AWS Gas Tungsten Arc Welding (GTAW/TIG) Fundamentals 2 — Questions and Answers

Question 1: For GTAW on titanium, which shielding gas is required, and why?

CO2 — it provides the best arc stability for titanium
100% Argon or Argon/Helium — titanium reacts with nitrogen and oxygen at any elevated temperature (Correct answer)
100% Helium — to prevent titanium hydride formation
CO2/Argon mix — same as stainless steel

Correct answer: 100% Argon or Argon/Helium — titanium reacts with nitrogen and oxygen at any elevated temperature

Titanium requires 100% argon or argon/helium shielding because it reacts with oxygen, nitrogen, and hydrogen above ~500°F, forming brittle interstitial compounds that embrittle the weld and HAZ.

Titanium readily dissolves oxygen, nitrogen, and hydrogen at elevated temperatures (above approximately 500°F/260°C for significant pickup). These interstitials cause weld metal and HAZ embrittlement, loss of ductility, and porosity (hydrogen). To prevent contamination: 100% argon shielding gas (100% helium or Ar/He mix for increased penetration); extended post-flow (gas lens, trailing shields); back purge; and often a containment box for highly reactive titanium alloys (Ti-6Al-4V). The weld color indicates contamination: silver = clean; gold, blue, purple, gray = increasing contamination levels; white = severe contamination requiring rejection.

Question 2: A gas lens in a GTAW torch replaces the standard collet body and provides:

A way to increase gas flow rate for thicker materials
A layered mesh screen that produces laminar shielding gas flow for better coverage at longer arc lengths (Correct answer)
A heat sink to reduce tungsten heating
An electrical lens that focuses the arc for deeper penetration

Correct answer: A layered mesh screen that produces laminar shielding gas flow for better coverage at longer arc lengths

A gas lens contains fine mesh screens that straighten and laminarize shielding gas flow, providing more uniform coverage, allowing the tungsten to extend further from the nozzle for better access.

Standard GTAW collet bodies produce turbulent gas flow that limits shielding effectiveness, requiring the nozzle to be close to the work. Gas lens assemblies contain multi-layer stainless steel mesh screens that break up turbulence, converting the gas flow from turbulent to laminar. Benefits: shielding coverage improves dramatically; tungsten can extend 1–1.5 times the nozzle diameter beyond the cup (vs. 0.25x without lens); wider nozzles can be used for broad coverage; allows welding in restricted spaces where standard setups can't maintain adequate shielding. Gas lenses are standard for titanium, stainless, and critical applications.

Question 3: When welding dissimilar metals by GTAW (e.g., 304 stainless steel to carbon steel), what filler metal is typically selected?

ER70S-6 (carbon steel filler — match the lower-strength material)
ER309L (austenitic stainless with higher chromium/nickel for dilution tolerance) (Correct answer)
ER308L (standard 304 match — same as welding 304 to 304)
No filler — autogenous weld only

Correct answer: ER309L (austenitic stainless with higher chromium/nickel for dilution tolerance)

ER309L is preferred for joining stainless to carbon steel because its higher Cr and Ni content provides a buffer against dilution with the carbon steel, maintaining a tough, crack-resistant austenitic weld deposit.

When austenitic stainless steel is joined to carbon steel, dilution with the carbon steel base metal can push the weld composition into a martensitic or ferritic region, causing cracking. ER309L has significantly higher Cr (~23%) and Ni (~13%) than ER308L, providing a safety margin against dilution. The 'L' (low carbon) designation limits carbide precipitation. Per AWS D1.6 and various process piping codes (ASME B31.3), ER309L or ER309LMo are the standard choices for dissimilar metal joints between austenitic stainless and carbon or low-alloy steel.

Question 4: In GTAW, 'tungsten inclusion' is caused by and results in:

Argon impurity depositing tungsten oxide — weld appears gray
Physical contact of the tungsten with the weld pool — hard, brittle white particles embedded in the weld (Correct answer)
Insufficient post-flow — tungsten oxidizes and flakes into the pool
Too high amperage — tungsten evaporates into the pool

Correct answer: Physical contact of the tungsten with the weld pool — hard, brittle white particles embedded in the weld

Tungsten inclusion occurs when the electrode tip contacts the molten pool, depositing tungsten particles in the weld metal. They appear as bright white, high-density inclusions on radiographs.

Tungsten inclusions occur when: the tungsten contacts the weld pool directly; excessive current melts the tungsten tip; spatter contacts and damages the tungsten; or improper arc initiation (scratch start) causes contamination. Tungsten has a melting point of 6192°F — much higher than steel — so tungsten particles don't dissolve; they remain as hard, brittle inclusions. On radiographs, tungsten inclusions appear as high-density white spots (unlike porosity, which is dark). They are unacceptable per all welding codes and require removal by grinding, gouging, or RTW, and the contaminated tungsten must be reground.

Question 5: Pulse GTAW (GTAW-P) provides which primary benefit on thin material?

Higher deposition rate than standard GTAW
Reduced average heat input controlling distortion and burn-through on thin sections (Correct answer)
Ability to weld without filler metal addition
Elimination of back purge requirements

Correct answer: Reduced average heat input controlling distortion and burn-through on thin sections

Pulsed GTAW alternates between high (peak) and low (background) current, reducing average heat input and allowing precise control of the weld pool — critical for preventing burn-through and distortion on thin materials.

Pulsed GTAW uses a programmable current waveform: peak current (high — forms and moves the weld pool), background current (low — maintains arc, allows pool to partially solidify), peak time, and background time. Average current is lower than conventional GTAW for equivalent penetration, reducing total heat input and heat-affected zone width. Benefits on thin materials: reduced distortion; prevention of burn-through; improved toe-to-toe fusion control; reduced warping on sheet metal. Pulsing also improves weld aesthetics ('stack of dimes' appearance) on visible welds. Pulse frequency, peak/background current ratio, and duty cycle are all WPS variables.

Question 6: What contamination does acetone in GTAW filler metal cleaning indicate, and how should the filler be handled?

Acetone is a flux applied to filler rod to improve fusion — standard practice
Acetone removes oil, fingerprints, and surface contamination from filler rods — filler must be wiped with acetone before use for critical welds (Correct answer)
Acetone is used to etch filler rods to verify chemistry — not related to contamination
Acetone cleaning is only needed for aluminum filler rods

Correct answer: Acetone removes oil, fingerprints, and surface contamination from filler rods — filler must be wiped with acetone before use for critical welds

Filler rods must be cleaned with acetone or isopropyl alcohol before use to remove oil, fingerprints, and surface oxides that would introduce contamination into the weld and cause porosity or reduced mechanical properties.

Contaminants on GTAW filler rods include: oil from handling (fingerprints), moisture, surface oxides, drawing lubricants, and packaging residue. For critical applications (titanium, stainless, aluminum, high-purity systems), filler rods must be cleaned with acetone or clean isopropyl alcohol immediately before use, using lint-free cloths or clean wipes. Oils decompose in the arc to release hydrogen and carbon, causing porosity, hydrogen embrittlement, and carbon contamination. AWS standards and ASME Section IX procedure requirements often mandate filler metal cleanliness procedures as part of the WPS.

AWS Gas Tungsten Arc Welding (GTAW/TIG) Fundamentals 2

For GTAW on titanium, which shielding gas is required, and why?