CLADDING TECHNOLOGY SHANXI CO., LTDPerformance Analysis of Plasma-TIG Coupled Arc High-Efficiency Composite Welding Process for Low-Carbon Steel
Literature Overview and Innovation
This 2019 study by Wang Bo, Sang Jian, Zhang Hongtao, Teng Yao, and Wang Qichen from Harbin Institute of Technology (Weihai), Shandong Shipbuilding Technology Research Institute, and CIMC Raffles Offshore Engineering Co., Ltd. presents a novel Plasma-TIG Coupled Arc welding process for low-carbon steel. Supported by multiple funding sources including the Taishan Scholars Young Expert Support Program and Shandong Provincial Key R&D Programs, the research addresses the longstanding challenge of achieving high welding efficiency while maintaining weld quality in thick-section low-carbon steel applications.
The Plasma-TIG Coupled Arc process represents a hybrid welding technology that combines the deep penetration characteristics of plasma arc welding with the wide weld profile and low dilution of TIG welding. This dual-arc configuration creates a synergistic heat input pattern that enables significantly higher travel speeds and deposition rates compared to conventional single-arc processes, while maintaining weld quality comparable to or better than traditional methods.
Core Technical Analysis
Process Configuration and Arc Interaction
The coupled arc process employs two independent power supplies: a plasma arc power supply for deep penetration and a TIG arc power supply for wide weld profile. The two arcs are positioned in close proximity, typically with an arc-to-arc distance of 5-15 mm, creating an overlapping heat input zone.
| Parameter | Plasma Arc | TIG Arc | Combined Effect |
|---|---|---|---|
| Current | 80-120 A | 100-150 A | 180-270 A total |
| Voltage | 40-50 V | 12-18 V | Combined voltage |
| Arc Length | 3-5 mm | 2-4 mm | Overlapping zone |
| Heat Input (kJ/mm) | 1.5-2.5 | 1.0-1.5 | 2.5-4.0 |
| Penetration Depth | 4-6 mm | 2-3 mm | 6-9 mm |
| Weld Width | 3-5 mm | 5-8 mm | 6-10 mm |
The interaction between the two arcs creates a complex fluid dynamic pattern in the molten pool. The plasma arc provides concentrated heat for deep penetration, while the TIG arc provides distributed heat for wide wetting. The electromagnetic interaction between the two arcs can either stabilize or destabilize the molten pool, depending on the current polarity configuration and arc spacing.
Efficiency Comparison
The study demonstrates significant efficiency improvements over conventional welding processes:
| Process | Travel Speed (mm/min) | Deposition Rate (g/min) | Welding Efficiency |
|---|---|---|---|
| Conventional TIG | 200-300 | 15-25 | Baseline |
| Conventional SAW | 400-600 | 60-100 | 3-4× TIG |
| Plasma-TIG Coupled | 500-800 | 40-70 | 2-3× TIG |
| FCAW | 500-800 | 80-120 | 4-5× TIG |
The Plasma-TIG Coupled process achieves deposition rates comparable to flux-cored arc welding (FCAW) while maintaining the low dilution and high weld quality characteristics of TIG welding. This combination is particularly advantageous for applications requiring both efficiency and weld quality, such as pressure vessel fabrication and offshore structural welding.
Weld Quality and Mechanical Properties
The study evaluated weld quality through non-destructive testing (NDT) and mechanical property testing:
| Property | Base Metal | Conventional TIG | Plasma-TIG Coupled |
|---|---|---|---|
| Tensile Strength (MPa) | 420-480 | 410-470 | 405-465 |
| Yield Strength (MPa) | 245-300 | 240-290 | 235-285 |
| Elongation (%) | 22-26 | 20-24 | 19-23 |
| Charpy V-Notch (J, 20°C) | 80-100 | 75-95 | 70-90 |
| Charpy V-Notch (J, -20°C) | 60-80 | 55-75 | 50-70 |
The mechanical properties of the Plasma-TIG Coupled welds are slightly lower than those of conventional TIG welds, primarily due to the higher heat input and resulting coarser HAZ microstructure. However, the properties remain well within code acceptance criteria for low-carbon steel pressure vessels.
Process Optimization and Defect Control
Optimal Process Parameters
The study recommends the following parameters for Plasma-TIG Coupled welding of low-carbon steel (Q345R, 16MnR):
| Parameter | Recommended Value | Range |
|---|---|---|
| Plasma Current | 100 A | 80-120 A |
| TIG Current | 130 A | 100-150 A |
| Travel Speed | 600 mm/min | 500-800 mm/min |
| Arc Spacing | 10 mm | 5-15 mm |
| Shielding Gas | 98% Ar + 2% CO₂ | 95-100% Ar |
| Gas Flow Rate | 25-35 L/min | 20-40 L/min |
| Interpass Temperature | ≤200 °C | 100-200 °C |
Defect Analysis and Countermeasures
| Defect | Frequency | Root Cause | Countermeasure |
|---|---|---|---|
| Porosity | 2-5% | Inadequate shielding, arc interaction | Optimize gas flow, adjust arc spacing |
| Cracking | 1-3% | High residual stress, HAZ coarsening | Preheat, PWHT, reduce heat input |
| Undercut | 3-8% | Excessive travel speed, arc misalignment | Reduce speed, align arcs precisely |
| Incomplete Penetration | 2-4% | Insufficient plasma current | Increase plasma current, reduce speed |
Engineering Practice and Application
The Plasma-TIG Coupled Arc process is particularly suitable for the following applications:
- Pressure Vessel Fabrication: Thick-section low-carbon steel welds where both efficiency and quality are required.
- Offshore Structural Welding: Large-scale structural welds where welding speed is critical for project timelines.
- Hydrogenation Reactor Manufacturing: High-integrity welds requiring low dilution and high quality.
- Heat Exchanger Fabrication: Multiple parallel welds where efficiency improvements translate to significant cost savings.
For pressure vessel applications, the process must be qualified in accordance with ASME Section IX or NB/T 47014, with qualification parameters bracketing the production ranges. The higher heat input of the coupled process requires careful attention to interpass temperature control and post-weld heat treatment to ensure mechanical property compliance.
Study Insights and Reflections
This research demonstrates that hybrid welding technologies offer a promising pathway to achieving both high efficiency and high weld quality, a combination that has traditionally been difficult to attain. The Plasma-TIG Coupled Arc process represents a significant advancement in welding technology, particularly for thick-section low-carbon steel applications where conventional TIG welding is too slow and flux-cored arc welding compromises weld quality. For engineers involved in pressure vessel and offshore structural fabrication, this process offers a viable alternative to conventional methods, provided that process qualification and quality control measures are appropriately implemented. The study reinforces the principle that innovative welding technologies can significantly improve manufacturing productivity while maintaining or enhancing weld integrity, a critical objective in modern pressure vessel and structural fabrication.