CLADDING TECHNOLOGY SHANXI CO., LTDTracking Analysis of Molten Pool Convection Forms During TIG Welding Process
Literature Overview and Research Context
This 2011 publication from the Journal of Welding, authored by Li Dongjie, Lu Shanping, Li Dianzhong, and Li Yiyi from the Shenyang National Laboratory for Materials Science at the Institute of Metal Research, Chinese Academy of Sciences, presents a systematic investigation into the convection mechanisms operating within the TIG (gas tungsten arc) weld pool. Funded by the National Natural Science Foundation of China (Grant No. 50874101), the National Major Science and Technology Project for High-End CNC Machine Tools and Basic Manufacturing Equipment (2009ZX04014-081), and the Graduate Innovation Fund of the Institute of Metal Research (1193002090), this work addresses a fundamental yet often underappreciated aspect of welding physics. The study employs particle image velocimetry (PIV) and dye tracing techniques to visualize and quantify the complex fluid flow patterns that govern heat transfer, mass transport, and ultimately the weld geometry and microstructure.
Core Technical Findings
The research identifies three primary convection mechanisms driving molten metal movement in the TIG weld pool: electromagnetic force-induced convection, buoyancy-driven natural convection, and surface tension (Marangoni) convection. Each mechanism dominates under different process parameter regimes, and their superposition creates the complex flow field observed in practice.
| Convection Mechanism | Dominant Condition | Direction of Flow | Influence on Weld Geometry |
|---|---|---|---|
| Electromagnetic force | High current, large arc | Inward from surface toward pool center | Wider, shallower weld |
| Buoyancy | Low current, large pool | Upward from bottom | Deeper penetration |
| Surface tension (Marangoni) | Sulfur/phosphorus content dependent | Outward from center (positive coefficient) or inward (negative coefficient) | Controls pool shape and width |
The study demonstrates that the Marangoni effect, governed by the surface tension gradient induced by temperature variation across the pool surface, is the most critical factor in determining weld bead profile. When the surface tension coefficient decreases with increasing temperature (negative coefficient, typical of sulfur-contaminated steel), the surface tension force drives molten metal outward from the pool center, producing a wide and shallow weld. Conversely, a positive surface tension coefficient drives metal inward, creating a narrow and deep weld. This finding has direct implications for weld quality control in cladding and overlay applications where penetration depth and dilution ratio are critical parameters.
Process Parameter Sensitivity
The researchers systematically varied welding current (80–200 A), travel speed (200–600 mm/min), and electrode diameter (2.4–4.0 mm) to map the transition boundaries between different convection regimes. Key observations include:
- At low currents below 100 A, buoyancy convection dominates and the pool is relatively symmetric in cross-section.
- Between 100–160 A, electromagnetic and Marangoni forces become comparable, producing asymmetric flow patterns with distinct recirculation zones.
- Above 160 A, electromagnetic stirring becomes the dominant mechanism, and the pool exhibits strong inward flow at the surface with outward return flow at the bottom.
Engineering Practice Integration
For cladding and weld overlay operations, understanding the convection regime directly informs process optimization. In electroslag welding overlay and submerged arc welding overlay of nickel-based alloys on carbon steel, the dilution ratio between the base metal and the overlay material is a critical quality parameter. The convection analysis reveals that the dilution ratio is not merely a function of the relative melting rates of the two materials but is strongly influenced by the depth of base metal entrainment into the active convection zone. When electromagnetic stirring penetrates deeply into the base metal, dilution increases significantly even if the thermal input appears moderate.
This insight is particularly relevant for hydrogenation reactor fabrication where the overlay layer of Inconel 625 or Hastelloy C276 must maintain composition integrity throughout its thickness. The study's findings support the practice of using multi-pass overlay with intermediate grinding to break up the dilution gradient, and they provide a physical basis for the industry rule of thumb that the first pass of overlay welding should use lower currents to minimize base metal entrainment.
Application to Bimetal Pressure Vessel Fabrication
In the fabrication of clad-plate pressure vessels per NB/T 47002 and ASME VIII Div.1, the weld overlay pass that bonds the cladding to the base plate is the most dilution-critical operation. The convection analysis suggests that using a slightly smaller electrode diameter (2.4 mm rather than 3.2 mm) at the same current level can reduce the electromagnetic stirring intensity, thereby limiting base metal entrainment. This is consistent with the practice of using 2.4 mm or 3.0 mm electrodes for the first overlay pass on stainless steel clad plates over carbon steel base plates.
Key Questions and Reflections
The study raises an important question regarding the role of impurity elements in controlling weld pool convection. The Marangoni coefficient is highly sensitive to sulfur, oxygen, and phosphorus content in the base metal and filler material. In practice, the sulfur content of carbon steel base plates can vary significantly between heats and even within a single plate due to segregation. This variability introduces an uncontrolled variable into the welding process that can cause inconsistent weld geometry and dilution from plate to plate.
The researchers propose that the convection regime can be predicted with reasonable accuracy using a dimensionless number that combines the electromagnetic force, buoyancy force, and surface tension force. However, the practical challenge lies in obtaining accurate values for the surface tension coefficient under welding conditions, which requires knowledge of the local composition at the pool surface. This remains an area requiring further investigation, particularly for dissimilar material welding where the composition of the pool surface is a complex function of both the base metal and filler metal compositions.
Study Insights and Implications for Cladding Engineers
The most significant practical takeaway from this research is the recognition that weld pool convection is not a passive background phenomenon but an active and controllable process variable. By understanding the dominant convection mechanism under given process conditions, engineers can make informed decisions about current selection, electrode configuration, and travel speed to achieve the desired weld geometry and dilution characteristics. For overlay welding of corrosion-resistant alloys on structural steels, the study provides a framework for minimizing dilution without resorting to excessive grinding between passes, thereby improving productivity while maintaining overlay layer integrity.
The research also highlights the importance of maintaining low sulfur content in both base materials and filler materials for overlay welding applications. Sulfur contamination shifts the Marangoni coefficient to negative values, promoting outward surface flow and shallow, wide welds that increase the risk of incomplete bonding between overlay layers. This is particularly critical in multi-layer overlay sequences where interpass bonding quality directly affects the corrosion resistance of the final overlay surface. Engineers should verify sulfur content specifications in material certifications and consider low-sulfur filler metals for critical overlay applications.