Cold-Body TIG Overlay Welding Process and Mechanical Properties of Copper Alloy Seal Rings on Steel Substrate

Literature Overview and Application Context

This study published in Welding (2006, No. 9, pp. 43-46) by Lv Shixiong, Yang Shiqin, Wang Haitao, Xue Chengbo, and Zheng Yonggang from Harbin Institute of Technology and Heilongjiang Hua'an Industry Group addresses a specialized manufacturing challenge: the production of copper alloy seal rings on steel substrates using cold-body tungsten inert gas (TIG) overlay welding. Copper seal rings are critical components in high-pressure hydraulic systems, steam turbines, and nuclear reactor coolant systems, where they provide a reliable seal between mating surfaces under extreme pressure and temperature conditions. The traditional approach of brazing or friction welding copper rings onto steel substrates has limitations in terms of joint strength and thermal compatibility. The cold-body TIG overlay welding method investigated here offers an alternative that can produce metallurgical bonds with improved mechanical properties.

HS201 Copper Alloy as the Seal Ring Material

HS201 is a copper-silver alloy known for its excellent electrical conductivity, thermal conductivity, and resistance to erosion corrosion. In the context of seal ring applications, HS201 provides the necessary combination of ductility, thermal stability, and sealing performance. The alloy composition typically includes approximately 0.5-1.0% silver in a copper matrix, with trace amounts of other elements for microstructural refinement.

Cold-Body TIG Overlay Welding Process Analysis

Principle and Advantages of Cold-Body TIG Welding

Cold-body TIG welding, also referred to as cold wire TIG or cold metal transfer TIG, differs from conventional TIG welding in that the filler wire is fed into the weld pool without being heated by the arc. Instead, the wire is mechanically advanced into the molten pool at the arc-cathode or arc-anode position, where it melts due to the thermal energy of the pool. This technique offers several advantages:

• Reduced heat input: Since the arc does not directly heat the filler wire, the overall heat input to the joint is lower compared to conventional TIG welding.
• Better control of dilution: The cold wire melts gradually in the pool, allowing more precise control of the dilution ratio between the filler metal and the base metal.
• Reduced distortion: Lower heat input results in less thermal distortion of the base material.
• Higher deposition rate: The cold wire can be fed at higher rates than the arc-heated wire, increasing productivity.

Process Parameters and Optimization

The study investigated the following process parameters for the cold-body TIG overlay welding of HS201 copper alloy onto steel substrate:

The optimization of these parameters is critical for achieving a high-quality overlay with minimal dilution, no defects, and adequate mechanical properties. The dilution ratio, defined as the percentage of base metal in the deposited metal, is a key quality indicator. For copper seal rings, a dilution ratio of 5-15% is generally acceptable, as excessive dilution would alter the copper alloy properties and compromise the sealing performance.

Mechanical Property Testing and Results

Tensile Testing

The tensile tests were conducted on specimens prepared from the overlay-welded seal rings. The tensile strength of the HS201 overlay on steel substrate was evaluated to determine whether the joint could withstand the operational loads in hydraulic and steam turbine applications. The results indicated that with appropriate process parameters, the tensile strength of the overlay joint was satisfactory, meeting the requirements for the intended service conditions.

Shear Testing

Shear testing is particularly relevant for seal ring applications, as the primary loading mode in a seal is shear stress at the interface between the copper ring and the steel housing. The shear strength of the overlay weld joint was measured to ensure that the bond could resist the forces generated during pressure cycling and thermal expansion. The results demonstrated that the cold-body TIG process produced joints with adequate shear strength for practical applications.

Hardness Testing

Hardness measurements were performed across the weld cross-section to characterize the hardness profile from the overlay surface to the base metal. The hardness of the HS201 overlay layer was expected to be lower than the steel substrate, reflecting the inherent softness of copper alloys. The hardness gradient across the weld zone provides information about the dilution and metallurgical bonding quality. A smooth hardness transition indicates good metallurgical bonding, while abrupt hardness changes may indicate incomplete fusion or interfacial defects.

Engineering Practice and Quality Assurance

Application-Specific Considerations

The cold-body TIG overlay welding of copper seal rings on steel substrates addresses specific challenges in the manufacturing of high-pressure seals:

• Thermal management: In applications such as steam turbine seals, the copper ring must accommodate thermal expansion differences between copper and steel during startup and shutdown cycles. The metallurgical bond provided by overlay welding, combined with the ductility of the copper alloy, allows for controlled deformation without cracking.
• Pressure resistance: The seal must maintain its integrity under pressures ranging from 5 to 25 MPa. The tensile and shear properties of the overlay joint directly determine the pressure rating of the seal.
• Corrosion resistance: In some applications, the seal environment may contain corrosive media. The copper-silver alloy provides good resistance to many corrosive environments, but the steel substrate must also be protected, either by the overlay itself or by additional coatings.

Non-Destructive Testing Requirements

For critical applications such as nuclear reactor seals or high-pressure hydraulic seals, the following non-destructive testing (NDT) methods are recommended:

• Visual inspection (VT): Surface examination for porosity, undercut, spatter, and other visible defects.
• Ultrasonic testing (UT): Detection of internal defects such as lack of fusion, cracks, and inclusions at the overlay-base metal interface.
• Dye penetrant testing (PT): Detection of surface-breaking defects such as cracks and porosity.
• Radiographic testing (RT): For thick sections where UT may be less effective, RT can reveal volumetric defects.

Study Insights and Independent Reflections

The research by Lv Shixiong and colleagues represents a practical contribution to the field of dissimilar metal joining, specifically addressing the challenge of bonding copper alloys to steel substrates in seal ring applications. The cold-body TIG welding technique offers a compelling alternative to traditional brazing methods, providing a true metallurgical bond with superior mechanical properties. The lower heat input characteristic of this process is particularly advantageous for thin-walled components where distortion control is critical.

From a metallurgical perspective, the cold-body TIG process creates a unique weld microstructure. The gradual melting of the cold wire in the pool results in a more uniform composition and microstructure compared to conventional TIG welding, where the arc-heated wire can lead to segregation and compositional variations. This uniformity is beneficial for the mechanical properties of the overlay layer, as it reduces the likelihood of localized weak spots.

However, the study's scope is limited in several respects. The mechanical property data presented focuses on static tensile and shear tests, without addressing fatigue behavior or thermal cycling performance. In practical seal applications, the component undergoes millions of pressure cycles and thermal cycles during its service life, and the long-term durability of the overlay weld joint under these conditions remains an open question. Additionally, the study does not address the effect of welding sequence on multi-pass overlay configurations, which is relevant for thicker seal rings.

The practical applicability of this technology is constrained by the need for specialized cold-body TIG welding equipment, which is not universally available in manufacturing environments. The investment in such equipment must be justified by the volume of production and the criticality of the application. For high-value components such as nuclear reactor seals or aerospace hydraulic seals, the investment is readily justified, but for lower-value industrial applications, alternative methods may be more economically viable.

This study provides a valuable foundation for the development of cold-body TIG welding as a manufacturing technology for dissimilar metal joints. Future research should expand the scope to include fatigue testing, thermal cycling testing, and long-term creep behavior under combined mechanical and thermal loading. The integration of this technology into existing manufacturing processes will require the development of standardized welding procedure specifications and qualification protocols, which can serve as the basis for quality assurance in industrial applications.