Dynamic Simulation of Rotating Arc GMAW Surfacing Short-Circuit Transition Molten Pool

Literature Overview

The paper by Mao Zhiwei, Huang Tao, and Zhou Shaoling, published in The Welding Journal (2020, Vol. 41, No. 1, pp. 44-49), presents a three-dimensional numerical simulation of the molten pool dynamics during rotating arc GMAW surfacing with short-circuit metal transfer. The authors are affiliated with Nanchang University and Jiangxi Industry Polytechnic College, and the research was supported by the National Natural Science Foundation of China (Grant No. 51265036). This study is significant because it provides a computational understanding of the complex fluid dynamics and heat transfer phenomena that occur in the molten pool during rotating arc surfacing, which is a process used for high-productivity surfacing of large flat surfaces and pipe interiors. The simulation uses Flow-3D software with a spherical rotating heat source model and considers the effects of gravity, droplet drag force, surface tension, and buoyancy on the molten pool flow and solidification.

Core Technical Content and Simulation Methodology

The simulation was conducted for a Q235 carbon steel substrate using the following methodology:

The spherical rotating heat source model is a key innovation in this study. In rotating arc GMAW, the welding torch rotates around the substrate surface while maintaining a constant standoff distance. The heat input is therefore distributed in a circular pattern on the substrate, and the molten pool geometry and flow patterns are fundamentally different from those in stationary arc welding. The spherical rotating heat source model captures this spatial distribution of heat input more accurately than a stationary Gaussian or double-ellipsoidal heat source model.

The VOF method is used to track the free surface of the molten pool as it forms, deforms, and solidifies. This is critical for predicting the final weld bead geometry, which is the primary quality metric for surfacing operations. The simulation also captures the transient behavior of the molten pool as each droplet impacts the pool surface, creating localized disturbances that propagate through the pool and affect the final bead shape.

Key Simulation Results and Physical Insights

The simulation results reveal several important physical phenomena:

1. Rotating droplet stirring effect: The rotating droplets that impact the molten pool surface act as stirrers, enhancing the convective flow within the pool. This stirring effect increases the velocity of the liquid metal and promotes a more uniform temperature distribution within the pool.
2. Pool volume and width increase: Compared to stationary arc welding, the rotating arc process produces a larger molten pool volume and a wider weld bead. This is attributed to the spatial distribution of heat input, which allows the heat to accumulate over a larger area before the torch moves away.
3. Enhanced liquid metal activity: The combination of droplet impact and rotating heat input creates a more active liquid metal environment, which promotes better mixing of the weld pool and potentially improves the homogeneity of the weld metal composition.
4. Good agreement with experiments: The predicted weld bead dimensions and shapes showed good agreement with experimental measurements, validating the simulation model and its parameterization.

The following table summarizes the comparison between simulation and experiment:

Technical Interpretation and Process Optimization

The simulation results have direct implications for process optimization. The rotating droplet stirring effect suggests that the rotating arc process may produce weld beads with better mechanical properties than stationary arc welding, due to the enhanced mixing and more uniform cooling. This is particularly relevant for surfacing applications where the weld metal composition and microstructure affect the corrosion resistance or wear resistance of the overlay. The wider bead width also implies higher deposition rates, which is a key productivity metric for surfacing operations.

However, the wider bead width also means that the overlap between adjacent passes must be adjusted. If the overlap is too large for the wider bead, excessive metal accumulation will occur at the pass boundaries. If the overlap is too small, gaps will form between passes. The simulation can be used to predict the optimal overlap for a given set of welding parameters, reducing the need for extensive trial-and-error experimentation.

The short-circuit transfer mode, which was simulated in this study, is characterized by the periodic contact of the wire with the molten pool, causing a short circuit that transfers the droplet. This mode is commonly used for thin-section welding and surfacing because it produces a relatively low heat input and a narrow weld bead. The simulation shows that even in short-circuit transfer, the rotating arc configuration produces a wider and more active molten pool than stationary arc welding, which is a useful insight for process development.

Engineering Practice Integration

For engineers working with rotating arc GMAW surfacing, the simulation results provide several practical guidelines:

1. Heat input management: The rotating arc process produces a wider heat-affected zone than stationary arc welding. For heat-sensitive base materials (e.g., aluminum alloys, thin stainless steel), the heat input must be carefully controlled to avoid excessive distortion or microstructural changes.
2. Overlap planning: The wider bead width requires a larger overlap than stationary arc welding. The overlap should be determined by simulation or experiment, not by empirical rules.
3. Deposition rate optimization: The rotating arc process can achieve higher deposition rates than stationary arc welding. Engineers should take advantage of this productivity benefit while ensuring that the weld quality is maintained.
4. Model validation: Before using the simulation for process optimization, the model must be validated against experimental data for the specific welding conditions being used. The authors' validation approach, comparing predicted and measured weld bead geometry, is a sound methodology.

Study Insights and Implications

This paper demonstrates the value of numerical simulation in understanding and optimizing complex welding processes. The rotating arc GMAW surfacing process is inherently three-dimensional and transient, making it difficult to analyze using purely empirical methods. The simulation provides a window into the internal dynamics of the molten pool, revealing phenomena such as droplet stirring and enhanced convective flow that are not directly observable in experiments. The good agreement between simulation and experiment validates the model and gives engineers confidence in using it for process optimization. The key insight is that the rotating arc configuration fundamentally changes the molten pool dynamics compared to stationary arc welding, producing a wider, more active pool with enhanced mixing. This has implications for weld metal quality, deposition rate, and process productivity. Engineers should consider using numerical simulation as a tool for process development and optimization, particularly for complex processes like rotating arc surfacing where experimental investigation is time-consuming and expensive.