Why Intermetallic Compounds Make Al-Cu Busbar Joining So Difficult

Joining aluminum and copper in EV battery systems fails predictably when the two metals are melted together: they form brittle intermetallic phases—including CuAl₂ and Cu₉Al₄—that degrade both mechanical strength and electrical conductivity at the joint interface. According to a 2023 review from the Shanghai Collaborative Innovation Center of Laser Advanced Manufacturing Technology, “more intermetallic compounds are formed, some of which can affect the microstructure, electrical and thermal properties of the joint.” This thermodynamic instability is not a manufacturing defect; it is an intrinsic property of the Al-Cu binary system, triggered whenever the two metals reach their liquidus temperatures simultaneously.

The challenge is compounded by the large differences in physical properties between aluminum and copper: their melting points, thermal conductivities, and electrical conductivities all differ substantially. This means that any fusion-based process—laser welding, resistance welding, arc welding—must simultaneously manage wildly different heat absorption and dissipation rates, making it difficult to avoid local hotspots where intermetallic formation accelerates. The consequence for battery pack designers is stark: a busbar joint that looks sound immediately after welding may exhibit rising electrical resistance and declining mechanical integrity within months of service, particularly under the thermal cycling loads typical of EV battery operation.

The University of Warwick’s comprehensive 2018 review of joining technologies for automotive battery systems used Manufacturing Readiness Levels (MRLs) and a Pugh matrix to benchmark FSW, laser welding, ultrasonic bonding, and mechanical joining across the full range of battery assembly tasks. The IMC problem emerges as a central discriminator: processes that avoid melting—FSW, ultrasonic bonding, diffusion bonding, and mechanical joining—all sidestep the IMC formation mechanism entirely, while fusion processes must compensate through process engineering, such as high-speed multi-pass strategies or beam shaping, to minimise melt pool residence time.

How FSW Suppresses IMC Formation and Manages Conductivity Mismatch

Friction stir welding resolves the IMC problem by keeping both aluminum and copper in a plasticized, sub-solidus state throughout the joining cycle—bulk melting never occurs. 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. Because neither metal reaches its liquidus temperature, the thermodynamic driving force for intermetallic phase formation is strongly suppressed, and the resulting microstructure retains much of the electrical and mechanical character of the parent materials.

“Because aluminum has lower electrical conductivity than copper, an adjustment of the cross-section is required to realize electrical conductors free of resistive losses.”

But suppressing IMCs is only half the engineering challenge. The second problem is the conductivity mismatch: aluminum carries current less efficiently than copper, so a busbar that simply replaces copper with aluminum on one side will introduce resistive losses even if the joint itself is perfect. Volkswagen AG’s 2018 study on dissimilar friction stir butt welding of aluminum and copper addresses this directly, investigating three geometric 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. The cross-section adjustment—thickening the aluminum side to compensate for its lower conductivity—is a design requirement specific to FSW busbar engineering, and one that distinguishes FSW-designed busbars from those produced by other processes.

Tool material selection is a third critical variable. Nachi-Fujikoshi Corp.’s 2021 Japanese patent on electrification components for batteries 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 a quantified electrical performance target: conductivity at the bonding part of 65% IACS or more. This figure is directly applicable as a design acceptance criterion in EV battery busbar manufacturing, and it illustrates how tool material selection is integral to meeting the low-resistance requirements of current-carrying joints. Standards bodies such as IEC use IACS (International Annealed Copper Standard) as the reference scale for electrical conductivity in conductor materials.

The combination of these three engineering levers—solid-state operation, cross-section adjustment, and nickel alloy tooling—gives FSW a distinctive technical profile for Al-Cu busbar joining that no single competing process fully replicates. Fusion processes avoid the cross-section engineering challenge (because both sides melt and flow together) but cannot suppress IMCs. Mechanical joining avoids both IMCs and conductivity mismatch concerns but introduces space and assembly complexity trade-offs. FSW sits at the intersection: it requires careful geometric design but delivers a metallurgically clean, low-resistance joint suitable for the current-carrying demands of EV battery packs. Research published in Nature and peer-reviewed materials science journals has increasingly highlighted solid-state joining as a frontier for dissimilar-metal electrical connections in energy storage systems.

FSW Process Variants: FSSW, HFDB, and Micro-FSW in Battery Contexts

Beyond conventional linear FSW, two specialized 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, and both have been directly evaluated for e-mobility current-carrying components.

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 eliminates the keyhole defect inherent to conventional FSSW—where the pin withdrawal leaves a void at the weld terminus—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. Notably, 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 joints rather than busbar-to-busbar connections, 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 up to module-level busbars.

FSW is also being evaluated at the module housing level for structural rather than electrical joints. Volvo Car Corporation’s 2022 study on high-speed FSW of AA6063-T6 alloy for lightweight EV battery trays reports defect-free welds in 3 mm thick extruded material 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 further application of FSW principles to dissimilar-alloy battery pack components is reported by the University of Naples Federico II (2022), addressing dissimilar aluminum alloys—extruded versus die-cast—in battery pack assembly for hybrid vehicles. While not an Al-Cu study, it evaluates weldability through microstructural analysis and Vickers microhardness mapping, identifying the influence of tool geometry, workpiece setup, and process parameter combinations. These findings are directly transferable methodology to the more challenging Al-Cu dissimilar case, and the study confirms that FSW process development for battery applications requires systematic parameter mapping rather than single-point optimization.

Patent Landscape: Key Assignees, Architectures, and Competing Technologies

Active patents claiming FSW in battery module busbar architectures span multiple jurisdictions and reflect distinct engineering philosophies. The most foundational FSW-specific patents in this space are assigned to Byun Sang-Won (US, active): the 2011 patent and its 2013 continuation both 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.

Kaswin Co., Ltd.’s 2025 Korean patent extends FSW into a hybrid process architecture specifically designed for eco-friendly vehicle battery packs. The process uses FSW to join an aluminum first plate and a copper second plate along their contact line, after which a cutting step 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. It is a pragmatic response to the manufacturing reality that resistance welding infrastructure is already widely deployed in battery production lines.

Nachi-Fujikoshi Corp.’s 2021 Japanese patent on electrification components for batteries targets the conductivity threshold of ≥65% IACS at the FSW bond interface, providing a quantified electrical performance specification that can serve as a design acceptance criterion. This is significant for IP professionals: the patent frames the nickel alloy tool as the enabling technology for meeting this threshold, creating a clear freedom-to-operate consideration for any manufacturer seeking to achieve equivalent conductivity performance using alternative tool materials. According to WIPO, solid-state joining patent filings for battery applications have grown substantially over the past decade, reflecting the broader electrification transition.

LG Chem’s two active EP patents (both 2020) address the Al-Cu busbar problem using a fundamentally different approach: clad overlay and buried connecting parts. Rather than welding aluminum to copper directly, LG Chem’s architecture uses pre-bonded bimetallic strips—produced by industrial rolling or cladding—and then performs conventional welding only between like metals on each side. This sidesteps the dissimilar-weld challenge entirely but introduces a supply chain dependency on bimetallic strip manufacturers and may limit geometric design freedom compared to FSW-based approaches.

Kobe Steel’s 2017 Hungarian patent 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. Like FSW, diffusion bonding avoids bulk melting and IMC formation, but it typically requires higher pressures and longer process times than FSW, and it is less amenable to in-line manufacturing integration. The European Patent Office has granted multiple patents in this space, reflecting the competitive intensity of dissimilar-metal joining for battery applications across European automotive suppliers.

Daimler AG’s 2018 (inactive) and 2019 (pending) German patents represent the laser welding and ultrasonic bonding approach at OEM scale. The laser variant employs high-speed multi-pass strategies to minimize melt pool residence time and reduce IMC formation—an engineering workaround rather than a fundamental solution to the IMC problem. The 2019 patent remains pending, suggesting continued development activity in this area at the time of filing.

Benchmarking FSW Against Laser Welding, Cladding, and Mechanical Joining

The competitive landscape for Al-Cu busbar joining in EV battery packs is genuinely multi-technology: no single process dominates across all application requirements, and the University of Warwick’s 2018 MRL/Pugh matrix analysis confirms that each technology has a distinct profile of strengths and limitations. Understanding where FSW leads—and where it does not—is essential for R&D engineers making process selection decisions.

Laser Welding: Speed Advantage, IMC Liability

Laser welding offers high processing speeds and contactless energy delivery, making it attractive for high-volume battery manufacturing. The 2022 University West study on laser welding of aluminum battery tabs to variable Al/Cu busbars in Li-ion battery joints documents the process parameters and joint geometries achievable in this configuration. The 2023 Bayerisches Laserzentrum GmbH study advances the technology further, highlighting that green laser radiation offers significantly increased absorptivity for copper relative to infrared, improving energy coupling and reducing IMC formation. However, even with green laser and optimized process parameters, fusion-based laser welding cannot eliminate IMC formation—it can only minimize it. For applications where long-term electrical stability under thermal cycling is paramount, this residual IMC risk is a meaningful liability.

Mechanical Joining: No IMCs, but Space Penalties

Injection lap riveting and double-sided self-pierce riveting are documented as viable alternatives for Al-Cu busbar joining. Studies from Universidade de Lisboa (2022) on injection lap riveting and the University of Lisbon (2023) on double-sided self-pierce riveting both confirm that mechanical joints can achieve lower electrical resistance than bolted references while avoiding IMC formation entirely. The trade-off is geometric: mechanical joining requires additional space considerations compared to welded joints, and the joint footprint may be incompatible with the increasingly compact cell-to-pack architectures being adopted by leading EV manufacturers. According to the IEC, electrical contact resistance specifications for battery interconnects are becoming more stringent as cell energy densities increase, further constraining the design space for mechanical joints.

FSW’s Distinctive Position

FSW occupies a distinctive position in this landscape: it is the only process that simultaneously avoids bulk melting (suppressing IMCs), enables cross-section-adjusted conductor design (managing conductivity mismatch), and produces a joint geometry compatible with compact busbar architectures. The hybrid FSW-plus-resistance welding approach documented in Kaswin’s 2025 patent extends this advantage by integrating FSW’s metallurgical benefits with the manufacturing maturity of resistance welding infrastructure already present in battery production lines. For IP professionals, the active patent portfolio around FSW busbar architectures—spanning the US, Korea, and Japan—represents both a freedom-to-operate consideration and a signal of sustained R&D investment by multiple independent organizations in this joining approach. The USPTO database confirms continued filing activity in FSW-based battery joining through 2025.

“Stacks of up to 100 foils can be joined in a few tenths of a second”—micro-FSSW’s speed and scalability credentials make FSW-based processes viable across the entire hierarchy of EV battery electrical connections.