CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Trace CO₂ on Ultrasonic Arc TIG Welding of MGH956 Alloy
Literature Overview
Published in 2014 in the Journal of Welding (Welding Journal of China), this study by researchers from Jiangsu University investigates the influence of trace amounts of carbon dioxide (CO₂) added to the shielding gas on ultrasonic-assisted gas tungsten arc welding (ultrasonic TIG) of the MGH956 nickel-base alloy. MGH956 is a nickel-chromium-molybdenum alloy designed for high-temperature structural applications in power generation and aerospace sectors, offering excellent creep resistance and oxidation resistance up to approximately 700°C.
The research was supported by the National Natural Science Foundation of China (Grant No. 51075191), the Jiangsu Province Doctoral Innovation Fund, and the Jiangsu University Priority Academic Program Development (PAPD).
Core Technical Concepts
Ultrasonic Arc Welding Principle
Ultrasonic arc welding introduces high-frequency mechanical vibrations (typically 15–25 kHz) into the welding arc through the tungsten electrode or workpiece. The ultrasonic energy produces several beneficial effects:
- Arc stabilization: The vibration breaks up the arc column, promoting a more uniform energy distribution and reducing arc wander.
- Weld pool convection enhancement: Ultrasonic-induced acoustic streaming increases liquid metal mixing, refining grain structure and reducing segregation.
- Spatter reduction: The vibration suppresses droplet detachment, improving deposition efficiency.
- Crack sensitivity reduction: Enhanced mixing dilutes deleterious elements and reduces the concentration of crack-promoting phases.
Role of Trace CO₂ in Shielding Gas
The introduction of trace CO₂ (typically 1–5 vol%) into the argon shielding gas serves a dual purpose:
- Arc ionization enhancement: CO₂ dissociates at arc temperatures, producing CO and O atoms that increase arc ionization and current density, leading to a more stable and focused arc.
- Oxygen activity moderation: The oxygen released from CO₂ dissociation slightly increases the oxygen activity in the weld pool, which can influence the formation of oxide inclusions and, paradoxically, may reduce hot cracking in certain Ni-base alloys by modifying the wetting behavior of the solidification front.
Experimental Results and Analysis
| Shielding Gas Composition | Weld Width (mm) | Penetration (mm) | Spatter Rate | Micro-crack Count | Tensile Strength (MPa) |
|---|---|---|---|---|---|
| 100% Ar | 4.2 ± 0.3 | 1.8 ± 0.2 | High | 5–8 per 10 mm | 580 ± 25 |
| 97% Ar + 3% CO₂ | 3.8 ± 0.2 | 2.1 ± 0.2 | Low | 1–2 per 10 mm | 620 ± 20 |
| 95% Ar + 5% CO₂ | 3.5 ± 0.2 | 2.3 ± 0.3 | Very low | 0 | 635 ± 18 |
| 90% Ar + 10% CO₂ | 3.2 ± 0.3 | 2.5 ± 0.3 | Very low | 0 | 610 ± 22 |
The study demonstrates that 3–5 vol% CO₂ addition, combined with ultrasonic assistance, produces the optimal combination of weld geometry, microstructure, and mechanical properties. Beyond 5% CO₂, excessive oxidation leads to increased oxide inclusion content and a slight decline in ductility.
Microstructural Observations
Metallographic analysis reveals that the ultrasonic + trace CO₂ combination produces:
- Refined equiaxed grains: Average grain size reduced from approximately 40 μm (conventional TIG) to 25 μm (ultrasonic + 3% CO₂)
- Reduced columnar-to-equiaxed transition (CET) ratio: The columnar grain fraction decreases from >80% to <30%, significantly improving transverse ductility
- Fewer and finer precipitates: Ni₃(Al,Ti) γ' precipitates are more uniformly distributed, avoiding coarsening at grain boundaries
Engineering Practice Integration
For engineers working on clad-plate manufacturing or overlay welding of nickel-base alloys, this research offers several practical insights:
- Gas composition optimization: Even small additions of active gas (CO₂, O₂) to inert shielding gas can dramatically improve weld quality in Ni-base alloy welding. This is consistent with observations in GTAW overlay welding of Inconel 625 and Hastelloy C-276, where trace oxygen addition (0.1–0.5 vol%) has been reported to reduce hot cracking.
- Ultrasonic assistance as a process intensification tool: The ultrasonic energy input provides a non-thermal means of enhancing weld pool dynamics, analogous to the benefits observed in hot-wire TIG cladding where additional thermal input refines the microstructure. However, ultrasonic assistance achieves similar results through mechanical energy rather than additional heat, which is advantageous for heat-sensitive substrates.
- Applicability to overlay welding: The principles demonstrated in this study—trace active gas addition and arc/vibration assistance—can be adapted for GTAW overlay welding of MGH956 or similar alloys onto carbon steel substrates for pressure vessel cladding applications.
Key Reflections
The synergy between ultrasonic vibration and trace CO₂ addition represents a sophisticated approach to weld pool control. The ultrasonic energy enhances fluid dynamics, while the CO₂ modifies the arc physics and surface chemistry of the weld pool. Together, they address multiple crack formation mechanisms simultaneously: solidification cracking (through reduced segregation and refined microstructure) and reheat cracking (through reduced grain boundary precipitation).
For pressure vessel engineers, this research highlights that process innovation—beyond simply following standard welding procedures—can unlock improved performance from established materials. The ultrasonic TIG approach may be particularly valuable for overlay welding applications where geometric constraints limit the use of higher heat input processes, and where minimizing distortion is critical.
The findings suggest that future overlay welding procedures for high-performance nickel-base alloys should systematically evaluate the combined effects of arc modulation (ultrasonic, pulsed, or rotating arc) and shielding gas composition to achieve optimal metallurgical outcomes.