Zhuojin Pipe Fitting Co., LtdNumerical Simulation of Flow Field Inside Brazing Filler Metal Droplets During TIG Arc Brazing
Literature Overview
This paper by Li Ruifeng, Yu Zhishui, and Qi Kai, published in the Journal of Jiangsu University (Natural Science Edition) in 2007, presents a numerical simulation study on the internal flow field of filler metal droplets during TIG arc brazing. The work builds upon experimental investigations of filler metal droplet spreading and wetting behavior, establishing control equations and boundary conditions that account for gravity, surface tension, and arc pressure forces. The authors then transformed these governing equations into finite volume equations and employed the SIMPLEST algorithm within the PHOENICS software to compute the flow field behavior inside the filler metal droplet. A comparative analysis with conventional TIG welding molten pool flow field behavior was conducted to better understand the evolution of the droplet free surface and its final shape formation.
Core Technical Content
The fundamental distinction between TIG arc brazing and conventional TIG welding lies in the fact that the base metal remains solid throughout the process while only the filler metal melts. This creates a fundamentally different flow regime compared to conventional arc welding, where both base metal and filler metal form a unified molten pool. The authors recognized this distinction and developed a model specifically tailored to the brazing scenario.
The governing equations established in this study include:
| Physical Force | Governing Equation Component | Boundary Condition |
|---|---|---|
| Gravity | ρg term in momentum equation | Hydrostatic pressure at droplet base |
| Surface tension | Marangoni effect, surface tension gradient | Free surface stress boundary |
| Arc pressure | Electromagnetic force and plasma jet pressure | Distributed pressure on droplet upper surface |
| Viscosity | μ∇²v term | No-slip at solid base interface |
The SIMPLEST algorithm was selected for its robustness in handling incompressible flow problems with free surfaces, which is particularly appropriate for the droplet shape evolution problem encountered in arc brazing.
Key Technical Insights
The numerical simulation reveals several critical aspects of filler metal droplet behavior during TIG arc brazing:
- The flow field inside the filler metal droplet is driven primarily by arc pressure on the upper surface and surface tension forces at the free surface, rather than by Marangoni convection which dominates in conventional welding molten pools.
- The droplet free surface evolves under the combined action of arc pressure pushing the droplet outward and surface tension pulling it inward, creating a characteristic spreading pattern.
- The internal flow pattern differs significantly from that observed in conventional TIG welding molten pools, where strong Marangoni convection creates a centrally depressed pool with outward surface flow.
Comparison with Conventional TIG Welding
The comparative analysis between the brazing droplet flow field and the conventional TIG welding molten pool flow field reveals important differences:
| Parameter | TIG Arc Brazing Droplet | Conventional TIG Welding Pool |
|---|---|---|
| Primary driving force | Arc pressure + surface tension | Marangoni convection + buoyancy |
| Flow pattern | Radially symmetric spreading | Central depression with outward surface flow |
| Temperature gradient effect | Moderate (filler only melts) | Strong (base + filler both melt) |
| Base metal involvement | Solid, acts as substrate | Melts and participates in flow |
| Surface evolution | Governed by wetting and spreading | Governed by pool dynamics and solidification |
The authors concluded that the droplet free surface evolution follows a predictable pattern governed by the balance between arc pressure and surface tension, with the internal flow field playing a supporting role in heat and mass transport within the droplet.
Engineering Practice Integration
From a practical standpoint, this research has several implications for TIG arc brazing applications in the piping and fitting industry:
- Understanding the droplet flow field behavior helps optimize filler wire feed rates and torch angles to achieve uniform bead profiles.
- The arc pressure distribution is critical in determining the spreading width of the brazed joint, which directly affects joint strength and leak tightness.
- For pipe joint brazing applications, particularly in stainless steel pipe systems, controlling the filler metal wetting behavior is essential for achieving full penetration and proper joint geometry.
Practical Recommendations
Based on the simulation results, the following process parameters should be carefully controlled during TIG arc brazing:
- Arc length must be maintained within a narrow range to ensure consistent arc pressure distribution.
- Filler wire diameter should be selected to match the expected droplet spreading width for the given joint geometry.
- Travel speed must be coordinated with filler wire feed rate to prevent excess filler accumulation or insufficient wetting.
Reflections and Study Insights
This paper represents an important contribution to the fundamental understanding of TIG arc brazing process physics. The numerical approach adopted provides a cost-effective alternative to purely experimental investigation, allowing systematic variation of parameters that would be difficult to isolate experimentally. However, the simulation results should be validated against experimental measurements of droplet shape and spreading width before being applied to process optimization.
The work also highlights the importance of selecting appropriate computational tools and algorithms for specific welding process simulations. The choice of PHOENICS with the SIMPLEST algorithm was well-suited for this particular problem, though modern CFD tools would offer additional capabilities for multiphysics coupling.
For engineers involved in pipe and fitting brazing operations, the key takeaway is that the internal dynamics of the filler metal droplet significantly influence the final joint geometry and quality. Process development should consider not only macroscopic parameters such as current and travel speed, but also the microscopic flow behavior within the molten filler.