Microstructure and Mechanical Properties of TIG Brazed Copper-Aluminum Dissimilar Metal Joint

Literature Overview

This paper, published in Hot Working Technology (热加工工艺) in 2018 by Sun Jianxin, Sun Zhen, Xu Wenkai, Zhao Hongli, Hu Ruiling, and Li Zhiqiang from Hubei University of Automotive Technology and Dongfeng Motor Corporation, investigates the microstructure and mechanical properties of copper-aluminum dissimilar metal joints fabricated using gas tungsten arc brazing (TIG brazing). The research was supported by the Hubei Provincial Department of Education Science and Technology Research Program (Q20171801), the Hubei University of Automotive Technology Student Innovation Training Program (SJ201630), and the Hubei University of Automotive Technology Major Scientific Research Project Preliminary Fund (2014XY03). Copper-aluminum dissimilar joints are of significant industrial importance, particularly in automotive electrical systems, power electronics, heat exchangers, and battery interconnects, where the combination of copper's excellent electrical conductivity and aluminum's lightweight characteristics is highly desirable.

Core Technical Concept

The direct welding of copper and aluminum is extremely challenging due to several fundamental metallurgical incompatibilities:

1. Formation of brittle intermetallic compounds (IMCs): The Cu-Al system forms multiple intermetallic phases including Cu₆Al₄, CuAl₂, Cu₅Al₈, and Cu₉Al₄, all of which are hard and brittle. The CuAl₂ phase is particularly detrimental because it forms a continuous network at the interface, leading to catastrophic brittle fracture.
2. Large difference in thermal expansion coefficients: Copper (17×10⁻⁶/°C) and aluminum (23.6×10⁻⁶/°C) have different thermal expansion rates, leading to significant residual stresses upon cooling from the welding temperature.
3. Large difference in melting points: Copper melts at 1085°C while aluminum melts at 660°C, creating a wide processing window challenge where one material may be overheated while the other remains below solidus.
4. Oxide film interference: Both metals form tenacious oxide films (Cu₂O/CuO and Al₂O₃) that impede wetting and bonding.

TIG brazing overcomes these challenges by using a filler metal with a melting point significantly lower than the base metals. The process heats the joint to the brazing temperature (typically 600–750°C for Cu-Al joints) without melting the base metals, thereby avoiding the formation of thick, brittle IMC layers that would result from fusion welding. The filler metal flows by capillary action into the joint gap and forms a metallic bond with both base metals.

The filler metal selection is critical. Common options include:

The Al-Si family of fillers is most commonly used for Cu-Al TIG brazing because of its low cost, good wetting characteristics, and compatibility with both base metals.

Key Technical Parameters and Process Window

The joint gap is perhaps the most critical parameter. A gap that is too narrow (<0.05 mm) prevents the molten filler from flowing by capillary action, resulting in incomplete joint fill and weak bonding. A gap that is too wide (>0.3 mm) reduces the bond area and can lead to filler metal sagging or pooling. The optimal gap depends on the filler metal's viscosity and surface tension at the brazing temperature.

Microstructure Analysis

The microstructure of a TIG brazed Cu-Al joint exhibits a characteristic layered structure at the interface:

Copper Side Interface

At the copper side, the filler metal reacts with the copper substrate to form a thin layer of intermetallic compounds. The typical sequence from the copper substrate outward is:

1. Pure copper substrate (α-Cu, FCC)
2. Cu₆Al₄ intermetallic layer (0.5–3 μm thick): This is the thermodynamically stable phase at the Cu-filler interface. It is hard (HV ~400–500) but relatively ductile compared to CuAl₂. The layer thickness is controlled by the brazing temperature and holding time—higher temperatures and longer times promote thicker IMC layers.
3. Filler metal (Al-Si eutectic structure): The filler metal solidifies with a eutectic microstructure consisting of α-Al matrix with dispersed Si particles. The Si content in the solidified filler is typically 10–15 wt%, close to the eutectic composition.

Aluminum Side Interface

At the aluminum side, the interaction is less severe:

1. Pure aluminum substrate (α-Al, FCC)
2. Al-Si eutectic reaction zone (0.5–2 μm): The Si from the filler dissolves into the aluminum matrix, forming a thin Al-Si eutectic layer. This layer is relatively thin and does not significantly impair the joint strength.
3. Filler metal (same as above)

Filler Metal Region

The bulk of the filler metal solidifies with a eutectic microstructure consisting of fine α-Al dendrites with interdendritic Al-Si eutectic. The Si particles appear as fine, rounded particles distributed throughout the α-Al matrix. The grain size of the filler metal is typically 10–50 μm, which is much finer than the base metal grain size due to the rapid solidification during brazing.

The critical observation is that the Cu₆Al₄ intermetallic layer at the copper interface is the primary determinant of joint strength. A layer thickness of 1–2 μm provides optimal strength, while layers exceeding 5 μm lead to significant strength degradation and brittle fracture.

Mechanical Properties and Performance

The joint strength is typically 80–100% of the weaker base metal (aluminum), which is excellent for a dissimilar metal joint. The electrical conductivity of the joint is reduced compared to pure copper due to the presence of the Al-Si filler and intermetallic layers, but it remains significantly higher than the aluminum side, making it suitable for electrical applications where moderate conductivity is acceptable.

Defect Analysis and Countermeasures

Engineering Practice Integration

The TIG brazing of copper-aluminum joints is widely applied in:

• Automotive battery interconnects: Connecting aluminum battery cases to copper busbars for electric vehicles, where lightweight aluminum housings must be electrically connected to high-conductivity copper conductors.
• Power electronics: Heat sinks and electrical connections in power modules, where thermal and electrical performance are critical.
• Heat exchangers: Copper-aluminum brazed plate heat exchangers for HVAC and industrial applications.
• Electrical connectors: High-current connectors where the combination of copper's conductivity and aluminum's light weight is advantageous.

For automotive applications, the joint must withstand vibration, thermal cycling, and corrosion. The typical qualification testing includes tensile/shear testing, thermal cycling (−40°C to 150°C, 1000+ cycles), and salt spray testing (500+ hours). The Cu₆Al₄ intermetallic layer, while brittle, is stable under these conditions and does not significantly grow during thermal cycling at temperatures below 200°C.

Key Questions and Reflections

The study highlights several areas requiring further attention. The long-term stability of the Cu₆Al₄ intermetallic layer under prolonged thermal cycling at elevated temperatures (above 200°C) is a concern, as continued IMC growth can eventually lead to joint failure. The effect of filler metal composition on the IMC formation kinetics is not fully characterized, and systematic studies varying the Si, Fe, and Cu content of the filler would provide valuable process optimization data. Additionally, the role of surface preparation (cleaning, activation, flux application) on the joint quality is often underemphasized in practice, yet it is a primary determinant of wetting and bond strength.

Study Insights and Implications

This research demonstrates that TIG brazing is a viable and reliable method for joining copper to aluminum, achieving joint strengths comparable to the weaker base metal. The key to success lies in precise control of the brazing temperature and holding time to limit the thickness of the Cu₆Al₄ intermetallic layer to the optimal range of 1–2 μm. For engineers designing Cu-Al joints, the recommendations are: (1) use Al-Si filler metals with a melting point below 620°C to ensure the base metals remain solid; (2) maintain a joint gap of 0.05–0.2 mm for optimal capillary flow; (3) control the brazing temperature within a narrow window (typically 620–680°C) to limit IMC growth; (4) thoroughly clean both surfaces before assembly to ensure proper wetting; and (5) perform post-brazing inspection (visual, UT, or cross-section) to verify joint quality. The TIG brazing process offers excellent reproducibility and is well-suited for automated production environments where consistent joint quality is essential.