Numerical Simulation of Dynamic Thermal Process in Double-Sided Asymmetrical TIG Backing Welding of Large Thick Plates

Literature Overview

This study by Liu Dianbao, Zhao Huihui, Li Fuquan, Zhang Guangjun, and Wu Lin from the National Key Laboratory of Science and Technology on Precision Heat Processing of Metals, Harbin Institute of Technology (published 2011 in China Welding) presents a numerical simulation of the dynamic thermal process during double-sided asymmetrical TIG backing welding of large thick plates. This research addresses a practical challenge in heavy industry where thick plate welding requires backing welds for root pass quality assurance.

Technical Background

Double-sided asymmetrical TIG welding involves welding from one side while simultaneously welding from the opposite side with different parameters. This technique is employed for:

Thick plate fabrication where single-sided welding cannot achieve full penetration
Applications where back-side access is limited but root quality must be assured
Pressure vessel and heat exchanger shell fabrication
Nuclear-grade component manufacturing where root weld integrity is critical

The asymmetrical nature of the process means that the front-side and back-side welding parameters differ, creating complex thermal interactions that are difficult to predict without numerical simulation.

Numerical Simulation Methodology

The study employs finite element analysis (FEA) to model the transient thermal field during welding:

Governing Equations

The heat transfer equation for the moving heat source:

∂(ρcT)/∂t + ∇·(k∇T) = Q(x, y, z, t)

Where:

ρ = density (kg/m³)
c = specific heat capacity (J/kg·K)
T = temperature (K)
k = thermal conductivity (W/m·K)
Q = heat source intensity (W/m³)

Heat Source Model

A double-Gaussian heat source model is used to represent the two TIG arcs:

Q(r) = (f·P)/(√(2π)·a·b) · exp(-r²/(2a²))

Where:

f = fraction of heat absorbed by the workpiece (typically 0.6–0.8)
P = arc power (W)
a, b = elliptical distribution parameters
r = distance from heat source center

Boundary Conditions

Convective heat transfer on exposed surfaces (h = 10–25 W/m²·K)
Radiative heat transfer at high temperatures (ε = 0.8–0.95)
Adiabatic conditions on symmetry planes

Simulation Results and Key Findings

The numerical simulation reveals several important characteristics of the thermal process:

Temperature Distribution

Location	Peak Temperature (°C)	Cooling Rate through 800°C (°C/s)
Front weld center	1800–2200	200–500
Back weld center	1500–1900	150–400
Interface between welds	1200–1600	100–300
HAZ (front)	800–1200	50–200
HAZ (back)	700–1100	40–150

Thermal Interaction Effects

The simulation demonstrates that:

Sequential welding effect: When the back-side weld follows the front-side weld with a time delay, the thermal history of the front weld is modified by the back weld's heat input
Temperature uniformity: Proper parameter asymmetry can achieve more uniform temperature distribution across the plate thickness
Residual stress prediction: The thermal gradients predict residual stress patterns that influence distortion and cracking susceptibility

Process Parameter Optimization

Based on simulation results, the following parameter ranges are recommended for thick plate backing welds:

Parameter	Front Side	Back Side	Notes
Current (A)	200–300	150–250	Back side lower to reduce burn-through
Travel speed (m/min)	0.3–0.6	0.3–0.6	Matched for thermal balance
Arc voltage (V)	18–24	16–22	Lower for back side
Shielding gas flow (L/min)	20–30	15–25	Adequate coverage
Time delay (s)	—	5–15	Depends on plate thickness
Electrode diameter (mm)	3.2–4.0	3.2–4.0	Matched for stability

Engineering Application Considerations

For practical implementation in heavy industry fabrication:

Thick plate definition: Plates exceeding 25 mm typically require backing weld strategies
Material selection: Low-alloy steels (SAE 1045, A516 Gr.70, 16MnR) are common for thick plate applications
Pre-heat requirements: 100–250°C depending on carbon equivalent and plate thickness
Distortion control: Symmetrical clamping and sequential welding sequences minimize angular and bow distortion
Quality assurance: Full RT inspection of root pass, UT for volumetric defects in thick sections

Defect Prevention Through Thermal Management

The simulation provides insight into defect prevention:

Porosity: Controlled by maintaining adequate shielding gas coverage during both front and back passes
Cracking: Reduced by managing cooling rates through parameter optimization
Incomplete fusion: Prevented by ensuring sufficient heat input at the interface
Burn-through: Avoided by controlling back-side heat input and using backing bars or consumable backing

Study Insights and Reflections

This numerical simulation work demonstrates the value of computational methods in understanding complex thermal processes that are difficult to measure experimentally. The double-sided asymmetrical approach offers a practical solution for thick plate fabrication where single-sided welding cannot achieve the required root quality.

Engineers should recognize that numerical simulation results must be validated against experimental data before full-scale implementation. The thermal model assumptions—particularly regarding heat source distribution and boundary conditions—must be refined based on actual welding conditions. The study's methodology provides a framework for process optimization that can be extended to other welding configurations and material systems.

The practical significance of this research lies in its application to heavy industry fabrication, where welding of thick plates is routine but quality assurance remains challenging. By understanding the thermal interactions between front and back welds, fabricators can optimize parameters to achieve superior root quality while minimizing distortion and residual stresses.