GAS METAL ARC WELDING (GMAW)

Transcription

1 GAS METAL ARC WELDING (GMAW) INTRODUCTION Gas Metal Arc Welding (GMAW) is also called Metal Inert Gas (MIG) arc welding. It uses consumable metallic electrode. There are other gas shielded arc welding processes utilizing the consumable electrodes like Flux cored Arc Welding (FCAW) all of which are termed under MIG. When thicker sheets are to be welded, the filler metal requirement makes GTAW difficult to use and hence GMAW comes handy. In GMAW process, consumable electrode is in the form of wire reel, whichisfedataconstantratethroughthefeedrollers.theweldingtorch is connected to the gas supply cylinder which supplies the necessary inert gas. The electrode and the workpiece are connected to the welding power supply and the power supply is constant voltage only. The current from the welding machine is changed by the rate of feeding of the electrode wire. Normally dc arc welding machines are used for GMAW with electrode positive. DCEP increases the metal deposition rate, produces stable arc and smooth electrode metal transfer. This is not possible with DCEN. Special electrodes having calcium and titanium oxide mixtures as coatingsarefoundtobegoodforweldingsteelwithdcen. 1

2 SCHEMATIC OF GMAW PROCESS TERMINOLOGY USED IN GMAW PROCESS METAL TRANSFER In the GMAW process, the filler metal is transferred from the electrode to the joint. Depending upon the current and the voltage used for given electrode, the metal transfer is done in different ways. They are Short circuit or dip transfer Globular transfer Spray transfer Pulsed spray transfer Rotating spray transfer Theabovearefoundtooccurwithvaryingratesofcurrent,thelowestrate giving the short circuit transfer with the rest arranged in the increased order of current. The SHORT CIRCUIT METAL TRANSFER occurs with relatively low current settingsoftheorderof75to175aforanelectrodediameterof0.9mm. Initially initiation of the arc starts. Under the intense heat of the spark, the electrode tip melts and forms a globule of molten metal at the tip. As the electrode wire is fed towards the work piece, the molten tip touches the weld metal pool and thus short circuits the electrode to the work piece. This reduces the voltage across the arc. 2

3 The metal tip then gets pinched by the surface tension of the weld metal pool as well as the magnetic force due to the current flow. Finally the metal is pinched away and the arc gets ignited away, the cycle is repeated all over again. The number of times that the pinching takes place depends on the inductance of the welding machine used and the parameters set. The inductance of the welding machine controls the rate at which the short circuiting current increases. Low inductance gives rise to very high short circuiting current and consequently high pinching rate. With high inductance, the short circuit current becomes low and results in some what lower pinching force. It has been observed that optimum response rates depends on the electrode wire size. Hence the welding machine to be used for short circuiting transfer makes use of variable choke system which can be tuned for different wire sizes. The frequency of metal transfer may be of the order of 50 to 500 per second. This rate also depends on the open circuit voltage and the wire feed rate employed. High open circuit voltage reduces frequency of short circuting, low wire feed rae can compensate for low frequency. The main advantage of using short circuit metal transfer is lower penetration (effective for welding thin sheets). This process is useful for poorly fed joints and difficult to reach weld positions. There is no splatter and less amount of metal is effected around weld joint because of small input heat. SHORT CIRCUIT METAL TRANSFER SPRAY TRANSFER 3

4 SPRAY TRANSFER of metal occurs when current is increased beyond globular transfer current. As the current is increased, magnetic pull on molten metal increases and therefore molten metal is detached from electrode tip by this pull. Irrespective of gravity force, the gravity has no effect on metal transfer. Spray transfer welding can be done in either horizontal, vertical or overhead. As the arc heat melts the electrode, the metal is continuously attracted towards theworkbymagneticforcelikeasprayfromthewaterjet. The current where metal transfer changes from globular to spray is called transition current. This transition current is different for different materials. It increases with the electrode diameter and decreases with an increase in the electrode extension. The spray of metal occurs when shielding gas contains atleast 90% Argon and no Carbon dioxide. The arc in spray transfer can be controlled and directed accurately and it provides deep penetration because metal spray pattern is concentrated and stable. This is the most common method of metal transfer in GMAW with Argon as shielding gas. SHIELDING GASES Shielding gases should always flow in a laminar manner without causing any turbulence. The turbulence causes weld contamination. When gas flow is too small, weld area is not sufficiently protected from atmospheric air. The gas flow rates to be used depend upon the thickness of the sheet being welded, the positionoftheweldaswellasthebasematerial. Thegasesthat can beused for GMAW areargon, helium, nitrogen, oxygen, carbon dioxideandamixtureofabovegasesinvariousproportions.eachgashasitseffect on formation of the bead and penetration. Argon reduces the spatter and concentrates the arc. It gives deep penetration welds. It ionises easily requiring smaller arc voltages. It has lower thermal conductivity and therefore conducts heat very slowly from the arc to the weld zone.itisgoodforweldingthinsheetsandoutofpositionwelding. Helium is the most expensive of all shielding gases. It has better thermal conductivity and hence is useful for thicker sheets as well as for metals having higher thermal conductivity (like copper and aluminium). The filler metal deposition rate by helium is higher than argon because of the higher current carrying capacity. 4

5 The arc in carbon dioxide shielding gas is unstable, therefore very short arc is used to reduce metal spatter. This is the least expensive of all shielding gases. Since about 7.5% of the carbon dioxide decomposes into carbon monoxide and oxygen in the arc, deoxidizers such as aluminium and silicon are used. It is a heavy gas and therefore covers the weld zone very well. The metal transfer is globular with the carbon dioxide shielding gas. The mixture of gases such as argon-helium, argon-carbon dioxide, argonoxygen are used for special applications where these constituent gases provide the difficult capabilities that are desired. ELECTRODES The electrode comes generally in the form of wire. Various wire compositions are available, depending upon the base metal composition. The normal diameter sizes are of the order of 0.5 mm to 3.2 mm. Wires of very small diameter are very expensive. The electrode wires to be used for steels generally have deoxidisers added to it. These help in reduction of oxides of the weld metal as well as porosity. But the cost of the electrode wire also increases. The electrode wire is produced by wire drawing where lubrication is necessary. Any lubricant film left on the wire will interfere with welding quality (unstable arc) and promote porosity. Therefore proper cleaning of wirebeforeuseismust. Electrode wire should be of proper diameter because variation of diameter causes wandering of the arc from the desired location. 5

6 WELDING TECHNIQUE The variables that effect the weld quality are the electrode-stick-out (extension), travel speed, the welding method, electrode diameter and welding machine parameters. A minimum electrode-stick-out of the order of 10 mm is to be maintained to safeguard the contact tube from burning. Larger electrode extensions decrease the melting rates and increase the penetration. The effect of electrode extension on the melting rate also depends on size of electrode. The melting rate in terms of various parameters are given as 2 ( a I ) + ( b L I ) MR = Where MR = melting rate(kg/hr) I=current(A) L = electrode extension(mm) a,b=constants With DCEP and wire size of 1.6 mm, the values of a & b for different materials are Material a b Aluminium 5.4 x x 10-6 Mild steel 8.6 x x 10-5 The electrode travel speed also affects the bead size and penetration. If the travel speedistoolow,itcausesalargedepositionoffillermetal.ifthespeedistoohigh, it does not give enough time for melting the base metal and results in less deposition. Different electrode require different optimum travel speeds which are provided by the manufacturer. The welding method determines the positioning of the gun with respect to the base metal. Three possible positioning of the welding gun are called forehand, perpendicular and backhand. Inforehandwelding,thetorchpointsinthedirectionoftheelectrodetravelwitha slant backwards from the vertical. Here the arc heat is forward directed, as a result, least penetration is observed. In the backhand technique, the gun is slanted in the direction of the travel by an angle of about 25. In this position, the arc heat is directed on the weld bead which gives best penetration. This is the most preferred technique. In perpendicular position, the penetration midway between the above two techniques. 6

7 POSITIONS OF THE WELDING GUN FLUX CORED ARC WELDING The Flux cored Arc Welding (FCAW) process is a modification of GMAW, where the solid electrode wire is replaced by a tubular electrode containing flux at the center of the electrode throughout its length. Because of the availability of flux in the welding zone, the weld metal of any required composition can be closely controlled as well as obtain a smooth weld bead. FCAW is normally used for welding structural steel and alloy steels. This process has the advantage of the stick electrodes in the form of flux available to remove the oxides and other containments in the form of slag and form protective coatings. Alloying elements and deoxidisers can be added to the joint. This process gives rise to deeper penetration. The equipment used for flux cored arc welding is similar to GMAW. Since the electrode is not a solid wire, the feeding unit has to be designed so as to put excess pressure on the electrode resulting in its flattening. 7

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