Cold-Body TIG Surfacing Process and Mechanical Properties of Copper Alloy Sealing Rings

Literature Overview

The paper by Lv Shixiong, Yang Shiqin, Wang Haitao, Xue Chengbo, and Zheng Yonggang, published in Welding (2006, No. 9, pp. 43-46), investigates the application of cold-body TIG (Tungsten Inert Gas) surfacing technology for manufacturing copper alloy sealing rings on steel substrates. The study uses HS201 as the ring body material and evaluates the tensile strength, shear strength, and hardness of the surfacing welds. The research was conducted at the State Key Laboratory of Modern Welding Technology, Harbin Institute of Technology, in collaboration with Heilongjiang Hua'an Industrial Group Co., Ltd.

Core Technical Content

The cold-body TIG surfacing process is a specialized welding technique designed for producing high-quality surfacing deposits with minimal dilution from the base metal. This is particularly important when depositing dissimilar materials, such as copper alloys onto steel substrates, where excessive dilution can degrade the functional properties of the overlay.

Process Characteristics

The cold-body TIG process operates with a lower heat input compared to conventional TIG welding, achieved through:

• Lower welding current settings
• Higher travel speed
• Use of a cold wire feed (if applicable) or pulsed current
• Precise control of arc length and torch angle

This reduced heat input minimizes the melting of the base metal, thereby reducing dilution and preserving the desired composition of the copper alloy overlay.

Mechanical Performance

The study evaluated three key mechanical properties:

1. Tensile strength: The weld joints demonstrated adequate tensile strength, indicating good metallurgical bonding between the copper overlay and steel substrate.
2. Shear strength: Shear testing confirmed that the interface strength was sufficient for sealing ring applications, where the ring is subjected to radial and axial loads.
3. Hardness: The hardness profile across the weld cross-section showed a gradient from the copper alloy overlay through the heat-affected zone to the base metal, with the overlay maintaining its characteristic hardness.

The results confirmed that with appropriate welding parameters, the cold-body TIG process can produce high-quality copper alloy sealing rings that meet the requirements for production on steel substrates.

Engineering Practice Integration

Copper alloy sealing rings are critical components in high-pressure applications, including:

• Hydraulic cylinders and pistons
• Compressor valves and seals
• High-pressure pump glands
• Nuclear reactor cooling system seals
• Aerospace hydraulic systems

The cold-body TIG process offers several advantages for these applications:

• Low dilution: Maintains the corrosion resistance and sealing properties of the copper alloy.
• Precise control: Enables deposition of thin, uniform layers suitable for sealing applications.
• Good weld quality: Produces sound welds with minimal porosity or lack of fusion.
• Scalability: Can be applied to both small components and large assemblies.

Key process parameters that must be optimized include:

1. Welding current (typically 80-150 A for thin overlays)
2. Travel speed (150-300 mm/min depending on layer thickness)
3. Arc length (3-5 mm for stable arc and good penetration)
4. Shielding gas flow rate (8-12 L/min for adequate protection)
5. Torch angle (10-15° from vertical for optimal molten pool control)

Key Technical Insights and Reflections

The successful application of cold-body TIG for copper-on-steel surfacing addresses a significant engineering challenge: the large difference in thermal conductivity and melting point between copper and steel. Copper has approximately 4-5 times the thermal conductivity of carbon steel, which tends to draw heat away from the weld zone and create an asymmetric molten pool. The cold-body TIG process mitigates this issue by:

• Reducing the overall heat input to minimize thermal distortion
• Using a lower current density to achieve a more controlled molten pool
• Allowing for precise control of the weld bead geometry

However, several challenges remain:

• Intermetallic compound formation: At the copper-steel interface, brittle iron-copper intermetallic compounds (such as Fe2Cu, FeCu) can form, which may reduce the ductility and toughness of the joint.
• Residual stress: The mismatch in thermal expansion coefficients between copper and steel can generate significant residual stresses at the interface.
• Porosity: Copper's high affinity for oxygen and hydrogen can lead to porosity if the shielding gas coverage is inadequate.

Study Implications and Outlook

This research demonstrates that the cold-body TIG process is a viable and effective method for producing copper alloy sealing rings on steel substrates. The process offers the precision and low-dilution characteristics required for high-performance sealing applications. For engineers, the key insight is that process parameter optimization is critical to achieving the desired mechanical properties and functional performance. The collaboration between academic research institutions and industrial partners (as exemplified by the Harbin Institute of Technology and Hua'an Industrial Group) highlights the importance of industry-academia partnerships in translating research findings into practical manufacturing solutions. Future work could explore the use of advanced monitoring techniques (such as acoustic emission or infrared thermography) to provide real-time feedback on weld quality, further enhancing the reliability of cold-body TIG surfacing for critical sealing applications.