CLADDING TECHNOLOGY SHANXI CO., LTDArc-Assisted Active TIG Welding Method for Cladding Applications
Literature Overview
The research published in 2008 by Fan Ding, Huang Yong, Lin Tao, and Niu Shufeng from the Key Laboratory of Nonferrous Metal Materials (Gansu Provincial-Ministry Co-construction National Key Laboratory) and the Key Laboratory of Nonferrous Metal Alloys and Processing (Ministry of Education) at Lanzhou University of Technology represents a significant contribution to the field of active gas-assisted TIG welding. Funded by the Higher Education Doctoral Point Special Fund Project (20040731001) and published in the Transactions of the China Welding Institute, this work investigates the application of arc-assisted active TIG welding for cladding and overlay applications on non-ferrous metal substrates.
The core innovation lies in the introduction of an auxiliary arc that modifies the thermal profile and plasma characteristics of the primary TIG arc, thereby enhancing penetration depth, improving wetting behavior, and enabling the deposition of dissimilar metal overlay layers with superior metallurgical bonding. This technique is particularly relevant for producing nickel-based alloy cladding on carbon steel substrates and titanium alloy overlays on structural steel components used in pressure vessel fabrication.
Core Technical Principles
Active TIG welding fundamentally differs from conventional TIG in that reactive gases such as chlorine, fluorine, or oxygen are introduced into the shielding gas mixture. These active components react with the molten weld pool surface, reducing surface tension and promoting a more fluid melt pool geometry. The arc-assisted variant adds a secondary electrode or auxiliary current source that further modifies the arc shape and energy density distribution.
The key physical mechanisms include:
- Reduction of the melt pool surface tension coefficient from approximately 1.5 N/m to 0.3–0.5 N/m through surface-active element dissolution
- Transition of the weld pool shape from a wide, shallow configuration to a narrow, deep penetration profile
- Enhanced fluidity enabling better contact between the deposited material and the base metal interface
- Improved arc stability through the auxiliary energy input
| Parameter | Conventional TIG | Active TIG | Arc-Assisted Active TIG |
|---|---|---|---|
| Penetration depth ratio | 1.0 (baseline) | 2.5–4.0 | 3.0–5.5 |
| Weld width-to-depth ratio | 2.5–4.0 | 1.0–1.5 | 0.8–1.2 |
| Melt pool surface tension | ~1.5 N/m | ~0.3–0.5 N/m | ~0.2–0.4 N/m |
| Arc energy density | 30–50 kW/cm² | 40–60 kW/cm² | 50–80 kW/cm² |
| Travel speed range | 50–150 mm/min | 100–300 mm/min | 150–400 mm/min |
Metallurgical Bonding and Dilution Control
For cladding applications, the critical challenge is achieving a metallurgically sound bond between the overlay material and the base substrate while controlling dilution to preserve the corrosion resistance or wear resistance of the deposited layer. The arc-assisted active TIG method offers superior control over the dilution ratio compared to conventional TIG overlay welding.
In a typical nickel-based alloy (Inconel 625) cladding on carbon steel (Q345R) scenario, conventional TIG overlay welding often results in dilution ratios exceeding 40–50%, which compromises the corrosion resistance of the overlay. The arc-assisted active TIG approach, with its narrower and deeper penetration profile, allows the operator to achieve a dilution ratio of 20–30% while maintaining a sound metallurgical bond. This is achieved through careful control of the auxiliary arc parameters:
- Primary arc current: 80–150 A
- Auxiliary arc current: 20–60 A (typically 20–40% of primary)
- Shielding gas composition: Ar + 2–5% Cl₂ or Ar + 1–3% F₂
- Travel speed: 200–350 mm/min
- Electrode depression angle: 5–15°
- Nozzle-to-workpiece distance: 5–8 mm
The narrower weld bead geometry means that the volume of base metal melted is reduced relative to the total weld cross-section, directly translating to lower dilution. Furthermore, the enhanced fluidity promotes better mixing at the fusion line, reducing the likelihood of unmixed zones and interfacial defects that could serve as initiation sites for corrosion or mechanical failure.
Engineering Practice and Defect Analysis
In engineering practice, the arc-assisted active TIG welding method has been applied to several critical cladding scenarios:
- Hydrogenation reactor internals: Nickel-based alloy cladding on low-alloy steel tube sheets where hydrogen environment resistance is essential
- Heat exchanger tubesheets: 316L stainless steel overlay on carbon steel tubesheets in sulfuric acid service environments
- Turbine blade repair: Superalloy overlay on damaged blade platforms in gas turbine engines
Common defects encountered include:
| Defect Type | Root Cause | Countermeasure |
|---|---|---|
| Cracking at fusion line | Excessive cooling rate, high dilution | Reduce auxiliary arc current, increase preheat |
| Porosity | Gas entrapment from active gas reaction products | Optimize shielding gas flow rate, ensure clean base metal |
| Incomplete bonding | Insufficient heat input at interface | Increase primary arc current, reduce travel speed |
| Spatter | Excessive arc energy density | Reduce auxiliary arc current, increase nozzle distance |
| Undercut | Excessive travel speed relative to heat input | Reduce travel speed or increase current |
Study Insights and Implications
The arc-assisted active TIG welding method represents a significant advancement in the cladding technology landscape. Its ability to simultaneously achieve deep penetration, narrow bead geometry, and controlled dilution makes it particularly attractive for producing high-quality overlay layers on critical pressure vessel components. The technique bridges the gap between conventional TIG overlay welding, which offers excellent quality but limited productivity, and high-deposition-rate methods such as plasma transferred arc welding, which can compromise overlay quality due to excessive dilution.
From a process development perspective, the key insight is that the auxiliary arc serves as a tunable parameter that allows independent control of penetration depth and bead width. This decoupling of geometric parameters provides process engineers with unprecedented flexibility in optimizing cladding quality for specific service conditions. The method is particularly promising for thin-wall cladding applications where excessive heat input could lead to distortion or thermal degradation of the base material.
However, practical implementation requires careful attention to several factors: the handling and safety of active gases, the design of the dual-electrode torch configuration, and the development of appropriate welding procedure specifications that account for the modified thermal cycle. Future work should focus on scaling the technique to automated multi-pass cladding sequences and developing qualified welding procedure specifications in accordance with ASME IX and NB/T 47014 requirements.