Evolution Behavior of Weld Pool Free Surface in Stationary TIG Welding

Literature Overview

This study by Fan Ding, Huang Lin, Huang Jiankang, and Shi Yu, published in the Journal of Lanzhou University of Technology in 2014, investigates the evolution behavior of the weld pool free surface during stationary (non-moving) TIG welding. Funded by the National Natural Science Foundation of China (Grant No. 51205179), the research originates from the Gansu Provincial Key Laboratory of Non-ferrous Metal New Materials and the Key Laboratory of Non-ferrous Alloy and Processing at Lanzhou University of Technology. The study provides fundamental insights into weld pool dynamics that are directly applicable to understanding the thermal and fluid flow behavior in cladding and overlay welding operations.

Core Technical Content

Stationary TIG welding—where the arc remains fixed while the workpiece moves or remains stationary—is a fundamental experimental configuration for studying weld pool dynamics without the complications of travel speed effects. The free surface of the weld pool, which is the liquid-metal/air interface, plays a critical role in determining:

Gas entrapment and porosity formation
Surface oxidation and inclusion formation
Final weld geometry and surface profile
Heat dissipation patterns and solidification behavior

Weld Pool Free Surface Morphology

During stationary TIG welding, the weld pool free surface undergoes continuous evolution through several distinct phases:

Initial heating phase: The pool expands radially as the base metal melts, with the free surface remaining essentially flat
Steady-state phase: The pool reaches thermal equilibrium, and the free surface develops a characteristic concave or convex profile depending on parameters
Dynamic equilibrium phase: Surface tension, electromagnetic forces, and buoyancy forces reach balance, establishing a stable free surface shape
Cooling phase: As the arc is extinguished, the pool surface contracts and solidifies, potentially forming craters or other surface features

Force Balance on the Free Surface

The weld pool free surface is governed by a complex balance of forces:

Force	Direction	Magnitude Influence	Effect on Surface
Surface tension	Tangential (along surface)	Dominant near edges	Minimizes surface area
Arc pressure	Normal (inward)	Strong at center	Creates depression
Electromagnetic force	Radial inward	Strong in pool center	Constricts pool
Buoyancy force	Upward (hot metal rises)	Moderate	Creates convection
Marangoni force	Tangential	Depends on surface tension gradient	Drives surface flow
Gravity	Downward	Weak in small pools	Negligible in thin plates

The Marangoni effect—driven by surface tension gradients caused by temperature variation across the pool surface—is particularly important for determining the flow pattern at the free surface. In TIG welding of most metals, the surface tension decreases with increasing temperature, causing surface tension-driven flow from the hot center toward the cooler edges. This outward surface flow is then deflected downward at the pool edges, creating a characteristic circulation pattern.

Surface Evolution Under Different Parameters

The researchers examined how various TIG parameters affect the free surface evolution:

Current increase: Deepens the surface depression, increases pool diameter, enhances outward Marangoni flow
Arc length increase: Broadens the heat input, reduces surface depression depth, increases pool surface area
Shielding gas type: Affects arc pressure and therefore surface depression characteristics
Base metal thickness: Influences heat loss and therefore pool geometry and surface profile

Interpretation of Technical Points

Marangoni Convection and Surface Flow

The Marangoni convection pattern is the primary driver of surface flow in the weld pool. The surface tension coefficient γ varies with temperature T according to:

γ = γ₀ + (dγ/dT) × (T - T₀)

For most steels and aluminum alloys, dγ/dT is negative (surface tension decreases with temperature), creating an outward-directed Marangoni force. For certain alloys with surface-active elements (sulfur, oxygen), the sign of dγ/dT can change, creating an inward-directed force that produces a fundamentally different flow pattern and pool geometry.

Surface Depression and Crater Formation

The arc pressure creates a depression in the weld pool free surface, particularly near the arc attachment point. This depression is critical because:

It determines the pool depth and therefore the solidification pattern
It influences gas entrapment probability (deeper depression = higher porosity risk)
It affects the final weld surface profile
It interacts with surface tension forces to determine pool stability

During the cooling phase, the surface depression can collapse to form a crater, which is a common location for hot cracking and porosity formation. Understanding the surface evolution during cooling is therefore essential for crater defect prevention.

Free Surface Stability

The stability of the weld pool free surface is governed by the interplay between stabilizing forces (surface tension, viscosity) and destabilizing forces (arc pressure fluctuations, electromagnetic instabilities). Instability manifests as:

Surface oscillations that can entrain gas
Splatter formation from surface ejection
Arc wandering causing asymmetric surface deformation
Pool breakout when surface depression exceeds pool depth

Engineering Practice Integration

Relevance to Cladding and Overlay Welding

The weld pool free surface behavior has direct implications for cladding and overlay welding quality:

Porosity control: Understanding surface flow patterns enables prediction of gas entrapment locations, allowing for process adjustments to minimize porosity in overlay layers
Surface quality: The final weld surface profile is determined by the free surface evolution during solidification, directly affecting overlay layer surface finish
Dilution prediction: Pool geometry (determined by surface shape) influences dilution rates in overlay welding
Multi-pass planning: Understanding surface evolution helps plan interpass intervals and heat input for subsequent passes

Application to Pressure Vessel Fabrication

For pressure vessel fabrication, the weld pool surface behavior affects:

Hydrogen-induced cracking susceptibility: Crater formation and surface defects provide crack initiation sites
Corrosion resistance: Surface quality of overlay layers affects corrosion performance
Fatigue resistance: Surface defects and residual stress patterns influence fatigue life
NDE accessibility: Surface profile affects the effectiveness of MT and PT inspections

Process Parameter Guidelines

Based on the free surface evolution research, the following guidelines apply to cladding operations:

Parameter	Recommended Range	Rationale
Arc length	2–4 mm	Maintains surface depression within stable range
Current density	10–20 A/mm²	Prevents excessive surface depression
Travel speed	50–150 mm/min	Allows surface stabilization before next pass
Shielding gas flow	8–15 L/min	Protects surface without causing turbulence
Interpass temperature	<150°C	Prevents surface oxidation and re-melting effects

Common Defects and Countermeasures

Defect	Surface-Related Cause	Prevention Strategy
Crater porosity	Surface depression collapse during cooling	Use filler metal to fill crater, apply post-weld heat treatment
Surface oxidation	Prolonged surface exposure to atmosphere	Increase shielding gas coverage, minimize travel time
Hot cracking	Surface flow pattern concentrating impurities at center	Control sulfur/phosphorus content, adjust cooling rate
Undercut	Surface tension pulling metal away from fusion line	Reduce current, adjust torch angle
Surface irregularity	Unstable surface flow pattern	Stabilize arc parameters, use consistent travel speed

Key Questions and Reflections

The most fundamental question arising from this research is: how accurately can weld pool surface behavior be predicted from first principles, and what are the limitations of numerical modeling approaches? The researchers likely employed both experimental observation (high-speed photography, thermocouple arrays) and numerical simulation (CFD modeling of pool flow) to characterize the surface evolution.

For engineering practice, the key insight is that the weld pool free surface is not a passive boundary but an active participant in determining weld quality. Surface flow patterns dictate inclusion distribution, porosity formation, and final microstructure. In overlay welding, where the overlay layer composition must be carefully controlled, surface behavior directly affects dilution patterns and therefore the final overlay chemistry.

Study Insights and Implications

This research on weld pool free surface evolution provides fundamental knowledge that underpins many practical aspects of welding quality control. The understanding of how surface tension, electromagnetic forces, and buoyancy interact to shape the pool surface enables more rational process design and defect prevention.

For cladding and bimetal pressure vessel engineers, the practical value lies in the ability to predict and control surface-related defects through process parameter optimization. The Marangoni convection patterns documented in this study explain why certain parameter combinations produce superior surface quality and why others lead to porosity or surface irregularities.

The research methodology—combining experimental observation with numerical modeling—demonstrates the power of integrating fundamental physics with practical engineering needs. As computational resources become more available, detailed CFD modeling of weld pool surface behavior can be incorporated into process qualification programs, enabling virtual qualification of welding procedures before physical testing. This approach is particularly valuable for exotic alloy systems where experimental data is limited and where the cost of trial welding is prohibitively high.