CLADDING TECHNOLOGY SHANXI CO., LTDNumerical Analysis of Molten Pool Behavior in Dual-Pulse TIG Welding
Literature Overview
This 2018 study by researchers from Henan Institute of Technology, supported by the Henan Provincial Science and Technology Project (182102210260), presents a numerical simulation of the molten pool dynamics in dual-pulse TIG welding. Dual-pulse TIG is an advanced variant of conventional TIG welding where two distinct pulse frequencies are superimposed on the welding current waveform, creating complex and controllable thermal cycles within the weld pool. The study was published in the Transactions of the China Welding Institute (焊接学报) and addresses the fluid dynamics, temperature field, and solidification behavior within the weld pool using finite element methods.
Core Technical Content
Dual-pulse TIG welding operates by combining a high-frequency, low-current pulse (sub-pulse) with a lower-frequency, higher-current pulse (main pulse). The sub-pulse maintains arc stability and promotes surface tension-driven flow, while the main pulse provides deep penetration. The superposition creates a periodic variation in heat input that influences convective flow patterns, pool geometry, and solidification morphology.
Numerical Model Parameters
| Parameter | Value/Range | Description |
|---|---|---|
| Main pulse frequency | 5–20 Hz | Controls penetration depth |
| Sub-pulse frequency | 50–200 Hz | Controls surface flow and bead shape |
| Main pulse current | 100–250 A | Determines peak heat input |
| Sub-pulse current | 20–80 A | Maintains minimum heat input |
| Travel speed | 3–10 cm/min | Influences pool elongation |
| Thermal conductivity | Material-dependent | Copper: 400 W/m·K; Steel: 50 W/m·K |
| Surface tension coefficient | 1.5–1.8 N/m | Drives Marangoni convection |
Key Simulation Findings
The numerical results revealed several important phenomena:
- Pool geometry: Dual-pulse TIG produces a deeper and narrower weld pool compared to single-pulse TIG at equivalent average current, due to the periodic concentration of energy input during main pulses.
- Flow patterns: The superposition of two frequencies creates a complex vortex structure within the pool. The main pulse generates a downward-directed jet flow from the arc center, while the sub-pulse induces lateral surface tension-driven flow along the pool surface.
- Solidification behavior: The periodic variation in cooling rate promotes fine grain formation at the fusion boundary. The thermal cycling reduces the width of the coarse grain zone (CGZ) in the HAZ by approximately 20–30% compared to continuous TIG.
- Weld pool oscillation: The pool boundary exhibits periodic expansion and contraction synchronized with the pulse frequencies, which can be exploited to control bead width and penetration ratio.
Process Optimization and Standards Analysis
The numerical model provides a basis for process optimization that aligns with practical welding procedure qualification requirements under NB/T 47014 or ASME IX. The key process windows identified are:
- For low-carbon steel (Q345R): Main pulse 150–200 A, sub-pulse 40–60 A, travel speed 5–8 cm/min yields optimal penetration-to-width ratio of 2.5–3.5.
- For stainless steel (304/316L): Reduced current (80–150 A) with higher sub-pulse ratio (60–80%) minimizes sensitization in the HAZ.
- For austenitic stainless steel overlay cladding: Dual-pulse parameters can be tuned to achieve columnar grain refinement, improving crack resistance in multi-pass overlay welds.
Study Insights and Reflections
This work represents a significant advancement in understanding the physics of advanced TIG welding processes. The numerical approach allows engineers to predict weld pool behavior without extensive trial-and-error experimentation, which is particularly valuable for exotic materials where test coupons are expensive. The practical implication for cladding applications is substantial: by controlling the pulse parameters, it is possible to tailor the solidification microstructure of overlay layers, directly influencing properties such as crack resistance, corrosion resistance, and bond strength. For engineers working with weld-overlay clad pressure vessels, the ability to predict and control HAZ grain structure through pulse waveform design offers a powerful tool for meeting the stringent requirements of GB/T 150 and ASME VIII Div.2 for post-weld heat treatment and mechanical property verification.