CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Excitation Current on Mechanical Properties of MGH956 Alloy Ultrasonic Arc TIG Weld Joints
Literature Overview
This study by Lei Yucheng, Cheng Long, Li Menggang, and Liang Shenyong from Jiangsu University examines the influence of excitation current amplitude on the mechanical properties of MGH956 nickel-based alloy weld joints produced by ultrasonic arc TIG welding. Funded by the National Natural Science Foundation of China (Grant 51075191), the Jiangsu Province Doctoral Innovation Fund (CXZZ11-0556), and the Jiangsu University Science and Technology Innovation Team Project (JD00019), this research explores an advanced welding technology that superimposes ultrasonic-frequency current pulses onto the conventional TIG welding current to modify weld pool dynamics and solidification behavior.
Ultrasonic Arc TIG Welding Technology
Ultrasonic arc TIG welding represents a novel approach to improving weld quality by introducing high-frequency current modulation to the welding arc. The excitation current, typically operating at frequencies of 20-100 kHz with adjustable amplitude, creates periodic variations in arc energy, electromagnetic forces, and weld pool fluid flow. This modulation can break up the stable weld pool surface, enhance stirring, refine grain structure, and reduce porosity formation.
| Process Parameter | Conventional TIG | Ultrasonic Arc TIG (Low Excitation) | Ultrasonic Arc TIG (High Excitation) |
|---|---|---|---|
| Base current (A) | 100-150 | 100-150 | 100-150 |
| Excitation current amplitude (A) | 0 | 5-15 | 20-35 |
| Excitation frequency (kHz) | N/A | 20-40 | 50-100 |
| Arc power fluctuation (%) | 0 | 3-8 | 10-20 |
| Weld width (mm) | 6.0-8.0 | 5.0-7.0 | 4.5-6.5 |
| Weld penetration (mm) | 1.5-2.5 | 2.0-3.5 | 2.5-4.5 |
| Weld pool stability | Stable | Moderately disturbed | Highly disturbed |
The excitation current amplitude was varied systematically from 0 A (conventional TIG) to 35 A to investigate the threshold beyond which weld quality degrades due to excessive arc instability.
Mechanical Property Results
The tensile properties of the weld joints showed a clear dependence on excitation current amplitude. At low excitation levels (5-15 A), the tensile strength improved by 8-15% compared to conventional TIG welding, reaching values of 750-820 MPa. The elongation at fracture also improved from 15-18% to 18-22%, indicating that the weld metal became both stronger and more ductile.
| Excitation Current (A) | UTS (MPa) | Yield Strength (MPa) | Elongation (%) | Hardness (HV) |
|---|---|---|---|---|
| 0 (conventional) | 650-700 | 450-490 | 15-18 | 230-250 |
| 5-10 | 690-740 | 480-520 | 17-20 | 240-260 |
| 15-20 | 750-820 | 530-580 | 18-22 | 255-280 |
| 25-30 | 700-760 | 490-540 | 14-17 | 250-275 |
| 30-35 | 620-680 | 420-470 | 10-14 | 230-260 |
The optimum excitation current range of 15-20 A produced the best combination of strength and ductility. Beyond this range, the increased arc instability led to incomplete fusion, porosity, and irregular solidification patterns that degraded mechanical properties.
Microstructural Analysis
The microstructural improvements at optimal excitation current levels are attributed to several mechanisms:
- Enhanced weld pool stirring: The periodic electromagnetic force variations caused by the excitation current create oscillatory fluid flow within the weld pool, promoting more uniform temperature and composition distribution.
- Grain refinement: The periodic thermal cycling within each ultrasonic cycle promotes the formation of multiple nucleation sites, reducing grain size by 20-35% compared to conventional TIG welds.
- Reduced columnar grain fraction: The oscillatory thermal field disrupts the directional solidification pattern, increasing the fraction of equiaxed grains from approximately 30% to 60-70%.
- Homogenized microsegregation: Enhanced fluid flow reduces compositional segregation at interdendritic regions, producing more uniform elemental distribution within the weld metal.
The grain size refinement and increased equiaxed grain fraction directly contribute to the improved ductility and toughness observed at optimal excitation current levels. The finer, more uniform microstructure also provides better resistance to cracking during solidification.
Application to Cladding and Pressure Vessel Fabrication
The ultrasonic arc TIG welding technology has significant potential for cladding applications involving nickel-based alloys:
- Improved cladding layer quality: The enhanced stirring and grain refinement produced by ultrasonic excitation can improve the homogeneity of multi-pass cladding layers, reducing compositional banding and improving corrosion resistance uniformity.
- Reduced dilution variation: The periodic electromagnetic stirring at the base metal-cladding interface can help maintain more consistent dilution levels across successive cladding passes.
- Enhanced bond strength: The refined microstructure at the cladding interface, with increased equiaxed grain fraction, can improve the mechanical bond between the cladding layer and backing material.
- Reduced cracking susceptibility: The improved ductility of ultrasonic arc TIG welds reduces the risk of hot cracking during cladding of nickel-based alloys on dissimilar base metals.
For bimetal pressure vessels fabricated with MGH956 cladding layers, the ultrasonic arc TIG welding technology could provide a means of achieving superior cladding layer properties without requiring changes to the base metal or filler material compositions. The ability to control weld pool dynamics through electrical parameters offers a non-invasive method of process optimization.
Study Insights and Practical Recommendations
This research demonstrates that ultrasonic arc TIG welding is a viable technology for improving the mechanical properties of nickel-based alloy welds, with an optimal excitation current range of 15-20 A for MGH956 alloy. The technology offers a practical means of enhancing weld quality without modifying material compositions or requiring complex equipment changes. For cladding engineers, the key insight is that process parameter optimization through electrical modulation can achieve results comparable to or better than material formulation changes, offering a flexible and cost-effective approach to improving cladding layer quality. The non-monotonic relationship between excitation current and weld properties underscores the importance of process window characterization, as both insufficient and excessive excitation can degrade weld quality. Future work should extend these findings to multi-pass cladding scenarios and investigate the effects of ultrasonic excitation on the long-term creep and corrosion performance of clad pressure vessel components.