02 Gas Metal Arc Welding Notes.
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Post on 16-Dec-2015
DESCRIPTIONAbout gas metal arc welding
<ul><li><p>Gas Metal Arc Welding & Flux Cored Arc Welding </p><p>GAS METAL ARC WELDING </p><p>Introduction: </p><p>Gas metal arc welding (GMAW), was initially referred to as metal inert gas (MIG metal </p><p>inert gas) welding is an arc welding process that uses an arc between a continuous filler </p><p>metal electrode and the weld pool. The process incorporates shielding from an externally </p><p>supplied gas and is used without the application of pressure. </p><p>MIG was primarily developed for welding aluminium and other non-ferrous materials in </p><p>the 1940s. However, further process developments, such as operating at low current </p><p>densities, pulsed current and the use of reactive gases (particularly carbon dioxide) or gas </p><p>mixtures led to its use for a broader range of materials. It was the use of both inert and </p><p>reactive gases or gas mixtures that made the term Gas Metal Arc Welding (GMAW) as </p><p>the formally accepted name of the process. </p><p>Further developments during the 1950s and 1960s gave the process more versatility and </p><p>as a result, it became a highly used industrial process. Presently, GMAW is commonly </p><p>used the automotive and shipbuilding industries, where it is preferred for its versatility </p><p>and speed. </p><p>The GMAW process can be implemented in Semi-automatic or Automated operations. </p><p>All commercially available important metals, listed below, can be welded in all positions </p><p>with this process by selecting the appropriate combination of shielding gas, electrodes </p><p>and welding process variables. </p><p>Materials welded by GMAW: </p><p> Carbon steels </p><p> High strength low alloy steels (HSLA) </p><p> Stainless steels </p><p> Aluminium and its alloys </p><p> Copper and its alloys </p><p> Nickel and Nickel based alloys </p><p> Cast irons </p></li><li><p>Principles or Operation of GMAW </p><p> Figure 1: GMAW Process </p><p>The GMAW Process utilizes the automated feeding of a continuous, consumable </p><p>electrode that is shielded by externally supplied gas. The process is illustrated </p><p>schematically in Figure 1. </p><p>The basic equipment components required are a welding gun and cable assembly, an </p><p>electrode (wire) feeder unit, a welding power source and the gas supply with the </p><p>associated apparatus. A typical semi-automatic GMAW set up is illustrated in the </p><p>schematic in Figure 2. </p><p>The gun guides the electrode, conducts electrical current and directs shielding gas to the </p><p>work-piece. Most commonly, a constant voltage power source, providing essentially a </p><p>flat volt-ampere (V-A) curve, is used in conjunction with a constant speed electrode feed </p><p>unit to regulate the arc length. Alternatively, a constant current power source (providing a </p><p>drooping V-A curve can also be used with an electrode feed unit that is arc-voltage </p><p>controlled. </p></li><li><p>Figure 2: GMAW Set up </p><p>The constant voltage power source & constant speed wire feed combination is self </p><p>regulatory. Any changes in the gun position caused by the welders hand movement, can </p><p>initiate a self regulating action to maintain the arc length. </p><p>For example, when the gun-to-workpiece distance is suddenly increased, the arc length </p><p>momentarily becomes longer and this causes a reduction in current, thereby momentarily </p><p>reducing the electrode melting rate. Since the feed remains at the same speed, the arc </p><p>length decreases and the current increases until the melting rate again equals the feed </p><p>rate. </p><p>This self regulation of the GMAW arc is depicted in figure 3. </p><p>Figure 3: Self regulation of the GMAW arc with Constant voltage power source and </p><p>constant speed wire feed </p></li><li><p>Metal Transfer Modes </p><p>The characteristics of GMAW are best described in terms of the basic means by which </p><p>metal is transferred from the electrode to the workpiece, or the mode of metal transfer as </p><p>this is referred to. </p><p>There are four primary methods of metal transfer in GMAW, called globular, short-</p><p>circuiting, spray, and pulsed-spray, each of which has distinct properties and </p><p>corresponding advantages and limitations. </p></li><li><p>The mode of transfer is determined by a number of factors: </p><p>1. Magnitude, type and polarity of welding current </p><p>2. Electrode diameter </p><p>3. Electrode composition </p><p>4. Electrode extension and </p><p>5. Shielding gas composition </p><p>Short-circuiting Transfer </p><p>This mode of transfer, known as short-circuiting or short-arc GMAW, occurs when </p><p>carbon dioxide is the shielding gas, the electrode diameter is smaller, and the current </p><p>density is low. The metal is transferred from the electrode only during the period in </p><p>which the electrode is in contact with the weld pool. No metal is transferred across the </p><p>arc. The electrode contacts the weld pool in the range of 20- to 200 times per second. </p><p>Molten droplets forms on the tip of the electrode, but instead of dropping to the weld </p><p>pool, they bridge the gap between the electrode and the weld pool as a result of the </p><p>greater wire feed rate. This causes a short circuit and extinguishes the arc, but it is </p><p>quickly reignited after the surface tension of the weld pool pulls the molten metal bead </p><p>off the electrode tip. This process is repeated 20 to 200 times per second, making the arc </p><p>appear constant to the human eye. </p><p>As a result of the lower current, the heat input for the short-arc variation is reduced, </p><p>making it possible to weld thinner materials while decreasing the amount of distortion </p><p>and residual stress in the weld area. This transfer is generally suited for joining of thin </p><p>sections, for out-of-position welding and bridging large root openings </p><p>This type of metal transfer is slow and it is difficult to maintain a stable arc, because it </p><p>depends on achieving a consistent and high short-circuiting frequency, which can only be </p><p>accomplished with a good power source, suitable welding conditions, and significant </p><p>welder skill. </p><p>The composition of the shielding gas has a dramatic effect on the surface tension of the </p><p>molten metal, arc characteristics and penetration. CO2 produces high spatter levels </p><p>compared to Argon and Helium but it also promotes deeper penetration. To achieve a </p><p>good compromise between spatter and penetration when welding carbon and low alloy </p><p>steels, mixtures of CO2 and argon are often used. Addition of Helium or Argon may </p><p>increase penetration in nonferrous metals. </p><p>Globular Transfer </p><p>GMAW with globular metal transfer is often considered the most undesirable of the four </p><p>major GMAW variations, because of its tendency to produce high heat, a poor weld </p><p>surface, and spatter. The method was originally developed as a cost efficient way to weld </p><p>steel using GMAW, because this variation uses carbon dioxide, a less expensive shielding </p><p>gas than Argon. </p></li><li><p>As the weld is made, a ball of molten metal from the electrode tends to build up on the </p><p>end of the electrode, often in irregular shapes with a larger diameter than the electrode </p><p>itself. When the droplet finally detaches either by gravity or short circuiting, it falls to the </p><p>workpiece, leaving an uneven surface and often causing spatter. </p><p>As a result of the large molten droplet, this mode of transfer is generally limited to flat </p><p>and horizontal welding positions. </p><p>Spray Transfer </p><p>Spray transfer GMAW occurs when the molten metal from the electrode is propelled </p><p>axially across the arc in the form of minute droplets. With Argon-rich gas shielding it is </p><p>possible to produce a very stable, spatter-free axial spray transfer mode. The mode </p><p>requires Direct current with a positive electrode (DCEP) and a current level above a </p><p>critical value termed the spray transition current. Below this level, the transfer is globular. </p><p>In this variation, molten metal droplets (with diameters smaller than the electrode </p><p>diameter) are rapidly passed along the stable electric arc from the electrode to the </p><p>workpiece, essentially eliminating spatter and resulting in a high-quality weld finish. </p><p>However, high amounts of voltage and current are necessary, which means that the </p><p>process involves high heat input and a large weld area and heat-affected zone. </p><p>As a result, it is generally used only on work-pieces of thicknesses above about 6 mm </p><p>(0.25 in). Because of the large weld pool, it is often limited to flat and horizontal welding </p><p>positions, but when a smaller electrode is used in conjunction with lower heat input, its </p><p>versatility increases. The maximum deposition rate for spray arc GMAW is relatively </p><p>high; about 60 mm /sec (150 in/min). </p><p>Pulsed-spray Transfer </p><p>A more recently developed method, the pulse-spray metal transfer mode is based on the </p><p>principles of spray transfer but uses a pulsing current to melt the filler wire. This allows </p><p>one small molten droplet to fall with each pulse. The pulses allow the average current to </p><p>be lower, decreasing the overall heat input and thereby decreasing the size of the weld </p><p>pool and heat-affected zone while making it possible to weld thin work-pieces. </p><p>The pulse provides a stable arc and no spatter, since no short-circuiting takes place. This </p><p>also makes the process suitable for nearly all metals, and thicker electrode wire can be </p><p>used as well. </p><p>The process also requires that the shielding gas be primarily argon with a low carbon </p><p>dioxide concentration. </p><p>Additionally, it requires a special power source capable of providing current pulses with a </p><p>frequency of between 30 and 400 pulses per second. </p></li><li><p>However, the method has gained popularity, since it requires lower heat input and can be </p><p>used to weld thin workpieces, as well as nonferrous materials. </p><p>Process Variables: </p><p>Power source characteristics: As stated before, a constant voltage power source is used for GMAW process. The V-A </p><p>characteristics of the power source are depicted in figure 5 below. </p><p>Figure 5: Constant voltage power supply V-A curve </p><p>As is seen in the figure, a small change in arc voltage will cause a large change in current </p><p>thereby increasing or decreasing the melt-off rate of the electrode. This compensates for </p><p>the variations in contact tip-to-workpiece distance by instantaneously increasing or </p><p>decreasing the welding current. </p><p>Shielding Gases </p><p>As we know, the primary function of the Shielding gas is to exclude the atmosphere from </p><p>contact with the molten metal. This is necessary since most metals, when molten from </p><p>oxides and to a lesser extent, nitrides. Oxygen also reacts with carbon in the molten metal </p><p>to form CO and CO2 gases. These reaction gases may produce defects or discontinuities </p><p>such as slag inclusions, porosity and weld metal embrittlement. </p><p>In addition to provide a protective environment, shielding gas and flow rate have a </p><p>pronounced effect on: </p><p> Arc characteristics </p><p> Modes of metal transfer </p></li><li><p> Penetration and weld bead profile </p><p> Speed of welding </p><p> Undercutting tendency </p><p> Cleaning action and weld metal mechanical properties </p><p>The choice of a shielding gas therefore depends on several factors, most importantly: </p><p> the type of material being welded and </p><p> the process variation being used </p><p>Figure 7: Bead contours and penetration patterns with various shielding gases </p><p>Bead contours and Penetration patterns for various shielding gases are shown in figure 7. </p><p>Carbon dioxide provides maximum penetration while Helium produces uniformly </p><p>distributed arc energy. </p><p>Pure inert gases such as argon and helium produce excellent results for nonferrous </p><p>welding and therefore are used with most nonferrous metals, However, with steel these </p><p>pure gases cause an erratic arc, encourage spatter (Helium) or do not provide adequate </p><p>weld penetration (Argon). </p><p>Pure carbon dioxide, on the other hand, allows for deep penetration welds but encourages </p><p>oxide formation, which adversely affect the mechanical properties of the weld. Its low </p><p>cost makes it an attractive choice, but the arc is harsh, spatter is unavoidable, making </p><p>welding thin materials difficult. </p></li><li><p>Figure 8: Relative effect of Oxygen versus Carbon dioxide additions to the Argon shield </p><p>Mixtures therefore are used to combine the properties to give optimum results. </p><p>Oxygen (1 5%) and Carbon dioxide (3 25%) additions to Argon produce noticeable </p><p>improvement in arc stability and reduces the tendency for undercut. Carbon dioxide </p><p>additions to Argon may also enhance the weld bead configuration by producing a well </p><p>defined pear-shaped profile as is seen in fillet welds shown in figure 8. </p><p>As a result, argon and carbon dioxide are frequently mixed in a 75%/25% or 80%/20% </p><p>mixture, which reduces spatter and makes it possible to weld thin steel workpieces. </p><p>Argon is also commonly mixed with other gases, such as oxygen, helium, hydrogen, and </p><p>nitrogen. The addition of up to 5% oxygen encourages spray transfer, which is critical for </p><p>spray-arc and pulsed spray-arc GMAW. </p><p>An argon-helium mixture is completely inert, and is used on nonferrous materials. </p><p>Gas Flow Rate </p><p>The desirable rate of gas flow depends primarily on weld geometry, speed, current, the </p><p>type of gas, and the metal transfer mode being utilized. </p><p>Welding flat surfaces requires higher flow than welding grooved materials, since the gas </p><p>is dispersed more quickly. </p><p>Faster welding speeds mean that more gas must be supplied to provide adequate </p><p>coverage. </p><p>Higher current requires greater flow, and generally, more helium is required to provide </p><p>adequate coverage than argon. </p><p>Gas flow rates used for different metal transfer modes are listed in the table below for a </p><p>quick reference. </p></li><li><p> Metal Transfer Mode Gas Flow required </p><p>Short circuiting & Pulsed spray </p><p>modes (small weld pools) </p><p>10 L/min (20 ft/h) </p><p>Globular transfer 15 L/min (30 ft/h) is preferred </p><p>Spray transfer (higher heat input, </p><p>larger weld pool) </p><p>20-25 L/min (4050 ft/h) </p><p>Electrode </p><p>The electrodes (filler metals) for GMAW are specified by various filler metal </p><p>specifications (AWS for instance) </p><p>They define requirements for sizes and tolerances, packaging, chemical composition, and </p><p>in some cases, mechanical properties. </p><p>The process variables that affect weld penetration, bead geometry and overall weld </p><p>quality are: </p><p> Welding amperage (electrode feed speed) </p><p> Polarity </p><p> Arc Voltage </p><p> Travel speed </p><p> Electrode extension beyond the contact tip (stick out) </p><p> Electrode orientation (work, travel angles) </p><p> Weld joint position </p><p> Electrode diameter and </p><p> Shielding gas composition and flow rate </p><p>Process Variations </p><p>The gas metal arc process has been adapted to provide specific characteristics for a wide </p><p>range of applications. Some of these variations are: </p><p> Gas Metal Arc Sport Welding </p><p> Narrow Groove Gas metal arc welding </p><p> Gas Metal Arc Brazing </p><p> Tandem gas metal arc welding </p><p>Only Gas metal arc brazing is discussed here within the scope of the lecture. </p></li><li><p>Gas Metal Arc Brazing: </p><p>In gas metal arc braze welding, a copper based electrode (e.g. aluminium bronze or </p><p>silicon bronze) is used instead of a steel electrode to join steel. </p><p>As the copper alloy has a lower melting temperature than steel, less heating of the base </p><p>metal is required in order to deposit a weld bead, and little or no melting of the base </p><p>metal occurs. </p><p>Because of the low heat input, braze welding is some times used to join heat sensitive </p><p>materials such as cast iron, for welding of thin sheet steel...</p></li></ul>
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