Effect of Preheating on TIG Weldability of Thick Copper Plates

Literature Overview

This 2005 paper by Yan Jiuchun and colleagues from Harbin Engineering University and Harbin Jiancheng Group provides a systematic investigation into the influence of preheating temperature on the TIG welding of thick copper plates. The study addresses the well-known challenge of hot cracking susceptibility in pure copper (red copper) welding, examining how preheating modifies microstructure, mechanical properties, and introduces secondary problems such as surface oxidation and grain coarsening.

Research Methodology and Experimental Design

The authors conducted TIG welding experiments on thick copper plates with varying preheating temperatures, systematically evaluating the weld joint characteristics at each temperature level. The experimental matrix included preheating temperatures ranging from ambient to high preheat levels, with corresponding post-weld examinations of microstructure, mechanical properties, and surface condition.

Experimental Parameters and Results

Preheat Temperature	Cracking Susceptibility	Tensile Strength	Surface Oxidation	Grain Size
No preheat (room temp)	High	Lower	Minimal	Fine
Moderate preheat (~200–300°C)	Reduced	Improved	Moderate	Slightly coarsened
High preheat (~400–600°C)	Low	Highest	Severe	Significantly coarsened

Core Findings and Technical Analysis

The study identified three principal effects of preheating on copper plate TIG welding:

Reduction of crystallization cracking tendency: Preheating lowers the cooling rate in the weld zone, extending the time available for liquid film healing at grain boundaries during solidification. This is the primary benefit of preheating in copper welding, as pure copper has a very narrow solidification range and is highly susceptible to hot cracking.
Improvement of tensile strength: The reduced cooling rate allows for more uniform solidification and reduced residual stress, contributing to improved joint strength. The tensile strength of the welded joint approaches that of the base metal at optimal preheat temperatures.
Introduction of secondary problems: At high preheat temperatures, the following adverse effects become pronounced:

Severe surface oxidation due to prolonged exposure of hot copper to atmospheric oxygen
Increased welding deformation and distortion
Significant grain growth in the base metal, heat-affected zone, and weld metal

Detailed Microstructural Analysis

The grain coarsening observed at high preheat temperatures is particularly concerning from a fatigue and creep resistance perspective. In the base metal and HAZ, the original fine-grained structure undergoes recrystallization and grain growth during the preheating cycle, resulting in grain sizes that can exceed the original by factors of 3 to 5. In the weld metal, the elevated starting temperature reduces the thermal gradient during solidification, promoting columnar grain growth and reducing the fraction of equiaxed grains.

The surface oxidation problem is especially problematic for copper because copper oxide (Cu₂O and CuO) has poor adhesion to the copper substrate and can spall during mechanical processing or thermal cycling. This creates surface defects that may act as stress concentration sites and corrosion initiation points.

Engineering Practice Integration

For practical application in copper pipe and fitting fabrication, the following recommendations emerge from this study:

Optimal preheat temperature window: A preheat temperature of approximately 200–300°C provides the best balance between crack suppression and avoidance of excessive grain growth and oxidation.
Oxygen protection enhancement: When preheating is applied, the shielding gas coverage must be augmented, particularly at the trailing edge of the weld pool where hot copper is exposed to atmospheric oxygen during cooling.
Post-weld cleaning procedures: A systematic post-weld cleaning protocol should be established to remove oxide layers, including mechanical brushing followed by chemical pickling with appropriate copper cleaning solutions.
Deformation control: For thick copper plates, preheating should be combined with proper fixture design and clamping strategy to minimize thermal distortion.

FMEA Analysis of Preheating-Related Risks

Failure Mode	Severity	Occurrence	Detection	RPN	Mitigation
Excessive grain growth	8	6	7	336	Limit preheat to ≤300°C
Surface oxidation	6	7	5	210	Enhanced gas shielding
Residual cracking	9	3	6	162	Adequate preheat application
Weld distortion	5	6	4	120	Fixture design optimization

Study Insights and Implications

This paper offers a valuable lesson in the principle of metallurgical trade-offs. Preheating is not an unconditionally beneficial practice; it introduces a spectrum of competing effects that must be carefully balanced for each specific application. The finding that high preheat temperatures cause severe grain coarsening in both the base metal and weld metal has direct implications for the long-term performance of copper components in service, particularly in applications subject to cyclic loading or elevated temperature exposure.

For engineers working with copper pipes, busbars, heat exchangers, and electrical conductors, this study reinforces the importance of establishing validated preheating protocols rather than relying on generic recommendations. The optimal preheat temperature depends on plate thickness, joint geometry, and the specific requirements of the final application. A systematic approach involving test welding, microstructural evaluation, and mechanical property verification is essential before committing to production parameters.

The paper also highlights the importance of considering the complete thermal cycle rather than focusing solely on the welding parameters. The preheating phase represents a significant portion of the total thermal history experienced by the component, and its effects on microstructure and properties must be evaluated with the same rigor as the welding parameters themselves.