Single-Side TIG Welding with Double-Sided Forming of Pure Copper

Literature Overview

This paper by Wang Yan, Liang Zhaoxu, Zhang Kehong, Zhou Qing, and Zhao Ming, published in Welding (2006, No. 8, pp. 56–57), presents a practical welding technology for achieving double-sided forming from a single-sided TIG weld on pure copper. The work originates from the Henan Provincial Boiler and Pressure Vessel Safety Inspection Research Institute and China Sixth Metallurgical Installation Company, reflecting a strong connection between inspection requirements and field welding practice.

Core Technical Content

Process Background and Challenges

Pure copper welding presents unique challenges:

High thermal conductivity (approximately 390 W/(m·K) at 20°C), which causes rapid heat dissipation and requires high heat input.
Low melting point (1085°C) combined with high thermal conductivity creates a wide temperature gradient in the weld pool.
Susceptibility to oxidation at elevated temperatures, forming Cu₂O and CuO.
Hydrogen porosity due to the solubility of hydrogen in molten copper, which decreases dramatically upon solidification.
Hot cracking tendency in the weld and HAZ due to the low melting point of Cu-Ag eutectic (770°C) in copper containing silver impurities.

The requirement for double-sided forming from a single-sided weld is common in pressure vessel and piping applications where access to the back side is limited. Achieving a sound, visually acceptable weld on both sides from a single pass requires precise control of heat input, travel speed, and filler metal deposition.

Process Parameters and Technique

The paper describes the practical implementation of single-sided TIG welding with double-sided forming for pure copper. Key process elements include:

Parameter	Typical Range	Notes
Shielding gas	Argon or Helium-Argon mixture	Helium improves arc stability and penetration
Gas flow rate	15–25 L/min	Adequate coverage for copper weld pool
Current type	DCEN	Deep penetration preferred
Electrode	Pure tungsten, 2.4–3.2 mm	Grounded to 3/16" or 1/4"
Filler wire	ER Cu or pure copper	Matching filler composition
Preheating	150–250°C	Reduces thermal gradient and hydrogen porosity
Interpass temperature	< 250°C	Prevents excessive grain growth

Key Technical Points

Preheating: Preheating the copper workpiece to 150–250°C is critical to reduce the thermal gradient, minimize hydrogen porosity, and prevent cracking. The preheat temperature should be controlled to avoid excessive oxidation.
Shielding gas selection: Pure argon is commonly used, but adding helium (75% He / 25% Ar) can improve arc stability, increase penetration, and reduce the required current. The higher thermal conductivity of helium helps maintain a concentrated weld pool.
Travel speed control: Maintaining a consistent travel speed is essential for achieving uniform weld bead geometry on both sides. Too slow a speed causes excessive heat input and weld burn-through; too fast a speed results in incomplete penetration.
Filler wire technique: The filler wire should be added in a continuous manner, with the wire tip maintained in the arc zone but not touching the electrode. The wire should be added on the leading edge of the weld pool to ensure proper penetration.
Back-side forming: The double-sided forming is achieved by controlling the heat input and travel speed to create a concave or convex back-side weld profile that meets visual inspection requirements. The back-side profile depends on the combination of current, travel speed, and filler wire addition rate.

Process Analysis and Standards Compliance

Weld Quality Requirements

For pressure vessel and piping applications, the weld must meet the following requirements:

Visual inspection: No cracks, porosity, undercut, or excessive reinforcement on either side.
Radiographic testing (RT): No defects exceeding the acceptance criteria of applicable standards (ASME Section V, GB/T 3323).
Mechanical properties: Tensile strength, hardness, and impact properties must meet the requirements of the applicable code.
Corrosion resistance: The weld should not be significantly more susceptible to corrosion than the base metal.

Comparison with Other Copper Welding Methods

Method	Penetration	Productivity	Equipment Cost	Quality
TIG (GTAW)	Moderate to deep	Moderate	Low	High
MIG (GMAW)	Moderate	High	Moderate	Moderate
Oxy-fuel	Shallow	Low	Low	Low
Submerged arc (SAW)	Deep	High	Moderate	High
Friction stir welding (FSW)	Full	Moderate	High	Very high

TIG welding offers the best combination of quality and flexibility for copper welding, particularly for thin to medium thickness sections where precise heat input control is required.

Engineering Practice Integration

Typical Applications

Single-sided TIG welding with double-sided forming of pure copper is commonly used in:

Electrical busbars and conductor joints in power generation and distribution.
Heat exchanger tubes and tube sheets in chemical processing.
Copper piping in refrigeration and air conditioning systems.
Decorative and architectural copper structures.
Nuclear reactor coolant system piping.

Quality Control Procedures

Pre-weld inspection: Verify base metal composition, surface cleanliness, and fit-up quality.
Welding procedure qualification (WPQ): Qualify the welding procedure in accordance with ASME Section IX or ISO 15614-1.
Weld performance qualification (WPQ): Qualify the welder's ability to produce sound welds under production conditions.
In-process monitoring: Monitor current, voltage, travel speed, and gas flow rate during welding.
Post-weld inspection: Perform visual inspection, RT, and mechanical testing as required by the applicable code.

Common Defects and Root Cause Analysis

Defect	Root Cause	Prevention/Correction
Hydrogen porosity	Hydrogen absorption from atmosphere or surface contamination	Preheat; use clean filler wire; improve shielding
Hot cracking	Low melting point Cu-Ag eutectic in grain boundaries	Control silver content; preheat; reduce travel speed
Undercut	Excessive arc force or travel speed	Reduce current; increase travel speed slightly
Excessive reinforcement	Excessive filler wire addition	Reduce filler wire deposition rate
Back-side burn-through	Excessive heat input	Reduce current; increase travel speed; use backing bar
Tungsten inclusion	Tungsten contamination of weld pool	Maintain proper electrode protrusion; avoid arc striking on tungsten

Study Insights and Implications

This paper provides valuable practical guidance for copper TIG welding in field conditions, where access limitations often require single-sided welding. The emphasis on preheating, shielding gas selection, and travel speed control reflects the critical process parameters that must be mastered for successful copper welding.

From a standards perspective, copper welding procedures must comply with applicable codes such as ASME B31.3 for process piping, ASME BPV Code Section VIII for pressure vessels, and NB/T standards for nuclear applications. The welding procedure specification (WPS) should clearly define the preheat temperature, interpass temperature, shielding gas composition and flow rate, and post-weld heat treatment requirements.

The double-sided forming technique described in this paper is particularly useful for applications where the back side of the weld must be visually inspected or where a smooth back-side profile is required for flow or stress distribution reasons. Engineers should note that achieving consistent double-sided forming requires skilled welders and well-controlled welding equipment.