TIG Arc Preheating Assisted Ultrasonic Seam Welding of Aluminum-Copper Dissimilar Metal Joints

Literature Overview

This research published in the Chinese Journal of Mechanical Engineering (2017, Vol. 53, No. 19, pp. 149-153) by Yu Jiang, Wang Bo, and colleagues from Harbin Institute of Technology (Weihai), Shandong Marine Technology Research Institute, and XCMG Group, investigates a novel hybrid approach to joining dissimilar metals—specifically aluminum and copper—using TIG arc preheating combined with ultrasonic seam welding. The study was supported by the Taishan Scholars Engineering Special Project (tsqn20161062). Dissimilar metal joining of aluminum and copper presents a formidable challenge in manufacturing due to the significant differences in thermal conductivity, melting point, and metallurgical compatibility between the two metals. Aluminum melts at 660°C while copper melts at 1085°C, and their direct fusion welding inevitably produces brittle intermetallic compounds (IMCs) such as Al₂Cu, AlCu, and Al₂Cu₃, which severely degrade joint strength and ductility. Ultrasonic seam welding offers a solid-state joining alternative that avoids melting, but conventional ultrasonic welding of aluminum-copper joints faces challenges related to insufficient plastic flow at the interface and poor bonding quality.

Technical Mechanism and Process Analysis

The proposed process combines two distinct energy inputs in a synergistic manner: TIG arc preheating softens the workpieces to reduce the force required for ultrasonic welding and promote plastic flow at the interface, while the ultrasonic vibration provides the localized frictional heating and mechanical agitation necessary for clean, oxide-free bonding. The process sequence involves applying a TIG arc to the joint region to raise the temperature to a controlled level, followed by ultrasonic seam welding at high frequency vibration.

Process Parameters and Their Effects

The study systematically investigated the effect of TIG current on weld quality, using 1 mm thick aluminum and copper sheets with a pure aluminum interlayer. The interlayer serves as a diffusion barrier to limit the formation of intermetallic compounds at the aluminum-copper interface.

The results demonstrate a clear threshold effect: below a critical TIG current, the preheating is insufficient to promote adequate plastic flow, resulting in poor bonding with visible non-bonded regions at the interface. At 30 A, the combination of arc heat and high-efficiency ultrasonic vibration energy produces sufficient plastic deformation at the interface to achieve intimate bonding with characteristic interlocking morphology. The maximum interface temperature of 446°C is well below the melting point of both aluminum (660°C) and copper (1085°C), confirming that the process operates in a solid-state regime.

Interface Microstructure and Metallurgical Analysis

Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) analysis of the bonded interface revealed several important microstructural features:

1. Absence of intermetallic compounds: At the optimal TIG current of 30 A, EDS analysis confirmed that no intermetallic compounds formed at the aluminum-copper interface. This is a critical finding because IMCs are the primary cause of joint failure in aluminum-copper welded joints. The pure aluminum interlayer acts as a diffusion barrier, and the limited temperature (446°C) is insufficient to drive significant interdiffusion over the short welding time.
2. Interlocking morphology: The interface exhibited "interlocking" and "continuous protrusion" features, indicating that plastic flow was sufficient to create mechanical interlocking between the two metals. This mechanical interlocking contributes significantly to joint strength, as evidenced by the fracture occurring in the copper base material rather than at the interface.
3. Oxide removal: The ultrasonic vibration effectively removed oxide films from the bonding surfaces through the mechanical agitation effect. This is essential for achieving metallurgical bonding, as aluminum oxide (Al₂O₃) is extremely stable and would otherwise prevent intimate contact between the two metals.

Effect of TIG Current on Interface Temperature

The relationship between TIG current and interface temperature is nonlinear, reflecting the complex heat transfer dynamics at the joint. As TIG current increases, the arc heat input rises, raising the interface temperature. However, the temperature increase is moderated by several factors: the high thermal conductivity of copper rapidly conducts heat away from the interface; the ultrasonic vibration promotes convective heat transfer within the molten or semi-solid material; and the short welding duration limits the time available for heat accumulation. The maximum interface temperature of 446°C at 30 A represents an optimal balance between sufficient plastic flow for bonding and avoidance of excessive temperature that could promote IMC formation or aluminum melting.

Engineering Practice Implications

The TIG arc preheating assisted ultrasonic seam welding process offers several practical advantages for aluminum-copper dissimilar metal joining:

1. Application domains: This process is particularly relevant for marine engineering (where aluminum hulls may require copper fastening or copper-containing components), electrical engineering (where aluminum conductors may be joined to copper busbars or terminals), and automotive applications (where lightweight aluminum structures may require copper electrical connections). The ability to achieve strong, IMC-free joints without melting makes this process attractive for applications where thermal distortion must be minimized.
2. Process control: The TIG current serves as the primary control parameter for process quality. Operators must carefully calibrate the TIG current to achieve the target interface temperature. Excessive current leads to aluminum melting and IMC formation, while insufficient current results in poor bonding. The use of a temperature monitoring system or pyrometer during production welding is recommended to ensure consistent quality.
3. Interlayer design: The pure aluminum interlayer is a critical process feature that must be carefully designed and applied. The interlayer thickness should be sufficient to act as a diffusion barrier but thin enough to avoid introducing excessive joint thickness. In practice, interlayer thicknesses of 0.1-0.5 mm are typical for this application.
4. Quality assurance: Joint quality should be verified through lap shear testing, microstructural examination (SEM/EDS) to confirm the absence of IMCs, and non-destructive testing (UT or acoustic emission) to detect bonding defects. The fracture location (in the copper base material rather than at the interface) serves as a positive quality indicator, indicating that the joint strength exceeds the base material strength.
5. Limitations and challenges: The process is currently demonstrated for thin sheets (1 mm) and may face challenges when applied to thicker materials due to increased thermal mass and reduced temperature uniformity. The ultrasonic welding equipment requires careful maintenance to ensure consistent vibration amplitude, and the TIG arc setup requires precise alignment with the ultrasonic horn.

Study Insights and Outlook

This research presents an innovative solution to the longstanding challenge of aluminum-copper dissimilar metal joining by combining arc preheating with ultrasonic welding in a synergistic hybrid process. The key insight is that the TIG arc does not serve as the primary welding energy source but rather as a preheating tool that reduces the force requirement and promotes plastic flow for the ultrasonic welding process. The maximum interface temperature of 446°C, well below the melting points of both metals, confirms the solid-state nature of the bonding and explains the absence of intermetallic compounds. The achievement of 2.58 kN lap shear strength with fracture in the copper base material demonstrates that the joint achieves strength comparable to or exceeding the base material. This hybrid approach represents a promising direction for dissimilar metal joining in industries where thermal distortion, IMC formation, and joint reliability are critical concerns.