Active TIG Welding Research Progress

Literature Overview

The comprehensive review by Peng Xiaoyang, Ling Zemin, Liao Juan, and Li Jinge from Chongqing University, published in Mechanical Engineering Materials in 2013, provides an in-depth analysis of Active TIG (aTIG) welding, a modified TIG process that introduces active gases (CO₂, CO, O₂, or H₂) into the primary shielding gas (Ar or He) to enhance weldability. This review is particularly relevant to cladding and overlay applications where improved wetting, reduced dilution, and enhanced mechanical properties of the overlay layer are desired.

Core Technical Content

Principle of Active Gas Addition

Conventional TIG welding uses inert shielding gases (Ar, He, or mixtures) that provide arc stability but do not participate in the welding reaction. Active TIG welding introduces reactive gases that:

Enhance arc constriction and energy density — Active gases have lower ionization potentials than argon, resulting in a more concentrated arc with higher current density
Improve surface tension of molten pool — Dissolved oxygen reduces surface tension, promoting wider bead profiles and better wetting
Modify microstructure — Oxygen and other active elements influence solidification behavior, grain refinement, and phase transformation
Reduce base metal dilution — The concentrated arc and improved wetting reduce the volume of base metal melted

Active Gas	Ionization Potential (eV)	Effect on Arc	Effect on Weld
Ar	15.76	Baseline	Baseline
He	24.59	Wider arc, lower penetration	Broader bead
CO₂	13.82	More constricted arc	Deeper penetration, oxidation
CO	14.04	Moderate constriction	Moderate oxidation
O₂	13.62	Strong constriction	Surface tension reduction
H₂	13.59	Enhanced penetration	Reduced surface tension

Typical Active Gas Compositions

Application	Shielding Gas Composition	Typical Current (A)	Travel Speed (mm/min)
Carbon steel	97% Ar + 3% CO₂	150–250	200–400
Stainless steel	98% Ar + 2% O₂	100–200	150–350
Aluminum alloys	95% Ar + 5% O₂	200–350	300–600
Low-alloy steel	97% Ar + 3% CO₂	200–350	250–500
Cladding (Ni-base)	98% Ar + 2% O₂	80–150	100–250

Microstructural Effects

The addition of active gases significantly influences the weld metal microstructure:

Grain refinement: Oxygen dissolves in the molten pool and acts as a heterogeneous nucleation site, reducing grain size by 30–50%
Phase transformation: In low-alloy steels, oxygen promotes acicular ferrite formation, improving toughness
Precipitate modification: In stainless steel welds, controlled oxygen levels can suppress δ-ferrite formation or promote its dissolution
Hardness distribution: Active TIG welds typically exhibit more uniform hardness profiles due to reduced dilution and refined microstructure

Process-Standard Analysis

Comparison of Conventional TIG vs. Active TIG for Cladding

Parameter	Conventional TIG (Ar)	Active TIG (Ar + 2% O₂)
Arc voltage	18–22 V	16–20 V
Penetration depth	1.5–2.5 mm	2.0–3.5 mm
Bead width	8–12 mm	10–15 mm
Dilution rate	15–25%	8–15%
Deposition efficiency	60–70%	75–85%
Bead profile	Convex	Flat to slightly concave
Surface oxidation	Minimal	Slight (controllable)
Cost	Moderate	Slightly higher (gas cost)

Standards Considerations

Active TIG welding introduces additional considerations for code compliance:

ASME IX: The WPS must specify the exact shielding gas composition. Any change in active gas percentage requires requalification if it affects the essential variables.
NB/T 47014: Chinese standards for welding procedure qualification require specific gas compositions to be documented and verified.
ASTM A263/A264: For stainless steel clad plate, the overlay weld procedure must demonstrate adequate corrosion resistance, which may be affected by oxygen pickup.

Key Questions and Reflections

The active TIG process offers several advantages for cladding applications, but also introduces challenges:

Oxygen pickup in weld metal: Excessive oxygen content can reduce corrosion resistance, particularly in austenitic stainless steel overlays. The oxygen content should be maintained below 0.02% for 316L and below 0.015% for 304L to ensure adequate corrosion performance.
Hot cracking susceptibility: The reduced surface tension from oxygen addition can increase hot cracking tendency in high-alloy overlays (e.g., Inconel 625, Hastelloy C276). Careful selection of active gas percentage and filler metal composition is essential.
Porosity: CO₂ addition can introduce nitrogen pickup and porosity if gas flow rates are inadequate. Backside protection with pure argon is recommended for thick-section cladding.
Repeatability: Active gas compositions must be precisely controlled. Gas blending equipment with flow control accuracy of ±0.5 L/min is recommended for consistent results.

Study Insights and Implications

Active TIG welding represents a significant advancement in TIG-based cladding technology. The ability to reduce dilution while increasing deposition rate makes it particularly attractive for overlay applications where maintaining the specified alloy composition of the cladding layer is critical. For bimetal pressure vessel fabrication, active TIG can be employed for the first few passes of weld overlay to establish a transition zone with controlled dilution, followed by conventional TIG or other processes for subsequent passes.

The review highlights the versatility of active TIG across multiple material systems. However, the specific active gas selection must be tailored to the base metal and overlay material combination. For nickel-based alloy cladding on carbon steel, a low-oxygen active gas (e.g., 99% Ar + 1% O₂) is preferred to minimize oxidation of the precious alloy elements while still providing the benefits of improved arc characteristics.