CLADDING TECHNOLOGY SHANXI CO., LTDTIG Welding of Copper Heat Exchanger Tubes to Stainless Steel Tube Sheets
Literature Overview
This 2009 study published in Electric Welding, authored by Huang Renguo, Zhang Guodong, Ren Shihong, and Zhang Jianxiao, addresses the practical challenges of TIG welding pure copper heat exchanger tubes to stainless steel tube sheets. The research involves collaboration between Wuhan University, Liuzhou Wuling Automobile United Development Co., Ltd., and Lanzhou Lanchi Machinery Equipment Co., Ltd. The topic is directly relevant to the fabrication of copper-alloy clad or copper-bonded heat exchangers used in chemical processing, power generation, and marine applications where thermal conductivity and corrosion resistance must be balanced.
Technical Challenges of Dissimilar Metal TIG Welding
The welding of copper to stainless steel presents a unique set of metallurgical challenges that distinguish it from more common dissimilar metal combinations such as stainless steel to carbon steel. The primary difficulties arise from the vastly different thermal conductivities of the two materials: copper has a thermal conductivity approximately five times that of austenitic stainless steel, which results in an asymmetric weld pool with the majority of heat being conducted away from the copper side.
| Property | Pure Copper (T2) | Stainless Steel (304/316L) |
|---|---|---|
| Thermal conductivity (W/m·K) | ~398 | ~16 |
| Coefficient of thermal expansion (μm/m·K) | ~17 | ~17 |
| Melting point (°C) | 1083 | 1450 |
| Electrical resistivity (μΩ·m) | 17.2 | 72 |
The thermal asymmetry means that without proper process control, the weld metal tends to flow toward the copper side, leaving the stainless steel side underpenetrated. Furthermore, the formation of brittle intermetallic compounds such as Cu-Zn, Cu-Fe, and Cu-Ni phases at the weld interface can severely compromise mechanical properties and corrosion resistance. The study examines filler metal selection, preheat strategies, and shielding gas composition as key levers for achieving sound welds.
Process Parameters and Weld Quality
The research identifies several critical process parameters for successful copper-to-stainless-steel TIG welding. The use of a silver-bearing filler metal or a nickel-based filler alloy is recommended to promote wetting on both sides and to dilute the formation of brittle intermetallics. Shielding gas flow rates must be increased on the copper side due to its higher thermal conductivity causing greater convective loss of the protective atmosphere. Back purging with argon or a mixture of argon and helium is essential to prevent oxidation of the copper root surface.
| Parameter | Recommended Value | Rationale |
|---|---|---|
| TIG current | 120-180 A | Sufficient for both materials |
| Travel speed | 4-8 cm/min | Slower for copper side penetration |
| Shielding gas flow | 15-25 L/min | Compensate for copper thermal sink |
| Back purge flow | 10-15 L/min | Prevent root oxidation |
| Preheat temperature | 150-250 °C | Reduce thermal gradient |
The study also discusses the importance of joint design, recommending a slight bevel on the copper tube end to compensate for the asymmetric melt flow. The weld geometry should be designed to ensure adequate fusion on both the copper and stainless steel sides, with the weld bead slightly offset toward the stainless steel side to compensate for the thermal asymmetry.
Engineering Practice and Quality Control
In heat exchanger fabrication, the copper-to-stainless-steel tube-to-tubesheet joint is a critical weld that must withstand thermal cycling, pressure, and potential corrosive media. Non-destructive testing of these joints typically involves visual inspection (VT), dye penetrant testing (PT) for surface defects, and ultrasonic testing (UT) for subsurface defects at the tube-to-tubesheet interface. For pressure-containing applications governed by GB/T 151 or ASME VIII Div. 1, the weld must demonstrate adequate bond strength and resistance to stress corrosion cracking under operating conditions.
The mechanical properties of the weld joint are particularly sensitive to the composition of the fusion zone. Intermetallic compounds formed during welding can reduce tensile strength by 30-50% compared to the base metal and can serve as initiation sites for corrosion. The study emphasizes the importance of post-weld inspection through metallographic examination of cross-sections to verify the absence of excessive intermetallic formation and to confirm full penetration on both sides of the joint.
Study Insights and Practical Recommendations
This research provides valuable guidance for practitioners who must fabricate copper-to-stainless-steel joints in heat exchangers and similar equipment. The emphasis on thermal management through preheat and joint design is particularly important, as these measures can significantly improve weld quality without requiring exotic filler metals or specialized equipment. The practical recommendations regarding shielding gas flow rates and back purging are directly applicable to shop-floor welding procedures. However, the study could benefit from additional data on long-term corrosion performance of the weld joints under aggressive chemical environments, which would be essential for applications in the chemical and petrochemical industries where such joints are commonly employed.