CLADDING TECHNOLOGY SHANXI CO., LTDArc Characteristics of Ultrasonic Assisted TIG Welding
Literature Overview
This study, published in China Welding in 2008 by Sun Qingjie, Lin Sanbao, Yang Chunli, Fan Yangyang, and Zhao Guoqing from the State Key Laboratory of Advanced Welding Production Technology at Harbin Institute of Technology, investigates how ultrasonic vibration applied to the welding torch affects the arc stability, arc shape, and energy distribution in TIG welding. Ultrasonic-assisted welding is an emerging technique that superimposes high-frequency mechanical oscillation onto the welding torch or electrode, aiming to refine grain structure, improve weld penetration, and reduce porosity. Understanding the arc characteristics under ultrasonic excitation is fundamental to optimizing this hybrid process for cladding and overlay applications.
Core Technical Points
The researchers examined arc voltage fluctuation, arc width, and arc length under various ultrasonic amplitudes and frequencies. The key finding is that ultrasonic vibration of the torch introduces periodic perturbation to the arc column, causing measurable changes in arc voltage waveform. Specifically:
- Arc voltage fluctuation amplitude increases proportionally with ultrasonic amplitude, reaching a peak variation of approximately 1.2 to 2.5 V for typical amplitudes of 20 to 50 micrometers.
- The arc constriction effect becomes more pronounced at higher ultrasonic frequencies (20 kHz range), leading to a narrower arc root and deeper penetration.
- Arc current density distribution becomes more uniform along the arc axis, which is beneficial for reducing arc blow and improving bead geometry.
| Parameter | Without Ultrasonic | With Ultrasonic (20 kHz, 30 μm) |
|---|---|---|
| Arc voltage fluctuation (V) | 0.3–0.5 | 1.2–2.0 |
| Arc root width (mm) | 4.2–4.8 | 3.5–4.0 |
| Penetration depth (mm) | 2.1–2.5 | 2.8–3.4 |
| Grain size in weld (μm) | 60–80 | 35–50 |
Interpretation of Technical Significance
From a cladding and overlay perspective, the arc constriction caused by ultrasonic assistance is particularly valuable. In overlay welding of nickel-based alloys such as Inconel 625 onto carbon steel substrates, excessive dilution is a primary concern. The narrower arc root and deeper, more focused energy input can potentially reduce lateral heat spread while maintaining adequate bond line integrity. However, the periodic voltage fluctuation must be carefully managed, as excessive arc instability can lead to spatter and porosity in the overlay layer.
The study also notes that ultrasonic vibration suppresses the formation of a large capillary hole in the molten pool, which is directly relevant to reducing hot cracking susceptibility in overlay deposits. This is consistent with the well-known mechanism whereby mechanical vibration breaks up dendrite arms and promotes equiaxed grain formation.
Engineering Practice Implications
For engineers involved in weld overlay cladding, the ultrasonic-assisted TIG process offers several practical advantages:
- Improved dilution control when overlaying dissimilar materials, such as 316 stainless steel on low-carbon steel pressure vessel components.
- Reduced need for multiple passes to achieve the required overlay thickness, thereby improving productivity.
- Enhanced metallurgical quality of the bond line, which is critical for pressure boundary applications governed by ASME VIII Div.1 or NB/T 47002.
However, the technology also presents challenges. The ultrasonic transducer must be integrated into the welding torch without compromising gas shielding effectiveness. Additionally, the equipment cost and complexity are higher than conventional TIG welding, which limits adoption in large-scale pressure vessel fabrication shops. Process qualification under NB/T 47014 or ASME IX would require demonstrating consistent weld quality across the full range of ultrasonic parameters.
Key Questions and Reflections
A critical question arising from this research is whether the benefits of ultrasonic assistance scale to thick overlay deposits. The study primarily examined single-pass welds on thin plates, and the interaction between ultrasonic vibration and multi-pass overlay welding remains largely unexplored. In pressure vessel fabrication, overlay layers often require multiple passes to achieve thicknesses of 3 to 6 mm or more. Understanding how ultrasonic effects accumulate or diminish with each successive pass is essential for practical implementation.
Another consideration is the interaction between ultrasonic vibration and flux or shielding gas composition. In submerged arc overlay welding, the flux layer may dampen ultrasonic transmission to the arc. This cross-process question warrants further investigation.
Study Insights and Summary
The fundamental contribution of this work lies in demonstrating that ultrasonic excitation of the TIG arc produces measurable and controllable changes in arc geometry and energy distribution. For the cladding engineer, this opens a pathway toward more precise control of dilution and bond line metallurgy. The arc constriction effect, in particular, holds promise for reducing the number of overlay passes required, which directly impacts fabrication cost and schedule. Nevertheless, the technology requires further development in terms of equipment robustness, process qualification protocols, and integration with existing quality assurance frameworks before it can be widely adopted in pressure vessel manufacturing. The research represents a solid foundation upon which future work on ultrasonic-assisted overlay welding can build.