Argon Arc Welding Process Research for Copper-Titanium Bimetallic Composite Pipes

Literature Overview

This study by Wang Yongfang, Wang Ge, Zhang Yanfei, and Guo Chongxiao, published in Hot Working Technology (2011, Vol. 40, No. 21, pp. 147-150), addresses the challenging problem of welding copper-titanium bimetallic composite pipes. The authors analyze the weldability of copper-titanium interfaces, propose a unique groove configuration and welding process, and comprehensively evaluate the resulting weld joint through radiographic testing, chemical analysis, mechanical testing, metallographic examination, microhardness profiling, and electrochemical corrosion testing. The proposed "TIG root pass + TIG brazing transition + TIG cap pass" process using BAg72Cu silver-based filler metal produced weld joints with excellent composition, microstructure, mechanical properties, and corrosion resistance.

Weldability Analysis of Copper-Titanium Systems

Fundamental Challenges

The welding of copper-titanium bimetallic composite pipes presents several fundamental metallurgical challenges:

The formation of brittle intermetallic compounds at the copper-titanium interface is the most critical concern. These phases, particularly Cu₄Ti₃ and CuTi, are extremely brittle and can cause catastrophic intergranular fracture under mechanical or thermal loading. The high thermal conductivity of copper relative to titanium creates an asymmetric heat flow pattern that further complicates the welding process.

Groove Configuration

The authors proposed a unique groove configuration specifically designed to accommodate the metallurgical differences between copper and titanium:

• The groove was asymmetric, with a wider opening on the copper side and a narrower opening on the titanium side.
• This configuration ensures that the copper side receives more filler metal and experiences greater dilution, while the titanium side is protected from excessive melting.
• The groove angle was optimized at 60° on the copper side and 45° on the titanium side.
• A root gap of 1.0-1.5 mm was maintained to facilitate full penetration.

Welding Process Development

Proposed Welding Sequence

The three-pass welding process was developed as follows:

The selection of BAg72Cu as the filler material is critical to the success of this process. BAg72Cu is a silver-copper brazing alloy containing approximately 72% Ag, 25% Cu, and 3% Ni. The high silver content provides excellent fluidity and wettability for both copper and titanium surfaces, while the copper content ensures good metallurgical compatibility with the copper side of the composite pipe. The nickel addition improves the strength and oxidation resistance of the filler alloy.

Process Parameters

The lower current in the transition pass is intentional: it minimizes melting of the base materials and relies primarily on the brazing action of the BAg72Cu filler to build up material at the interface. This approach effectively prevents excessive intermetallic formation by limiting the time and temperature exposure of the copper-titanium interface.

Weld Joint Characterization

Radiographic Testing

X-ray radiographic testing (RT) was performed on all weld specimens using the appropriate film technique and exposure parameters. The results showed:

• No detectable lack of fusion, porosity, or cracking in any of the weld specimens.
• The root pass exhibited complete penetration with a smooth, uniform reinforcement profile.
• The transition pass showed good wetting and flow of the BAg72Cu filler into the root bead, creating a continuous metallurgical bond.
• The cap pass produced a smooth, convex bead profile with no undercut or spatter.
• The overall weld quality met the acceptance criteria of ASME Section V, Article 2, for radiographic examination.

Chemical Composition Analysis

Chemical analysis of the weld metal at various locations revealed:

The chemical gradient across the weld joint is expected and manageable. The low titanium content in the weld metal (1-4%) is well below the threshold for extensive intermetallic formation, which typically requires Ti concentrations above 5-10% to produce significant embrittlement.

Mechanical Properties

The tensile properties are limited by the titanium side of the joint, as titanium has lower tensile strength than copper. However, the values achieved are adequate for most structural applications involving copper-titanium composite pipes.

Microhardness Distribution

Microhardness measurements across the weld cross-section revealed a characteristic gradient:

The hardness gradient is relatively gradual, without sharp transitions that would indicate brittle intermetallic formation. The slight increase in hardness toward the titanium side is due to the presence of titanium solid solution strengthening in the weld metal.

Electrochemical Corrosion Testing

Electrochemical corrosion testing was performed in a 3.5% NaCl solution to evaluate the corrosion resistance of the weld joint:

The titanium base exhibits the highest corrosion resistance, as expected, due to the formation of a stable passive oxide layer. The weld metal shows moderate corrosion resistance, which is acceptable for most service environments. The copper base is the least corrosion-resistant, but still adequate for non-aggressive environments.

Engineering Practice and Recommendations

This study provides a practical and effective welding solution for copper-titanium bimetallic composite pipes, which find applications in heat exchangers, condenser tubes, and specialized chemical processing equipment. The proposed three-pass process with BAg72Cu filler metal offers a reliable method for producing weld joints with acceptable mechanical and corrosion properties.

Key Engineering Considerations

1. Surface preparation: Both copper and titanium surfaces must be thoroughly cleaned to remove oxide layers, oils, and contaminants. Titanium surfaces should be pickled and passivated before welding.
2. Gas protection: High-purity argon (99.99% minimum) is essential to prevent titanium oxidation. A trailing gas cup should be used to protect the hot weld zone during cooling.
3. Heat input control: Minimizing heat input is critical to prevent excessive intermetallic formation. The three-pass approach with controlled parameters achieves this objective.
4. Post-weld inspection: Both radiographic testing and metallographic examination are recommended to verify weld quality and check for intermetallic formation at the interface.
5. Service temperature limitation: The weld joint should not be used at temperatures above 400°C, where intermetallic formation and titanium oxidation become significant concerns.

Study Insights

This research demonstrates that the welding of dissimilar bimetallic composite pipes, particularly those involving copper and titanium, requires careful process design and material selection. The proposed approach of using a silver-based brazing alloy as a transition material between the TIG root and cap passes is an elegant solution to the metallurgical incompatibility problem. By limiting the direct copper-titanium interaction to a minimum and using a compatible intermediate layer, the process effectively prevents the formation of brittle intermetallic compounds.

The comprehensive characterization approach employed in this study—combining radiographic testing, chemical analysis, mechanical testing, metallographic examination, microhardness profiling, and electrochemical corrosion testing—provides a thorough assessment of weld joint quality. This multi-faceted evaluation approach is recommended for all dissimilar metal welding applications, as no single test method can fully characterize the complex metallurgical and mechanical behavior of the weld joint.

For engineers working with bimetallic composite pipes, this study provides a proven welding process that can be adapted to similar dissimilar metal combinations. The key principles—controlled heat input, appropriate filler material selection, and comprehensive quality assessment—are universally applicable to dissimilar metal welding challenges across various industries.