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© 2012 Delmar, Cengage Learning Chapter 15 Gas Tungsten Arc Welding Equipment, Setup, Operation, and Filler Metals
© 2012 Delmar, Cengage Learning Objectives Describe the gas tungsten arc welding process and list the other terms used to describe it Explain what makes tungsten a good electrode Tell how tungsten erosion can be limited Discuss how the various types of tungsten electrodes are used Tell how to shape the end of the tungsten electrode and how to clean it
© 2012 Delmar, Cengage Learning Objectives (cont'd.) Demonstrate how to grind a point on a tungsten electrode using an electric grinder Demonstrate how to remove a contaminated tungsten end Demonstrate how to melt the end of the tungsten electrode into the desired shape Compare water-cooled GTA welding torches to air-cooled torches
© 2012 Delmar, Cengage Learning Objectives (cont'd.) Tell the purposes of the three hoses connecting a water-cooled torch to the welding machine Discuss how to choose an appropriate nozzle for the job Tell what procedures must be followed to get an accurate reading on a flowmeter Compare the three types of welding current used for GTA welding
© 2012 Delmar, Cengage Learning Objectives (cont'd.) Discuss the shielding gases used in the GTA welding process Define preflow and postflow Explain the problems that can occur as a result of an incorrect gas flow rate Demonstrate how to properly set up a GTA welder Demonstrate how to establish a GTA welding arc
© 2012 Delmar, Cengage Learning Introduction Gas tungsten arc welding (GTAW) process –Sometimes referred to as a TIG or heliarc TIG: short for tungsten inert gas –Arc is established between a nonconsumable tungsten electrode and base metal –Inert gas provides needed arc characteristics and protects the molten weld pool –Became more common when Argon became plentiful
© 2012 Delmar, Cengage Learning Tungsten Properties –High tensile strength –Hardness –High melting temperature –High boiling temperature –Good electrical conductivity Produced mainly by reduction of its trioxide with hydrogen
© 2012 Delmar, Cengage Learning Tungsten (cont'd.) Best choice for a nonconsumable electrode –High melting temperature –Good electrical conductivity Electrode becomes hot –Arc between electrode and work stabilizes –Because of the intense heat, some erosion of the electrode will occur
© 2012 Delmar, Cengage Learning FIGURE 15-1 Some tungsten will erode from the electrode, be transferred across the arc, and become trapped in the weld deposit. © Cengage Learning 2012
© 2012 Delmar, Cengage Learning Tungsten (cont'd.) Several ways to limit erosion –Good mechanical and electrical contact –Use as low a current as possible –Use a water-cooled torch –Use as large as a tungsten electrode as possible –Use DCEN current –Use as short as an electrode extension as possible –Use the proper shape electrode –Use an alloyed tungsten electrode
© 2012 Delmar, Cengage Learning Tungsten (cont'd.) Collet –Cone-shaped sleeve –Holds electrode in the torch Large-diameter electrodes –Conduct more current Current-carrying capacity at DCEN –About ten times greater than at DCEP Preferred electrode tip shape –Impacts temperature and erosion of tungsten
© 2012 Delmar, Cengage Learning FIGURE 15-2 Irregular surface of a cleaned tungsten electrode (poor heat transfer to collet). © Cengage Learning 2012
© 2012 Delmar, Cengage Learning Types of Tungsten Electrodes Pure tungsten –Excellent nonconsumable electrode –Can be improved by adding: Cerium Lanthanum Thorium Zirconium
© 2012 Delmar, Cengage Learning Types of Tungsten Electrodes (cont'd.) Classifications –Pure Tungsten, EWP –1% thorium tungsten, EWTh-1 –2% thorium tungsten, EWTh-2 –¼% to ½% zirconium tungsten, EWZr –2% cerium tungsten, EWCe-2 –1% lanthanum tungsten, EWLa-1 –Alloy not specified, EWG
© 2012 Delmar, Cengage Learning Table 15-1 Tungsten Electrode Types and Identification.
© 2012 Delmar, Cengage Learning Shaping the Tungsten Methods to obtain desired end shape –Grinding –Breaking –Remelting the end –Using chemical compound
© 2012 Delmar, Cengage Learning Grinding Often used to clean a contaminated tungsten or to point the end –Should have a fine, hard stone Coarse grinding stone with result in more tungsten breakage –Should be used for grinding tungsten only Metal particles will quickly break free when the arc is started, causing contamination
© 2012 Delmar, Cengage Learning FIGURE 15-8 Correct way of holding a tungsten when grinding. Larry Jeffus
© 2012 Delmar, Cengage Learning Breaking and Remelting Tungsten is hard but brittle –Struck sharply: will break without bending Holding against a sharp corner and hitting –Results in a square break After breaking squarely: melt back the end
© 2012 Delmar, Cengage Learning Chemical Cleaning and Pointing Tungsten –Can be cleaned and pointed using one of several compounds Heated by shorting it against the work Dipped in the compound Removed, cooled, and cleaned End will be tapered to a fine point –Contaminated electrode Chemical compound will dissolve the tungsten and allow contamination to fall free
© 2012 Delmar, Cengage Learning Pointing and Remelting Tapered tungsten with a balled end –Made by first grinding or chemically pointing the electrode Ball should be made large enough so color of the end stays dull red and bright red Increase ball size by applying more current Surface tension pulls molten tungsten up onto the tapered end
© 2012 Delmar, Cengage Learning GTA Welding Equipment Torches –Water-cooled: more efficient –Air-cooled: more portable –Hoses Water-cooled: three hoses connect it Air-cooled: may have one hose for shielding gas –Nozzle Directs shielding gas directly on welding zone –Flowmeter Regulates rate of gas flow
© 2012 Delmar, Cengage Learning FIGURE Schematic of a GTA welding setup with a water-cooled torch. © Cengage Learning 2012
© 2012 Delmar, Cengage Learning FIGURE Schematic of a GTA welding setup with an air-cooled torch. © Cengage Learning 2012
© 2012 Delmar, Cengage Learning Types of Welding Current GTA welding –All three types can be used DCEN: concentrates about 2/3 of its welding heat on the work DCEP: concentrates about 1/3 of its welding heat on the work AC: concentrates about 1/2 of its heat on the work
© 2012 Delmar, Cengage Learning FIGURE Electrons collect under the oxide layer during the DCEP portion of the cycle and lift the oxides from the surface. © Cengage Learning 2012
© 2012 Delmar, Cengage Learning Shielding Gases GTA welding process shielding gases –Argon (Ar) –Helium (He) –Hydrogen (H) –Nitrogen (N) –Mixture of two or more gases
© 2012 Delmar, Cengage Learning Shielding Gases (cont'd.) Key concepts –Argon Effectively shields welds in deep grooves in flat positions –Helium Advantage of deeper penetration –Hydrogen Not an inert gas Not used as primary shielding gas
© 2012 Delmar, Cengage Learning Shielding Gases (cont'd.) –Nitrogen Not an inert gas Has been used as an additive to argon –Hot start Allows a surge of welding current
© 2012 Delmar, Cengage Learning Preflow and Postflow Preflow –Time gas flows to clear out air in the nozzle or weld zone –Some machines do not have preflow Postflow –Time gas continues flowing after welding current has stopped –Protects molten weld pool, filler rod, and tungsten electrode as they cool
© 2012 Delmar, Cengage Learning Shielding Gas Flow Rate Measured in cubic feet per hour (CFH) or in metric measure as liters per minute (L/min) –Rate of flow should be as low as possible and still give adequate coverage TABLE 15-4 Suggested Argon Gas Flow Rate for Given Cup Sizes.
© 2012 Delmar, Cengage Learning Remote Controls Can be used to: –Start the weld –Increase current –Decrease current –Stop the weld Remote –Foot-operated or hand-operated device
© 2012 Delmar, Cengage Learning Summary Prime considerations for gas tungsten arc welding –Equipment cleanliness Everything is clean: welding process proceeds more easily –Tungsten end or tip shape –Contamination can be very frustrating Tungsten contamination is part of the process
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