Fig. 6.28. Gas Tungsten Arc Welding (GTAW) Schematic
Fig. 6.28. Gas Tungsten Arc Welding (GTAW) Schematic a V or U groove. For material thicker than 0.375 in., a double V or U groove joint design is recommended.
In GTAW, the welding heat is provided by an arc maintained between a non-consumable tungsten electrode and the workpiece. In GTAW, as shown in Fig. 6.28, the power supply is direct current with a negative electrode (DCEN). The negative electrode is cooler than the positive weld joint, enabling a small electrode to carry a large current, resulting in a deep weld penetration with a narrow weld bead. A high frequency circuit for assistance in starting the arc, and a current decay unit for slowly stopping the arc, should also be used. The high frequency circuit eliminates the need to contact the work with the electrode to start the arc. Contact starting can damage the electrode tip and also result in tungsten inclusions in the weld metal. A current decay unit gradually lowers the current before the arc is broken, to reduce the puddle size and end the bead smoothly.
The weld puddle and adjacent HAZ on the weld face are protected by the nozzle gas. Trailing shields are used to protect the hot solidified metal and the HAZ behind the weld puddle. Back-up shielding protects the root of the weld and its adjacent HAZ. Recommended shielding gases are helium, argon, or a mixture of the two. For welding thin material without the addition of filler metal, helium has the advantages over argon of reduced porosity and increased welding speed. Welding speeds can be increased as much as 40% over those achieved with argon; however, the arc voltage for a given arc length is about 40% greater with helium, and the heat input is therefore greater. Since welding speed is a function of heat input, the hotter arc permits higher speeds. The arc is more difficult to start and maintain in helium when the welding current is below about 60 amps. When low currents are required for joining small parts or thin material, either argon shielding gas should be used, or a high frequency current arc starting system should be added. Shielding gas flow rate is critical. Low rates will not protect the weld while high rates can cause turbulence and aspirate air, destroying the gas shield.
Tungsten electrodes, or those alloyed with thorium, are normally used for GTAW. A 2% thoria electrode will give good results for most welding applications; however, thoria tungsten electrodes are mildly radioactive, so alternate electrodes with ceria (2% cerium oxide) or lanthana (1-2% lanthanum oxide) are available. Although the initial cost of alloyed electrodes is greater, their longer life resulting from lower vaporization and cooler operation, along with their greater current carrying capacity, makes them more cost effective in the long term. The electrode will become contaminated if it contacts the weld metal or the base metal surface during the welding operation. If this occurs, the electrode should be cleaned and reshaped by grinding. In addition, the welder should stop and grind out the electrode debris from the weld metal.
Gas metal arc welding uses a consumable electrode, rather than a non-consumable electrode, as used in the GTAW process. In GMAW, as shown in Fig. 6.29, the power supply is direct current with a positive electrode (DCEP). The positive electrode is hotter than the negative weld joint ensuring complete fusion of the wire in the weld joint. GMAW has the advantage of more weld metal deposited per unit time and unit of power consumption. For plates 0.5 in. and thicker, it is a more cost-effective process than GTAW. However, poor arc stability can cause appreciable spatter during welding, which reduces its efficiency.
Filler metals are normally similar in chemical composition to the base metals with which they are used. Because of high arc currents and high puddle temperatures, filler metals often contain small additions of alloying elements to deoxidize the weldment, and thus help to prevent solidification cracking and hot cracking.
Direct Current, Electrode Positive (DCEP)
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