Numerical Simulation of Droplet Transition in Ultrasonic-Assisted MIG Welding of Q235 Galvanized Steel Plate

Literature Overview

Li Hua, Jia Hao, Yuan Haitao, and Ma Guohong, from the School of Advanced Manufacturing at Nanchang University and the Jiangxi Provincial Key Laboratory of Lightweight High-Strength Structural Materials, published this study in Journal of Nanchang University (Engineering Science) (2023, Vol. 45, No. 1, pp. 65-70). Funded by the National Natural Science Foundation of China (Grant No. 51665037), the paper presents a numerical simulation of droplet transition behavior during ultrasonic-assisted MIG welding of Q235 galvanized steel plate, validated by high-speed camera experiments.

Core Technical Points

Ultrasonic-Assisted Welding Principle

Ultrasonic-assisted welding introduces mechanical vibration (typically 20-40 kHz) into the welding zone through either the electrode, the workpiece, or both. The ultrasonic vibration affects the welding process through several mechanisms:

For galvanized steel, the ultrasonic vibration is particularly beneficial because the zinc coating creates additional challenges: zinc vaporization during welding produces toxic fumes and can cause spatter, while the zinc-rich oxide layer on the surface can interfere with arc stability and weld metal quality.

Numerical Model Development

The simulation is based on the Volume of Fluid (VOF) model in FLUENT, which tracks the interface between the molten metal droplet and the surrounding gas. The governing equations include:

1. Navier-Stokes equations for momentum conservation, incorporating the effects of electromagnetic forces (Lorentz force, surface tension force), gravity, and the ultrasonic vibration force.
2. VOF transport equation for tracking the liquid-gas interface.
3. Energy equation for heat transfer within the droplet and surrounding gas.
4. Maxwell stress tensor for electromagnetic force calculation.

The ultrasonic vibration is modeled as a periodic force applied to the droplet surface, with frequency and amplitude corresponding to the experimental conditions.

Simulation Results: Effect of Ultrasonic Vibration on Droplet Transition

The results clearly demonstrate that ultrasonic vibration:

• Increases the droplet transition frequency by 38-73% compared to conventional MIG welding.
• Reduces the neck radius at detachment, indicating more complete and rapid droplet separation.
• Promotes a finer, more stable droplet transfer mode.
• Reduces spatter, which is particularly beneficial for galvanized steel where zinc spatter is a significant concern.

Experimental Validation

High-speed camera imaging (typically 10,000-100,000 fps) was used to capture the droplet transition process under identical welding parameters. The experimental observations confirmed the simulation predictions:

The close agreement between simulation and experiment validates the numerical model and confirms the effectiveness of ultrasonic assistance for improving droplet transfer in MIG welding.

Engineering Practice Considerations

For Q235 galvanized steel plate welding, several practical considerations must be addressed:

1. Zinc vapor management: The ultrasonic vibration may increase zinc vaporization due to enhanced molten pool turbulence. Adequate fume extraction and proper ventilation are essential for worker safety.
2. Weld metal quality: The finer droplet transfer and enhanced mixing promoted by ultrasonic vibration should result in a more homogeneous weld metal with reduced porosity and improved mechanical properties.
3. Equipment requirements: Ultrasonic-assisted welding requires additional hardware: an ultrasonic transducer, power amplifier, and vibration coupling mechanism. This increases equipment cost and complexity.
4. Parameter optimization: The ultrasonic frequency and amplitude must be optimized for each specific welding configuration. Too high an amplitude may destabilize the arc or cause excessive electrode vibration.

Comparison with Conventional Galvanized Steel Welding

Applications in Pipe and Fitting Manufacturing

While the study focuses on flat plate welding, the principles of ultrasonic-assisted MIG welding are applicable to pipe and fitting fabrication:

• Galvanized steel pipe welding: The ultrasonic assistance can reduce zinc-induced porosity and spatter in welded joints of galvanized steel pipes.
• Thin-wall stainless steel pipe: Ultrasonic-assisted TIG or MIG welding can improve wetting and reduce oxide inclusion in thin-wall pipe welds.
• Aluminum alloy pipe: The enhanced droplet transfer can reduce HAZ softening in aluminum alloy pipe welding by enabling lower heat input.

However, practical implementation on cylindrical geometries introduces additional challenges: the ultrasonic vibration must be coupled effectively to a curved surface, and the droplet transfer dynamics are modified by the joint geometry (butt, fillet, plug welds).

Study Insights

This paper demonstrates the value of combining numerical simulation with experimental validation in welding process research. The VOF-based model provides detailed insight into the droplet transition dynamics that are difficult to observe experimentally at the required time and spatial resolution. The finding that ultrasonic vibration significantly promotes droplet detachment by reducing the neck radius is consistent with the theoretical understanding that mechanical vibration reduces the effective surface tension holding the droplet to the electrode tip.

For engineers working with galvanized steel or other coated metals, the ultrasonic-assisted approach offers a promising solution to the persistent challenges of zinc-induced defects. However, the technology is not yet widely adopted in industrial practice due to equipment cost, process complexity, and the need for parameter optimization for each specific application. As ultrasonic welding equipment becomes more compact and cost-effective, its adoption in pipe and fitting manufacturing is likely to increase, particularly for applications where weld quality is critical and conventional methods produce unacceptable defect rates.

Overall Summary

These five studies collectively illustrate the breadth and depth of welding process research conducted in China, spanning aluminum alloy tanker fabrication, intelligent control of pulse MIG welding, PLC-based automatic welding system development, hybrid welding heat source modeling, and ultrasonic-assisted droplet transition simulation. Each paper addresses a specific technical challenge with a rigorous approach combining theoretical analysis, numerical modeling, and experimental validation. For engineers in the steel pipe, pipe fitting, and welding industry, these works provide valuable technical insights that can be directly applied to process development, quality improvement, and productivity enhancement in their respective manufacturing environments. The common thread across all five studies is the recognition that understanding the fundamental physics of the welding process—whether it is droplet transfer dynamics, arc interaction in hybrid welding, or heat source distribution—is essential for developing reliable and efficient welding procedures that meet the demanding quality requirements of modern industrial applications.