Comparison of the Physical Performance between Infrared and Hybrid Welding of Copper Wire Hairpins

Comparison of the Physical Performance between Infrared and Hybrid Welding of Copper Wire Hairpins | 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 Research Article Comparison of the Physical Performance between Infrared and Hybrid Welding of Copper Wire Hairpins Tim Pasang, Shumpei Fujio, Pai-Chen Lin, Zheng-Da Wang, Roni Rountree, and 7 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7140646/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract With the rise in demand for electric vehicles, hairpin welding is gaining popularity for its efficient manufacturing of critical EV motor components. Due to its high thermal conductivity, copper is commonly used. The tip of the hairpin can be joined by laser welding, micro TIG, and resistance brazing. In this paper, hairpin welding using infrared and hybrid (infrared combined with blue diode laser) technology is presented. When using infrared-only with a power of 1000 W, a shallow weld joint was produced. A decent weld join (weld bead) was achieved when the power of the infrared was increased to 1300 W. However, when the hybrid method was employed with an infrared power of 1000 W and blue diode laser power of 750 W, a very satisfactory weld joint was achieved, i.e., comparable to that of infrared only with a power of 1300 W. The electrical resistivity results indicated relatively low resistivity of the welded hairpin samples. Peel test results also suggest that high force is required to break the decent weld joints, especially the samples welded with IR 1300 W and the hybrid method. The results showed that hybrid laser technology is very promising for hairpin welding. Copper Hairpin Infrared Hybrid Welding Resistivity Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 1. Introduction Hairpins - the name known for this component due to its identical shape to the hairpin - are critical components in an electric vehicle motor. The electric motor in an electric vehicle, which converts energy from a battery into mechanical energy to power the wheels, contains two essential components: the rotor and the stator [ 1 , 2 ]. A stator is the stationary part that has copper coils, which will create a magnetic field, while the rotor is the rotating part. There may be hundreds of connections in the stator, and they must be reliable as they provide both the mechanical and electrical connections. Riedel et al. [ 3 ] laid out the general manufacturing steps and challenges in hairpin production. One of the steps is connecting each wire to a hairpin stator. Aside from strengths, the connection must also have low electrical resistance. The tip of a hairpin is typically joined by welding. Several methods are commonly used to weld the hairpin, including electron beam welding, laser welding, micro TIG welding, and resistance brazing [ 4 ]. The weld must be of high quality, as it provides both mechanical and electrical contact. It should be strong, stable, and last over long periods of time. Various publications related to hairpin welding are reviewed in the following section. Glaessel et al. employed an infrared disk laser with a power of 2500–7000 W, using a sample size of 3.15 × 1.6 mm and a cross-sectional area of 5 mm 2 . The authors reported that a welding depth of 4.1 mm was achieved in 0.11 seconds. The electrical resistance was measured to be approximately 90 µΩ for samples with a weld depth of 4.1 mm and approximately 110 µΩ for those with a depth of 1.3 mm. Their results showed that the deeper the weld depth, the lower the resistance. The results demonstrate the suitability of the infrared for hairpin welding [ 5 ]. Feve et al. investigated the welding of coil terminals or hairpins using a 500 W continuous-wave blue laser system and a beam diameter of 230 µm on 1 × 1 mm copper pins. They managed to obtain a weld depth of around 1 mm. The authors claimed that the welding process did not produce any spatter, and the resulting weldments were strong. Additionally, the authors successfully welded 0.25 mm-thick copper foils at a power of 275 W and a speed of 1 mm/s. Successful welding was then also demonstrated with a 1 mm thick copper plate when power was increased to 600 W [ 6 ]. Glaessel et al. reported a welding strategy using rectangular copper wire with dimensions of 4.2 x 2.5 mm and a cross-sectional area of around 10.5 mm 2 . The machine used was a TruDisk 8001 disk laser, featuring a power of 7 kW, a wavelength of λ = 1030 nm, and a fiber diameter of 150 µm. The resistance was measured to be in the range of 36 to 40.7 µΩ, which is comparable to the resistance of the reference material, around 37 µΩ. The results suggested that laser beam technology has the potential for hairpin welding applications [ 7 , 8 ]. Toth et al. conducted welding experiments on rectangular copper wires with varying oxygen content. A vacuum electron beam welding (EBW) machine with a maximum output power of 15 kW was used. The samples had a dimension of 4.3 x 2.33 mm, which gives a cross-sectional area of around 10 mm 2 . The resulting weld depth was measured to be around 3 mm. Using both X-ray computed tomography and metallography examination, the authors were able to locate the porosity and determine the volume and number of porosities in the weld joint area. The authors also claimed that no spatter was observed during welding [ 9 ]. Dimatteo et al. employed a continuous-wave (CW) nLight Alta 3.0 kW Yb: Fiber laser with a near-infrared wavelength of 1070 nm. The fiber core diameter was 50 µm, and the spot size was 68 µm. The power used varied from 1500 to 2500 W. The pure copper samples used had a length of about 100 mm and a cross-sectional area of 7 mm 2 . Each sample was coated with insulating lacquer. Before welding, approximately 10 mm of the insulation material was removed from one end of each sample, where welding and joining were to occur. The welded samples were subjected to resistance (R) measurements and peel testing. The R was measured to be between 95 and 113 µΩ, and the force was 190–490 N. For comparison, the R value for the unwelded (base metal) was approximately 114 µΩ [ 10 ]. Ning and Zhang conducted welding on Cu hairpins using an ultrasonic-assisted laser machine. The sample size was 1mm x 4 mm, with welding times ranging from 2 seconds to 6 seconds, and power ranging from 1200 W to 1500 W. The authors claimed that laser spot welding, performed with a 5-second duration and 1500 W power under argon protection, resulted in well-formed joints, excellent welding depth, and good repeatability. Microhardness data indicated higher hardness values in the fusion zone (FZ) than in the heat-affected zone (HAZ) or the base metal (BM). Tensile test results showed that welding for 4 seconds had higher strength than that of 5 seconds, i.e., 716 N vs 661 N. However, the electrical resistance of the welded samples showed a weld time of 5 seconds being the closest to that of the base metal, i.e., 165 µΩ, compared with the base metal of around 170 µΩ [ 11 ]. Demir et al. performed laser welding on pure Cu with Power ranging from 2100 W to 6000 W, on rectangular samples with dimensions of 3.87 x 2.66 mm. The wavelengths of the laser were near-infrared (1030–1070 nm), mid-green (515 nm), and mid-blue laser (455 nm). From the peel tests, it was revealed that the highest force was produced by near-infrared radiation with a power of 6000 W, resulting in a force of around 400 ± 50 N, which was comparable to that of the base metal. The authors concluded that near-infrared light, with its relatively low intensity, was beneficial in producing high-quality welds and reducing spatter [ 12 ]. D’Arcangelo et al. performed laser welding using a near-infrared disk laser with a power of 4 kW, operating at two different wavelengths (1070 nm and 1030 nm) and two different spot sizes (215 µm and 150 µm, respectively). The rectangular copper wire had a dimension of 3.87 x 2.66 mm. The welding time was 135 ms. The depth of the weld joints reached up to 4 mm. The force required to break the weld joints ranged from 200 to 500 N, with an overall average of approximately 320 N [ 13 ]. More recently, the blue diode laser (BDL) has been introduced and reported to successfully weld copper [ 14 – 26 ]; therefore, it could be an alternative for welding hairpins. The BDL can provide better weld quality due to the good stability of the melt pool and less spatter [ 23 , 26 ]. Zijue et al. employed a blue laser with a power of 1950 W, a spot diameter of 0.6 mm, and an argon gas supply of 15 L/s to weld rectangular copper wires measuring 4 x 2 x 40 mm. Welding times ranged from 0.3 to 1.1 seconds. Additionally, several offset distances were tested, but the offset of 0.5 mm and welding time of 0.7 seconds were found to be the most ideal, resulting in an optimal near-spherical weld bead with a width of 5.4 mm and a depth of around 4 mm. Regarding electrical resistance, a longer weld time is associated with lower resistance [ 27 ]. The use of infrared, green laser, and blue diode laser individually showed that they require high power and/or long welding times. Otherwise, a good weld joint with a decent weld bead may not be achieved due to the relatively low absorption of copper by the laser. At room temperature, the absorption of laser light by copper is approximately 10%, 40%, and 60% for infrared (IR), green laser (GL), and blue diode laser (BDL) wavelengths, respectively. Therefore, increasing power is one way to obtain deep penetration. However, an unstable melt pool, in addition to excessive molten metal, may lead to the formation of the welding defects [ 24 ]. A hybrid method, combining IR with GL or BDL, could improve efficiency [ 24 , 30 , 31 ]. Hess et al. combined infrared and green lasers with wavelengths of 1030 nm and 515 nm, respectively. The authors performed welding on copper alloys at a speed of 25 m/min. Individually, both the infrared and green lasers were only able to achieve weld depths of less than 100 µm. With the hybrid method, a weld depth of 300 µm (0.3 mm) was found [ 31 ]. Fujio et al. reported the effect of a single-mode fiber laser (infrared) vs a hybrid laser combining a blue diode laser and a fiber laser. Their finding suggested that the use of blue diode lasers accelerates copper melting [ 24 , 28 ]. In this paper, a comparison was performed between infrared and hybrid (infrared coupled with a blue diode laser - BDL) methods on rectangular copper wires to create hairpins. Hairpin weld quality was assessed on the mechanical (tensile and hardness) and electrical (resistivity) properties of welds produced using each method. Metallographic analysis was also performed to assess weld penetration, shape, and structure and correlate the resulting mechanical and electrical properties to the weld microstructure for each processing condition. 2. Materials and Methods 2.1. Material The material used in these experiments was Oxygen-free copper (> 99.96%) shaped into rectangular strips measuring 2.5 x 3 x 100 mm (thickness x width x length), resulting in a cross-sectional area of approximately 7.5 mm². The original strips were covered with insulating material. About 10 mm of the insulation was removed from one end of the strip samples, where weld joints were made. 2.2. Welding Experiments The hairpin samples were welded for 0.6 seconds using two methods: infrared-only (IR #1 and IR #2) and hybrid (a combination of IR and BDL) methods. The power (P) used for the infrared was 1000 W and 1300 W for IR #1 and IR #2, respectively. For the hybrid method, the combination employed was IR power of 1000 W and BDL power of 750 W. The wavelengths were 1070 nm for IR and 450 nm for BDL. More details about the welding parameters are presented in Table 1 . Argon gas with a flow of 15L/min was used as a shielding to protect the samples from the surrounding atmosphere. A schematic diagram of the welding experiments is shown in Fig. 1 . Table 1 Welding parameters employed in this investigation No Method Power (W) Spot size (mm) Time (s) Heat Input (J) 1 IR #1 1000 0.055 0.6 600 2 IR #2 1300 0.055 0.6 780 3 Hybrid: IR BDL 1000 750 0.055 0.3 0.6 1050 2.3. Electrical Resistivity Measurements The electrical resistivity, ρ, was measured using the four-point probe method due to the low resistivity of the copper strips. The Keithley 2400 source measurement unit was configured to supply a current of 100 mA and a compliance of 2.1 V, therefore, providing an accuracy of 3 mΩ. A custom-made four-point measurement housing was designed for the dimensions of the copper strip and contained spring-actuated probes. The housing was attached to Mark-10’s ESM301 Force Gauge to apply the same probe pressure to each sample. This measurement takes about 45 seconds for each sample. For comparison and validation of the measurements, the resistivity of unwelded copper material was also measured. 2.4. Microstructure and Hardness 2.4.1 Metallography Stereo microscopy was used to perform metallographic analysis to observe the relatively large weld region and base metal while maintaining high enough resolution to observe the grain structure of each region. Preparation was carried out by first sectioning the welded hairpins transverse to the weld bead. Following sample sectioning, each sample was mounted in Bakelite resin and grinded sequentially using SiC paper from 100 to 2400 grit. After grinding, samples were sequentially polished using a Buehler AutoMet250 auto polisher, operating in concentric mode with 6, 3, and 1 µm diamond polishing media. Samples were then etched to reveal the weld microstructure using a solution of 50 mL of nitric acid and 50 mL of distilled water. Samples were then submersed for 4 s and immediately cleansed with water and ethanol. Upon drying, micrographs of the samples were then recorded using a SZX10 Stereo Microscope (Evident Scientific, formerly owned subsidiary of Olympus Corporation). 2.4.2. Microhardness Vickers micro-hardness tests were performed using a LM248 series micro indenter machine (Leco LM) with a load of 300 g. The hardness indentations were placed at a constant distance from the base metal to the weld seams to produce a hardness profile. The hardness of the fractured samples, following the peel tests, was also examined to assess the mechanical properties of the local area around the weldments. 2.5. Mechanical Testing 2.5.1. Bending Bending of the samples was conducted after welding with a special fixture which was prepared according to ISO 14270 to ensure that bending processes did not affect the quality of the weldments [ 31 ]. 2.5.2. Tensile/Peel Testing Tensile/peel testing was performed in accordance with ISO 14270:2016(E) [ 29 ] using a Shimadzu AG-IS machine (1, Nishinokyo Kuwabara-cho, Nakagyo-ku, Kyoto 604–8511, Japan) equipped with a 10 KN load cell. The crosshead speed used in this investigation was 4.2mm/min. Following peel testing, the fractured samples were also prepared metallographically (as described above) to examine the exact fracture surface. 2.6. Scanning Electron Microscopy To understand the fracture mechanism, scanning electron microscopy analysis was performed using a JEOL JSM-IT200 (JEOL Ltd, 3-1-2, Musashino, Akishima, Tokyo 196–8558, Japan) on the fracture surfaces following the peel tests. All fracture surfaces were soaked in ethanol and cleaned with an ultrasonic cleaner. 3. Results and Discussion 3.1. Weldments The resulting hairpin welds are presented in Fig. 3 . The IR #1 samples have very little weld bead, indicating poor penetration or shallow fusion due to insufficient heat input (Fig. 3 a). Figures 3 b (IR #2) and 3c (Hybrid) exhibited larger weld beads characteristic of quality welds. All these experiments were conducted for 0.6 seconds, and all of them provided consistent results based on their welding parameters. During welding, minimal spatter (if any) was observed with each method. 3.2. Microstructure and Microhardness Figure 4 shows the cross-sectional views of the welded hairpin samples following metallography examination. The sample welded using infrared with a power of 1000 W (IR #1) reached a weld depth of around 2.4 mm (Fig. 4 a). However, the weld zone has a V-shaped appearance, indicating there was not enough melting on both hairpin stripsand suggesting that power from the heat source was insufficient. It can also be observed from Fig. 4 a that the grain solidification occurred at an angle pointing to the center of the heat source, indicative of non-uniform cooling due to a large thermal gradient under IR #1 conditions. Further, several small pores are featured under IR #1 conditions, likely resulting from non-uniform cooling. The samples welded with 1300 W and hybrid showed completely different grain structures. Some of the grains spanned the entire weld zone. The direction of these columnar grains indicates the uniform cooling/solidification direction towards the heat source as they do not converge to the center as occurred under IR #1 conditions. The samples welded using infrared with a power of 1300 W (IR #2) had a weld depth ranging from 2.9 to 3.1 mm. The weld shape is excellent, resembling a mushroom (Fig. 4 b). However, there is a reasonable size of porosity at the bottom of the weld zone. The sample welded using the hybrid method, which combined infrared (P = 1000 W) and blue diode laser (P = 750 W) sources, showed a weld depth between 2.7 and 2.9 mm, with a small amount of porosity at the bottom of the weld zone (Fig. 4 c). Regarding the heat-affected zone (HAZ) – the area between WZ and BM - the HAZ width was approximately 1.5 mm for IR #1 (1000 W) and just over 3 mm for IR #2. On the hybrid sample, however, the width of the HAZ was around 4.3 mm although the IR power was only 1000 W. This shows the effect of employing BDL as a preheating source [ 24 , 28 ]. The grain size of the base metal was less than 10µm. The grain size at the heat-affected zones was between 20 and 30 µm, and they were equiaxed with many twin grains, a typical grain structure of Copper. The weld zone has a completely different grain structure. The grains were columnar and their length could be up to the entire length of the weld zone, e.g., 3 mm. Hardness profiles are shown in Fig. 5 . It can be observed that there may be a slight decrease in hardness in the heat-affected zone and the fusion zone, due to the annihilation of dislocations in these areas, as well as the larger grain size compared to the base metal. This is consistent with what was previously reported by Dimatteo et al. [ 10 ]. 3.3. Peel Tests Prior to the tensile peel tests, all the welded samples were bent according to ISO 14270 (as explained in the Experimental Procedure section) . Figure 6 shows the bent samples prior to peel testing. An example of a fractured sample following the peel test is shown in Fig. 7 . As mentioned earlier, samples welded with an IR power of 1000 W exhibited little penetration (Fig. 3 a) compared to those with an IR power of 1300 W (Fig. 3 b) and the hybrid (Fig. 3 c). This is also supported by the peel test results. The results of peel tests are presented on Table 2 , while a plot showing force vs. elongation is presented in Fig. 8 . All samples had more than 10% elongation. With the relatively good weld depth and excellent size of the melt/fusion zone, the force required to break the samples was rather high, ranging from 400 to 785 N. The highest force was recorded for the IR#2 samples. This is higher than what was previously reported by Dimatteo [ 10 ], i.e., up to 500 N, and by Ning and Zhang [ 11 ], with a maximum load of 717 N and a welding time of up to 6 s, compared to 0.6 s in this investigation. Joints with high strength are required as the vehicle would experience vibrations when in operation, which may cause premature fatigue failure, particularly if porosity is present. Table 2 Peel test results of the hairpin welded samples. Samples Parameters Force (N) Elongation (%) IR #1 P = 1000 W; t = 0.6 s 410 ± 15 11 ± 2 IR #2 P = 1300 W; t = 0.6 s 770 ± 15 12 ± 3 Hybrid IR_P = 1000 W and BDL_P = 750 W; t = 0.6 s 720 ± 20 13 ± 3 3.4. Resistivity Measurements The results from resistivity measurements are shown in Fig. 9 . The resistivity of the unwelded copper strip (reference sample) was approximately 1.9 × 10^ (-4) Ω cm. The welded copper strips had resistivity of 1.2x10 -3 – 6.2x10 -3 Ω.cm, 2.3x10 -4 − 4x10 -4 Ω.cm and 2.6x10 -4 – 6.1x10 -4 Ω.cm for IR #1, IR #2, and Hybrid, respectively. IR #1 samples had high resistivity, i.e., 5 to 31 times higher than that of the unwelded sample. Both IR #2 and hybrid samples had lower resistivity compared with that of IR #1 and were only up to three times higher than those of unwelded samples. The reason why IR #1 had very high resistivity may be related to the high number of defects and porosity, as well as the small area of the weld zones, i.e., only 20–30% of the sample size. These results are consistent with what had been reported by Dimatteo et al. [ 10 ] and Ning and Zhang [ 11 ], where great weld depth, hence, stronger joints are associated with low resistivity. 3.5. Scanning Electron Micrographs The SEM images of the fracture surfaces (Fig. 10 ) indicated a relatively small fracture surface area on the IR #1 sample. In contrast, both HR #2 and Hybrid samples exhibited large fracture surfaces, indicating a large joint area. All samples fractured with a ductile mechanism, characterized by microvoid coalescence or dimples. This agrees with the elongation of more than 10% for all peel test samples (Table 2 and Fig. 8 ). From the macrographs (Fig. 10 ), it is evident that the weld zone of IR #1 is reasonably narrow, while both IR #2 and Hybrid showed a much larger weld zone (larger fracture surface area). It is worth noting that, even though the cross-sectional area of the samples in this investigation (based on the size of the sample) was around 7.5 mm 2 , the cross-sectional area of the welded area varied from 2.5 to 7.8 mm 2 where IR #1 samples had the smallest area followed by the Hybrid samples and IR #2, respectively. This also explains why IR #2 samples required the highest force to break during peel testing compared to samples from other batches. 4. Conclusions Based on the present investigation, it can be concluded that both IR-only and Hybrid (IR + BDL) methods can be employed to produce a reliable hairpin weld joint. The preliminary results indicated that a weld time of 0.6 seconds is sufficient to achieve a reliable weld joint. This is demonstrated by the resistivity measurement results, which show that the resistivity of the welded joints was only marginally higher than that of the unwelded copper. From the peel test, the results were consistent with the resistivity measurements data, i.e., IR #2 and Hybrid samples required higher force to break than IR #1 samples due to the good weld quality of the former samples. Although the results from this investigation, in terms of strengths and resistivity, are very promising, further experiments are needed with different parameters to achieve high quality weld joints with the industry goal of 0.1 seconds per weld. Declarations Acknowledgements We would like to thank Denso Corporation for providing the pure copper strip samples. Data availability The data that support the findings of this study are available from the corresponding author on reasonable request. Contributions Tim Pasang: methodology, formal analysis, writing—original draft, writing—review and editing. Shumpei Fujio: experiments and formal analysis. Pai-Chen Lin: testing, analysis and resources. Zheng-Da Wang: testing and analysis. Roni Rountree: metallography, analysis, writing-review and editing. Tony Hanson: analysis and methodology. Jie Xiong: analysis and methodology. Jacob Nyholm: metallography and analysis. Wojtek Misiolek: writing-review, methodology, resources and editing. Poppy Puspitasari: analysis and methodology. Yuji Sato: methodology, resources and supervision. Masahiro Tsukamoto: resources, supervision and funding acquisition. Corresponding author Correspondence to Tim Pasang: [email protected] Funding This research work was supported by OU Master Plan Implementation Project, The University of Osaka Competing interests The authors declare no competing interests. References Rahman K, Jurkovic S, Hawkins S, Tarnowsky SA, Savagian P. Propulsion system design of a battery electric vehicle. IEEE Electrif Mag 2014;2(June (2)):14–24. https://doi.org/10.1109/MELE.2014.2316977. Melancon, S., What is a Hairpin Motor - Benefits & Assembly Process, February 23, 2023., Laserax.com Riedel A, Masuch M, Weigelt M, Glaessel T, Kuehl A, Reinstein S, Franke J. Challenges of the hairpin technology for production techniques. In: 2018 21 st International Conference on electrical machines and systems (ICEMS); 2018, p. 2471–6. 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Opt Laser Technol 2024;168: 109971 Shumpei Fujio, Yuji Sato, Keisuke Takenaka,Rika Ito, Masatoshi Ito, Masayuki Harada, Takashi Nishikawa, Tetsuo Suga and Masahiro Tsukamoto, “Welding of pure copper wires using a hybrid laser system with a blue diode laser and a single-mode fiber laser”, J. Laser Appl. 33, 042056 (2021); doi: 10.2351/7.0000502 Rutering M. Hybrid Solution Moves Boundaries of Copper Welding, Photonics Views 5/2019, © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, www.photonicsviews.com ISO 14270:2016-11. Resistance welding – destructive testing of welds – specimen dimensions and procedure for mechanized peel testing resistance spot, seam and embossed projection welds. 2016. A. Hess, R. Schuster, A. Heider, R. Weber, and T. Graf, “Continuous wave laser welding of copper with combined beams at wavelengths of 1030 nm and of 515 nm, LiM 2011,” Phys. Proc. 12, 88–94 (2011). 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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-7140646","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":492989071,"identity":"05bfb143-b9da-431c-8d87-6e03a376e828","order_by":0,"name":"Tim Pasang","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA20lEQVRIiWNgGAWjYDACZgTrAAODAUMCA4jEB3gQWtgSiNSCxASrJKzFnp07gbmgpk7enH/NN6kbBXZ5DOzN2yTwO4x3A/OMY4cNd854u006xyC5mIHnWBlhLTxsBxg33Di77XaOwYHEBokcMyK0/Kuz33DjzDOIFvk3RGjhbWNO3HC+hw1qCw8BLYd5Nxye2Xc4ecMNNvPfQL8ktvGkFVvg08Lef3bj44JvdbYbzh9+bJzzxy6xn/3wxhv4tIDAYTApkQDhsRFSDgKQBMB/gBi1o2AUjIJRMBIBAPxGSCU6O/NlAAAAAElFTkSuQmCC","orcid":"","institution":"Western Michigan University","correspondingAuthor":true,"prefix":"","firstName":"Tim","middleName":"","lastName":"Pasang","suffix":""},{"id":492989072,"identity":"fe5e9933-869c-4aab-8cbd-a9f52221ea82","order_by":1,"name":"Shumpei Fujio","email":"","orcid":"","institution":"Osaka University: Osaka Daigaku","correspondingAuthor":false,"prefix":"","firstName":"Shumpei","middleName":"","lastName":"Fujio","suffix":""},{"id":492989073,"identity":"419de1c7-1255-426a-8b6a-c02b9633edf8","order_by":2,"name":"Pai-Chen Lin","email":"","orcid":"","institution":"National Chung Cheng University","correspondingAuthor":false,"prefix":"","firstName":"Pai-Chen","middleName":"","lastName":"Lin","suffix":""},{"id":492989074,"identity":"6e692972-d940-4804-9e19-6c0b4d6ce5d1","order_by":3,"name":"Zheng-Da Wang","email":"","orcid":"","institution":"National Chung Cheng University","correspondingAuthor":false,"prefix":"","firstName":"Zheng-Da","middleName":"","lastName":"Wang","suffix":""},{"id":492989075,"identity":"3abd5b29-a3d6-49fc-9b7c-0fe990f6f969","order_by":4,"name":"Roni Rountree","email":"","orcid":"","institution":"Lehigh University","correspondingAuthor":false,"prefix":"","firstName":"Roni","middleName":"","lastName":"Rountree","suffix":""},{"id":492989076,"identity":"02176819-4886-461a-a1fa-4102cbfc1086","order_by":5,"name":"Tony Hanson","email":"","orcid":"","institution":"Western Michigan University","correspondingAuthor":false,"prefix":"","firstName":"Tony","middleName":"","lastName":"Hanson","suffix":""},{"id":492989077,"identity":"d2dc599f-362d-4516-8057-c42d3fdf4112","order_by":6,"name":"Jie Xiong","email":"","orcid":"","institution":"Western Michigan University","correspondingAuthor":false,"prefix":"","firstName":"Jie","middleName":"","lastName":"Xiong","suffix":""},{"id":492989078,"identity":"dfe7261d-5cf3-4aed-8faf-16f22f9af58c","order_by":7,"name":"Jacob Nyholm","email":"","orcid":"","institution":"Lehigh University","correspondingAuthor":false,"prefix":"","firstName":"Jacob","middleName":"","lastName":"Nyholm","suffix":""},{"id":492989079,"identity":"363de887-bb78-48da-879e-8a20442e0b8f","order_by":8,"name":"Wojciech Misiolek","email":"","orcid":"","institution":"Lehigh University","correspondingAuthor":false,"prefix":"","firstName":"Wojciech","middleName":"","lastName":"Misiolek","suffix":""},{"id":492989080,"identity":"19651ac3-6ab2-499f-a112-c6307b5e5135","order_by":9,"name":"Poppy Puspitasari","email":"","orcid":"","institution":"Universitas Negeri Malang","correspondingAuthor":false,"prefix":"","firstName":"Poppy","middleName":"","lastName":"Puspitasari","suffix":""},{"id":492989081,"identity":"c8e06229-b35b-4d25-ad5c-5c8fc4298ae6","order_by":10,"name":"Yuji Sato","email":"","orcid":"","institution":"Osaka University: Osaka Daigaku","correspondingAuthor":false,"prefix":"","firstName":"Yuji","middleName":"","lastName":"Sato","suffix":""},{"id":492989082,"identity":"bef46b7c-6e5a-4b64-8f24-dd413bede180","order_by":11,"name":"Masahiro Tsukamoto","email":"","orcid":"","institution":"Osaka University: Osaka Daigaku","correspondingAuthor":false,"prefix":"","firstName":"Masahiro","middleName":"","lastName":"Tsukamoto","suffix":""}],"badges":[],"createdAt":"2025-07-16 13:46:29","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-7140646/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-7140646/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":88269007,"identity":"33924dcf-5fb7-4605-9fec-48de83cfc5a7","added_by":"auto","created_at":"2025-08-04 16:52:47","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":655179,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Schematic diagram of the hairpin welding set-up, and (b) the real welding set-up for hairpin. Inset (a) shows the top view and the size/position of both IR and BDL on the rectangular Cu strip samples.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-7140646/v1/5af311746e04fca2ee5fa899.png"},{"id":88268904,"identity":"3ad3f52a-8a4f-4f39-b050-486f094d153e","added_by":"auto","created_at":"2025-08-04 16:52:43","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":537022,"visible":true,"origin":"","legend":"\u003cp\u003eFour-point probe measurement technique showing (a) schematic diagram, (b) set up of the resistivity measurement, and (c) close-up view of (b).\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-7140646/v1/fa2beb154f37536fad8751f6.png"},{"id":88269009,"identity":"39985514-357d-49fc-9694-8b8e12b64a4a","added_by":"auto","created_at":"2025-08-04 16:52:47","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":500651,"visible":true,"origin":"","legend":"\u003cp\u003eHairpin samples welded for 0.6 seconds using (a) IR with Power of 1000 W – IR #1, (b) IR with Power of 1300 W – IR #2, and (c) Hybrid with IR Power of 1000 W and BDL Power of 750 W. Insets: higher magnification of the weld joint area.\u003c/p\u003e","description":"","filename":"floatimage31.png","url":"https://assets-eu.researchsquare.com/files/rs-7140646/v1/b8c76ec0495efddd111a7d9e.png"},{"id":88268896,"identity":"1ed73ec7-8b6b-4ba5-af31-46268468a251","added_by":"auto","created_at":"2025-08-04 16:52:42","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":679288,"visible":true,"origin":"","legend":"\u003cp\u003eOptical macrographs showing the cross-sections of the welded hairpin samples using (a) IR with Power of 1000 W – IR #1, (b) IR with Power of 1300 W – IR #2, and (c) Hybrid with IR Power of 1000 W and BDL Power of 750 W. Dashed lines indicate the weld lines.\u003c/p\u003e","description":"","filename":"floatimage41.png","url":"https://assets-eu.researchsquare.com/files/rs-7140646/v1/cfb528aa1774df6e6c00751f.png"},{"id":88268858,"identity":"3524452c-fe0b-4f42-ac67-5bb1390875f9","added_by":"auto","created_at":"2025-08-04 16:52:34","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":22621,"visible":true,"origin":"","legend":"\u003cp\u003eHardness profiles of all samples were measured from the base metal to the heat-affected zone (HAZ) and the weld zone.\u003c/p\u003e","description":"","filename":"floatimage51.png","url":"https://assets-eu.researchsquare.com/files/rs-7140646/v1/552e74f2b7f05c13d9e69aec.png"},{"id":88268947,"identity":"37c55cc3-7e78-47b5-85e7-b5e0e4f7aaea","added_by":"auto","created_at":"2025-08-04 16:52:44","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":156579,"visible":true,"origin":"","legend":"\u003cp\u003e(a) General picture of a bent sample, and (b) close-up view of a weldment after bending\u003c/p\u003e","description":"","filename":"floatimage6.png","url":"https://assets-eu.researchsquare.com/files/rs-7140646/v1/b3b927ed56ae56d92425c148.png"},{"id":88268890,"identity":"f4430217-ace8-4abb-b2cb-655b7140a91f","added_by":"auto","created_at":"2025-08-04 16:52:41","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":51728,"visible":true,"origin":"","legend":"\u003cp\u003ePhotos showing an example of a sample after testing. The inset shows a higher magnification at the fracture location\u003c/p\u003e","description":"","filename":"floatimage7.png","url":"https://assets-eu.researchsquare.com/files/rs-7140646/v1/1b298cf753a1c4bf4b8b6f18.png"},{"id":88269005,"identity":"b58e2c8f-f46e-4fde-8c12-0f4665af76bd","added_by":"auto","created_at":"2025-08-04 16:52:47","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":29480,"visible":true,"origin":"","legend":"\u003cp\u003eForce-Elongation diagrams from three batches of evaluated samples\u003c/p\u003e","description":"","filename":"floatimage8.png","url":"https://assets-eu.researchsquare.com/files/rs-7140646/v1/7d97dbcdab8795744e58874b.png"},{"id":88268903,"identity":"d4c6953f-488e-43cc-9eb5-fc2fffd8b443","added_by":"auto","created_at":"2025-08-04 16:52:43","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":23202,"visible":true,"origin":"","legend":"\u003cp\u003eThe plot of resistivity data shows consistency for IR #2 and Hybrid samples, while IR #1 samples show inconsistent results. The dashed line indicates the resistivity of unwelded/reference samples, i.e., around 1.9\u003csup\u003e \u003c/sup\u003e× 10- 4 Ω. cm.\u003c/p\u003e","description":"","filename":"floatimage9.png","url":"https://assets-eu.researchsquare.com/files/rs-7140646/v1/668e2118e4601b97c4752bca.png"},{"id":88269003,"identity":"de2425f6-5a7a-4eac-b7f7-704b137e0f88","added_by":"auto","created_at":"2025-08-04 16:52:47","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":479991,"visible":true,"origin":"","legend":"\u003cp\u003eScanning electron macro and micrographs showing fracture surfaces of samples related to (a\u003csub\u003e1-2\u003c/sub\u003e) IR #1, (b\u003csub\u003e1-2\u003c/sub\u003e) IR #2 and (c\u003csub\u003e1-2\u003c/sub\u003e) Hybrid\u003c/p\u003e","description":"","filename":"floatimage10.png","url":"https://assets-eu.researchsquare.com/files/rs-7140646/v1/3a0815affa5cdec0b2f6523b.png"},{"id":103507398,"identity":"dbf21775-f848-469e-b429-1c82ac7a3a49","added_by":"auto","created_at":"2026-02-26 13:41:13","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":4557024,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7140646/v1/cf6c20d5-2d63-46ad-8204-6052203a8fe7.pdf"}],"financialInterests":"","formattedTitle":"Comparison of the Physical Performance between Infrared and Hybrid Welding of Copper Wire Hairpins","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eHairpins - \u003cem\u003ethe name known for this component due to its identical shape to the hairpin\u003c/em\u003e - are critical components in an electric vehicle motor. The electric motor in an electric vehicle, which converts energy from a battery into mechanical energy to power the wheels, contains two essential components: the rotor and the stator [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e, \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. A stator is the stationary part that has copper coils, which will create a magnetic field, while the rotor is the rotating part. There may be hundreds of connections in the stator, and they must be reliable as they provide both the mechanical and electrical connections.\u003c/p\u003e\u003cp\u003eRiedel et al. [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e] laid out the general manufacturing steps and challenges in hairpin production. One of the steps is connecting each wire to a hairpin stator. Aside from strengths, the connection must also have low electrical resistance. The tip of a hairpin is typically joined by welding. Several methods are commonly used to weld the hairpin, including electron beam welding, laser welding, micro TIG welding, and resistance brazing [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]. The weld must be of high quality, as it provides both mechanical and electrical contact. It should be strong, stable, and last over long periods of time. Various publications related to hairpin welding are reviewed in the following section.\u003c/p\u003e\u003cp\u003eGlaessel et al. employed an infrared disk laser with a power of 2500\u0026ndash;7000 W, using a sample size of 3.15 \u0026times; 1.6 mm and a cross-sectional area of 5 mm\u003csup\u003e2\u003c/sup\u003e. The authors reported that a welding depth of 4.1 mm was achieved in 0.11 seconds. The electrical resistance was measured to be approximately 90 \u0026micro;Ω for samples with a weld depth of 4.1 mm and approximately 110 \u0026micro;Ω for those with a depth of 1.3 mm. Their results showed that the deeper the weld depth, the lower the resistance. The results demonstrate the suitability of the infrared for hairpin welding [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]. Feve et al. investigated the welding of coil terminals or hairpins using a 500 W continuous-wave blue laser system and a beam diameter of 230 \u0026micro;m on 1 \u0026times; 1 mm copper pins. They managed to obtain a weld depth of around 1 mm. The authors claimed that the welding process did not produce any spatter, and the resulting weldments were strong. Additionally, the authors successfully welded 0.25 mm-thick copper foils at a power of 275 W and a speed of 1 mm/s. Successful welding was then also demonstrated with a 1 mm thick copper plate when power was increased to 600 W [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]. Glaessel et al. reported a welding strategy using rectangular copper wire with dimensions of 4.2 x 2.5 mm and a cross-sectional area of around 10.5 mm\u003csup\u003e2\u003c/sup\u003e. The machine used was a TruDisk 8001 disk laser, featuring a power of 7 kW, a wavelength of λ\u0026thinsp;=\u0026thinsp;1030 nm, and a fiber diameter of 150 \u0026micro;m. The resistance was measured to be in the range of 36 to 40.7 \u0026micro;Ω, which is comparable to the resistance of the reference material, around 37 \u0026micro;Ω. The results suggested that laser beam technology has the potential for hairpin welding applications [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e, \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. Toth et al. conducted welding experiments on rectangular copper wires with varying oxygen content. A vacuum electron beam welding (EBW) machine with a maximum output power of 15 kW was used. The samples had a dimension of 4.3 x 2.33 mm, which gives a cross-sectional area of around 10 mm\u003csup\u003e2\u003c/sup\u003e. The resulting weld depth was measured to be around 3 mm. Using both X-ray computed tomography and metallography examination, the authors were able to locate the porosity and determine the volume and number of porosities in the weld joint area. The authors also claimed that no spatter was observed during welding [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]. Dimatteo et al. employed a continuous-wave (CW) nLight Alta 3.0 kW Yb: Fiber laser with a near-infrared wavelength of 1070 nm. The fiber core diameter was 50 \u0026micro;m, and the spot size was 68 \u0026micro;m. The power used varied from 1500 to 2500 W. The pure copper samples used had a length of about 100 mm and a cross-sectional area of 7 mm\u003csup\u003e2\u003c/sup\u003e. Each sample was coated with insulating lacquer. Before welding, approximately 10 mm of the insulation material was removed from one end of each sample, where welding and joining were to occur. The welded samples were subjected to resistance (R) measurements and peel testing. The R was measured to be between 95 and 113 \u0026micro;Ω, and the force was 190\u0026ndash;490 N. For comparison, the R value for the unwelded (base metal) was approximately 114 \u0026micro;Ω [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]. Ning and Zhang conducted welding on Cu hairpins using an ultrasonic-assisted laser machine. The sample size was 1mm x 4 mm, with welding times ranging from 2 seconds to 6 seconds, and power ranging from 1200 W to 1500 W. The authors claimed that laser spot welding, performed with a 5-second duration and 1500 W power under argon protection, resulted in well-formed joints, excellent welding depth, and good repeatability. Microhardness data indicated higher hardness values in the fusion zone (FZ) than in the heat-affected zone (HAZ) or the base metal (BM). Tensile test results showed that welding for 4 seconds had higher strength than that of 5 seconds, i.e., 716 N vs 661 N. However, the electrical resistance of the welded samples showed a weld time of 5 seconds being the closest to that of the base metal, i.e., 165 \u0026micro;Ω, compared with the base metal of around 170 \u0026micro;Ω [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]. Demir et al. performed laser welding on pure Cu with Power ranging from 2100 W to 6000 W, on rectangular samples with dimensions of 3.87 x 2.66 mm. The wavelengths of the laser were near-infrared (1030\u0026ndash;1070 nm), mid-green (515 nm), and mid-blue laser (455 nm). From the peel tests, it was revealed that the highest force was produced by near-infrared radiation with a power of 6000 W, resulting in a force of around 400\u003csup\u003e\u0026plusmn;\u0026thinsp;50\u003c/sup\u003e N, which was comparable to that of the base metal. The authors concluded that near-infrared light, with its relatively low intensity, was beneficial in producing high-quality welds and reducing spatter [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]. D\u0026rsquo;Arcangelo et al. performed laser welding using a near-infrared disk laser with a power of 4 kW, operating at two different wavelengths (1070 nm and 1030 nm) and two different spot sizes (215 \u0026micro;m and 150 \u0026micro;m, respectively). The rectangular copper wire had a dimension of 3.87 x 2.66 mm. The welding time was 135 ms. The depth of the weld joints reached up to 4 mm. The force required to break the weld joints ranged from 200 to 500 N, with an overall average of approximately 320 N [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eMore recently, the blue diode laser (BDL) has been introduced and reported to successfully weld copper [\u003cspan additionalcitationids=\"CR15 CR16 CR17 CR18 CR19 CR20 CR21 CR22 CR23 CR24 CR25\" citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e]; therefore, it could be an alternative for welding hairpins. The BDL can provide better weld quality due to the good stability of the melt pool and less spatter [\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e, \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e]. Zijue et al. employed a blue laser with a power of 1950 W, a spot diameter of 0.6 mm, and an argon gas supply of 15 L/s to weld rectangular copper wires measuring 4 x 2 x 40 mm. Welding times ranged from 0.3 to 1.1 seconds. Additionally, several offset distances were tested, but the offset of 0.5 mm and welding time of 0.7 seconds were found to be the most ideal, resulting in an optimal near-spherical weld bead with a width of 5.4 mm and a depth of around 4 mm. Regarding electrical resistance, a longer weld time is associated with lower resistance [\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eThe use of infrared, green laser, and blue diode laser individually showed that they require high power and/or long welding times. Otherwise, a good weld joint with a decent weld bead may not be achieved due to the relatively low absorption of copper by the laser. At room temperature, the absorption of laser light by copper is approximately 10%, 40%, and 60% for infrared (IR), green laser (GL), and blue diode laser (BDL) wavelengths, respectively. Therefore, increasing power is one way to obtain deep penetration. However, an unstable melt pool, in addition to excessive molten metal, may lead to the formation of the welding defects [\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eA hybrid method, combining IR with GL or BDL, could improve efficiency [\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e, \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e, \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e]. Hess et al. combined infrared and green lasers with wavelengths of 1030 nm and 515 nm, respectively. The authors performed welding on copper alloys at a speed of 25 m/min. Individually, both the infrared and green lasers were only able to achieve weld depths of less than 100 \u0026micro;m. With the hybrid method, a weld depth of 300 \u0026micro;m (0.3 mm) was found [\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e]. Fujio et al. reported the effect of a single-mode fiber laser (infrared) vs a hybrid laser combining a blue diode laser and a fiber laser. Their finding suggested that the use of blue diode lasers accelerates copper melting [\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e, \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e]. In this paper, a comparison was performed between infrared and hybrid (infrared coupled with a blue diode laser - BDL) methods on rectangular copper wires to create hairpins. Hairpin weld quality was assessed on the mechanical (tensile and hardness) and electrical (resistivity) properties of welds produced using each method. Metallographic analysis was also performed to assess weld penetration, shape, and structure and correlate the resulting mechanical and electrical properties to the weld microstructure for each processing condition.\u003c/p\u003e"},{"header":"2. Materials and Methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\u003ch2\u003e2.1. Material\u003c/h2\u003e\u003cp\u003eThe material used in these experiments was Oxygen-free copper (\u0026gt;\u0026thinsp;99.96%) shaped into rectangular strips measuring 2.5 x 3 x 100 mm (thickness x width x length), resulting in a cross-sectional area of approximately 7.5 mm\u0026sup2;. The original strips were covered with insulating material. About 10 mm of the insulation was removed from one end of the strip samples, where weld joints were made.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e\u003ch2\u003e2.2. Welding Experiments\u003c/h2\u003e\u003cp\u003eThe hairpin samples were welded for 0.6 seconds using two methods: infrared-only (IR #1 and IR #2) and hybrid (a combination of IR and BDL) methods. The power (P) used for the infrared was 1000 W and 1300 W for IR #1 and IR #2, respectively. For the hybrid method, the combination employed was IR power of 1000 W and BDL power of 750 W. The wavelengths were 1070 nm for IR and 450 nm for BDL. More details about the welding parameters are presented in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. Argon gas with a flow of 15L/min was used as a shielding to protect the samples from the surrounding atmosphere. A schematic diagram of the welding experiments is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e.\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eWelding parameters employed in this investigation\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"6\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eNo\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eMethod\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003ePower (W)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003eSpot size (mm)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e\u003cp\u003eTime (s)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c6\"\u003e\u003cp\u003eHeat Input (J)\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eIR #1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e1000\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0.055\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e0.6\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e600\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e2\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eIR #2\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e1300\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0.055\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e0.6\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e780\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e3\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eHybrid:\u003c/p\u003e\u003cp\u003eIR\u003c/p\u003e\u003cp\u003eBDL\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e1000\u003c/p\u003e\u003cp\u003e750\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.055\u003c/p\u003e\u003cp\u003e0.3\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e0.6\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e1050\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e\u003ch2\u003e2.3. Electrical Resistivity Measurements\u003c/h2\u003e\u003cp\u003eThe electrical resistivity, ρ, was measured using the four-point probe method due to the low resistivity of the copper strips. The Keithley 2400 source measurement unit was configured to supply a current of 100 mA and a compliance of 2.1 V, therefore, providing an accuracy of 3 mΩ. A custom-made four-point measurement housing was designed for the dimensions of the copper strip and contained spring-actuated probes. The housing was attached to Mark-10\u0026rsquo;s ESM301 Force Gauge to apply the same probe pressure to each sample. This measurement takes about 45 seconds for each sample. For comparison and validation of the measurements, the resistivity of unwelded copper material was also measured.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec6\" class=\"Section2\"\u003e\u003ch2\u003e2.4. Microstructure and Hardness\u003c/h2\u003e\u003cdiv id=\"Sec7\" class=\"Section3\"\u003e\u003ch2\u003e2.4.1 Metallography\u003c/h2\u003e\u003cp\u003eStereo microscopy was used to perform metallographic analysis to observe the relatively large weld region and base metal while maintaining high enough resolution to observe the grain structure of each region. Preparation was carried out by first sectioning the welded hairpins transverse to the weld bead. Following sample sectioning, each sample was mounted in Bakelite resin and grinded sequentially using SiC paper from 100 to 2400 grit. After grinding, samples were sequentially polished using a Buehler AutoMet250 auto polisher, operating in concentric mode with 6, 3, and 1 \u0026micro;m diamond polishing media. Samples were then etched to reveal the weld microstructure using a solution of 50 mL of nitric acid and 50 mL of distilled water. Samples were then submersed for 4 s and immediately cleansed with water and ethanol. Upon drying, micrographs of the samples were then recorded using a SZX10 Stereo Microscope (Evident Scientific, formerly owned subsidiary of Olympus Corporation).\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec8\" class=\"Section3\"\u003e\u003ch2\u003e2.4.2. Microhardness\u003c/h2\u003e\u003cp\u003eVickers micro-hardness tests were performed using a LM248 series micro indenter machine (Leco LM) with a load of 300 g. The hardness indentations were placed at a constant distance from the base metal to the weld seams to produce a hardness profile.\u003c/p\u003e\u003cp\u003eThe hardness of the fractured samples, following the peel tests, was also examined to assess the mechanical properties of the local area around the weldments.\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Sec9\" class=\"Section2\"\u003e\u003ch2\u003e2.5. Mechanical Testing\u003c/h2\u003e\u003cdiv id=\"Sec10\" class=\"Section3\"\u003e\u003ch2\u003e2.5.1. Bending\u003c/h2\u003e\u003cp\u003eBending of the samples was conducted after welding with a special fixture which was prepared according to ISO 14270 to ensure that bending processes did not affect the quality of the weldments [\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e].\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec11\" class=\"Section3\"\u003e\u003ch2\u003e2.5.2. Tensile/Peel Testing\u003c/h2\u003e\u003cp\u003eTensile/peel testing was performed in accordance with ISO 14270:2016(E) [\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e] using a Shimadzu AG-IS machine (1, Nishinokyo Kuwabara-cho, Nakagyo-ku, Kyoto 604\u0026ndash;8511, Japan) equipped with a 10 KN load cell. The crosshead speed used in this investigation was 4.2mm/min. Following peel testing, the fractured samples were also prepared metallographically \u003cem\u003e(as described above)\u003c/em\u003e to examine the exact fracture surface.\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Sec12\" class=\"Section2\"\u003e\u003ch2\u003e2.6. Scanning Electron Microscopy\u003c/h2\u003e\u003cp\u003eTo understand the fracture mechanism, scanning electron microscopy analysis was performed using a JEOL JSM-IT200 (JEOL Ltd, 3-1-2, Musashino, Akishima, Tokyo 196\u0026ndash;8558, Japan) on the fracture surfaces following the peel tests. All fracture surfaces were soaked in ethanol and cleaned with an ultrasonic cleaner.\u003c/p\u003e\u003c/div\u003e"},{"header":"3. Results and Discussion","content":"\u003cdiv id=\"Sec14\" class=\"Section2\"\u003e\u003ch2\u003e3.1. Weldments\u003c/h2\u003e\u003cp\u003eThe resulting hairpin welds are presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. The IR #1 samples have very little weld bead, indicating poor penetration or shallow fusion due to insufficient heat input (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea). Figures\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb (IR #2) and 3c (Hybrid) exhibited larger weld beads characteristic of quality welds. All these experiments were conducted for 0.6 seconds, and all of them provided consistent results based on their welding parameters. During welding, minimal spatter (if any) was observed with each method.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec15\" class=\"Section2\"\u003e\u003ch2\u003e3.2. Microstructure and Microhardness\u003c/h2\u003e\u003cp\u003eFigure \u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e shows the cross-sectional views of the welded hairpin samples following metallography examination. The sample welded using infrared with a power of 1000 W (IR #1) reached a weld depth of around 2.4 mm (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea). However, the weld zone has a V-shaped appearance, indicating there was not enough melting on both hairpin stripsand suggesting that power from the heat source was insufficient. It can also be observed from Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea that the grain solidification occurred at an angle pointing to the center of the heat source, indicative of non-uniform cooling due to a large thermal gradient under IR #1 conditions. Further, several small pores are featured under IR #1 conditions, likely resulting from non-uniform cooling.\u003c/p\u003e\u003cp\u003eThe samples welded with 1300 W and hybrid showed completely different grain structures. Some of the grains spanned the entire weld zone. The direction of these columnar grains indicates the uniform cooling/solidification direction towards the heat source as they do not converge to the center as occurred under IR #1 conditions. The samples welded using infrared with a power of 1300 W (IR #2) had a weld depth ranging from 2.9 to 3.1 mm. The weld shape is excellent, resembling a mushroom (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb). However, there is a reasonable size of porosity at the bottom of the weld zone. The sample welded using the hybrid method, which combined infrared (P\u0026thinsp;=\u0026thinsp;1000 W) and blue diode laser (P\u0026thinsp;=\u0026thinsp;750 W) sources, showed a weld depth between 2.7 and 2.9 mm, with a small amount of porosity at the bottom of the weld zone (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ec). Regarding the heat-affected zone (HAZ) \u0026ndash; the area between WZ and BM - the HAZ width was approximately 1.5 mm for IR #1 (1000 W) and just over 3 mm for IR #2. On the hybrid sample, however, the width of the HAZ was around 4.3 mm although the IR power was only 1000 W. This shows the effect of employing BDL as a preheating source [\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e, \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e].\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe grain size of the base metal was less than 10\u0026micro;m. The grain size at the heat-affected zones was between 20 and 30 \u0026micro;m, and they were equiaxed with many twin grains, a typical grain structure of Copper. The weld zone has a completely different grain structure. The grains were columnar and their length could be up to the entire length of the weld zone, e.g., 3 mm.\u003c/p\u003e\u003cp\u003eHardness profiles are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e. It can be observed that there may be a slight decrease in hardness in the heat-affected zone and the fusion zone, due to the annihilation of dislocations in these areas, as well as the larger grain size compared to the base metal. This is consistent with what was previously reported by Dimatteo et al. [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e].\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec16\" class=\"Section2\"\u003e\u003ch2\u003e3.3. Peel Tests\u003c/h2\u003e\u003cp\u003ePrior to the tensile peel tests, all the welded samples were bent according to ISO 14270 \u003cem\u003e(as explained in the Experimental Procedure section)\u003c/em\u003e. Figure\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e shows the bent samples prior to peel testing. An example of a fractured sample following the peel test is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eAs mentioned earlier, samples welded with an IR power of 1000 W exhibited little penetration (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea) compared to those with an IR power of 1300 W (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb) and the hybrid (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ec). This is also supported by the peel test results. The results of peel tests are presented on Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e, while a plot showing force vs. elongation is presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e. All samples had more than 10% elongation. With the relatively good weld depth and excellent size of the melt/fusion zone, the force required to break the samples was rather high, ranging from 400 to 785 N. The highest force was recorded for the IR#2 samples. This is higher than what was previously reported by Dimatteo [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e], i.e., up to 500 N, and by Ning and Zhang [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e], with a maximum load of 717 N and a welding time of up to 6 s, compared to 0.6 s in this investigation. Joints with high strength are required as the vehicle would experience vibrations when in operation, which may cause premature fatigue failure, particularly if porosity is present.\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003ePeel test results of the hairpin welded samples.\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"4\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\"\u0026plusmn;\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\"\u0026plusmn;\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eSamples\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eParameters\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eForce (N)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003eElongation (%)\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eIR #1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eP\u0026thinsp;=\u0026thinsp;1000 W; t\u0026thinsp;=\u0026thinsp;0.6 s\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c3\"\u003e\u003cp\u003e410\u003csup\u003e\u0026plusmn;\u0026thinsp;15\u003c/sup\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c4\"\u003e\u003cp\u003e11\u003csup\u003e\u0026plusmn;\u0026thinsp;2\u003c/sup\u003e\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eIR #2\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eP\u0026thinsp;=\u0026thinsp;1300 W; t\u0026thinsp;=\u0026thinsp;0.6 s\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c3\"\u003e\u003cp\u003e770\u003csup\u003e\u0026plusmn;\u0026thinsp;15\u003c/sup\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c4\"\u003e\u003cp\u003e12\u003csup\u003e\u0026plusmn;\u0026thinsp;3\u003c/sup\u003e\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eHybrid\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eIR_P\u0026thinsp;=\u0026thinsp;1000 W and BDL_P\u0026thinsp;=\u0026thinsp;750 W; t\u0026thinsp;=\u0026thinsp;0.6 s\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c3\"\u003e\u003cp\u003e720\u003csup\u003e\u0026plusmn;\u0026thinsp;20\u003c/sup\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c4\"\u003e\u003cp\u003e13\u003csup\u003e\u0026plusmn;\u0026thinsp;3\u003c/sup\u003e\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec17\" class=\"Section2\"\u003e\u003ch2\u003e3.4. Resistivity Measurements\u003c/h2\u003e\u003cp\u003e\u003cdiv class=\"BlockQuote\"\u003e\u003cp\u003eThe results from resistivity measurements are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e. The resistivity of the unwelded copper strip (reference sample) was approximately 1.9 \u0026times; 10^ (-4) Ω cm. The welded copper strips had resistivity of 1.2x10\u003csup\u003e-3\u003c/sup\u003e \u0026ndash; 6.2x10\u003csup\u003e-3\u003c/sup\u003e Ω.cm, 2.3x10\u003csup\u003e-4\u003c/sup\u003e \u0026minus;\u0026thinsp;4x10\u003csup\u003e-4\u003c/sup\u003e Ω.cm and 2.6x10\u003csup\u003e-4\u003c/sup\u003e \u0026ndash; 6.1x10\u003csup\u003e-4\u003c/sup\u003e Ω.cm for IR #1, IR #2, and Hybrid, respectively.\u003c/p\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"BlockQuote\"\u003e\u003cp\u003eIR #1 samples had high resistivity, i.e., 5 to 31 times higher than that of the unwelded sample. Both IR #2 and hybrid samples had lower resistivity compared with that of IR #1 and were only up to three times higher than those of unwelded samples. The reason why IR #1 had very high resistivity may be related to the high number of defects and porosity, as well as the small area of the weld zones, i.e., only 20\u0026ndash;30% of the sample size. These results are consistent with what had been reported by Dimatteo et al. [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e] and Ning and Zhang [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e], where great weld depth, hence, stronger joints are associated with low resistivity.\u003c/p\u003e\u003c/div\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec18\" class=\"Section2\"\u003e\u003ch2\u003e3.5. Scanning Electron Micrographs\u003c/h2\u003e\u003cp\u003e\u003cdiv class=\"BlockQuote\"\u003e\u003cp\u003eThe SEM images of the fracture surfaces (Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e) indicated a relatively small fracture surface area on the IR #1 sample. In contrast, both HR #2 and Hybrid samples exhibited large fracture surfaces, indicating a large joint area. All samples fractured with a ductile mechanism, characterized by microvoid coalescence or dimples. This agrees with the elongation of more than 10% for all peel test samples (Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e and Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e). From the macrographs (Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e), it is evident that the weld zone of IR #1 is reasonably narrow, while both IR #2 and Hybrid showed a much larger weld zone (larger fracture surface area). It is worth noting that, even though the cross-sectional area of the samples in this investigation (based on the size of the sample) was around 7.5 mm\u003csup\u003e2\u003c/sup\u003e, the cross-sectional area of the welded area varied from 2.5 to 7.8 mm\u003csup\u003e2\u003c/sup\u003e where IR #1 samples had the smallest area followed by the Hybrid samples and IR #2, respectively. This also explains why IR #2 samples required the highest force to break during peel testing compared to samples from other batches.\u003c/p\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e"},{"header":"4. Conclusions","content":"\u003cp\u003e\u003cdiv class=\"BlockQuote\"\u003e\u003cp\u003eBased on the present investigation, it can be concluded that both IR-only and Hybrid (IR\u0026thinsp;+\u0026thinsp;BDL) methods can be employed to produce a reliable hairpin weld joint. The preliminary results indicated that a weld time of 0.6 seconds is sufficient to achieve a reliable weld joint. This is demonstrated by the resistivity measurement results, which show that the resistivity of the welded joints was only marginally higher than that of the unwelded copper. From the peel test, the results were consistent with the resistivity measurements data, i.e., IR #2 and Hybrid samples required higher force to break than IR #1 samples due to the good weld quality of the former samples. Although the results from this investigation, in terms of strengths and resistivity, are very promising, further experiments are needed with different parameters to achieve high quality weld joints with the industry goal of 0.1 seconds per weld.\u003c/p\u003e\u003c/div\u003e\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eWe would like to thank Denso Corporation for providing the pure copper strip samples.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData availability\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe data that support the findings of this study are available from the corresponding author on reasonable request.\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eContributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eTim Pasang: methodology, formal analysis, writing\u0026mdash;original draft, writing\u0026mdash;review and editing. Shumpei Fujio: experiments and formal analysis. Pai-Chen Lin: testing, analysis and resources. Zheng-Da Wang: testing and analysis. Roni Rountree: metallography, analysis, writing-review and editing. Tony Hanson: analysis and methodology. \u0026nbsp;Jie Xiong: analysis and methodology. Jacob Nyholm: metallography and analysis. Wojtek Misiolek: writing-review, methodology, resources and editing. Poppy Puspitasari: analysis and methodology. Yuji Sato: methodology, resources and supervision. Masahiro Tsukamoto: resources, supervision and funding acquisition.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCorresponding author\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eCorrespondence to Tim Pasang: [email protected]\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis research work was supported by OU Master Plan Implementation Project, The University of Osaka\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting interests\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare no competing interests.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eRahman K, Jurkovic S, Hawkins S, Tarnowsky SA, Savagian P. Propulsion system design of a battery electric vehicle. 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Development of high intensity multibeam laser metal deposition system with blue diode lasers for additively manufacturing of copper rod. \u003cem\u003eJ. Laser Appl.\u003c/em\u003e 2021, \u003cem\u003e33\u003c/em\u003e, 042014.\u003c/li\u003e\n\u003cli\u003eTang Zijue, Wan Le, Yang Huihui, Ren Pengyuan, Zhu Changlong, Wu Yi, Wang Haowei, Wang Hongze. Stable Conduction mode welding of conventional high-reflectivity metals with 2000 W blue laser. Opt Laser Technol 2024;168: 109971\u003c/li\u003e\n\u003cli\u003eShumpei Fujio, Yuji Sato, Keisuke Takenaka,Rika Ito, Masatoshi Ito, Masayuki Harada, Takashi Nishikawa, Tetsuo Suga and Masahiro Tsukamoto, \u0026ldquo;Welding of pure copper wires using a hybrid laser system with a blue diode laser and a single-mode fiber laser\u0026rdquo;, J. Laser Appl. 33, 042056 (2021); doi: 10.2351/7.0000502\u003c/li\u003e\n\u003cli\u003eRutering M. Hybrid Solution Moves Boundaries of Copper Welding, Photonics Views 5/2019, \u0026copy; 2019 WILEY-VCH Verlag GmbH \u0026amp; Co. KGaA, Weinheim, www.photonicsviews.com\u003c/li\u003e\n\u003cli\u003eISO 14270:2016-11. Resistance welding \u0026ndash; destructive testing of welds \u0026ndash; specimen dimensions and procedure for mechanized peel testing resistance spot, seam and embossed projection welds. 2016.\u003c/li\u003e\n\u003cli\u003eA. Hess, R. Schuster, A. Heider, R. Weber, and T. Graf, \u0026ldquo;Continuous wave laser welding of copper with combined beams at wavelengths of 1030 nm and of 515 nm, LiM 2011,\u0026rdquo; Phys. Proc. 12, 88\u0026ndash;94 (2011).\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":true,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Copper, Hairpin, Infrared, Hybrid, Welding, Resistivity","lastPublishedDoi":"10.21203/rs.3.rs-7140646/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7140646/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eWith the rise in demand for electric vehicles, hairpin welding is gaining popularity for its efficient manufacturing of critical EV motor components. Due to its high thermal conductivity, copper is commonly used. The tip of the hairpin can be joined by laser welding, micro TIG, and resistance brazing. In this paper, hairpin welding using infrared and hybrid \u003cem\u003e(infrared combined with blue diode laser)\u003c/em\u003e technology is presented. When using infrared-only with a power of 1000 W, a shallow weld joint was produced. A decent weld join (weld bead) was achieved when the power of the infrared was increased to 1300 W. However, when the hybrid method was employed with an infrared power of 1000 W and blue diode laser power of 750 W, a very satisfactory weld joint was achieved, i.e., comparable to that of infrared only with a power of 1300 W. The electrical resistivity results indicated relatively low resistivity of the welded hairpin samples. Peel test results also suggest that high force is required to break the decent weld joints, especially the samples welded with IR 1300 W and the hybrid method. The results showed that hybrid laser technology is very promising for hairpin welding.\u003c/p\u003e","manuscriptTitle":"Comparison of the Physical Performance between Infrared and Hybrid Welding of Copper Wire Hairpins","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-08-04 16:49:24","doi":"10.21203/rs.3.rs-7140646/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"aa85aa6b-9eca-4a5f-a350-ef6f8bb476b3","owner":[],"postedDate":"August 4th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2026-02-26T05:35:11+00:00","versionOfRecord":[],"versionCreatedAt":"2025-08-04 16:49:24","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-7140646","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7140646","identity":"rs-7140646","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}