© 2012 Delmar, Cengage Learning Chapter 15 Gas Tungsten Arc Welding Equipment, Setup, Operation,...

© 2012 Delmar, Cengage Learning

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



• 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



• 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 – 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


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 – 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:– 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

© 2012 Delmar, Cengage Learning Chapter 15 Gas Tungsten Arc Welding Equipment, Setup, Operation,...

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