Zhuojin Pipe Fitting Co., LtdTIG Brazing of Titanium and Copper Dissimilar Material Joints
Literature Overview
This 1991 technical note published in Welding Technology (焊接技术) by Bai Jinsheng and Zhou Jianming from the Tianjin Welding Research Institute documents a practical engineering solution for joining titanium (TA1) to copper (Cu) in a specialized equipment application. The component consists of a titanium container (50 mm diameter, 2 mm wall thickness) with a copper lead-out tube (8 mm diameter, 2 mm wall thickness). The challenge of joining these dissimilar materials through fusion welding is formidable due to their vastly different thermal properties, melting points, and metallurgical compatibility. The authors selected TIG brazing as the optimal joining process after evaluating alternatives.
Metallurgical Challenges of Titanium-Copper Joining
The fusion welding of titanium to copper is essentially impractical for several fundamental reasons. Titanium has a melting point of 1668 °C while copper melts at 1085 °C, creating a 583 °C difference that makes simultaneous melting impossible without excessive heating of the copper side. More critically, titanium and copper form brittle intermetallic compounds (TiCu, Ti₂Cu, TiCu₂) that are hard, brittle, and prone to cracking under thermal cycling. These intermetallics form readily during fusion welding and cannot be avoided through filler metal selection or process parameter adjustment.
| Material Property | Titanium (TA1) | Copper (Cu) | Implication for Joining |
|---|---|---|---|
| Melting Point (°C) | 1668 | 1085 | 583 °C difference |
| Thermal Conductivity (W/m·K) | 21.9 | 398 | Extreme thermal mismatch |
| Coefficient of Thermal Expansion (×10⁻⁶/K) | 8.6 | 16.5 | Residual stress on cooling |
| Oxidation Behavior | Forms TiO₂ above 400 °C | Forms Cu₂O/CuO | Different protection requirements |
| Intermetallic Formation | Ti-Cu intermetallics are brittle | — | Fusion welding impractical |
The thermal conductivity mismatch is particularly severe: copper conducts heat approximately 18 times faster than titanium. During fusion welding, the copper side would absorb and dissipate heat rapidly, making it difficult to achieve proper fusion on the titanium side without overheating the copper. The coefficient of thermal expansion difference would generate substantial residual stresses upon cooling, potentially causing joint failure.
TIG Brazing Process Selection and Implementation
TIG brazing was selected based on the following advantages:
- Low heating temperature (typically 300-400 °C for copper-base brazing fillers)
- Minimal microstructural changes in both base metals
- Capability to join dissimilar materials without intermetallic formation
- Good gas-tightness for containment applications
- Precise heat input control through TIG arc
The brazing process requires careful selection of the filler metal. For titanium-copper joints, copper-base or silver-copper brazing fillers are typically employed, with melting points well below the solidus of both base metals. The brazing temperature must be maintained below 400 °C to prevent titanium oxidation and intermetallic formation. The joint design must ensure proper capillary action for filler metal flow, typically requiring a lap joint or sleeve joint configuration with a controlled gap of 0.05-0.15 mm.
Key process parameters for TIG brazing of Ti-Cu joints include:
- Arc current: 15-30 A (low current to limit heat input)
- Travel speed: 50-100 mm/min
- Shielding gas: Argon or argon-hydrogen mixture (98% Ar + 2% H₂)
- Filler wire: Silver-copper or copper-base brazing alloy
- Joint gap: 0.05-0.15 mm
- Preheat: 100-150 °C to reduce thermal shock
Engineering Practice Considerations
The success of this brazing approach depends on meticulous surface preparation. Both titanium and copper surfaces must be cleaned to bare metal to ensure proper wetting by the brazing filler. Titanium surfaces require acid pickling or mechanical cleaning followed by immediate brazing to prevent re-oxidation. The use of argon-hydrogen shielding gas provides additional deoxidizing capability for the titanium surface during brazing.
The gas-tightness requirement for this application (a container with a lead-out tube) necessitates post-brazing leak testing. Helium leak detection or pressure decay methods should be employed to verify joint integrity. The brazed joint should be capable of withstanding the service pressure and temperature without leakage.
This technical note, while brief, encapsulates a fundamental engineering principle: when fusion welding is metallurgically incompatible, brazing provides a viable alternative that preserves the base metal properties while achieving the required joint integrity. The approach is particularly relevant for specialized equipment where material combinations are dictated by functional requirements rather than joining convenience.