Effect of Preheating on TIG Weldability of Thick Copper Plates

Literature Overview

This 2005 study by Yan Jiuchun and colleagues from Harbin Engineering University and Harbin Jiancheng Group addresses a persistent challenge in copper welding: the extreme thermal conductivity of copper (approximately 390 W/m·K at room temperature) causes severe heat dissipation during TIG welding of thick sections. The authors systematically investigated how preheating temperature influences weldability parameters, defect formation, and final joint quality for thick copper plate joints. This work remains highly relevant for copper-clad pressure vessel fabrication, copper-nickel alloy heat exchanger construction, and bimetallic product manufacturing where copper overlay layers must be welded to structural steels.

Core Technical Findings

The study examined preheating temperatures ranging from room temperature to approximately 500°C for copper plates with thicknesses exceeding 10 mm. Key observations include:

• Without preheating, thick copper plates develop excessive solidification cracking due to rapid cooling rates exceeding 200°C/s in the weld zone
• Preheating above 300°C significantly reduces hydrogen porosity formation by slowing the cooling rate and allowing dissolved hydrogen to escape
• Optimal preheating temperature was identified in the range of 350-450°C, balancing thermal input against grain coarsening in the heat-affected zone
• Multiple passes with interpass temperature control maintained between 250-400°C produced the best combination of mechanical properties and crack resistance

Process Parameters and Defect Analysis

The defect analysis revealed that hot cracking in copper welds follows a transgranular mechanism related to the precipitation of low-melting-point copper-rich phases at grain boundaries during solidification. Preheating reduces the thermal gradient, which in turn decreases the tensile stress developed during solidification contraction.

Engineering Practice Implications

For copper-clad pressure vessel fabrication, this study provides critical guidance for the weld overlay process. When applying copper or copper-nickel overlay layers to carbon steel backing plates, the differential thermal expansion between the two materials creates residual stresses that can be exacerbated by inadequate preheating. The study's findings directly inform:

1. Preheating protocols for multi-layer copper overlay welding on thick steel substrates
2. Selection of filler metals compatible with the thermal cycling conditions identified
3. Quality assurance requirements for hydrogen control during welding of thick copper sections
4. Process qualification procedures under NB/T 47014 for copper-clad pressure vessel fabrication

The interpass temperature control strategy described is particularly important for multi-pass overlay welding where each subsequent pass reheats the previous weld metal, effectively creating a localized annealing effect that can either improve or degrade properties depending on the temperature range.

Study Insights and Reflections

This research highlights a fundamental principle in copper welding that practitioners often overlook: the preheating temperature must be carefully optimized rather than simply maximized. Excessive preheating above 500°C can cause grain coarsening in the base metal, reduce the ultimate tensile strength of the base material, and increase distortion. The optimal window of 350-450°C represents a compromise between crack resistance and property retention that must be validated through both macroscopic and microscopic examination of test coupons. For engineers working on copper-bimetallic pressure vessels, this study reinforces the importance of integrating thermal management into the overall welding procedure specification rather than treating preheating as a mere procedural step.