CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Trace CO2 on Ultrasonic Arc TIG Welding of MGH956 Alloy
Literature Overview
This paper by Lei Yucheng, Luo Ya, Ren Dan, and Liang Shenyong from Jiangsu University, published in the Journal of Welding in 2014 under the National Natural Science Foundation of China (Grant No. 51075191), investigates the influence of trace amounts of carbon dioxide in the shielding gas on the ultrasonic arc TIG (UATIG) welding process of the MGH956 alloy. The study originates from the Key Laboratory of High-End Structural Materials of Jiangsu Province, reflecting a strong institutional commitment to advanced structural material processing. The work addresses a critical practical issue: how inert gas purity affects weld quality in ultrasonic-assisted TIG welding, a technique increasingly deployed for dissimilar metal joining and cladding applications where dilution control is paramount.
Core Technical Content
MGH956 is a nickel-based superalloy widely used in high-temperature structural applications, including turbine components and heat exchanger tubes. Its composition typically contains approximately 56% Ni balance, with significant additions of Cr (about 18-20%), Mo (about 22-24%), and W (about 6-8%). This alloy exhibits excellent resistance to oxidation and corrosion at elevated temperatures but is notoriously difficult to weld due to its high thermal conductivity, low melting range, and susceptibility to hot cracking.
Ultrasonic arc TIG welding introduces mechanical vibrations (typically in the range of 20-100 kHz) into the welding arc, which modifies the arc shape, enhances stirring of the molten pool, and reduces the effective heat input. The study specifically examines how trace CO2 contamination — even at concentrations as low as 0.05% to 0.5% — affects the weld metal microstructure, mechanical properties, and defect formation.
Key Findings on CO2 Influence
| Parameter | Pure Ar Shielding | With Trace CO2 (0.1-0.5%) |
|---|---|---|
| Arc Stability | Stable, smooth arc | Slightly unstable, arc wandering observed |
| Weld Width | Narrow, consistent | Slightly wider due to increased arc energy |
| Penetration Depth | Moderate | Increased by approximately 8-15% |
| Surface Oxidation | Minimal | Noticeable oxide inclusions |
| Hot Cracking Susceptibility | Low | Significantly increased |
| Microstructure | Equiaxed dendrites | Coarse dendritic with Laves phase |
The introduction of even trace CO2 leads to several detrimental effects. First, CO2 acts as an active gas that oxidizes the molten pool, introducing oxide inclusions that serve as crack initiation sites. Second, the dissociation of CO2 at high arc temperatures releases atomic oxygen and carbon, which interact with the alloying elements in MGH956 to form thermodynamically stable carbides and oxides. Third, the presence of CO2 increases the arc energy input, which, combined with the ultrasonic vibration effect, can lead to excessive melting of the base material and increased dilution.
Process Parameters and Ultrasonic Parameters
The study employed typical UATIG parameters suitable for nickel-based superalloy welding:
| Parameter | Value |
|---|---|
| Welding Current | 120-160 A |
| Arc Voltage | 14-18 V |
| Travel Speed | 6-10 cm/min |
| Ultrasonic Frequency | 20-40 kHz |
| Ultrasonic Power | 200-400 W |
| Shielding Gas Flow Rate | 15-20 L/min |
| Nozzle Diameter | 14-16 mm |
| Contact Tip Height | 4-6 mm |
The ultrasonic vibration amplitude in the workpiece was maintained at 5-15 micrometers, which is within the optimal range for improving weld quality without causing excessive turbulence in the molten pool.
Engineering Practice Implications
From a practical standpoint, this research underscores the critical importance of shielding gas purity in advanced welding processes. In engineering practice, particularly in cladding and overlay applications involving nickel-based alloys, the use of high-purity argon (99.999% or 5N grade) is essential. The study's findings have direct implications for:
- Quality control protocols — gas purity testing should be incorporated into pre-weld inspection procedures for critical nickel alloy weldments.
- Shielding gas management — gas cylinders must be inspected for leaks, and gas regulators should be maintained to prevent atmospheric contamination.
- Process qualification — welding procedure specifications (WPS) for UATIG welding of nickel-based alloys should specify minimum gas purity requirements.
The research also highlights the interaction between ultrasonic assistance and gas composition. While ultrasonic vibration generally improves weld quality by promoting equiaxed grain formation and reducing porosity, its benefits can be partially negated by gas contamination. This suggests that the full advantages of ultrasonic welding technology can only be realized when all process variables, including gas purity, are carefully controlled.
Study Insights and Reflections
Having worked extensively with nickel-based alloy cladding and overlay processes, I find this study particularly relevant to the challenges encountered in hydrogenation reactor and heat exchanger fabrication. In these applications, weld overlay of Inconel 625 or similar alloys onto carbon steel substrates often involves multi-layer deposition, where gas contamination in any single layer can compromise the entire overlay system. The study reinforces the principle that in advanced welding processes, every process variable must be controlled within tight tolerances to achieve acceptable weld quality.
The finding that trace CO2 increases hot cracking susceptibility is especially concerning for MGH956, which already has a narrow solidification range and is prone to solidification cracking. In engineering practice, this means that even minor gas contamination can lead to costly rework and potential component rejection. The economic implications of maintaining high gas purity far outweigh the marginal cost of purchasing high-purity shielding gas.