Gas Tungsten Arc Welding (GTAW) (TIG)

Uses high quality welds - good seal root bead for heavy pipe in petroleum, chemical industry flange joints designed for no filler metal seldom used for metals over 1/4” thick - if so, just a root pass metals as thin as .003”

Filler metal added manually occasionally automatically

Requires more welder skill - handle electrode and torch

Adv: small HAZ - less weakening no metal transfer through arc no splattering quiet high quality

Power supply AC / DCEP / DCEN (most common)

DCEN (most common) 70% at work piece while 30% at electrode = deeper penetration

DCEP - less heat - ok for thin metal

Breaking up surface oxides occurs with DCEP and 1/2 AC cycle + charged shielding gas ions are flowing from nozzle to (-) charged work these ions strike the work surface with enough force to break up oxides works well for magnesium and aluminum

Always done with Constant Current power supply same as for SMAW varying arc length allows slight variations in the current Current setting determined by size of electrode metal thickness shielding gas used type of current supplied

Torch - 4 functions 1. hold electrode 2. electrical contact with electrode 3. direct shielding gas to weld 4. provide means to cool electrode

Torch nozzles ceramic exit dia of nozzle and lengths vary depending on mfgr. single digit number ID’s the nozzle exit dia. in 1/16” increments a #6 = 6 x 1/16” = 3/8” a #8 = 8 x 1/16” = 1/2” nozzle dia must allow complete coverage of weld area with shield gas

Small exit diameter

Other torch parts Collet - supports electrode

Shield gas protects electrode from contamination oxygen, nitrogen in air, dirt **allow gas to flow after arc has stopped - 10 - 15 seconds

Shielding gases argon - smoother, quieter, better cleaning, heavier than air helium - more heat at work piece, gives deeper penetration mixtures of the two

Gas flow rates being equal at 14 CFPH #8 nozzle - smooth flow #4 nozzle - rapid flow about 35 CFPH recommended for mild steel

Tungsten electrodes 100% tungsten - green band - tungsten with 1% thoria added - yellow band tungsten with 2% thoria added - red band (common) tungsten with .15 to .40 % zirconia added - brown band pure tungsten with a core of 1 to 2% thoria added - blue band

Thoria & zirconia helps to harden the electrode facilitate arc starting help maintain a stable arc

Electrode sizes diameters .010” to .025” lengths 6 - 7” up to 24”

Electrode extension at end of nozzle or up to 1/2” extension determined by shape of joint longer extension = more gas flow rate

Electrode Tips

Conical - for DCEN produces a concentrated, hotter arc - deeper penetration 70% of heat developed at work so tip will not melt sharpen with 80 to 120 grit

Rounded - used for AC and DCEP Welding al. or magnesium electrode forms a ball at tip - arc flows more easily smoother transition through cycle from cleaning to penetration smoother weld bead

Filler metal bare wire - coils or precut lengths common dia of 1/16” to 1/4” cannot withdraw from shielding gas envelope while molten

Torch and filler position

Method of adding filler metal

Joint designs same as for SMAW

Hot shortness metals like al. become very weak and melt through use backing plate

Methods of starting arc 1. scratch 2. superimposed high frequency current 3. high voltage

Safety same as for SMAW and GMAW infrared rays may be more intense when using larger electrodes

Automation