Narrow-Gap TIG Horizontal Welding Sidewall Fusion Behavior

Literature Overview

This paper by Wang Meng, Lü Xiaochun, Liang Xiaomei, and He Shi, published in 2016 in the Transactions of the China Welding Institution, investigates the fusion behavior of the upper and lower sidewalls in narrow-gap TIG welding in the horizontal (5G) position. Conducted at the Harbin Welding Research Institute of the Chinese Academy of Machinery Science, this study provides detailed process parameter guidelines for achieving uniform fusion in one-pass narrow-gap welding—a technique widely used in large-diameter pipe manufacturing.

Core Technical Points

Narrow-gap welding is a one-pass welding technique used primarily for thick-walled pipes (typically 20–60 mm wall thickness) where the groove is prepared with a narrow root gap (typically 2–4 mm) and minimal bevel angle (1–3°). In the horizontal position, gravity significantly affects the molten pool flow, creating asymmetric fusion between the upper and lower sidewalls.

The key process parameters studied include:

Tungsten electrode position: The vertical offset of the tungsten tip relative to the centerline of the groove.
Welding current: The base (non-pulsed) current level.
Wire feed speed: The rate of filler wire deposition.
Pulse frequency: The modulation frequency of the welding current.

Parameter Influence Analysis

The study systematically evaluates each parameter's effect on upper and lower sidewall fusion. The following table summarizes the critical findings:

Parameter	Optimal Range	Effect on Lower Sidewall	Effect on Upper Sidewall
Tungsten position	-1.5 to -2.5 mm	Stable fusion achieved	No undercut when ≤ 1.0 mm
Base current	150–250 A	Good fusion in this range	Minor effect
Wire feed speed	< 3000 mm/min	Angle α approaches 90° above 3000 mm/min (poor fusion)	Angle with sidewall decreases above 3000 mm/min
Pulse frequency	> 10 Hz	Surface curvature minimized, α approaches 90°	Minor effect
Arc voltage	Standard range	Minor effect	Significant effect on fusion

Tungsten Electrode Position

The tungsten electrode position is the most critical parameter for lower sidewall fusion. When the tungsten is positioned 1.5–2.5 mm below the groove centerline (negative position), the arc energy is directed toward the lower sidewall, compensating for gravitational sag of the molten pool. If the tungsten is positioned too high (positive offset > 1.5 mm), the lower sidewall receives insufficient heat input, resulting in incomplete fusion. Conversely, a tungsten position of +1.5 mm (above centerline) causes undercut on the upper sidewall due to excessive arc energy concentration.

Wire Feed Speed and Pulse Frequency

At wire feed speeds exceeding 3000 mm/min, the deposited metal forms a convex surface with angle α approaching 90° relative to the sidewall. This geometry indicates that the deposited metal is not flowing into the root of the groove, creating a risk of incomplete fusion at the lower sidewall. Similarly, pulse frequencies above 10 Hz reduce the dwell time at peak current, minimizing molten pool sag and producing a flatter weld surface—but this also reduces the driving force for metal to flow into the lower root.

Engineering Practice Integration

For large-diameter pipe manufacturing, narrow-gap TIG welding offers significant advantages over multi-pass conventional welding:

Reduced welding time: One-pass welding eliminates the need for multiple layers, reducing production time by 60–80%.
Lower heat input: Reduced total heat input minimizes HAZ softening and distortion.
Improved metallurgical properties: A single weld bead has a more uniform microstructure compared to multi-layer welds.

However, the horizontal position presents unique challenges:

Molten pool sag: Gravity causes the molten pool to sag, creating asymmetric fusion.
Gas shielding difficulty: Proper shielding gas coverage of the upper portion of the groove is challenging.
Filler wire placement: The wire must be positioned to direct deposited metal toward the lower sidewall.

Defect Analysis and Countermeasures

Based on the study findings, the following defect modes and countermeasures are identified:

Defect	Root Cause	Countermeasure
Lower sidewall incomplete fusion	Insufficient tungsten offset, low current	Increase tungsten offset to -2.0 mm, raise current to 200–250 A
Upper sidewall undercut	Excessive tungsten offset above centerline	Reduce tungsten position to ≤ 1.0 mm above centerline
Excessive convexity (angle α > 85°)	High wire feed speed, high pulse frequency	Reduce wire feed speed to < 2500 mm/min, pulse frequency to 5–8 Hz
Root concavity	Excessive wire feed speed	Reduce wire feed speed, increase base current

Study Insights and Implications

This research provides actionable process windows for narrow-gap TIG welding in the horizontal position, which is one of the most challenging configurations in industrial pipe welding. The systematic parameter study methodology—varying one parameter at a time while holding others constant—provides clear cause-and-effect relationships that can be directly applied to welding procedure qualification.

For pipe manufacturers, the findings suggest that a balanced approach is necessary: the tungsten offset should be set to approximately -2.0 mm to ensure lower sidewall fusion, while the wire feed speed should be kept below 2500 mm/min to maintain adequate root filling. The pulse frequency should be selected in the 5–10 Hz range to provide sufficient peak current for fusion without excessive molten pool sag.

The study also highlights the importance of real-time monitoring of weld geometry during production. Techniques such as optical sensing or acoustic emission monitoring could be employed to detect deviations in fusion behavior and trigger automatic parameter adjustment, ensuring consistent quality across the full production cycle.