Zhuojin Pipe Fitting Co., LtdNumerical Simulation of Thermal Process in Plasma-Tungsten Double-Sided Arc Welding
Literature Overview
This paper, published in Acta Metallurgica Sinica (2003, Vol. 39, No. 5, pp. 499-504), authored by Sun Junsheng from Shandong University and Wu Chuansong from Harbin Institute of Technology, presents a comprehensive numerical simulation of the thermal process in double-sided arc welding (DSAW) using a combined plasma arc (PAW) and tungsten inert gas (TIG) arc configuration. The work was supported by the US National Science Foundation (Grant DMI 9812981). The study addresses the thermal mechanics of a single-power-source double-sided welding process that achieves high depth-to-width ratio welds with improved efficiency and reduced cost compared to conventional single-sided processes.
Core Technical Content and Findings
The DSAW process described in this paper utilizes a single power source to simultaneously generate a plasma arc on one side of the workpiece and a TIG arc on the opposite side. The plasma arc creates a keyhole that enables deep penetration, while the TIG arc on the back side provides support, stabilizes the molten pool, and prevents sagging or collapse of the weld root. This configuration eliminates the need for backing gas or backing material, simplifying the welding setup and reducing production costs.
Mathematical Model Development
The authors developed a coupled mathematical model that accounts for the keyhole formation process by incorporating the mechanical factors influencing the plasma arc keyhole, including plasma flow force, gravity, and surface tension. The model establishes the governing equations for current density distribution and heat transfer in the DSAW process. The keyhole geometry is determined by the balance between the plasma jet's momentum flux and the surface tension of the molten metal, with gravity playing a secondary but non-negligible role in the molten pool shape.
The heat transfer model employs a moving heat source approach, with the double-sided arc represented as a distributed heat input on both surfaces of the workpiece. The model accounts for conduction, convection, and phase change effects within the molten pool, as well as convective and radiative heat losses from the workpiece surface.
Numerical Results and Heat Transfer Analysis
The numerical simulation results reveal several important thermal characteristics of the DSAW process. The combined action of the plasma arc and TIG arc produces a significantly larger molten pool volume compared to either process used alone. The peak temperature in the molten pool is concentrated near the plasma arc side, where the keyhole forms, while the TIG arc side exhibits a broader, lower-temperature distribution.
The heat-affected zone (HAZ) in DSAW is characterized by a wider but shallower thermal gradient compared to conventional PAW alone. This is attributed to the additional heat input from the TIG arc on the back side, which moderates the peak cooling rates at the fusion boundary. The simulation demonstrates that DSAW can achieve penetration depths 1.5 to 2 times greater than TIG welding alone, while maintaining a HAZ width comparable to or slightly wider than PAW welding.
Comparison with Conventional PAW
A comparative simulation of conventional PAW welding highlights the advantages of the DSAW configuration. In conventional PAW, the deep, narrow keyhole results in very high cooling rates at the fusion boundary, which can lead to microstructural coarsening and potential cracking susceptibility in certain alloys. The DSAW process, by adding the TIG arc on the back side, effectively reduces the peak cooling rate while maintaining deep penetration, thereby improving the microstructural quality of the weld and HAZ.
Process Parameters and Thermal Response
| Parameter | Typical Value | Effect on Thermal Process |
|---|---|---|
| Plasma arc current | 100-300 A | Primary driver of keyhole formation and penetration |
| TIG arc current | 50-150 A | Provides back-side support and moderates cooling rate |
| Travel speed | 5-20 mm/s | Controls molten pool volume and solidification rate |
| Plate thickness | 3-12 mm | Determines required penetration depth |
| Shielding gas | Argon (both sides) | Ensures adequate protection and stable arc |
The numerical model provides a quantitative basis for optimizing these parameters. For example, increasing the plasma arc current beyond a certain threshold leads to keyhole instability and excessive spatter, while increasing the TIG arc current beyond the optimal range can cause excessive dilution and HAZ widening. The model thus serves as a valuable tool for establishing process windows before physical trials are conducted.
Engineering Practice Integration
From an engineering perspective, the DSAW process offers several practical advantages for production welding of thick-section plates and pipes. The elimination of backing material reduces material costs and simplifies fixture design. The improved penetration-to-width ratio allows for single-pass welding of thicker sections, reducing the number of passes and consequently the total welding time and distortion. The moderated cooling rates in the HAZ reduce the susceptibility to cold cracking in high-strength steels and improve the toughness of welded joints in austenitic stainless steels.
However, the process requires careful alignment of the two arcs and stable workpiece positioning to ensure consistent weld quality. In automated welding systems, this can be achieved through precise torch positioning and closed-loop control of arc parameters. The numerical model developed in this study provides the theoretical foundation for such control strategies.
Key Questions and Reflections
One limitation of the model is the assumption of steady-state heat transfer, which may not accurately represent the transient conditions at the start and end of a weld, or during interruptions and restarts. Future work should incorporate transient thermal analysis and coupled thermal-mechanical modeling to predict residual stress and distortion more accurately. Additionally, the model does not explicitly account for the electromagnetic forces generated by the welding current, which can influence the molten pool shape and flow patterns, particularly in the keyhole region.
Study Insights and Implications
This paper represents a significant advancement in the understanding of double-sided arc welding thermal processes. The development of a coupled mathematical model that accounts for keyhole mechanics and heat transfer provides a rigorous analytical framework for process optimization. For engineering practice, the findings support the use of DSAW as a viable alternative to conventional PAW and multi-pass TIG welding for thick-section applications, particularly where high penetration, moderate cooling rates, and cost efficiency are required. The numerical methodology established in this work can be extended to other hybrid welding processes and serves as a template for future simulation-based process development.