Zhuojin Pipe Fitting Co., LtdTIG Brazing of Copper-Aluminum Dissimilar Metal Joints: Microstructure and Mechanical Properties
Literature Overview
The paper by Sun Jianxin, Sun Zhen, Xu Wenkai, Zhao Hongli, Hu Ruiling, and Li Zhiqiang (2018), published in Hot Working Technology (热加工工艺), Vol. 47, No. 21, investigates TIG brazing of pure copper to pure aluminum lap joints using Zn-5wt%Al flux-cored wire. Funded by Hubei Provincial Department of Education (Grant No. Q20171801) and Hubei University of Automotive Technology (Grant Nos. SJ201630, 2014XY03), this research addresses the challenging problem of joining copper and aluminum, two metals that form brittle intermetallic compounds when directly fused welded.
Core Technical Findings
The study systematically examines the effect of TIG current on brazing performance, including wetting angle, brazed joint microstructure, and mechanical properties. The key findings are presented below:
| TIG Current | Wetting Angle (Cu side) | Wetting Angle (Al side) | Average Lap Shear Strength |
|---|---|---|---|
| Low current | Larger | Larger | Lower |
| 70 A (Optimal) | Minimum | Minimum | >90 MPa (75% of pure Al tensile strength) |
| High current | Increased | Increased | Decreased |
The optimal TIG current of 70 A produces the best mechanical performance with an average lap shear strength exceeding 90 MPa, which represents approximately 75% of the tensile strength of industrial pure aluminum plate.
Microstructural Analysis
The brazed joint microstructure reveals several important features:
Brazing seam microstructure:
- Primary phase: Zn-Al eutectic structure
- No significant intermetallic compound formation observed in the brazing seam
- Good metallurgical bonding between filler and both base metals
Interface microstructure:
- Cu interface: 3-4 μm thick Zn-rich diffusion layer
- Al interface: 3-4 μm thick Zn-rich diffusion layer
- Uniform diffusion layer thickness on both sides
The formation of Zn-Al eutectic as the primary phase in the brazing seam, without significant intermetallic compound formation, is a critical success factor. Unlike Cu-Al direct fusion welding where brittle CuAl₂ and Cu₅Al₈ intermetallics form, the Zn-based filler metal provides a diffusion-mediated bonding mechanism that avoids these brittle phases.
Wetting Behavior Analysis
The wetting angle analysis reveals a non-linear relationship with TIG current:
- Increasing current (up to 70 A): Wetting angle decreases due to:
- Higher heat input improving filler metal fluidity
- Enhanced oxide removal at the metal surfaces
- Increased capillary action in the lap joint gap
- Exceeding optimal current (>70 A): Wetting angle increases due to:
- Excessive filler metal evaporation (Zn boiling point: 907°C)
- Surface tension changes in the molten filler
- Possible oxide reformation at high temperatures
Process Parameters and Quality Control
For engineering application, the following process parameters are critical:
| Parameter | Recommended Value | Rationale |
|---|---|---|
| TIG Current | 70 A | Optimal wetting and mechanical properties |
| Filler Wire | Zn-5wt%Al flux-cored | Provides self-fluxing and good wettability |
| Joint Configuration | Lap joint | Maximizes bonding area for shear loading |
| Gap Control | Uniform (typical brazing gap) | Ensures consistent capillary action |
| Shielding Gas | Argon | Prevents oxidation of filler and base metals |
Engineering Application Considerations
Copper-aluminum dissimilar metal joints are common in:
- Electrical connections: Busbar systems, switchgear, and power distribution equipment.
- Heat exchangers: Copper-aluminum heat exchange tubes in marine and automotive applications.
- Piping systems: Transition joints between copper and aluminum piping in HVAC systems.
- Aerospace: Lightweight structural joints combining copper conductivity with aluminum lightness.
The 75% strength retention relative to pure aluminum is acceptable for many non-structural applications but must be evaluated against specific design codes and service conditions.
Key Questions and Reflections
Several important technical questions emerge from this research:
- How does the Zn-rich diffusion layer affect long-term corrosion resistance in humid or marine environments?
- What is the electrical contact resistance at the Cu-Zn and Al-Zn interfaces, which is critical for electrical applications?
- How does thermal cycling affect the stability of the diffusion layers and overall joint integrity?
- Can this brazing process be adapted for pipe joint applications with different geometries and wall thicknesses?
- What are the creep resistance characteristics of the joint under sustained load at elevated temperatures?
The study demonstrates that TIG brazing provides a practical solution for copper-aluminum joining that avoids the brittle intermetallic formation inherent in fusion welding. However, the relatively thin diffusion layers (3-4 μm) may be susceptible to degradation under prolonged thermal or mechanical loading, which requires further investigation for critical applications.
Summary and Outlook
This research establishes that Zn-5wt%Al flux-cored wire TIG brazing at 70 A current provides an effective method for joining copper to aluminum with average lap shear strength exceeding 90 MPa (75% of pure aluminum tensile strength). The formation of Zn-Al eutectic without significant intermetallic compounds, combined with uniform 3-4 μm diffusion layers at both interfaces, demonstrates the metallurgical compatibility of this process. For engineers working with dissimilar metal piping and electrical connections, this technology offers a viable alternative to fusion welding, particularly where avoiding brittle intermetallics is critical. Future development should focus on long-term durability testing, thermal cycling qualification, and adaptation to pipe joint geometries with comprehensive evaluation under relevant industry standards such as ASME B31.3 for process piping or ASTM B815 for brazing filler metals.