Chapter 15 Gas Tungsten Arc Welding Equipment, Setup, Operation, and Filler Metals

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

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

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

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

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

Tungsten Properties High tensile strength Hardness High melting temperature High boiling temperature Good electrical conductivity Produced mainly by reduction of its trioxide with hydrogen

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

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

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

Tungsten (cont'd.) Collet Large-diameter electrodes 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

FIGURE 15-2 Irregular surface of a cleaned tungsten electrode (poor heat transfer to collet). © Cengage Learning 2012

Types of Tungsten Electrodes Pure tungsten Excellent nonconsumable electrode Can be improved by adding: Cerium Lanthanum Thorium Zirconium

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

Table 15-1 Tungsten Electrode Types and Identification.

Shaping the Tungsten Methods to obtain desired end shape Grinding Breaking Remelting the end Using chemical compound

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

FIGURE 15-8 Correct way of holding a tungsten when grinding. Larry Jeffus

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

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

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

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

FIGURE 15-21 Schematic of a GTA welding setup with a water-cooled torch. © Cengage Learning 2012

FIGURE 15-22 Schematic of a GTA welding setup with an air-cooled torch. © Cengage Learning 2012

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

FIGURE 15-29 Electrons collect under the oxide layer during the DCEP portion of the cycle and lift the oxides from the surface. © Cengage Learning 2012

Shielding Gases GTA welding process shielding gases Argon (Ar) Helium (He) Hydrogen (H) Nitrogen (N) Mixture of two or more gases

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

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

Preflow and Postflow Preflow Postflow 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

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.

Remote Controls Can be used to: Remote Start the weld Increase current Decrease current Stop the weld Remote Foot-operated or hand-operated device

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