Zhuojin Pipe Fitting Co., LtdUltrasonic Arc Excitation Current Effects on Porosity and Tensile Properties of MGH956 Alloy TIG Welds
Literature Overview
This paper by Lei Yucheng and colleagues, published in the Welding Journal (2013, Vol. 34, No. 4), investigates the effects of ultrasonic arc excitation current on porosity formation and tensile properties of MGH956 alloy TIG welds. MGH956 is a nickel-based superalloy widely used in aerospace and power generation applications where high-temperature strength and creep resistance are critical. The research was supported by the National Natural Science Foundation of China (Grant No. 51075191) and the Jiangsu Province Doctoral Innovation Program (CXLX12_0638). The authors employed high-frequency modulation of the TIG welding arc to generate ultrasonic vibrations within the arc plasma, and systematically varied the excitation current to evaluate its effects on weld quality.
Core Technical Findings
The study presents a clear threshold behavior in the relationship between excitation current and porosity reduction:
| Excitation Current (A) | Porosity Behavior | Tensile Strength (MPa) | Fracture Mode |
|---|---|---|---|
| 0 (conventional TIG) | Baseline porosity | Baseline | Brittle |
| 5 | Porosity size increases; quantity unchanged | Slight improvement | Brittle |
| 10 | Porosity size increases; quantity unchanged | Moderate improvement | Brittle |
| 20 | Porosity quantity drastically reduced | 550 (maximum) | Brittle-ductile mixed |
| 30 | Porosity quantity drastically reduced | Slightly below maximum | Brittle-ductile mixed |
The maximum tensile strength of 550 MPa achieved at 20 A excitation current represents 76% of the base metal strength, which is a significant improvement over the conventional TIG weld. The transition from complete brittle fracture to a brittle-ductile mixed fracture mode indicates enhanced weld ductility, which is particularly important for fatigue and fracture resistance in service.
Mechanism Analysis
The mechanism by which ultrasonic arc excitation reduces porosity can be understood through several interrelated physical processes. First, the ultrasonic vibrations generated within the arc plasma increase the arc stability and reduce arc oscillation, leading to a more stable weld pool. Second, the ultrasonic energy transmitted to the weld pool creates acoustic streaming effects that promote the coalescence and removal of gas bubbles from the molten metal. Third, the increased arc pressure associated with ultrasonic excitation drives deeper penetration, which may alter the weld pool geometry and gas entrapment mechanisms.
The threshold behavior observed at 20 A excitation current suggests that a minimum ultrasonic energy level is required to overcome the surface tension forces that stabilize gas bubbles within the molten weld pool. Below this threshold (5-10 A), the ultrasonic energy is insufficient to effectively disrupt the gas bubble interfaces, and in fact, the increased arc pressure may compress existing bubbles to larger sizes without promoting their removal. Above this threshold (20-30 A), the ultrasonic energy is sufficient to fragment and remove gas bubbles, resulting in a dramatic reduction in porosity.
Metallurgical Implications for Nickel-Based Superalloys
Nickel-based superalloys such as MGH956 are known to be susceptible to porosity formation during welding due to several factors: high affinity of the alloying elements (particularly chromium and titanium) for oxygen and nitrogen, high melting point leading to rapid solidification, and the tendency for gas solubility to decrease sharply during solidification. The ultrasonic arc excitation technique offers a promising approach to mitigating these inherent porosity tendencies without requiring changes to the welding consumables or process parameters that might compromise other weld properties.
The improvement in tensile strength and fracture mode is also significant from a metallurgical perspective. The brittle fracture mode observed in conventional TIG welds of MGH956 alloy is often associated with microcrack formation along grain boundaries or at interdendritic regions during solidification. The transition to a brittle-ductile mixed fracture mode suggests that the ultrasonic excitation may have refined the weld microstructure, reduced solidification cracking susceptibility, or improved the homogeneity of the weld metal composition.
Quality Control Considerations
For engineering applications involving MGH956 alloy welds, the following quality control measures should be considered when implementing ultrasonic arc excitation:
- Porosity acceptance criteria: The reduction in porosity quantity achieved at 20-30 A excitation current must be evaluated against applicable acceptance criteria, such as those specified in ASME Section IX or EN 12072. The size and distribution of remaining porosity must be documented and verified through radiographic testing or computed tomography.
- Mechanical property verification: The improved tensile strength and fracture mode must be confirmed through standardized mechanical property tests, including tensile testing, hardness profiling, and fractographic analysis. The weld metal and heat-affected zone properties must be evaluated separately to ensure that the ultrasonic excitation does not adversely affect the HAZ.
- Process reproducibility: The excitation current must be precisely controlled and monitored throughout the welding process to ensure consistent weld quality. Process monitoring systems should be implemented to detect and correct deviations in excitation current, arc voltage, and travel speed.
- Long-term performance evaluation: For high-temperature applications, the creep properties, thermal fatigue resistance, and oxidation resistance of the ultrasonic arc excitation welds must be evaluated under simulated service conditions. The potential for stress corrosion cracking in aggressive environments should also be assessed.
Study Insights and Implications
This research demonstrates the potential of ultrasonic arc excitation as a process enhancement technique for welding difficult-to-weld alloys such as nickel-based superalloys. The threshold behavior observed in the porosity reduction provides valuable guidance for process parameter optimization: excitation currents below 20 A should be avoided as they may not provide meaningful porosity reduction and could potentially worsen porosity size. The optimal excitation current of 20 A represents a balance between porosity reduction, tensile strength improvement, and process stability.
For pipe welding applications involving nickel-based alloy pipes (such as those used in petrochemical and power generation industries), the ultrasonic arc excitation technique could be particularly beneficial for achieving high-quality welds in thick-walled sections where porosity formation is a persistent challenge. The technique should be further investigated for its compatibility with various welding positions and for its effects on weld geometry in pipe joint configurations.