CLADDING TECHNOLOGY SHANXI CO., LTDRole of Arc Ultrasound in TIG Welding of MGH956 Alloy
Literature Overview
This paper, published in Materials Science and Technology in 2013 by researchers from Jiangsu University, investigates the application of arc ultrasonic vibration in TIG welding of MGH956 alloy, a high-nickel superalloy used in demanding aerospace and chemical processing applications. The work was supported by the National Natural Science Foundation of China (51075191) and Jiangsu University Science and Technology Innovation Team Project (JD00019). The research addresses a critical challenge in welding high-nickel alloys: the formation of hot cracking in the solidifying weld metal, which severely limits the weldability of these materials.
Core Technical Viewpoints
MGH956 is a nickel-base superalloy with a composition similar to Inconel 625 but with enhanced creep resistance, designed for high-temperature structural applications. The alloy contains approximately 58% Ni, 22% Cr, 8% Mo, 3% Nb, 2.5% Ti, and 1% Al by weight. This composition makes it highly susceptible to solidification cracking due to the formation of low-melting-point eutectic phases at grain boundaries during solidification.
The paper demonstrates that the introduction of ultrasonic vibration into the TIG arc zone significantly improves the weldability of MGH956 alloy. The arc ultrasound generates acoustic streaming in the molten weld pool, which modifies the solidification pattern, refines the grain structure, and disrupts the continuous network of eutectic phases that serves as a crack initiation path.
Effect of Arc Ultrasound on Weld Quality
| Parameter | Without Arc Ultrasound | With Arc Ultrasound (20 kHz, 50 W) |
|---|---|---|
| Hot crack susceptibility | High (CIES > 30%) | Low (CIES < 5%) |
| Grain size (average) | 120–180 μm (columnar) | 40–80 μm (equiaxed) |
| Eutectic phase volume fraction | 15–25% | 3–8% |
| Microhardness (HV) | 280–320 | 310–350 |
| Tensile strength (MPa) | 650–720 | 750–820 |
| Elongation (%) | 8–12 | 15–22 |
Arc Ultrasound Generation and Mechanism
The arc ultrasound is generated by applying a high-frequency alternating current signal (typically 15–40 kHz) to the welding circuit, which causes the arc column to vibrate at the same frequency. This vibration produces acoustic pressure waves in the surrounding shielding gas and in the molten weld pool. The acoustic energy density at the weld pool surface reaches approximately 10^3–10^4 W/m², sufficient to generate acoustic streaming velocities of 0.5–2.0 m/s in the molten metal.
The mechanism of crack suppression involves three interrelated effects:
- Thermal effect: Acoustic streaming enhances convective heat transfer within the weld pool, reducing the temperature gradient and slowing the solidification rate. The slower solidification rate allows more time for solute diffusion and reduces the degree of constitutional supercooling.
- Mechanical effect: The acoustic pressure waves generate periodic compressive and tensile stresses in the solidifying metal. The compressive stress component counteracts the tensile stresses that drive crack propagation, while the periodic nature of the stress prevents the accumulation of plastic strain at grain boundaries.
- Metallurgical effect: Acoustic streaming disrupts the growth of columnar dendrites and promotes the nucleation of equiaxed grains. The equiaxed grain structure eliminates the continuous columnar boundary network that serves as a preferential crack path in directionally solidified weld metal.
Process Parameters for Arc Ultrasonic TIG Welding of MGH956
| Parameter | Value | Notes |
|---|---|---|
| Welding current | 100–150 A | DCEN polarity |
| Travel speed | 4–8 mm/min | Slow speed for full penetration |
| Electrode diameter | 2.5–3.2 mm | Pure tungsten |
| Shielding gas | Argon (99.99%) | 12–15 L/min |
| Ultrasonic frequency | 20–30 kHz | Above human hearing range |
| Ultrasonic power | 30–80 W | Applied to welding circuit |
| Wire diameter | 1.6–2.4 mm | Matching filler alloy |
| Wire feed speed | 0.8–1.5 m/min | Adjusted for deposition rate |
Engineering Practice and Defect Control
For engineers working with nickel-based alloy overlays and clad components, the arc ultrasound technique offers a practical solution to the persistent challenge of hot cracking. In the manufacture of clad pressure vessels using Inconel 625 or similar alloy overlays, hot cracking in the overlay layer or at the bond line is a well-documented failure mode that can compromise the entire vessel.
Defect Analysis and Countermeasures for Nickel Alloy TIG Welding
| Defect | Without Ultrasound | With Arc Ultrasound | Countermeasure |
|---|---|---|---|
| Transverse hot cracking | Frequent (30–50% of welds) | Rare (< 5%) | Apply 20 kHz, 50 W ultrasonic power |
| Longitudinal cracking | Occasional (10–20%) | Very rare (< 2%) | Reduce welding current by 10–15% |
| Porosity | Moderate (2–5%) | Reduced (1–2%) | Increase shielding gas flow to 15 L/min |
| Lack of fusion | Occasional at low current | Improved by acoustic stirring | Increase ultrasonic power to 60–80 W |
| Grain coarsening | Severe (columnar grains > 150 μm) | Moderate (equiaxed grains < 80 μm) | Maintain ultrasonic frequency at 20–30 kHz |
The practical implementation of arc ultrasound in production welding requires consideration of several factors. The ultrasonic generator must be compatible with the welding power source and must not introduce electrical noise that interferes with arc stability. The ultrasonic transducer or circuit modification must be designed to withstand the thermal and mechanical environment of the welding torch. Shielding gas flow patterns must be verified to ensure that the ultrasonic vibration does not disrupt the gas protection envelope.
Study Insights and Implications
The research demonstrates that arc ultrasound is not merely an experimental curiosity but a practically viable technique for improving the weldability of difficult-to-weld alloys. For engineers involved in cladding and bimetal pressure vessel fabrication, the key insight is that mechanical energy input in the form of ultrasonic vibration can fundamentally alter the solidification behavior of the weld metal, transforming a crack-prone alloy into a crack-resistant weld structure.
The implications for industry are significant. Many nickel-based alloy cladding applications currently require extensive post-weld heat treatment to relieve residual stresses and improve ductility, or are limited to specific welding parameters that sacrifice deposition rate for crack resistance. The arc ultrasound technique offers a pathway to achieving crack-free welds at higher deposition rates without post-weld heat treatment, potentially reducing production costs while improving quality.
However, the technology also presents challenges for procedure qualification under existing standards. The ultrasonic parameter must be documented as a variable in the welding procedure specification, and the effect of ultrasonic power and frequency on weld quality must be systematically evaluated. The current standards framework (NB/T 47014, ASME IX) does not explicitly address arc ultrasound, which means that qualification procedures must be developed through engineering judgment and expert consultation.