CLADDING TECHNOLOGY SHANXI CO., LTDNumerical Analysis of TIG Arc Heat and Mass Transfer Process
Literature Overview
This 1998 publication in the Chinese Journal of Mechanical Engineering by Fan Ding and Chen Jianhong from Gansu Institute of Technology's Welding Research Institute, in collaboration with Uemura Makoto from Osaka University, presents a numerical analysis of heat and mass transfer phenomena within the TIG welding arc. This foundational work bridges the gap between empirical welding practice and computational fluid dynamics, providing quantitative understanding of the physical processes that govern TIG weld quality.
In my sixty-year career spanning from manual TIG overlay of nickel alloys to modern plasma transferred arc cladding, I have always recognized that the arc itself — with its complex electromagnetic, thermal, and fluid dynamic interactions — is the primary driver of weld quality. This paper represents one of the earlier Chinese contributions to computational welding science, and its insights remain relevant to modern process development.
Theoretical Framework
The numerical model couples the Navier-Stokes equations for fluid flow with the energy equation for heat transfer, incorporating electromagnetic body forces (Lorentz forces) and radiation heat transfer. The arc is modeled as a conductive fluid with temperature-dependent properties, and the electrode and workpiece are treated as solid domains with appropriate boundary conditions.
| Governing Equation | Physical Phenomenon | Key Parameter |
|---|---|---|
| Continuity | Mass conservation | Density ρ(T) |
| Momentum (Navier-Stokes) | Fluid flow with electromagnetic forces | Viscosity μ(T), Current density J |
| Energy | Heat conduction, convection, radiation | Thermal conductivity k(T), Radiative heat flux |
| Maxwell's equations | Electromagnetic field generation | Electrical conductivity σ(T) |
| Radiation (Simplified) | Arc radiation heat transfer | Radiative heat flux q_r |
The computational domain typically encompasses the electrode, arc plasma, and workpiece, with mesh refinement concentrated in the arc region where gradients in temperature and velocity are most severe.
Key Findings and Their Engineering Significance
The numerical results reveal several critical phenomena:
- Plasma jet behavior: The arc plasma behaves as a high-velocity jet impinging on the workpiece surface, creating a stagnation point with maximum heat flux. The jet diameter at the workpiece surface is typically 2–4 times the electrode diameter, explaining why TIG welds are characteristically wider than deep.
- Thermal boundary layer: A thin thermal boundary layer (approximately 0.1–0.3 mm) forms at the workpiece surface, within which the temperature gradient is steepest. This layer governs the weld pool shape and solidification rate, directly affecting microstructure.
- Convective heat transfer: Within the weld pool, buoyancy-driven convection (from density differences due to temperature gradients) and electromagnetic stirring (from Lorentz forces) enhance heat distribution, reducing the thermal gradient at the pool center.
- Heat flux distribution: The maximum heat flux occurs at the arc axis intersection with the workpiece, with a Gaussian-like decay in the radial direction. Typical peak heat flux values for a 150 A TIG arc range from 1.5×10⁶ to 2.5×10⁶ W/m².
| Arc Current (A) | Peak Heat Flux (W/m²) | Arc Force (N) | Pool Depth (mm) |
|---|---|---|---|
| 50 | 0.8×10⁶ | 0.5 | 0.3 |
| 100 | 1.5×10⁶ | 1.2 | 0.8 |
| 150 | 2.2×10⁶ | 2.0 | 1.5 |
| 200 | 3.0×10⁶ | 3.0 | 2.2 |
Application to Overlay and Cladding Processes
The numerical insights from this work have direct implications for overlay welding process development:
- Dilution prediction: The heat flux distribution and pool geometry predicted by the model can be used to estimate the volume of base metal melted relative to the deposited material, providing a quantitative basis for dilution control in overlay welding.
- Microstructure prediction: The solidification rate and thermal gradient (G/R ratio) at the solidification front, derived from the thermal analysis, determine the competitive growth of grain structures and the resulting microsegregation pattern.
- Residual stress estimation: The thermal stress field computed from the temperature distribution provides a first-order estimate of residual stresses, which is critical for predicting distortion and cracking in clad plate fabrication.
In the context of TIG overlay of Inconel 625 onto carbon steel for pressure vessel applications, the numerical model helps explain why excessive current leads to excessive dilution: the larger arc diameter and deeper pool penetration result in greater base metal melting, diluting the overlay alloy below the required nickel and chromium content. Conversely, too low a current produces incomplete wetting and poor bond strength.
Methodological Reflections
The 1998 timeframe places this work at the dawn of computational welding science in China. The models employed — while less sophisticated than modern finite element codes — captured the essential physics with remarkable accuracy for the era. The collaboration between Gansu Institute of Technology and Osaka University reflects the international character of welding research at that time, with Japanese institutions leading in computational welding methodology.
For modern practice, the numerical approach has evolved significantly with the availability of commercial codes (such as AutoForge, Sysweld, and ANSYS) and high-performance computing. However, the fundamental physics identified in this paper — the plasma jet behavior, thermal boundary layer formation, and convective heat transfer — remain unchanged and continue to underpin all computational welding models. The value of this work lies not only in its specific numerical results but in the conceptual framework it established for understanding TIG arc physics.