Interlocking Interfaces with Nanostructured Intermetallics Enable Enhanced Performance in Aluminum-Copper Hybrid Rotors for Next-Generation Electric Motors

Interlocking Interfaces with Nanostructured Intermetallics Enable Enhanced Performance in Aluminum-Copper Hybrid Rotors for Next-Generation Electric Motors | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Interlocking Interfaces with Nanostructured Intermetallics Enable Enhanced Performance in Aluminum-Copper Hybrid Rotors for Next-Generation Electric Motors Arun Devaraj, Hrishikesh Das, Shivakant Shukla, Zehao Li, Mitch Blocher, and 4 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7851957/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted You are reading this latest preprint version Abstract Electric drives and other vehicle motors have renewed interest in aluminum-copper (Al-Cu) hybrid rotors, which combine aluminum’s low weight with copper’s superior conductivity. Die-casting of Al-Cu rotors, however, suffers from the formation of brittle intermetallic compound (IMC) layers that limit joint strength and long-term reliability. Here, we demonstrate for the first time-the use of friction stir welding (FSW) to fabricate Al-Cu hybrid rotor joints for induction motors. Optimized welding schemes produced robust metallurgical bonds, achieving pull-out loads of 3272 ± 60 N, approximately 20% higher than die-cast counterparts. Microstructural characterization by scanning transmission electron microscopy revealed nanoscale IMCs, primarily Al₂Cu and Al₄Cu₉, with Ni interlayers further refining interface chemistry and improving load capacity. Full rotor assemblies fabricated with the optimized FSW process sustained maximum loads of 185 kN, representing nearly a two-fold improvement in mechanical performance relative to conventional designs. Electrical resistance of FSW joints (14.9 ± 0.7 µΩ) was comparable to that of bimetallic Al-Cu joints, confirming their suitability for traction applications. By overcoming longstanding challenges in IMC formation and joint integrity, this study establishes FSW as a scalable and transformative manufacturing method for next-generation hybrid rotors, underscoring its promise as a pathway for enhancing electric motor performance and advancing the future of mobility. Physical sciences/Materials science/Structural materials/Metals and alloys Physical sciences/Engineering/Mechanical engineering Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Introduction All vehicles, independent of the propulsion mode have intensified the need for traction motor technologies that combine efficiency, cost-effectiveness, and performance. Two motor technologies dominate this space: induction motors and direct current (DC) brushless permanent magnet motors. Although DC brushless motors are more efficient relative to induction motors of comparable size, their reliance on rare-earth elements [ 1 , 2 ] poses challenges including supply chain and cost volatility. This has resulted in renewed interest in rare-earth-free induction motors [ 3 ] that are lightweight, affordable, and high-performing. A critical component of induction motors, the squirrel cage rotor, is traditionally fabricated using either aluminum (Al) or copper (Cu) [ 4 ]. Aluminum rotors, produced through die casting, are 30–40% lighter but have ~ 60% lower electrical conductivity compared to pure Cu [ 4 ]. In contrast, Cu rotors excel in conductivity yet are challenging to manufacture due to their high melting point and higher cost. These challenges have inspired alternative engineering solutions, such as hybrid Al-Cu rotors [ 4 ], to balance weight, cost, and electrical performance. The hybrid Al-Cu rotor offers a practical compromise, incorporating the lightweight advantages of Al with the superior electrical conductivity of Cu. For example, General Motors (GM) patented designs [ 5 ] where Al end-rings interlocked over Cu bars within magnetic steel laminate stack. However, diecasting these designs have inherent drawbacks. First, it produces heterogeneous and brittle intermetallic compounds (IMCs) at the Al-Cu interface due to high-operating temperature (705–870°C), with thicknesses from 10 to 200 µm [ 6 ]. While thin IMCs (< 20 µm) can be tolerated, thicker layers reduce joint strength and cause brittle failure [ 4 ]. Persistent challenges-including weak interfacial bonding, thermal mismatch between Al (660°C) and Cu (1084°C), and the brittleness of Al-Cu IMCs limit die-cast rotor performance [ 4 ]. Although some efforts have extended die-casting to hybrid designs, fundamental issues of joint integrity and robustness remain unresolved. Friction stir welding (FSW), a solid-state joining process, has garnered significant attention as a method to overcome the challenges associated with Al-Cu joining. Unlike traditional fusion-based methods, FSW facilitates bonding below the melting points of the constituent materials, effectively reducing IMC formation and thermal stresses. Over the past two decades, FSW has been extensively studied for dissimilar material welding, including Al-Cu systems [ 7 , 8 ], with considerable exploration into butt joint configurations [ 9 ]. Key variables such as tool design [ 10 ], offset distance [ 11 ], and IMC thickness [ 12 ] have been shown to strongly influence joint performance. However, research on Al-Cu lap or T-joints, which are more representative of rotor manufacturing, remains sparse. Existing literature also highlights the role of thin IMC layers (often within sub-micrometer to micrometer scales) such as Al 2 Cu and Al 4 Cu 9 in enhancing joint strength, with strategies like the use of cold-sprayed Ni interlayers [ 13 ] showing further promise. The contrast between conventional die casting and the proposed FSW-based assembly is illustrated in Fig. 1 . Cu and Al rotors are typically produced by die casting. Die life is especially limited when casting Cu rotors, with molds sustaining only a restricted number of shots before failure due to Cu’s high melting temperature and reactivity [ 4 ]. This further increases costs and complicates large-scale manufacturing. In our approach, aluminum end caps are joined to Cu shorting bars using FSW, as shown in the semi-exploded view of the hybrid rotor assembly (Fig. 1 ). The right-hand schematic in Fig. 1 highlights the FSW process, underscoring the transition from casting-based to solid-state joining methods for hybrid rotor fabrication. No prior work has focused specifically on leveraging FSW for the manufacturing of Al-Cu hybrid rotors for induction motors. Recognizing this opportunity, the Pacific Northwest National Laboratory (PNNL), in collaboration with an original equipment manufacturer (OEM), has developed an innovative approach using FSW to fabricate Al-Cu hybrid rotors. This study presents and validates FSW as an innovative method for rotor assembly, showcasing enhanced joint strength and electrical performance compared to conventional die-casting. Multi-modal characterization highlights the interfacial microstructure's influence on mechanical behavior, providing valuable insights for leveraging FSW in next-generation induction motors and addressing rotor manufacturing challenges. Results and discussion To overcome the challenges associated with die-casting and increase the joint integrity in hybrid Al-Cu rotor designs, we explored friction stir welding (FSW) as a transformative alternative. By systematically optimizing key parameters, including tool offset and penetration depth, our approach aims to establish robust metallurgical bonds that address the limitations of conventional fabrication techniques and enable superior performance in rotor assemblies. To optimize the FSW process, three tool offset schemes (Fig. 2 a-d) were tested: (1) 3 mm offset from the Cu bar, (2) 0.5 mm penetration into the Cu bar, and (3) 1.25 mm penetration. The measured peak temperature during welding was ~ 550°C, well below die-casting conditions (705–870°C). [ 4 ]. Tensile pull-out tests showed that the 0.5 mm penetration configuration delivered the highest load capacity, outperforming both; the offset (insufficient Cu interaction) and the deeper penetration (excessive Cu removal) (Fig. 2 e). X-ray computed tomography confirmed that this scheme promoted Cu flow along the tool direction and created interlocking features between the Cu bar and the Al matrix (Fig. 2 f). Dispersed Cu-rich particles within the Al were also observed, consistent with particle flow patterns reported in friction stir assisted scribe welding ([ 14 ]. Together, these results demonstrate that a 0.5 mm penetration into the Cu bar provides the most favorable balance of metallurgical bonding and mechanical interlocking, and this scheme was selected for subsequent experiments. Two types of Cu shorting bars-uncoated and nickel (Ni)-coated, were used for the welds fabricated using the scheme 3 (0.5 mm into Cu), followed by pull-out tests to evaluate the effect of the Ni coating on joint strength. The Ni-coated Cu shorting bars significantly enhance the pull-out load capacity of the joints. FSW joints with Ni-coated Cu bars exhibited an average pull-out load of 3272 ± 60 N, which is approximately 400 N higher than joints produced with uncoated Cu bars (2871 ± 87 N) [Fig. 2 (e)]. Furthermore, Ni-coated Cu bars FSW joints also demonstrated minimal reduction in average pull-out load (3070 ± 39 N) during high-temperature testing at 100°C. Both variants of FSW joints demonstrated superior mechanical performance compared to conventional die-cast Al-Cu joints (2717 ± 265 N) in terms of pull-out strength, showing smaller standard deviations and excellent repeatability across more than 30 samples. This underscores the reliability of the FSW process for fabricating Al-Cu hybrid rotor joints. Furthermore, the pull-out strength of FSW joints for both uncoated and Ni-coated Cu bars surpassed the minimum performance criterion of 2500 N required for acceptable fatigue resistance. Fracture analysis revealed a typical Cu failure mechanism occurring outside the Al weld region, showing the robust mechanical interlocking and joint structure induced by the FSW process [Fig. 2 (e)]. The electrical resistance of the high-performance FSWed Al with Ni-coated Cu joints was determined to be 14.88 ± 0.74 µΩ, based on measurements taken across a 20 mm gauge length in a single T-configuration joint (average of 11 samples). The resistance values are comparable to those previously reported for bimetallic Al-Cu joints [ 15 ]. The associated profiles of the Al and Cu bars have different cross-sections, with the Cu bar also having a non-standard slotted cross-section. These factors prevent a more direct comparison of the estimated electrical resistance with those mentioned in existing literature of bimetallic Al/Cu joints with standard cross-sections. Nevertheless, the low standard deviation and comparable values of electrical resistance of the Ni-coated Al-Cu joints indicate improved mechanical performance without affecting its electrical performance. For a deeper insight into the joining mechanism and effect of Ni coating at the interfacial chemistry, scanning transmission electron microscopy - energy dispersive spectroscopy (STEM/EDS) and selected area electron diffraction (SAED) have been performed to identify fine scale IMC formation at the interface as scanning electron microscopy (SEM) EDS is not conclusive. Figure 3 a shows the high angle annular dark field (HAADF)-STEM image of the joint interface obtained from the Al/uncoated Cu sample (same process condition). Two continuous reaction layers, denoted by I and II, are observed at the interface between Al and Cu base metals. The layer I, composed of columnar grains, has a thickness of ~ 400 nm, which is larger than that of layer II with irregular shaped grains with an average size of ~ 270 nm. STEM-EDS elemental maps show that both reaction layers are enriched in Al and Cu, while the Cu concentration in the layer I at ~ 31.2 ± 2.1 at. % is much lower than that in the layer II, 68.1 ± 1.3 at. % (see Fig. 3 b). Combined with SAED pattern analyses (Fig. 3 c), these two layers are determined to be Al 2 Cu (I, I 4/ mcm , a = 0.606 nm, c = 0.487nm) and Al 4 Cu 9 (II, P \(\:\stackrel{\text{-}}{\text{4}}\) 3 m , a = 0.871 nm), respectively, which agree with previous reports [ 16 , 17 ]. In addition, Al contains a large number of Cu-rich and Mg/Si-rich particles along the Al/Al 2 Cu interface, which are identified as Al 2 Cu (III) and Mg 2 Si (IV, Fm \(\:\stackrel{\text{-}}{\text{3}}\) m , a = 0.633 nm), respectively. Figure 4 a shows the HAADF-STEM image of the joint region taken from the region with Ni interlayer between Al/Cu. The Ni interlayer consisting of equiaxed grains exhibits gradually increased porosity from the Al6061 to the Cu side [ 18 ]. The interface between Al and Ni interlayer is relatively flat; in contrast, a wavy interface with a reaction layer of ~ 270 nm thickness is formed at the Ni/Cu interface. STEM-EDS elemental maps obtained from Reg. 1 (marked by dashed-line rectangular frame) show no obvious inter-diffusion between Al and Ni, Fig. 4 b, indicating good wettability between Al and Ni interlayer. However, a uniform distribution of Cu (~ 3.3 ± 1.2 at. %) is also detected within the Ni interlayer, which suggests the Cu diffusion occurred during the FSW process. Combined with the SAED pattern analysis, the crystal structure of the Ni interlayer is determined to remain a face-centered cubic (FCC) structure (II, Fm \(\:\stackrel{\text{-}}{\text{3}}\) m , a = 0.362 nm), Fig. 4 d, indicating the formation of Ni-Cu solid solution. In addition to the Mg 2 Si particles along the Al/Ni interface, the rectangular-shaped particle in the Al matrix is identified as Q phase (III, Al 4 Mg 8 Si 7 Cu 2 , P \(\:\stackrel{\text{-}}{\text{6}}\) , a = 1.039 nm, c = 0.402nm). On the other hand, the reaction layer at the Ni/Cu interface is enriched in Al and Cu, which is indexed to be Al 4 Cu 9 (IV), Figs. 4 c and d. Building upon the improved understanding of process development for joining Al and Cu, and the effect of Ni coating on Cu shorting bars, a full-scale hybrid Al-Cu rotor was successfully fabricated using the optimized FSW scheme. The rotor, featuring 56 spokes, was subjected to a full rotor tension test using an indigenously developed fixturing system to evaluate the bonding strength between the Al end caps and Cu bars. Remarkably, the maximum load recorded for the rotor reached 185 kN, demonstrating the robust mechanical integrity of the welded assembly. The FSWed hybrid rotor exhibited a two-fold improvement in pull-out strength compared to its die-cast counterpart [Fig. 5 (a)]. At the individual bar level, the pull-out force for the FSWed rotor was 3304 ± 64 N, which is approximately 600 N higher than that of a single bar within a conventional die-cast rotor. The fracture analysis of the FSWed rotor revealed that the Cu bars experienced failures similar to those observed in single-bar pull-out testing. This phenomenon indicates a strong bond at the interface between the Cu bars and the Al end-ring joint. A representative schematic of cross-sectional view of the joint interface, is depicted in Fig. 5 (b). The superior joint strength in FSW motor compared to conventional die-casting in both linear and rotor configurations can be attributed to two complimentary mechanisms: (a) the formation of a metallurgical bond characterized by a nanometer-scale intermetallics at the interface and (b) the mechanical interlocking of Cu with the Al matrix. These synergistic factors transform the Al-Cu joint into a robust hybrid structure, resulting in an FSW motor outperforming its die-cast counterpart in mechanical reliability and structural integrity. In dissimilar material joining, processing temperature is an essential determinant of interfacial phenomena. During FSW of Al-Cu joints, the measured peak temperature reached approximately 550°C, which facilitates atomic-level diffusion of Al and Cu toward the weld interface. This temperature regime, along with the high strain rates inherent to FSW, accelerates the uphill diffusion of elements while limiting the exposure time. The transient nature of the welding process (milliseconds to seconds) results in rapid cooling rates, constraining diffusion kinetics to create localized, nanoscale intermetallic compounds (IMCs) at the interface. These nanometer-scale IMCs exhibit minimal brittleness, exceptional mechanical compatibility, and superior load transfer capacity-hallmarks of strong metallurgical bonds [ 14 ]. Furthermore, fine-scale interfacial chemistry, attributable to localized thermodynamics and deformation mechanisms, leads to spatial variations in IMC formation, further optimizing joint performance. In contrast, die-casting involves a higher processing temperature range (705–870°C) and longer exposure times, facilitating extensive atomic diffusion resulting in thicker and microscale brittle IMC layers (20–200 µm). Thicker brittle layers are prone to crack initiation under mechanical loading, reducing load-bearing capacity and fatigue performance. This explains the comparative underperformance and inconsistency of die-cast Al-Cu joints in high-stress applications. Previous investigations by the authors into dissimilar material systems-particularly Al-to-steel [ 19 ] and immiscible systems such as magnesium (Mg)-to-steel [ 14 ] have revealed that reducing interfacial IMC thickness to the nanoscale regime significantly enhances joint strength by a bridging layer between the two system via lattice mismatching [ 14 ]. The inherent ability of FSW to produce ultra-thin, nanoscale IMC layers-coupled with geometric mechanical interlocking marks a paradigm shift in achieving high mechanical performance Al-Cu joints. These findings highlight the critical interplay between temperature, processing time scale, and deformation parameters in modulating interfacial structure and chemistry, with FSW presenting a clear technological advantage over conventional die-casting for hybrid rotor applications. Conclusion This work demonstrates the successful application of friction stir welding (FSW) to the fabrication of Al-Cu hybrid rotors, offering a practical, industrially scalable alternative to conventional die-casting. Process optimization enabled the identification of parametric window that favored the formation of nanoscale Al-Cu intermetallic compounds and effective mechanical interlocking at the Al-Cu interface, leading to significantly improved joint performance. The use of Ni-coated Cu bars further enhanced pull-out strength, with FSW joints showing a pull-out force exceeding 3200 N and maintaining performance stability at elevated temperature. At the rotor level, the FSW approach achieved maximum loads of 185 kN and nearly doubled the mechanical performance compared with die-cast rotors. These results confirm that FSW can reliably produce strong, fatigue-resistant Al-Cu joints suitable for traction motor applications. Beyond demonstrating improved strength and consistency, this study establishes FSW as a scalable and industrially relevant manufacturing route for hybrid rotors, providing a path toward more efficient and rare-earth-free motor technologies. Experimental Methods The schematic of the disassembled and fully assembled hybrid aluminum-copper (Al-Cu) rotor is presented in Fig. 1 . The rotor design consists of aluminum end caps fabricated from five sheets of 2 mm-thick AA6xxx-series alloy stacked in a lap configuration. The manufacturing challenge addressed here is notably complex and unique, requiring simultaneous (a) T-configuration joining of aluminum to copper and (b) lap-shear joining of the stacked aluminum sheets during the T-joint formation. To evaluate joint performance, single Al-Cu pull-out test specimens were precisely machined using electrical discharge machining techniques to isolate the weld region. Pull-out tensile tests were subsequently conducted at room temperature using an Instron 5582 universal tensile testing machine, with a controlled crosshead displacement rate of 1.27 mm/min to ensure consistent loading conditions. For full rotor evaluation, tension tests on completed hybrid rotors were performed using a custom-designed fixture system to quantify the bonding strength between Al end caps and Cu bars. The hybrid rotor, featuring two external nuts positioned along the middle section of its 56-spoke design, demonstrated remarkable structural integrity during testing [ 4 ]. To elucidate microstructural evolution and interfacial characteristics, advanced imaging techniques were employed to investigate the bonding interface between Al and Cu, including the presence and distribution of intermetallic compounds (IMCs). Scanning Transmission Electron Microscopy (STEM) imaging was conducted using a Cs-corrected FEI Titan 80–300 environmental TEM equipped with high-angle annular dark-field (HAADF) and energy dispersive X-ray spectroscopy (EDS) detectors, enabling nanoscale visualization and elemental mapping of the bonding interface. Thin-lamellae specimens for STEM imaging were meticulously prepared using the focused ion beam (FIB) lift-out technique on a dual-beam plasma FIB/SEM system (FEI Helios 5 Hydra UX) to maintain the integrity of the bonded regions. In addition, electrical conductivity testing was performed to assess the performance of the welded joints at the interface. Measurements utilized the 4-wire test method, employing a Keithley 6221 power supply coupled with a Keithley 2182A nanovoltmeter for precise data acquisition. With the gauge length set at 20 mm, tests directly targeted the T-configuration interface of the Al-Cu bimetallic joints to ensure accurate determination of electrical conductivity under controlled conditions (room temp. 23°C). Declarations Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Declaration of Generative AI and AI-assisted Technologies in the Writing Process During the preparation of this work the author(s) used AI Incubator Chat (based on GPT-4) provided by the Pacific Northwest National Laboratory, to only rectify grammar and sentence structure issues. After using this tool/service, the author(s) reviewed and edited the content as needed and take (s) full responsibility for the content of the published article. Acknowledgement This project was funded by the U.S. Department of Energy's Office of Energy Efficiency and Renewable Energy, through the Vehicle Technologies Office's Powertrain Materials Core Program (PMCP 1.0). We thank DOE technology manager Christopher Schooler as well as PNNL’s staff members Rob J Seffens and Michael Blazon for their contributions to sample preparation and testing. We also thank Blair Carlson and John Agapiou from General Motors (GM) for their support. The Pacific Northwest National Laboratory (PNNL) is operated by Battelle Memorial Institute under DOE contract DE-AC05-76RL01830. References Induction motors with die-cast copper, motors with die-cast copper rotors. (2006) Kim D (2010) Design and Comparison between IM and PMSM for Hybrid Electrical Vehicles. Digests of the 14th Biennial IEEE Conference on Electromagnetic Field Computation Cui J (2025) Scientists develop rare earth free magnet for use in industrial motors. AMES National Laboratory Report Agapiou JS (2023) Development of manufacturing technology for a hybrid induction rotor. Manuf Lett 35:277–288 Qigui Wang MAO, Eugene C, Tuohy (2014) Methods of manufacturing induction rotors with conductor bars having high conductivity. USA, GM Global Technology Operations LLC. https://patents.google.com/patent/US8701270B2/ en ,U.S. Patent, Editor Tang J, Wu LSC, Liu X, Zhao Y (2024) Improving bonding strength of medium-thick Al-Cu dissimilar joint by a novel splat cooling assisted double side friction stir welding. Mater Charact, 211 Abdollah-Zadeh A, Sazgari TSB (2008) Microstructural and mechanical properties of friction stir welded aluminum/copper lap joints. J Alloys Compd 46(1–2):535–538 Wiedenhoft AG, de Rosendo HdAT, Tier MAD, Reguly A (2018) Effect of heat input on the mechanical behaviour of Al-Cu FSW lap joints. Mater Res 21:1–9 Wentao Hou ZS, Huda N, Oheil M, Shen Y, Jahed H (2021) Gerlich Enhancing metallurgical and mechanical properties of friction stir butt welded joints of Al–Cu via cold sprayed Ni interlayer . Mater Sci Engineering: A 809:140992 Zhou L, Li RXZGH, Zhou WL, Huang YX, Song XG (2018) Effect of pin profile on microstructure and mechanical properties of friction stir spot welded Al-Cu dissimilar metals. J Manuf Process 36:1–9 Bhattacharya TK, Das H, Jana SS (2017) Numerical and experimental investigation of thermal history, material flow and mechanical properties of friction stir welded aluminium alloy to DHP copper dissimilar joint. Int J Adv Manuf Technol 88:847–861 Galvão I, Loureiro RMLA, Rodrigues DM (2010) Material flow in heterogeneous friction stir welding of aluminium and copper thin sheets. Sci Technol Weld Joining 14:654–660 Wentao Hou ZS, Huda N, Oheil M, Shen Y, Jahed H, Gerlich AP (2021) Enhancing metallurgical and mechanical properties of friction stir butt welded joints of Al–Cu via cold sprayed Ni interlayer. Materials Science and Engineering: A, 809(30) Hrishikesh Das PU, Wang T (2021) Bharat Gwalani & Xiaolong Ma Interfacial reaction during friction stir assisted scribe welding of immiscible Fe and Mg alloy system . Sci Rep, 11 Aksel Elkjaer JAS, Geir Ringen R, Bjørge Øystein, Grong (2022) Electrical and thermal stability of Al-Cu welds: Performance benchmarking of the hybrid metal extrusion and bonding process. J Manuf Process 79:626–638 Furuya HS, Kokawa YSSH, Huang T, Xiao RS (2018) Metall Mater Trans A 49(12):6215–6223 Kurabayashi K, Sato STYS (2022) Metals 12(3) Hou W, Huda ZSN, Oheil M, Shen Y, Jahed H, Gerlich AP (2021) Mater Sci Engineering: A 809 Piyush Upadhyay YH, Saumyadeep Jana LS, Fifield (2017) Joining Dissimilar Materials Using Friction Stir Scribe Technique. J Manuf Sci Eng, 139 (3) Additional Declarations There is NO Competing Interest. Cite Share Download PDF Status: Under Review Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-7851957","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":534657501,"identity":"878a36ff-946b-438f-8f53-c634bb230e2f","order_by":0,"name":"Arun Devaraj","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA5klEQVRIie3RMYvCMBTA8VcCyRJ0zS31K7wSUA5OP4tF6KQiFDo79abDteD3EEeh4PQo3H5DDsG5q5NWK7rFugnmPyRvyG8ID8Dletl4fWFZHa2nSJDd54aEySakvaSugeQrXCxzE/fXf1MO4ncnLUQV4x5CEYVZEaGe0D7mIGNtI0Cyq7w010BQkTQPU5DRR2YRnZocdYdEqT+bEKzJxq8G1N6FiK0qLSQgnqhhMfIDkrPghyrCJEMb8YmtVJkMpE9ihYd1Hi7E978Z2r5/7vqAozeH83bwEbjFzIWAMI2Jy+VyvUUnIjxGVbBaafMAAAAASUVORK5CYII=","orcid":"https://orcid.org/0000-0003-1314-6212","institution":"Pacific Northwest National Laboratory","correspondingAuthor":true,"prefix":"","firstName":"Arun","middleName":"","lastName":"Devaraj","suffix":""},{"id":534657502,"identity":"149645f2-35d1-470e-9974-c9016ad23900","order_by":1,"name":"Hrishikesh Das","email":"","orcid":"","institution":"Pacific Northwest National Laboratory","correspondingAuthor":false,"prefix":"","firstName":"Hrishikesh","middleName":"","lastName":"Das","suffix":""},{"id":534657503,"identity":"2d6d751e-8fc2-4705-908b-75bf1f98b51f","order_by":2,"name":"Shivakant Shukla","email":"","orcid":"","institution":"Pacific Northwest National Laboratory","correspondingAuthor":false,"prefix":"","firstName":"Shivakant","middleName":"","lastName":"Shukla","suffix":""},{"id":534657504,"identity":"340ff04e-bf4d-4bcb-ba4f-cd4437675db4","order_by":3,"name":"Zehao Li","email":"","orcid":"","institution":"Pacific Northwest National Laboratory","correspondingAuthor":false,"prefix":"","firstName":"Zehao","middleName":"","lastName":"Li","suffix":""},{"id":534657505,"identity":"7ddc94f9-1207-427a-8bd8-2c0fa7e7af6e","order_by":4,"name":"Mitch Blocher","email":"","orcid":"","institution":"Pacific Northwest National Laboratory","correspondingAuthor":false,"prefix":"","firstName":"Mitch","middleName":"","lastName":"Blocher","suffix":""},{"id":534657506,"identity":"d483bcd5-6f66-49f0-aa29-156bca153e14","order_by":5,"name":"Aditya Nittala","email":"","orcid":"https://orcid.org/0000-0002-4108-1957","institution":"Pacific Northwest National Laboratory","correspondingAuthor":false,"prefix":"","firstName":"Aditya","middleName":"","lastName":"Nittala","suffix":""},{"id":534657507,"identity":"0983f7f7-69f2-4c86-83db-02adec8ebacc","order_by":6,"name":"Keerti Kappagantula","email":"","orcid":"","institution":"Pacific Northwest National Laboratory","correspondingAuthor":false,"prefix":"","firstName":"Keerti","middleName":"","lastName":"Kappagantula","suffix":""},{"id":534657508,"identity":"378c71ef-119f-41f7-8f9d-174459e85e90","order_by":7,"name":"Glenn Grant","email":"","orcid":"","institution":"Pacific Northwest National Laboratory","correspondingAuthor":false,"prefix":"","firstName":"Glenn","middleName":"","lastName":"Grant","suffix":""},{"id":534657509,"identity":"112c3157-bd6e-429d-9ae2-4784c6efe6a1","order_by":8,"name":"Piyush Upadhyay","email":"","orcid":"","institution":"Pacific Northwest National Laboratory","correspondingAuthor":false,"prefix":"","firstName":"Piyush","middleName":"","lastName":"Upadhyay","suffix":""}],"badges":[],"createdAt":"2025-10-13 19:30:30","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-7851957/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-7851957/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":94588436,"identity":"3898dfb7-4889-4bd7-98af-8885ee677375","added_by":"auto","created_at":"2025-10-28 18:19:21","extension":"docx","order_by":0,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":1762807,"visible":true,"origin":"","legend":"","description":"","filename":"FinalversionManuscriptSubmitted101325.docx","url":"https://assets-eu.researchsquare.com/files/rs-7851957/v1/ade9372ef0c5ae5c1e6d1251.docx"},{"id":94588842,"identity":"d170b841-66d8-4a45-bb86-25004722327e","added_by":"auto","created_at":"2025-10-28 18:19:50","extension":"json","order_by":1,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":10313,"visible":true,"origin":"","legend":"","description":"","filename":"NCOMMS2581040.json","url":"https://assets-eu.researchsquare.com/files/rs-7851957/v1/45308309a7e77e9116a2695e.json"},{"id":94588820,"identity":"8bf3a34c-3c24-4e8f-8f5a-e03eea9ebfd4","added_by":"auto","created_at":"2025-10-28 18:19:47","extension":"xml","order_by":2,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":61925,"visible":true,"origin":"","legend":"","description":"","filename":"NCOMMS25810400enriched.xml","url":"https://assets-eu.researchsquare.com/files/rs-7851957/v1/4827559ab0d6380cda323fab.xml"},{"id":94588577,"identity":"4b74df73-8710-4a57-82a6-867f519472b5","added_by":"auto","created_at":"2025-10-28 18:19:33","extension":"png","order_by":8,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":124222,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-7851957/v1/69d2d4a8245ec0fcb643c2fb.png"},{"id":94589284,"identity":"edf72f4a-100d-4921-a5a9-b1703167e5b4","added_by":"auto","created_at":"2025-10-28 18:20:06","extension":"png","order_by":9,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":237010,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-7851957/v1/c80f45fe7cafa64b20b001f1.png"},{"id":94589832,"identity":"d86be054-9ac9-4fe5-b8ff-2ba006d84c6a","added_by":"auto","created_at":"2025-10-28 18:20:39","extension":"png","order_by":10,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":88186,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-7851957/v1/424487401f2395017ab4e03d.png"},{"id":94589517,"identity":"c796fc3a-0818-46b9-af17-056c5594d4a7","added_by":"auto","created_at":"2025-10-28 18:20:23","extension":"png","order_by":11,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":144637,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-7851957/v1/7322782e797e4aa3e6a05cb6.png"},{"id":94589353,"identity":"00f56a96-3cc8-4e83-8089-09fdab538456","added_by":"auto","created_at":"2025-10-28 18:20:09","extension":"png","order_by":12,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":34920,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-7851957/v1/58bca3d80d245cf46ed5bf64.png"},{"id":94596427,"identity":"48016224-79f8-4f52-bc21-5a1b524a6c87","added_by":"auto","created_at":"2025-10-28 18:42:05","extension":"xml","order_by":13,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":58681,"visible":true,"origin":"","legend":"","description":"","filename":"NCOMMS25810400structuring.xml","url":"https://assets-eu.researchsquare.com/files/rs-7851957/v1/697ffeebcf9fdaca53bad52a.xml"},{"id":94589548,"identity":"18541ca9-b3c8-4b35-9550-6f2d1463c7af","added_by":"auto","created_at":"2025-10-28 18:20:26","extension":"html","order_by":14,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":67579,"visible":true,"origin":"","legend":"","description":"","filename":"earlyproof.html","url":"https://assets-eu.researchsquare.com/files/rs-7851957/v1/ea912ce202df670af6275768.html"},{"id":94589900,"identity":"fbad4f0f-fe7b-487b-af9c-79291fb8fd03","added_by":"auto","created_at":"2025-10-28 18:20:46","extension":"jpeg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":213272,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic comparison of rotor fabrication methods. Conventional die-cast Cu (left) and Al (center-left) rotors are contrasted with the proposed Al-Cu hybrid rotor assembled by friction stir welding (right). The exploded view in the middle shows aluminum end caps joined to Cu shorting bars. The insets illustrate the FSW process (top and side views).\u003c/p\u003e","description":"","filename":"floatimage1.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-7851957/v1/dc5e404a46b61615fa85cdb5.jpeg"},{"id":94589370,"identity":"5406d49f-52f9-4282-9c42-14603c588d1b","added_by":"auto","created_at":"2025-10-28 18:20:11","extension":"jpeg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":906869,"visible":true,"origin":"","legend":"\u003cp\u003eOptimization of FSW tool offset schemes for Al–Cu joints. (a) Schematic of the FSW process and (b-d) cross-sections showing Scheme 1: 3 mm offset, Scheme 2: 0.5 mm penetration, and Scheme 3: 1.25 mm penetration into the Cu bar. (e) Pull-out strength comparison showing that 0.5 mm penetration produced the highest load capacity. (f) X-ray CT reveals mechanical interlocking and Cu particle dispersion in the 0.5 mm configuration, explaining its superior performance. Together, these results identify 0.5 mm penetration as the optimal scheme for subsequent experiments.\u003c/p\u003e","description":"","filename":"floatimage2.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-7851957/v1/e45040342e847f04aac0d0ee.jpeg"},{"id":94588961,"identity":"f61a4647-8975-4035-9125-c267fa3205a7","added_by":"auto","created_at":"2025-10-28 18:19:54","extension":"jpeg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":341581,"visible":true,"origin":"","legend":"\u003cp\u003eInterfacial structure of Al–Cu joints without Ni coating. (a) HAADF-STEM image shows two distinct reaction layers at the interface. (b) STEM-EDS elemental maps reveal Al–Cu enrichment in both layers. (c) SAED patterns taken from regions I–IV in (a) confirm that layer I corresponds to Al₂Cu and layer II to Al₄Cu₉, while particles along the Al/Al₂Cu boundary are identified as Al₂Cu and Mg₂Si. Together, these results demonstrate that uncoated joints form nanoscale IMCs at the interface, which influence mechanical performance.\u003c/p\u003e","description":"","filename":"floatimage3.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-7851957/v1/02de62aebeb88ee19d8c6698.jpeg"},{"id":94588885,"identity":"cd36db49-843d-484b-8331-f0f8b6934634","added_by":"auto","created_at":"2025-10-28 18:19:51","extension":"jpeg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":448080,"visible":true,"origin":"","legend":"\u003cp\u003eEffect of Ni coating on Al-Cu joint interfaces. (a) HAADF-STEM image of the Al/Ni/Cu region shows a flat Al-Ni boundary and a wavy Ni–Cu boundary with a ~270 nm reaction layer. (b,c) STEM-EDS maps indicate limited Al-Ni interdiffusion but measurable Cu diffusion into the Ni interlayer. (d) SAED confirms FCC Ni-Cu solid solution and identifies the Al₄Cu₉ phase at the Ni–Cu boundary. These results demonstrate that Ni coatings refine interfacial chemistry and improve joint stability.\u003c/p\u003e","description":"","filename":"floatimage4.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-7851957/v1/30f22e98721ed52f25522aa9.jpeg"},{"id":94596480,"identity":"59392d49-71ab-43a0-b777-ae75cf45091f","added_by":"auto","created_at":"2025-10-28 18:42:13","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":202993,"visible":true,"origin":"","legend":"\u003cp\u003eMechanical performance of FSWed full size hybrid rotors. (a) Pull-out strength comparison shows that FSWed rotors reach ~185 kN maximum load, nearly double that of die-cast counterparts [4]. Individual bar pull-out forces also exceed those of die-cast rotors by ~600 N. (b) Fracture analysis reveals Cu failure outside the weld region, confirming strong interfacial bonding. These results establish FSW as a scalable method for fabricating high-strength Al-Cu hybrid rotors.\u003c/p\u003e","description":"","filename":"floatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-7851957/v1/e8c44b8b035363b3d38f923f.png"},{"id":94598286,"identity":"d4ca6b6f-4bbd-411e-b81f-44db2955d540","added_by":"auto","created_at":"2025-10-28 18:52:22","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2568536,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7851957/v1/3f4e332a-533d-4ebf-846b-0f61443d6190.pdf"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"Interlocking Interfaces with Nanostructured Intermetallics Enable Enhanced Performance in Aluminum-Copper Hybrid Rotors for Next-Generation Electric Motors","fulltext":[{"header":"Introduction","content":"\u003cp\u003eAll vehicles, independent of the propulsion mode have intensified the need for traction motor technologies that combine efficiency, cost-effectiveness, and performance. Two motor technologies dominate this space: induction motors and direct current (DC) brushless permanent magnet motors. Although DC brushless motors are more efficient relative to induction motors of comparable size, their reliance on rare-earth elements [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e, \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e] poses challenges including supply chain and cost volatility. This has resulted in renewed interest in rare-earth-free induction motors [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e] that are lightweight, affordable, and high-performing. A critical component of induction motors, the squirrel cage rotor, is traditionally fabricated using either aluminum (Al) or copper (Cu) [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]. Aluminum rotors, produced through die casting, are 30\u0026ndash;40% lighter but have ~\u0026thinsp;60% lower electrical conductivity compared to pure Cu [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]. In contrast, Cu rotors excel in conductivity yet are challenging to manufacture due to their high melting point and higher cost. These challenges have inspired alternative engineering solutions, such as hybrid Al-Cu rotors [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e], to balance weight, cost, and electrical performance.\u003c/p\u003e\u003cp\u003eThe hybrid Al-Cu rotor offers a practical compromise, incorporating the lightweight advantages of Al with the superior electrical conductivity of Cu. For example, General Motors (GM) patented designs [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e] where Al end-rings interlocked over Cu bars within magnetic steel laminate stack. However, diecasting these designs have inherent drawbacks. First, it produces heterogeneous and brittle intermetallic compounds (IMCs) at the Al-Cu interface due to high-operating temperature (705\u0026ndash;870\u0026deg;C), with thicknesses from 10 to 200 \u0026micro;m [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]. While thin IMCs (\u0026lt;\u0026thinsp;20 \u0026micro;m) can be tolerated, thicker layers reduce joint strength and cause brittle failure [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]. Persistent challenges-including weak interfacial bonding, thermal mismatch between Al (660\u0026deg;C) and Cu (1084\u0026deg;C), and the brittleness of Al-Cu IMCs limit die-cast rotor performance [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]. Although some efforts have extended die-casting to hybrid designs, fundamental issues of joint integrity and robustness remain unresolved.\u003c/p\u003e\u003cp\u003eFriction stir welding (FSW), a solid-state joining process, has garnered significant attention as a method to overcome the challenges associated with Al-Cu joining. Unlike traditional fusion-based methods, FSW facilitates bonding below the melting points of the constituent materials, effectively reducing IMC formation and thermal stresses. Over the past two decades, FSW has been extensively studied for dissimilar material welding, including Al-Cu systems [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e, \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e], with considerable exploration into butt joint configurations [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]. Key variables such as tool design [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e], offset distance [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e], and IMC thickness [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e] have been shown to strongly influence joint performance. However, research on Al-Cu lap or T-joints, which are more representative of rotor manufacturing, remains sparse. Existing literature also highlights the role of thin IMC layers (often within sub-micrometer to micrometer scales) such as Al\u003csub\u003e2\u003c/sub\u003eCu and Al\u003csub\u003e4\u003c/sub\u003eCu\u003csub\u003e9\u003c/sub\u003e in enhancing joint strength, with strategies like the use of cold-sprayed Ni interlayers [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e] showing further promise.\u003c/p\u003e\u003cp\u003eThe contrast between conventional die casting and the proposed FSW-based assembly is illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. Cu and Al rotors are typically produced by die casting. Die life is especially limited when casting Cu rotors, with molds sustaining only a restricted number of shots before failure due to Cu\u0026rsquo;s high melting temperature and reactivity [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]. This further increases costs and complicates large-scale manufacturing. In our approach, aluminum end caps are joined to Cu shorting bars using FSW, as shown in the semi-exploded view of the hybrid rotor assembly (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). The right-hand schematic in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e highlights the FSW process, underscoring the transition from casting-based to solid-state joining methods for hybrid rotor fabrication. No prior work has focused specifically on leveraging FSW for the manufacturing of Al-Cu hybrid rotors for induction motors. Recognizing this opportunity, the Pacific Northwest National Laboratory (PNNL), in collaboration with an original equipment manufacturer (OEM), has developed an innovative approach using FSW to fabricate Al-Cu hybrid rotors. This study presents and validates FSW as an innovative method for rotor assembly, showcasing enhanced joint strength and electrical performance compared to conventional die-casting. Multi-modal characterization highlights the interfacial microstructure's influence on mechanical behavior, providing valuable insights for leveraging FSW in next-generation induction motors and addressing rotor manufacturing challenges.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e"},{"header":"Results and discussion","content":"\u003cp\u003eTo overcome the challenges associated with die-casting and increase the joint integrity in hybrid Al-Cu rotor designs, we explored friction stir welding (FSW) as a transformative alternative. By systematically optimizing key parameters, including tool offset and penetration depth, our approach aims to establish robust metallurgical bonds that address the limitations of conventional fabrication techniques and enable superior performance in rotor assemblies. To optimize the FSW process, three tool offset schemes (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea-d) were tested: (1) 3 mm offset from the Cu bar, (2) 0.5 mm penetration into the Cu bar, and (3) 1.25 mm penetration. The measured peak temperature during welding was ~\u0026thinsp;550\u0026deg;C, well below die-casting conditions (705\u0026ndash;870\u0026deg;C). [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eTensile pull-out tests showed that the 0.5 mm penetration configuration delivered the highest load capacity, outperforming both; the offset (insufficient Cu interaction) and the deeper penetration (excessive Cu removal) (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ee). X-ray computed tomography confirmed that this scheme promoted Cu flow along the tool direction and created interlocking features between the Cu bar and the Al matrix (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ef). Dispersed Cu-rich particles within the Al were also observed, consistent with particle flow patterns reported in friction stir assisted scribe welding ([\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]. Together, these results demonstrate that a 0.5 mm penetration into the Cu bar provides the most favorable balance of metallurgical bonding and mechanical interlocking, and this scheme was selected for subsequent experiments.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eTwo types of Cu shorting bars-uncoated and nickel (Ni)-coated, were used for the welds fabricated using the scheme 3 (0.5 mm into Cu), followed by pull-out tests to evaluate the effect of the Ni coating on joint strength. The Ni-coated Cu shorting bars significantly enhance the pull-out load capacity of the joints. FSW joints with Ni-coated Cu bars exhibited an average pull-out load of 3272\u0026thinsp;\u0026plusmn;\u0026thinsp;60 N, which is approximately 400 N higher than joints produced with uncoated Cu bars (2871\u0026thinsp;\u0026plusmn;\u0026thinsp;87 N) [Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(e)]. Furthermore, Ni-coated Cu bars FSW joints also demonstrated minimal reduction in average pull-out load (3070\u0026thinsp;\u0026plusmn;\u0026thinsp;39 N) during high-temperature testing at 100\u0026deg;C. Both variants of FSW joints demonstrated superior mechanical performance compared to conventional die-cast Al-Cu joints (2717\u0026thinsp;\u0026plusmn;\u0026thinsp;265 N) in terms of pull-out strength, showing smaller standard deviations and excellent repeatability across more than 30 samples. This underscores the reliability of the FSW process for fabricating Al-Cu hybrid rotor joints. Furthermore, the pull-out strength of FSW joints for both uncoated and Ni-coated Cu bars surpassed the minimum performance criterion of 2500 N required for acceptable fatigue resistance. Fracture analysis revealed a typical Cu failure mechanism occurring outside the Al weld region, showing the robust mechanical interlocking and joint structure induced by the FSW process [Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(e)]. The electrical resistance of the high-performance FSWed Al with Ni-coated Cu joints was determined to be 14.88\u0026thinsp;\u0026plusmn;\u0026thinsp;0.74 \u0026micro;Ω, based on measurements taken across a 20 mm gauge length in a single T-configuration joint (average of 11 samples). The resistance values are comparable to those previously reported for bimetallic Al-Cu joints [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]. The associated profiles of the Al and Cu bars have different cross-sections, with the Cu bar also having a non-standard slotted cross-section. These factors prevent a more direct comparison of the estimated electrical resistance with those mentioned in existing literature of bimetallic Al/Cu joints with standard cross-sections. Nevertheless, the low standard deviation and comparable values of electrical resistance of the Ni-coated Al-Cu joints indicate improved mechanical performance without affecting its electrical performance.\u003c/p\u003e\u003cp\u003eFor a deeper insight into the joining mechanism and effect of Ni coating at the interfacial chemistry, scanning transmission electron microscopy - energy dispersive spectroscopy (STEM/EDS) and selected area electron diffraction (SAED) have been performed to identify fine scale IMC formation at the interface as scanning electron microscopy (SEM) EDS is not conclusive. Figure\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea shows the high angle annular dark field (HAADF)-STEM image of the joint interface obtained from the Al/uncoated Cu sample (same process condition). Two continuous reaction layers, denoted by I and II, are observed at the interface between Al and Cu base metals. The layer I, composed of columnar grains, has a thickness of ~\u0026thinsp;400 nm, which is larger than that of layer II with irregular shaped grains with an average size of ~\u0026thinsp;270 nm. STEM-EDS elemental maps show that both reaction layers are enriched in Al and Cu, while the Cu concentration in the layer I at ~\u0026thinsp;31.2\u0026thinsp;\u0026plusmn;\u0026thinsp;2.1 at. % is much lower than that in the layer II, 68.1\u0026thinsp;\u0026plusmn;\u0026thinsp;1.3 at. % (see Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb). Combined with SAED pattern analyses (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ec), these two layers are determined to be Al\u003csub\u003e2\u003c/sub\u003eCu (I, \u003cem\u003eI\u003c/em\u003e4/\u003cem\u003emcm\u003c/em\u003e, \u003cem\u003ea\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.606 nm, \u003cem\u003ec\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.487nm) and Al\u003csub\u003e4\u003c/sub\u003eCu\u003csub\u003e9\u003c/sub\u003e (II, \u003cem\u003eP\u003c/em\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\stackrel{\\text{-}}{\\text{4}}\\)\u003c/span\u003e\u003c/span\u003e3\u003cem\u003em\u003c/em\u003e, \u003cem\u003ea\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.871 nm), respectively, which agree with previous reports [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e, \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]. In addition, Al contains a large number of Cu-rich and Mg/Si-rich particles along the Al/Al\u003csub\u003e2\u003c/sub\u003eCu interface, which are identified as Al\u003csub\u003e2\u003c/sub\u003eCu (III) and Mg\u003csub\u003e2\u003c/sub\u003eSi (IV, \u003cem\u003eFm\u003c/em\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\stackrel{\\text{-}}{\\text{3}}\\)\u003c/span\u003e\u003c/span\u003e\u003cem\u003em\u003c/em\u003e, \u003cem\u003ea\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.633 nm), respectively.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea shows the HAADF-STEM image of the joint region taken from the region with Ni interlayer between Al/Cu. The Ni interlayer consisting of equiaxed grains exhibits gradually increased porosity from the Al6061 to the Cu side [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]. The interface between Al and Ni interlayer is relatively flat; in contrast, a wavy interface with a reaction layer of ~\u0026thinsp;270 nm thickness is formed at the Ni/Cu interface. STEM-EDS elemental maps obtained from Reg. 1 (marked by dashed-line rectangular frame) show no obvious inter-diffusion between Al and Ni, Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb, indicating good wettability between Al and Ni interlayer. However, a uniform distribution of Cu (~\u0026thinsp;3.3\u0026thinsp;\u0026plusmn;\u0026thinsp;1.2 at. %) is also detected within the Ni interlayer, which suggests the Cu diffusion occurred during the FSW process. Combined with the SAED pattern analysis, the crystal structure of the Ni interlayer is determined to remain a face-centered cubic (FCC) structure (II, \u003cem\u003eFm\u003c/em\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\stackrel{\\text{-}}{\\text{3}}\\)\u003c/span\u003e\u003c/span\u003e\u003cem\u003em\u003c/em\u003e, \u003cem\u003ea\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.362 nm), Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ed, indicating the formation of Ni-Cu solid solution. In addition to the Mg\u003csub\u003e2\u003c/sub\u003eSi particles along the Al/Ni interface, the rectangular-shaped particle in the Al matrix is identified as Q phase (III, Al\u003csub\u003e4\u003c/sub\u003eMg\u003csub\u003e8\u003c/sub\u003eSi\u003csub\u003e7\u003c/sub\u003eCu\u003csub\u003e2\u003c/sub\u003e, \u003cem\u003eP\u003c/em\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\stackrel{\\text{-}}{\\text{6}}\\)\u003c/span\u003e\u003c/span\u003e, \u003cem\u003ea\u003c/em\u003e\u0026thinsp;=\u0026thinsp;1.039 nm, \u003cem\u003ec\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.402nm). On the other hand, the reaction layer at the Ni/Cu interface is enriched in Al and Cu, which is indexed to be Al\u003csub\u003e4\u003c/sub\u003eCu\u003csub\u003e9\u003c/sub\u003e (IV), Figs.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ec and d.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eBuilding upon the improved understanding of process development for joining Al and Cu, and the effect of Ni coating on Cu shorting bars, a full-scale hybrid Al-Cu rotor was successfully fabricated using the optimized FSW scheme. The rotor, featuring 56 spokes, was subjected to a full rotor tension test using an indigenously developed fixturing system to evaluate the bonding strength between the Al end caps and Cu bars. Remarkably, the maximum load recorded for the rotor reached 185 kN, demonstrating the robust mechanical integrity of the welded assembly. The FSWed hybrid rotor exhibited a two-fold improvement in pull-out strength compared to its die-cast counterpart [Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e(a)]. At the individual bar level, the pull-out force for the FSWed rotor was 3304\u0026thinsp;\u0026plusmn;\u0026thinsp;64 N, which is approximately 600 N higher than that of a single bar within a conventional die-cast rotor.\u003c/p\u003e\u003cp\u003eThe fracture analysis of the FSWed rotor revealed that the Cu bars experienced failures similar to those observed in single-bar pull-out testing. This phenomenon indicates a strong bond at the interface between the Cu bars and the Al end-ring joint. A representative schematic of cross-sectional view of the joint interface, is depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e (b).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe superior joint strength in FSW motor compared to conventional die-casting in both linear and rotor configurations can be attributed to two complimentary mechanisms: (a) the formation of a metallurgical bond characterized by a nanometer-scale intermetallics at the interface and (b) the mechanical interlocking of Cu with the Al matrix. These synergistic factors transform the Al-Cu joint into a robust hybrid structure, resulting in an FSW motor outperforming its die-cast counterpart in mechanical reliability and structural integrity. In dissimilar material joining, processing temperature is an essential determinant of interfacial phenomena. During FSW of Al-Cu joints, the measured peak temperature reached approximately 550\u0026deg;C, which facilitates atomic-level diffusion of Al and Cu toward the weld interface. This temperature regime, along with the high strain rates inherent to FSW, accelerates the uphill diffusion of elements while limiting the exposure time. The transient nature of the welding process (milliseconds to seconds) results in rapid cooling rates, constraining diffusion kinetics to create localized, nanoscale intermetallic compounds (IMCs) at the interface. These nanometer-scale IMCs exhibit minimal brittleness, exceptional mechanical compatibility, and superior load transfer capacity-hallmarks of strong metallurgical bonds [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]. Furthermore, fine-scale interfacial chemistry, attributable to localized thermodynamics and deformation mechanisms, leads to spatial variations in IMC formation, further optimizing joint performance. In contrast, die-casting involves a higher processing temperature range (705\u0026ndash;870\u0026deg;C) and longer exposure times, facilitating extensive atomic diffusion resulting in thicker and microscale brittle IMC layers (20\u0026ndash;200 \u0026micro;m). Thicker brittle layers are prone to crack initiation under mechanical loading, reducing load-bearing capacity and fatigue performance. This explains the comparative underperformance and inconsistency of die-cast Al-Cu joints in high-stress applications. Previous investigations by the authors into dissimilar material systems-particularly Al-to-steel [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e] and immiscible systems such as magnesium (Mg)-to-steel [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e] have revealed that reducing interfacial IMC thickness to the nanoscale regime significantly enhances joint strength by a bridging layer between the two system via lattice mismatching [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]. The inherent ability of FSW to produce ultra-thin, nanoscale IMC layers-coupled with geometric mechanical interlocking marks a paradigm shift in achieving high mechanical performance Al-Cu joints. These findings highlight the critical interplay between temperature, processing time scale, and deformation parameters in modulating interfacial structure and chemistry, with FSW presenting a clear technological advantage over conventional die-casting for hybrid rotor applications.\u003c/p\u003e"},{"header":"Conclusion","content":"\u003cp\u003eThis work demonstrates the successful application of friction stir welding (FSW) to the fabrication of Al-Cu hybrid rotors, offering a practical, industrially scalable alternative to conventional die-casting. Process optimization enabled the identification of parametric window that favored the formation of nanoscale Al-Cu intermetallic compounds and effective mechanical interlocking at the Al-Cu interface, leading to significantly improved joint performance. The use of Ni-coated Cu bars further enhanced pull-out strength, with FSW joints showing a pull-out force exceeding 3200 N and maintaining performance stability at elevated temperature. At the rotor level, the FSW approach achieved maximum loads of 185 kN and nearly doubled the mechanical performance compared with die-cast rotors. These results confirm that FSW can reliably produce strong, fatigue-resistant Al-Cu joints suitable for traction motor applications. Beyond demonstrating improved strength and consistency, this study establishes FSW as a scalable and industrially relevant manufacturing route for hybrid rotors, providing a path toward more efficient and rare-earth-free motor technologies.\u003c/p\u003e"},{"header":"Experimental Methods","content":"\u003cp\u003eThe schematic of the disassembled and fully assembled hybrid aluminum-copper (Al-Cu) rotor is presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. The rotor design consists of aluminum end caps fabricated from five sheets of 2 mm-thick AA6xxx-series alloy stacked in a lap configuration. The manufacturing challenge addressed here is notably complex and unique, requiring simultaneous (a) T-configuration joining of aluminum to copper and (b) lap-shear joining of the stacked aluminum sheets during the T-joint formation. To evaluate joint performance, single Al-Cu pull-out test specimens were precisely machined using electrical discharge machining techniques to isolate the weld region. Pull-out tensile tests were subsequently conducted at room temperature using an Instron 5582 universal tensile testing machine, with a controlled crosshead displacement rate of 1.27 mm/min to ensure consistent loading conditions. For full rotor evaluation, tension tests on completed hybrid rotors were performed using a custom-designed fixture system to quantify the bonding strength between Al end caps and Cu bars. The hybrid rotor, featuring two external nuts positioned along the middle section of its 56-spoke design, demonstrated remarkable structural integrity during testing [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]. To elucidate microstructural evolution and interfacial characteristics, advanced imaging techniques were employed to investigate the bonding interface between Al and Cu, including the presence and distribution of intermetallic compounds (IMCs). Scanning Transmission Electron Microscopy (STEM) imaging was conducted using a Cs-corrected FEI Titan 80\u0026ndash;300 environmental TEM equipped with high-angle annular dark-field (HAADF) and energy dispersive X-ray spectroscopy (EDS) detectors, enabling nanoscale visualization and elemental mapping of the bonding interface. Thin-lamellae specimens for STEM imaging were meticulously prepared using the focused ion beam (FIB) lift-out technique on a dual-beam plasma FIB/SEM system (FEI Helios 5 Hydra UX) to maintain the integrity of the bonded regions. In addition, electrical conductivity testing was performed to assess the performance of the welded joints at the interface. Measurements utilized the 4-wire test method, employing a Keithley 6221 power supply coupled with a Keithley 2182A nanovoltmeter for precise data acquisition. With the gauge length set at 20 mm, tests directly targeted the T-configuration interface of the Al-Cu bimetallic joints to ensure accurate determination of electrical conductivity under controlled conditions (room temp. 23\u0026deg;C).\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003ch2\u003eDeclaration of Competing Interest\u003c/h2\u003e\u003cp\u003eThe authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.\u003c/p\u003e\u003c/p\u003e\u003cp\u003e\u003cstrong\u003eDeclaration of Generative AI and AI-assisted Technologies in the Writing Process\u003c/strong\u003e\u003cp\u003eDuring the preparation of this work the author(s) used AI Incubator Chat (based on GPT-4) provided by the Pacific Northwest National Laboratory, to only rectify grammar and sentence structure issues. After using this tool/service, the author(s) reviewed and edited the content as needed and take (s) full responsibility for the content of the published article.\u003c/p\u003e\u003c/p\u003e\u003ch2\u003eAcknowledgement\u003c/h2\u003e\u003cp\u003eThis project was funded by the U.S. Department of Energy's Office of Energy Efficiency and Renewable Energy, through the Vehicle Technologies Office's Powertrain Materials Core Program (PMCP 1.0). We thank DOE technology manager Christopher Schooler as well as PNNL\u0026rsquo;s staff members Rob J Seffens and Michael Blazon for their contributions to sample preparation and testing. We also thank Blair Carlson and John Agapiou from General Motors (GM) for their support. The Pacific Northwest National Laboratory (PNNL) is operated by Battelle Memorial Institute under DOE contract DE-AC05-76RL01830.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003e\u003cem\u003eInduction motors with die-cast copper, motors with die-cast copper rotors.\u003c/em\u003e (2006)\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eKim D (2010) \u003cem\u003eDesign and Comparison between IM and PMSM for Hybrid Electrical Vehicles.\u003c/em\u003e Digests of the 14th Biennial IEEE Conference on Electromagnetic Field Computation\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eCui J (2025) Scientists develop rare earth free magnet for use in industrial motors. AMES National Laboratory Report\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eAgapiou JS (2023) Development of manufacturing technology for a hybrid induction rotor. Manuf Lett 35:277\u0026ndash;288\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eQigui Wang MAO, Eugene C, Tuohy (2014) Methods of manufacturing induction rotors with conductor bars having high conductivity. USA, GM Global Technology Operations LLC. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://patents.google.com/patent/US8701270B2/\u003c/span\u003e\u003cspan address=\"https://patents.google.com/patent/US8701270B2/\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e \u003cem\u003een\u003c/em\u003e,U.S. Patent, Editor\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eTang J, Wu LSC, Liu X, Zhao Y (2024) Improving bonding strength of medium-thick Al-Cu dissimilar joint by a novel splat cooling assisted double side friction stir welding. Mater Charact, 211\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eAbdollah-Zadeh A, Sazgari TSB (2008) Microstructural and mechanical properties of friction stir welded aluminum/copper lap joints. J Alloys Compd 46(1\u0026ndash;2):535\u0026ndash;538\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eWiedenhoft AG, de Rosendo HdAT, Tier MAD, Reguly A (2018) Effect of heat input on the mechanical behaviour of Al-Cu FSW lap joints. Mater Res 21:1\u0026ndash;9\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eWentao Hou ZS, Huda N, Oheil M, Shen Y, Jahed H (2021) Gerlich \u003cem\u003eEnhancing metallurgical and mechanical properties of friction stir butt welded joints of Al\u0026ndash;Cu via cold sprayed Ni interlayer\u003c/em\u003e. Mater Sci Engineering: A 809:140992\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eZhou L, Li RXZGH, Zhou WL, Huang YX, Song XG (2018) Effect of pin profile on microstructure and mechanical properties of friction stir spot welded Al-Cu dissimilar metals. J Manuf Process 36:1\u0026ndash;9\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eBhattacharya TK, Das H, Jana SS (2017) Numerical and experimental investigation of thermal history, material flow and mechanical properties of friction stir welded aluminium alloy to DHP copper dissimilar joint. Int J Adv Manuf Technol 88:847\u0026ndash;861\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eGalv\u0026atilde;o I, Loureiro RMLA, Rodrigues DM (2010) Material flow in heterogeneous friction stir welding of aluminium and copper thin sheets. Sci Technol Weld Joining 14:654\u0026ndash;660\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eWentao Hou ZS, Huda N, Oheil M, Shen Y, Jahed H, Gerlich AP (2021) \u003cem\u003eEnhancing metallurgical and mechanical properties of friction stir butt welded joints of Al\u0026ndash;Cu via cold sprayed Ni interlayer.\u003c/em\u003e Materials Science and Engineering: A, 809(30)\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eHrishikesh Das PU, Wang T (2021) Bharat Gwalani \u0026amp; Xiaolong Ma \u003cem\u003eInterfacial reaction during friction stir assisted scribe welding of immiscible Fe and Mg alloy system\u003c/em\u003e. Sci Rep, 11\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eAksel Elkjaer JAS, Geir Ringen R, Bj\u0026oslash;rge \u0026Oslash;ystein, Grong (2022) Electrical and thermal stability of Al-Cu welds: Performance benchmarking of the hybrid metal extrusion and bonding process. J Manuf Process 79:626\u0026ndash;638\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eFuruya HS, Kokawa YSSH, Huang T, Xiao RS (2018) Metall Mater Trans A 49(12):6215\u0026ndash;6223\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eKurabayashi K, Sato STYS (2022) Metals 12(3)\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eHou W, Huda ZSN, Oheil M, Shen Y, Jahed H, Gerlich AP (2021) Mater Sci Engineering: A 809\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003ePiyush Upadhyay YH, Saumyadeep Jana LS, Fifield (2017) Joining Dissimilar Materials Using Friction Stir Scribe Technique. J Manuf Sci Eng, 139 (3)\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"nature-portfolio","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"","title":"Nature Portfolio","twitterHandle":"","acdcEnabled":false,"dfaEnabled":false,"editorialSystem":"ejp","reportingPortfolio":"","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-7851957/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7851957/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eElectric drives and other vehicle motors have renewed interest in aluminum-copper (Al-Cu) hybrid rotors, which combine aluminum’s low weight with copper’s superior conductivity. Die-casting of Al-Cu rotors, however, suffers from the formation of brittle intermetallic compound (IMC) layers that limit joint strength and long-term reliability. Here, we demonstrate for the first time-the use of friction stir welding (FSW) to fabricate Al-Cu hybrid rotor joints for induction motors. Optimized welding schemes produced robust metallurgical bonds, achieving pull-out loads of 3272 ± 60 N, approximately 20% higher than die-cast counterparts. Microstructural characterization by scanning transmission electron microscopy revealed nanoscale IMCs, primarily Al₂Cu and Al₄Cu₉, with Ni interlayers further refining interface chemistry and improving load capacity. Full rotor assemblies fabricated with the optimized FSW process sustained maximum loads of 185 kN, representing nearly a two-fold improvement in mechanical performance relative to conventional designs. Electrical resistance of FSW joints (14.9 ± 0.7 µΩ) was comparable to that of bimetallic Al-Cu joints, confirming their suitability for traction applications. By overcoming longstanding challenges in IMC formation and joint integrity, this study establishes FSW as a scalable and transformative manufacturing method for next-generation hybrid rotors, underscoring its promise as a pathway for enhancing electric motor performance and advancing the future of mobility.\u003c/p\u003e","manuscriptTitle":"Interlocking Interfaces with Nanostructured Intermetallics Enable Enhanced Performance in Aluminum-Copper Hybrid Rotors for Next-Generation Electric Motors","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-10-28 16:41:43","doi":"10.21203/rs.3.rs-7851957/v1","editorialEvents":[],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"nature-communications","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"NCOMMS","sideBox":"Learn more about [Nature Communications](http://www.nature.com/ncomms/)","snPcode":"","submissionUrl":"https://mts-ncomms.nature.com/","title":"Nature Communications","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"ejp","reportingPortfolio":"Nature Communications","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"a6f47b1a-adc9-458d-99a5-72ebc579b1b1","owner":[],"postedDate":"October 28th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[{"id":56848742,"name":"Physical sciences/Materials science/Structural materials/Metals and alloys"},{"id":56848743,"name":"Physical sciences/Engineering/Mechanical engineering"}],"tags":[],"updatedAt":"2026-05-13T04:20:20+00:00","versionOfRecord":[],"versionCreatedAt":"2025-10-28 16:41:43","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-7851957","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7851957","identity":"rs-7851957","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}