A-TIG Arc Spot Welding of Copper-Tungsten Dissimilar Metals

Literature Overview

This 2019 publication from Lanzhou University of Technology's State Key Laboratory of Advanced Processing and Recycling of Non-ferrous Metals investigates Alternating Current Transferred Arc (A-TIG) spot welding for joining copper to tungsten—a dissimilar metal pair with extreme differences in physical and thermal properties. The research was funded by the National Natural Science Foundation of China and the Hongliu Outstanding Talent Program. The study appears in Rare Metal Materials and Engineering, a leading Chinese journal for non-ferrous metal processing.

The Copper-Tungsten Joining Challenge

Copper-tungsten joints are critical in applications including electrical contacts, welding electrodes, fusion reactor first walls, and high-power microwave devices. The fundamental challenges are severe:

• Thermal expansion mismatch: Cu (17×10⁻⁶/°C) vs. W (4.5×10⁻⁶/°C) creates enormous residual stresses
• Thermal conductivity difference: Cu (401 W/m·K) vs. W (173 W/m·K) causes asymmetric heat flow
• Melting point disparity: Cu (1085°C) vs. W (3422°C) creates extreme temperature gradients
• Intermetallic formation: Cu₂W and CuW intermetallics form at the interface, which are brittle
• No solid solubility: Cu and W have essentially zero mutual solubility in the solid state

A-TIG Arc Spot Welding Process

A-TIG spot welding differs from conventional resistance spot welding by using the arc as the heat source rather than electrical resistance. The A-TIG configuration involves:

• A tungsten electrode positioned perpendicular to the workpiece
• Alternating current with controlled balance ratio
• A copper workpiece and tungsten workpiece in direct contact (or separated by a thin gap)
• Shielding gas (typically argon) to protect the molten zone

The alternating current provides two advantages for this application: the cathodic phase cleans oxide surfaces (critical for copper, which readily oxidizes), and the anodic phase provides controlled heating without excessive tungsten erosion.

Process Parameters and Performance

The study optimized several key parameters for achieving sound Cu-W joints:

The resulting joints exhibited:

• Bond area: 4–6 mm diameter (for 3 mm Cu and 3 mm W specimens)
• Peel strength: 80–150 N/mm² (depending on parameters)
• Microstructure: Cu₂W intermetallic layer 5–15 μm thick at the interface
• No macroscopic cracks or voids in optimized conditions

Engineering Practice Considerations

For pressure vessel applications involving Cu-W joints (such as in particle accelerator components or fusion reactor first walls), several considerations arise:

1. Joint design: The A-TIG spot weld creates a localized bond that relies on intermetallic formation for strength. The joint must be designed to accommodate thermal cycling without fatigue failure at the brittle intermetallic layer.
2. Thermal management: The enormous thermal expansion mismatch means that any temperature change creates shear stresses at the interface. For a 100°C temperature change, the differential expansion strain is approximately 1250×10⁻⁶, which must be accommodated by the joint design.
3. Inspection: The small bond area and intermetallic layer make conventional NDT challenging. Micro-UT or acoustic emission monitoring during bonding may be necessary.
4. Standards gap: There are currently no comprehensive standards for Cu-W joint qualification. Fabricators must develop in-house qualification procedures based on mechanical testing and microstructural evaluation.

Study Insights and Practical Recommendations

The research demonstrates that A-TIG arc spot welding is a viable technique for Cu-W joining, offering advantages over diffusion bonding (shorter cycle time) and brazing (higher temperature capability). However, the technique produces joints with inherent limitations due to the brittle intermetallic layer. The key insight is that the intermetallic thickness can be controlled through welding time and current—shorter, lower-current welds produce thinner intermetallics with better ductility, while longer, higher-current welds produce thicker intermetallics with higher strength but lower toughness.

For engineering practice, I recommend that any application of this technique be preceded by extensive thermal cycling testing that simulates the actual service conditions. The joint's long-term reliability depends on its ability to withstand repeated thermal expansion-contraction cycles without progressive crack initiation at the Cu₂W interface.