Why the Al-Cu Interface Is a Critical Bottleneck in EV Battery Assembly

The aluminum-copper joint is the single most electrically sensitive connection in a battery electric vehicle pack. Aluminum and copper are the two dominant conductor materials in battery modules, and connecting them introduces a well-documented set of challenges: large differences in melting point, thermal conductivity, and electrical conductivity, combined with a thermodynamic tendency to form brittle intermetallic compounds (IMCs) such as CuAl₂ and Cu₉Al₄ at the joint interface. When these phases accumulate, both mechanical strength and electrical conductivity at the joint degrade—raising resistance, generating heat, and shortening pack service life.

The transition from internal combustion engines to battery electric vehicles has intensified demand for reliable, low-resistance electrical connections between battery cells and modules. As documented in a 2023 review from the Shanghai Collaborative Innovation Center of Laser Advanced Manufacturing Technology, when aluminum and copper are melted together, “more intermetallic compounds are formed, some of which can affect the microstructure, electrical and thermal properties of the joint.” This constraint makes solid-state joining processes—those that never bring either metal to its melting point—intrinsically attractive for the Al-Cu busbar connection.

The dataset analysed here spans more than a dozen directly relevant sources, including patents from Korea, the United States, Germany, and Japan, as well as peer-reviewed papers from institutions including Technische Universität Ilmenau, Volkswagen AG, the University of Naples Federico II, the University of Warwick, and Volvo Car Corporation. Across this body of work, friction stir welding and its derivative processes—friction stir spot welding (FSSW) and hybrid friction diffusion bonding (HFDB)—emerge as the most technically nuanced solid-state approach for Al-Cu busbar joining.

How FSW Suppresses Intermetallics and Enables Cross-Section Engineering

Friction stir welding resolves the IMC problem by keeping both materials in a plasticized, sub-solidus state throughout the joining cycle. A rotating tool with a shoulder and pin generates frictional heat, softens the workpiece material, and mechanically intermixes the two metals along the weld seam—without ever reaching the melting point of either aluminum or copper. Because no liquid phase is created, the thermodynamic conditions for bulk IMC formation are never met.

“An adjustment of the cross-section is required to realize electrical conductors free of resistive losses”—Volkswagen AG (2018), on FSW butt welding of 4.7 mm aluminum to 3 mm copper blanks for current-carrying components.

However, suppressing IMCs is only half the engineering challenge. Because aluminum has lower electrical conductivity than copper, a straight butt weld between equal-thickness plates would introduce resistive losses into the current path. Research from Volkswagen AG (2018) explicitly addresses this: the study investigated three approaches to produce sound butt welds between 4.7 mm aluminum and 3 mm copper blanks at a welding speed of 450 mm/min, evaluating both mechanical properties and electrical resistance. This direct acknowledgment of cross-section engineering distinguishes FSW busbar design from other welding approaches and establishes a design requirement that must be satisfied before FSW-joined busbars can be deployed in production vehicles.

Tool material selection is equally critical to meeting electrical performance requirements. A patent from Nachi-Fujikoshi Corp. (Japan, 2021) demonstrates that using a nickel alloy friction stir bonding tool—rather than conventional tool steel—when joining aluminum alloy to copper alloy components reduces abrasion particle contamination at the bond interface. The result is conductivity at the bonding part of 65% IACS or more. This is a quantified electrical performance target directly applicable to busbar-level EV battery components, and it illustrates how tool material selection is integral to meeting the low-resistance requirements of current-carrying joints, as catalogued in PatSnap’s innovation intelligence resources.

A Korean patent from Kaswin Co., Ltd. (2025) describes a hybrid process that further illustrates how FSW is being integrated into production-oriented architectures. FSW is first used to join an aluminum first plate and a copper second plate along their contact line; a cutting step then removes the pin groove at the weld terminus; and resistance welding is subsequently applied at designated joint sections compatible with each electrode material. This sequence—FSW for the Al-Cu interface, followed by resistance welding for the homogeneous electrode connections—reflects an emerging hybrid process architecture that leverages FSW’s superior IMC suppression while using established resistance welding for cell-level contacts.

FSSW, Pinless Tools, and Micro-FSW: Process Variants for Battery Contexts

Beyond conventional linear FSW, two specialised variants are gaining traction in battery assembly: friction stir spot welding (FSSW) and micro-friction stir spot welding (micro-FSSW). Both are driven by the need to join thin, highly conductive sheets without the thermal damage associated with laser or arc processes—a requirement that maps directly onto the thin aluminum and copper conductors found in e-drive busbars and cell-internal connections.

Research from Technische Universität Ilmenau (2021) provides the most direct evidence for FSSW in EV current-carrying components. The study focuses on pure aluminum and copper sheets with relatively thin cross-sections representative of the thermally and mechanically sensitive materials used in e-drive busbars. Critically, it examines pinless tools operating in what the authors term a “hybrid friction diffusion bonding” (HFDB) mode. This variant eliminates the keyhole defect inherent to conventional FSSW—a significant quality advantage—and is especially suited to thin-sheet applications where penetration depth must be tightly controlled. The study also establishes that tool downscaling—reducing shoulder and pin diameter proportionally—has measurable effects on both the process thermal cycle and joint electrical properties, making tool geometry a key process parameter for e-mobility current-carrying joints.

At the cell-internal level, research from the Technical University of Munich (2022) demonstrates that micro-FSSW can join electrode carrier foils to current collectors with high mechanical strength, and that stacks of up to 100 foils can be joined in a few tenths of a second. While this application targets cell-internal (foil-to-arrester) rather than busbar-to-busbar joints, it establishes the process speed and scalability credentials needed for high-volume EV production and confirms that FSW-based processes can address the entire hierarchy of battery electrical connections—from cell-internal foil stacks through to module-level busbars.

FSW is also being evaluated at the module housing level. Research from Volvo Car Corporation (2022), published with reference to IEEE manufacturing standards, reports defect-free welds in 3 mm thick extruded AA6063-T6 at welding speeds of 4.0–4.5 m/min using tool rotation speeds of 3500–4500 rpm and plunge forces of 8.5–10.5 kN, achieving a maximum joint efficiency of 72%. Although this work addresses similar-material (Al-Al) battery tray assembly rather than dissimilar Al-Cu busbar joining, it establishes process speed benchmarks relevant to manufacturing line integration and demonstrates that FSW can be adapted for high-throughput EV battery manufacturing contexts.

A preliminary study from the University of Naples Federico II (2022) directly targets battery pack assembly, evaluating FSW weldability of dissimilar aluminium alloys (extruded vs. die-cast) through microstructural analysis and Vickers microhardness mapping, identifying the influence of tool geometry, workpiece setup, and process parameter combinations. The findings on tool geometry and parameter sensitivity are directly transferable to the more challenging Al-Cu dissimilar case.

Patent Landscape: Key Assignees, Architectures, and Competing Technologies

The active patent space for FSW in Al-Cu battery busbar joining spans multiple jurisdictions and reflects distinct engineering philosophies. The most directly relevant patents to FSW in Al-Cu busbar joining for EVs are two US active patents assigned to Byun Sang-Won (filed 2011 and 2013). These claim a battery module in which the first terminal plate (aluminum, connected to the positive terminal) and the second terminal plate (copper, connected to the negative terminal) are joined to the bus bar via friction stir welds. The architecture is notable: the bus bar and the aluminum terminal plate share the same material, while the copper terminal plate bridges the dissimilar-metal interface. The FSW joint thus occurs at the aluminum-copper terminal plate interface, providing a solid-state bond that eliminates galvanic and IMC concerns at the critical electrical connection point.

Competing technologies occupy distinct positions in the IP landscape. LG Chem holds two active European patents (both 2020) addressing the Al-Cu busbar problem using clad overlay approaches and buried connecting parts—avoiding FSW entirely and instead using homogeneous-metal welding zones on each side of a pre-bonded bimetallic strip. This sidesteps the dissimilar-weld challenge by pre-joining the metals industrially through rolling or cladding, then performing conventional welding only between like metals.

Kobe Steel’s active Hungarian patent (2017) takes a diffusion bonding approach: an aluminum electrode section and a copper busbar section are integrated through solid-state diffusion bonding, yielding a bimetallic component that resists electrolytic corrosion and maintains low electrical resistance. Daimler AG’s German patents (2018, 2019) represent the laser welding and ultrasonic bonding approach at OEM scale, with the laser variant employing high-speed multi-pass strategies to minimise melt pool residence time and reduce IMC formation—an approach benchmarked against international standards tracked by WIPO and ISO.

Comparing Joining Technologies for Dissimilar Al-Cu Busbar Joints

The University of Warwick (2018) mapped the broader joining technology landscape for EV battery systems using Manufacturing Readiness Levels (MRLs) and a Pugh matrix to benchmark FSW, laser welding, ultrasonic bonding, and mechanical joining against each other for the full range of battery assembly joining tasks. This framework provides a structured basis for comparing the trade-offs that R&D engineers and IP professionals must navigate.

Laser welding with green radiation represents one of the most active competing approaches. Green laser radiation offers significantly increased absorptivity for copper relative to infrared, improving energy coupling and reducing IMC formation, as documented by Bayerisches Laserzentrum GmbH (2023). A separate study from University West (2022) reviewed laser welding of aluminum battery tabs to variable Al-Cu busbars in lithium-ion battery joints, establishing a comprehensive performance baseline for the fusion-welding alternative. The laser welding literature is also comprehensively reviewed in a 2023 paper from the Shanghai Collaborative Innovation Center, which notes that IMC formation remains a persistent challenge even with optimised laser parameters—a constraint that FSW avoids by design, as noted in databases maintained by EPO.

“Mechanical joints can achieve lower electrical resistance than bolted references while avoiding IMC entirely”—but require additional space considerations compared to welded joints (University of Lisbon, 2023).

Mechanical joining alternatives—notably injection lap riveting and double-sided self-pierce riveting—are documented by research groups at Universidade de Lisboa (2022) and the University of Lisbon (2023). These studies confirm that mechanical joints can achieve lower electrical resistance than bolted references while avoiding IMC entirely, but require additional space considerations compared to welded joints. This trade-off—no IMC risk but greater spatial footprint—positions mechanical joining as viable for certain module architectures where space constraints are less acute.

The table below summarises the key differentiators across the four main technology families, drawn from the sources analysed. FSW’s principal advantage—solid-state operation eliminating IMC formation—is shared with diffusion bonding and mechanical joining, but FSW alone enables continuous seam welds suitable for the butt-joint geometry required in busbar-to-busbar connections, while also being scalable to the production speeds demonstrated in the Volvo Car Corporation battery tray study. For R&D teams and IP professionals tracking this space through platforms such as PatSnap Eureka, the hybrid FSW-plus-resistance welding architecture emerging from the Korean patent literature represents the most significant near-term development to monitor.