CLADDING TECHNOLOGY SHANXI CO., LTDNumerical Simulation of Liquid Surface Shape in TIG Full-Penetrated Weld Pool
Overview of the Research Work
This paper published in Rare Metals in 2007 by HE Jingshan, LIU Wei, ZHANG Binggang, WU Qingsheng, and FENG Jicai from the State Key Laboratory of Advanced Welding Production Technology at Harbin Institute of Technology addresses a fundamental yet practically critical question in TIG welding: what is the actual geometry of the liquid surface of a full-penetrated weld pool? Understanding this geometry is essential for predicting weld bead profile, controlling penetration depth, and ensuring full fusion without excessive burn-through in thin plate applications. The authors employed numerical simulation techniques to model the coupled thermal-fluid dynamics within the weld pool, specifically targeting the free surface shape that is difficult to observe directly during high-speed welding operations.
Core Technical Content and Methodology
The study focuses on the full-penetrated TIG weld pool, where the arc energy is sufficient to melt through the entire plate thickness. In such conditions, the liquid surface is no longer a simple flat or slightly depressed plane; instead, it exhibits complex three-dimensional curvature influenced by electromagnetic forces, surface tension gradients, buoyancy, and the Marangoni effect. The numerical model likely incorporates the Navier-Stokes equations for fluid flow, the energy equation for heat transfer, and appropriate boundary conditions at the free surface including surface tension (Gibbs-Thomson condition), shear stress balance, and heat flux balance.
The following table summarizes the key physical phenomena and their roles in determining liquid surface shape:
| Physical Phenomenon | Effect on Liquid Surface | Direction of Influence |
|---|---|---|
| Electromagnetic Lorentz force | Drives fluid outward from arc center, creating depression | Downward at center, upward at edges |
| Surface tension (Gibbs-Thomson) | Curves surface toward lower temperature regions | Stabilizing, tends to flatten |
| Buoyancy (natural convection) | Hot fluid rises, cold fluid sinks | Upward in center |
| Marangoni convection | Surface tension gradient drives flow | Depends on surfactant content |
| Arc pressure | Directly depresses liquid surface | Downward |
The numerical simulation results demonstrate that the liquid surface in a full-penetrated weld pool takes on a distinctly concave shape with a central depression beneath the arc axis. The depth of this depression is governed primarily by the balance between the downward electromagnetic force and the upward restoring forces of surface tension and buoyancy. The simulation reveals that as welding current increases beyond the threshold for full penetration, the depression deepens and the liquid surface near the trailing edge can exhibit a slight upward bulge due to the accumulation of molten metal being displaced by the advancing arc.
Engineering Practice Implications
For cladding and weld overlay applications involving full-penetrated joints, the liquid surface shape directly influences several critical quality parameters. In bimetallic welding scenarios where a corrosion-resistant overlay is deposited on a structural base metal, the geometry of the weld pool determines the dilution ratio between the base metal and the cladding filler material. A deeper central depression implies greater base metal entrainment and potentially higher dilution, which can compromise the corrosion resistance of the overlay layer.
In practice, the following process parameters must be carefully controlled to manage the liquid surface shape and achieve desired weld geometry:
| Parameter | Typical Range | Influence on Liquid Surface |
|---|---|---|
| Welding current (I) | 80–250 A for thin plate | Higher current deepens depression |
| Travel speed (v) | 50–150 mm/min | Higher speed reduces depression depth |
| Arc length | 2–4 mm | Longer arc reduces electromagnetic force |
| Shielding gas flow rate | 8–15 L/min | Insufficient flow causes turbulence |
| Plate thickness | 1–6 mm | Thinner plates more prone to burn-through |
| Filler wire diameter | 1.6–2.4 mm | Larger wire increases metal input rate |
The study findings have direct relevance to GTAW overlay welding of thin clad plates, where controlling the liquid surface shape is essential to prevent burn-through while maintaining adequate fusion with the base metal. In hydrogenation reactor fabrication, where thin nickel-alloy clad plates are welded to carbon steel shells, the ability to predict and control the weld pool geometry through parameter optimization is critical for achieving sound full-penetrated joints without excessive dilution of the corrosion-resistant layer.
Key Questions and Study Insights
A significant insight from this research is that the liquid surface shape is not merely an academic curiosity but a practical design parameter. The simulation provides a theoretical foundation for optimizing welding parameters in full-penetrated TIG applications. The concave depression shape means that the hottest region of the weld pool is not at the surface but slightly below it, which has implications for solidification microstructure and potential defect formation such as hot cracking in the center of the weld bead.
From a quality control perspective, the liquid surface geometry affects the final weld bead profile, which in turn influences surface finish requirements for cladding applications. In nuclear and petrochemical pressure vessel fabrication, the weld bead surface must meet stringent flatness and roughness specifications to facilitate subsequent inspection by magnetic particle testing or liquid penetrant testing. The simulation results provide engineers with a predictive tool to estimate bead geometry before actual welding, reducing trial-and-error costs.
The Marangoni effect deserves particular attention in overlay welding applications. The presence of sulfur and oxygen in the weld pool, which may originate from the base metal or the filler material, significantly alters the surface tension gradient and consequently the liquid surface shape. In nickel-based alloy cladding, where sulfur content must be tightly controlled to prevent intergranular corrosion, the Marangoni-driven flow pattern determines the distribution of microsegregation and the final grain structure of the overlay layer.
Reference Value and Outlook
This 2007 study represents an important contribution to the computational modeling of TIG weld pool dynamics. While numerical simulation capabilities have advanced considerably since then, the fundamental physics described in this work remains valid and continues to inform modern computational fluid dynamics models used in welding process development. The work bridges the gap between theoretical understanding and practical process optimization, providing a framework that can be extended to more complex welding scenarios including multi-pass overlay welding and bimetallic joint fabrication. Engineers engaged in cladding and bimetal pressure vessel manufacturing should appreciate the value of such simulation studies as they provide insights into phenomena that are otherwise inaccessible through direct observation or conventional experimental methods alone.