electrodes prefer DCRP. Penetration vary with polarity: test to find what suits you best. GTAW is usually performed using DC Straight Polarity (electrode negative) on steels and also on stainless, heat resisting, nickel and copper but AC is also applicable. Titanium is GTA (Tig) welded with DCSP. GMAW (Mig) and FCAW (Flux cored) use preferably DCRP. SAW (Submerged Arc) uses DCRP (at low current) or AC (at high current). PAW (Plasma) employs DCSP and is used for steel, stainless, heat resisting, refractory metals, nickel, copper and titanium. Although possible, aluminum is not generally welded by this process. An Article on Filler Metal Resources was published in issue 31 of Practical Welding Letter for March 2006.
To see the article click on PWL#031.
Finally... STUD WELDING, the last of the Welding-arc processes reviewed here, consists of a method whereby studs or similar fasteners are welded to a surface by an electric arc struck between the two elements, and then brought rapidly in contact under pressure. Because of this, the process is not a pure FUSION welding one. All sequential operations are automatically controlled. Stud welding is preferentially applied to weld fasteners onto a surface in a controlled and repetitive way. The process can be adapted to fast feeding of studs to the gun for high welding rate. As with most welding processes, surface areas must be clean, especially aluminum alloys and stainless steels, which form a tenacious oxide layer by just being exposed to air at normal temperature. The thickness of the base material should be such that no burn through occurs. An Article on Electro Slag Welding has been published in our Practical Welding Letter No. 07, issue of March 2004. An Article on Applications of Electroslag Welding was published (11) in Issue 49 of Practical Welding Letter for September 2007. To see the article click on PWL#049.
An Article on Weld Deposition Rate was published in Section 2, in Issue 29 of Practical Welding Letter for January 2006. To read the article click on PWL#029. An Article on ElectroGas Welding was published in issue 31 of Practical Welding Letter for March 2006.
To see the article click on PWL#031.
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