Effect of Electric Pulse Treatment on the Interfacial Properties of Copper/304 Stainless Steel Composite Thin Strips Fabricated by Roll Bonding

Abstract

Annealing is a commonly used post-processing method for composite thin strips but suffers from drawbacks such as long processing time, high energy consumption, and susceptibility to oxidation. Replacing annealing with electric pulse treatment (EPT) can address these issues. In this study, a specially designed fixture was used to investigate the effects of pulsed current on the bonding strength of T2 copper (Cu)/304 stainless steel (SS) composite thin strips. The initial strip, with a 50% reduction rate, was prepared using a two-high mill, resulting in a Cu/SS composite strip with a thickness of 0.245 mm. Pulsed current treatment was applied with peak temperatures ranging from 350 °C to 600 °C. The results showed that EPT significantly improved the bonding strength. A pulsed current of 55 A resulted in the highest average peel strength of 10.66 ± 0.93 N/mm, with a maximum Fe content on the Cu side of 7.39 ± 0.84%, while a pulsed current of 65 A resulted in the highest Cu content on the SS side, reaching 57.54 ± 2.06%. This study demonstrates that EPT effectively controls the deformation behavior and interface state of composite strips, producing Cu/SS composite thin strips with high bonding strength.

1. Introduction

Metal composite thin strips usually integrate the unique properties of each constituent to achieve superior performance over single-metal materials [1,2,3,4,5]. Copper/stainless steel (Cu/SS) ultrathin composite strips, known for their extremely thin dimensions, high specific surface area, excellent electrical and thermal conductivity, superior mechanical strength, and outstanding corrosion resistance, are promising in advanced applications such as sensor diaphragms and flexible display substrates [6,7,8,9].

However, the significant differences in the physical and chemical properties of Cu and SS hinder interdiffusion, resulting in weak interfacial bonding [10]. The interface cracking of the composite ultrathin strips limits further processing and applications. Fabrication methods such as spray deposition [11], magnetron sputtering [12], and diffusion bonding [13] each present inherent limitations, including process complexity, high costs, and low efficiency. Roll bonding is a scalable and reliable method for fabricating ultrathin composite strips but often relies on prolonged annealing to improve interfacial bonding [14,15]. However, the high costs, extended processing times, and harmful emissions of annealing present challenges to sustainable production [16]. For example, Yanyang Qi et al. [17] prepared ultrathin stainless steel/copper composite materials by heating and cooling them with the furnace and holding them at the target temperature for 5 min. Chen Wang et al. [18] Cu/Al annealed composite thin strips in a tube furnace for 1 h to achieve a higher bonding strength.

Current-assisted processing, particularly pulsed current, has emerged as a promising alternative due to its high instantaneous energy density, enabling improved deformation behaviors [19], microstructural refinement, and interfacial bonding [20]. Tingting Zhang et al. [21] successfully prepared AZ31B Mg/5052 Al alloy composite plates with a thickness of 3.8 mm by pulsed current-assisted rolling welding. Xiongwei Guo et al. [22] applied a pulse current to the TA1/304 composite plate during the rolling process, which effectively improved the bonding strength of the composite plate. In the current research of Niu X. et al. [10], there are problems such as long time and high energy consumption, but electrical treatment can greatly shorten the treatment time of Cu/SS composite thin strips, reduce energy consumption, and greatly improve their bonding strength. However, in the current research, the influence mechanism of current processing parameters on the interface bonding state of composite thin bands is not clear. Therefore, it is necessary to study the effect of current on the bonding strength and microstructure of T2 Cu/304 SS composite thin strip.

This study investigates the impact of short-time pulsed current on the bonding performance and interfacial microstructure of T2 Cu/304 SS composite ultrathin strips. Results reveal that pulsed current significantly enhances interfacial bonding strength, offering insights into the mechanisms of bonding improvement. These findings provide a theoretical foundation for developing efficient, sustainable post-treatment methods and expanding the applications of composite materials in advanced industries.

2. Experimental

2.1. Fabrication of the Composite Thin Strips

The raw materials selected for this study include T2 pure Cu sheets measuring 100 mm × 30 mm × 0.3 mm and 304 SS sheets measuring 100 mm × 30 mm × 0.2 mm. The raw materials were subjected to annealing before roll bonding. The T2 pure copper strip underwent stress relief annealing at a low temperature of 260 °C for 1 h, while the 304 stainless steel strip underwent homogenized solution annealing at 1050 °C for 4 h. The chemical composition of the original 304 stainless steel strip and T2 pure copper used was detected by spectroscopic analysis, as shown in

Table 1. Chemical compositions of the strips (wt%).

Materials	Cr	Ni	C	O	Mn	Si	P	S	Fe	Cu
SS304	18.16	7.97	0.06	—	1.20	0.58	0.43	0.02	Bal.	—
T2 Copper	—	≤0.004	—	≤0.005	—	—	—	≤0.005	—	Bal.

. Cu/SS composite thin strips were prepared using a combination of cold rolling and EPT. The cold rolling was performed on a two-high rolling mill equipped with custom tungsten steel rollers, with a roller diameter of Φ150 mm and a roller speed of 37.7 rad/s and the rolling force was 90 kN.

The preparation and post-treatment processes of the Cu/SS composite thin strips are depicted in Figure 1. Prior to assembly, the oxide layer and contaminants on the bonding surfaces were removed using a steel wire brush. The SS strip (upper layer) and Cu strip (lower layer) were then stacked and fed into a ZJ150 two-high rolling mill (XFRMM Ltd., Shenyang, China). To achieve a 50% reduction rate in the Cu/SS composite thin strip, the rolling force was set to 90 kN. The rolled Cu/SS composite strip was subsequently cut into 8 mm-wide strips using wire electrical discharge machining. These strips served as specimens for electric pulse treatment (EPT), while two additional groups underwent corresponding heat treatments (HTs) as control groups. During the EPT process, thermocouples connected to a temperature recorder with the model TCP-500XL (MEASURE FINE Ltd., Hangzhou, China) were placed in contact with the SS strip to monitor the temperature of the Cu/SS composite thin strips. Finally, peeling tests were performed to evaluate the bonding strength under varying parameters.

2.2. EPT and HT Experiments

The Cu/SS composite thin strips after rolling and EPT are shown in Figure 2. To ensure an initial bonding strength after a single-pass rolling process, a high reduction rate was required to achieve preliminary bonding. The reduction rate was set at 50%, resulting in a post-rolling thickness of the composite thin strips ranging from 0.245 mm to 0.25 mm. The thickness of Cu is about 0.14 mm, and the thickness of SS is about 0.11 mm, and their reduction rates are 53.3% and 45%, respectively. Due to the mismatch in deformation behavior between pure Cu and SS, significant warping occurred in the composite thin strips after cold rolling. However, the application of EPT eliminated internal stresses, resulting in a flattened composite thin strip.

Based on our preliminary experiments, a fixture capable of providing stable current conduction for composite thin strips was designed. As shown in Figure 3, the fixture adopts a sequentially moving configuration with positive and negative electrodes, where the distance between the two electrodes is 12 mm. By moving the electrodes, the composite thin strip is continuously subjected to pulsed current. The positive electrode contacts the SS side, while the negative electrode contacts the Cu side. When the pulse power supply is activated, the higher electrical conductivity of copper compared to SS causes the current to flow from the positive electrode through the steel strip and then immediately to the copper side. Since the Cu and SS strips are preliminarily bonded by the first pass of cold rolling, a small portion of the current also flows through the bonding interface and the SS strip.

The composite thin strips were placed into the fixture and connected to a low-frequency pulse power supply. Uniform sequential current conduction was applied, with each conduction cycle lasting 5 s. The pulse current frequency was set to 500 Hz with a duty cycle of 20%. The bonding strength of copper/stainless steel composite thin strips was investigated under different current densities. In this study, HT-450 and HT-600 refer to annealing at 450 °C and 600 °C, respectively, while EPT-350, EPT-400, EPT-450, EPT-500, EPT-550, and EPT-600 represent the electrical parameters corresponding to the composite thin strips under different peak temperatures induced by varying EPT currents. The detailed HT and EPT parameters are summarized in

Table 2. Parameters of HT and EPT.

No.	Reduction Rate	Peak Temperature (°C)	Effective Current (A)	Processing Time (s)
As-rolled	50%	—	—	—
HT-450 HT-600		450 600	—	100
EPT-350 EPT-400 EPT-450 EPT-500 EPT-550 EPT-600		350 400 450 500 550 600	45 50 55 59 62 65	5

.

The temperature–time curves for different HT and EPT parameters are shown in Figure 4. The maximum temperature achieved after 5 s of pulse current for each EPT parameter was recorded (Figure 4b). The highest bonding strength was observed when the maximum temperature reached 450 °C, while at 600 °C, the composite exhibited reasonable metallurgical bonding. Annealing at 450 °C and 600 °C (HT-450 and HT-600) served as control groups for comparison.

2.3. Mechanical Test and Microscopic Characterization

The peeling test, conducted using an INSTRON 5969 universal testing machine (ITW Inc., Norwood, MA, USA), evaluated the interfacial bonding strength of composite thin strips. Static tensile forces were simultaneously applied to the Cu and SS sides at the opening of the strips, with a peeling rate of 25 mm/min, enabling progressive separation along the longitudinal interface. The peeling strength was determined from the stable region of the peeling curve over the last 20 mm of a total 70 mm peeling length. Five samples were tested under each parameter to obtain the average peel strength. Sample dimensions and testing procedures followed the GB/T 2792-2014 standard [23].

Microscopic characterization was performed using an electron microscope with the model JSM-IT 500 (JEOL Ltd., Tokyo, Japan). Scanning Electron Microscopy (SEM) was used to examine the bonding interface morphology in as-rolled, HT, and EPT states. Energy Dispersive Spectroscopy (EDS) was used to characterize elemental distributions at the interfaces. The microstructural analysis focused on unpeeled regions of the peeled specimens. Sample preparation included water-bath polishing with quartz sandpaper, mechanical polishing with a diamond suspension, and extracting 1 cm sections from both Cu and SS sides of the peeled region to study the bonding interface features. The Vickers microhardness of SS and Cu side surfaces was measured using an HVT-1000 microscopic Vickers hardness tester (SCTMC Ltd., Shanghai, China) in the unpeeled area. Three samples were taken under each parameter, and three points were taken for each sample to be averaged. The residence time was 10 s with a load of 500 g.

3. Experimental Results

3.1. Bonding Properties of the Specimen

The rolled composite thin strips exhibit a porous bonding interface and low bonding strength, with an average peel strength of 0.44 ± 0.10 N/mm. As shown in Figure 5, both EPT and HT significantly influence bonding strength. Increasing the HT temperature enhances the metallurgical bonding effect. Specifically, HT-450 results in a peel strength of only 0.96 ± 0.39 N/mm, while HT-600 increases the peel strength to 4.00 ± 0.49 N/mm.

EPT of 5 s can rapidly enhance the bonding strength of the composite thin strips. The peel strength exhibits a two-phase trend, initially increasing and then decreasing. In the first phase, with current parameters between 45 A and 55 A, the bonding strength increases sharply as the pulsed current rises. Notably, under the EPT-450 condition, the average peel strength at the bonding interface reaches its highest value of 10.66 ± 0.93 N/mm. In the second phase, with current parameters between 55 A and 65 A, the bonding strength decreases as the pulse current increases. This is due to the damage to the copper substrate, which lowers the deformation resistance of copper. As a result, the composite strip primarily exhibits fracture of the Cu substrate during the peel test. Under the EPT-600 condition, the average peel strength of the composite thin strip reaches its lowest value of 5.29 ± 0.40 N/mm.

A comparison of HT-450 with EPT-450 and HT-600 with EPT-600 reveals that, while both processes reach the highest temperature before post-treatment, EPT has a significantly greater effect on the bonding interface than annealing. When the peak temperature is 450 °C, the peel strength of EPT-450 is 11.1 times that of HT-450, while the peel strength of EPT-600 is 1.3 times that of HT-600. The increase in peel strength at 600 °C when applying EPT compared to HT at the same temperature is insignificant, which is attributed to the instantaneous high temperature generated by the large pulsed current, leading to the formation of a strong metallurgical bond at the interface [24]. Peel failure occurs on the Cu substrate, which makes the bond strength lower. In contrast, HT at 600 °C accelerates element diffusion [17], promoting metallurgical bonding in localized regions, at which time the bonding strength of the bonding interface is weaker than that of the copper substrate, and peel failure occurs at the Cu/SS interface.

3.2. Interfacial Structures of the As-Rolled and HT Specimens

Figure 6 shows the SEM images and corresponding EDS line scan spectra of the Cu/SS composite thin strip in both as-rolled and HT states. Figure 6a,c,e, represent the composite interface states in the as-rolled and annealed conditions and the yellow line represents the line scan path, while Figure 6b,d,f show the diffusion thickness of Cu atoms towards the SS side. No plateau was observed in the EDS line scan spectra at the bonding interface of the samples, indicating that no intermetallic compounds (IMCs) were formed at the Cu/SS bonding interface [14]. As shown in Figure 6a,c,e, defects can be observed at the composite interface in both the as-rolled and HT conditions. From Figure 6a, it is evident that there are significant long gaps and pores at the composite interface (marked by red dashed boxes), indicating poor bonding in the as-rolled Cu/SS composite thin strip. From Figure 6b, it can be seen that the thickness of the diffusion layer is only 1.022 μm. From Figure 6c, it can be seen that an incomplete hardened layer exists at the composite interface, which was caused by the breakage of the SS hardened layer under the rolling force, damaging the Cu substrate surface. The rolling process caused fresh Cu to bond with the SS substrate, resulting in a mechanical bond at the interface. However, pores remain at the interface, suggesting that annealing at 450 °C did not significantly improve the bonding strength, and the diffusion layer thickness slightly increased to 1.046 μm (Figure 6d). As shown in Figure 6e, at 600 °C annealing, most of the pores are eliminated, and the bonding state improves. As shown in Figure 6f, the thickness of the diffusion layer increased to 1.272 μm.

By observing the morphology of the peeled Cu/SS composite thin strips in both the as-rolled and HT states, EDS elemental analysis was performed on the Cu elements remaining on the SS surface after peeling. As shown in Figure 7a, the EDS scan reveals that the Cu remaining on the SS side in the as-rolled composite thin strip is insignificant, with a content of only 1.46%. As shown in Figure 7b,c, with the increase in annealing temperature, the diffusion area at the composite interface enlarges. After HT at 450 °C and 600 °C, the Cu content retained on the SS side slightly increases, reaching 5.09% and 10.30%, respectively. This Cu is mainly distributed in the cracks of the fractured hardened layer. A partial enlargement of Figure 7c (marked by red boxes), a small metallurgical bonding region can be observed, accompanied by the appearance of tiny ductile dimples, indicating that plastic fracture of the Cu substrate occurs in this area, and the bonding strength has been improved to some extent.

3.3. Interface Structure Analysis for EPT

Figure 8a,c,e,g,i,l represent the composite interface states after EPT, while Figure 8b,d,f,h,j,k show the thickness of Cu atom diffusion towards the SS side. As shown in Figure 8a,c, although pores are still observed at the composite interface under the conditions of EPT-350 and EPT-400, there are also many areas with strong bonding, indicating that the interface bonding state has improved compared to the as-rolled condition. As shown in Figure 8e,g,i,l, the interface bonding is tight, with no obvious cracks, voids, or delamination defects, and the bonding interface forms a jagged structure, which tends to become smoother with increasing effective current. This improvement is likely due to the local high temperatures generated by the pulsed current, which causes partial melting at the interface. The diffusion of Cu and Fe atoms, combined with the slight clamping force from the electrical contacts, facilitates the reorganization of the hardened layer and effectively fills the defects and voids at the interface, significantly relieving the residual stress in the hardened layer, thereby making the layer smoother.

The EDS line scan results in Figure 8b,d,f,h,i,k show that the concentration of Cu rapidly decreases from the Cu side to the SS side, while the concentration of Fe increases sharply. No plateau was observed at the bonding interface of all samples, indicating that the pulsed current does not lead to the formation of IMCs. Therefore, IMCs do not affect the bonding quality and performance of the Cu/SS composite thin strip. Additionally, the thickness of the diffusion layer increases with the increase in effective current. Under the condition of EPT-450, the diffusion layer thickness is 1.361 μm, and under EPT-600, the diffusion layer thickness reaches its maximum value of 1.612 μm.

By observing the morphology of the peeled Cu/SS composite thin strip after pulsed current treatment, EDS elemental analysis was conducted on the Cu elements remaining on the SS surface after peeling. As shown in Figure 9a–f, with the increase in effective current, the Cu content remaining on the SS side of the composite thin strip gradually increases. As seen in Figure 9a–c, with the increase in current, the Cu content progressively rises, and the Cu remaining on the SS side is distributed in dot-like metallurgical bonding regions. Under the EPT-450 condition, larger Cu flakes appear on the SS side, with a Cu content of 35.23%, and the bonding strength reaches its maximum. This suggests a positive correlation between Cu content and bonding strength.

From Figure 9d–f, it can be observed that starting from EPT-500, the exposed fresh SS metal surface is almost entirely covered by strip-like Cu sheets and scattered dot-like metallurgical bonding regions. In this stage, although the Cu content is high, the peel strength decreases with the increasing Cu content. This is because the Cu/SS composite thin strip forms large, localized, strong metallurgical bonding regions. The strength of the bonding interface of the Cu/SS composite thin strip is greater than the strength of the fractured Cu substrate at the interface. During peeling, microcracks generated at the bonding interface propagate along the Cu grain boundaries, and when the bonding interface strength becomes weaker than that of the Cu substrate, peeling occurs along the interface. Since large areas of the Cu substrate are peeled off, and plastic fracture occurs in the Cu substrate at this point, larger ductile dimples are formed (Figure 9d). The highest Cu content is observed under the EPT-600 condition, reaching 55.57%. At this point, the entire Cu/SS composite thin strip is almost entirely in a metallurgical bonding state, and after peeling, a uniform copper layer adheres to the SS side.

As shown in Figure 10a,c–g, EDS elemental analysis was performed on the Fe content of the Cu surface of the composite thin strip after different post-treatments and peeling. From Figure 10a, it can be seen that the Cu surface in the as-rolled state contains 0.63% Fe, which is due to the rolling pressure and friction during the composite process, causing the Cu and SS metals to be in close contact and flow together, thus promoting the diffusion of Fe elements into the Cu side and Cu elements into the SS side.

Figure 10b shows that with the increase in annealing temperature, the Fe content on the Cu surface and the Cu content on the SS surface both increases. At 600 °C, the Fe content on the Cu surface reaches 2.82%, while the Cu content on the SS side is 10.3%. As the pulsed current increases, the Cu content on the SS side surface increases. The Fe content on the Cu surface, however, shows a positive correlation with the pulsed current up to EPT-450. This is because, at this stage, peeling occurs primarily at the bonding interface. Under the EPT-450 condition, peeling happens from the entire bonding interface of the composite thin strip with maximum bonding strength, resulting in the highest Fe content on the Cu surface, which is 6.61%. At this point, the bonding strength of the composite thin strip is at its highest (10.66 ± 0.93 N/mm).

However, after EPT-500, the Fe content on the Cu surface becomes negatively correlated with the pulsed current. This is because, at this stage, peeling mainly occurs on the Cu substrate. Under the EPT-600 condition, since the SS side is entirely covered by a layer of Cu sheet, the Fe content on the Cu surface is only 1.47%.

4. Discussion

4.1. Effects of HT and EPT on Cu/SS Composite Interface Morphology

As shown in Figure 11, during the cold-rolling composite process, an effective mechanical bond is formed to some extent, while some unbonded areas (pores) also remain. This bonding is mainly physical and typically does not form metallurgical bonding at room temperature [10]. Research has shown that as the annealing temperature increases, atomic diffusion at the interface becomes stronger [25,26,27]. During annealing, atoms near the pre-bonding points are activated, and elements from both the Cu and SS substrates diffuse at the interface, creating new bonding points. This results in the closure of pores and the formation of dot-like metallurgical bonding regions, thus improving the bonding strength of the composite strip. Gondcharton P [28] and others studied the void phenomena in copper–copper bonding layers at temperatures between 300 and 400 °C and found that at certain temperatures, vacancy accumulation promotes atomic diffusion.

When the annealing temperature reaches 450 °C, the Cu and Fe atoms receive relatively low energy, leading to some degree of diffusion, but at a slow rate, resulting in fewer metallurgical bonding regions and a low bonding strength. At 600 °C, the higher temperature provides more energy for atomic diffusion, significantly accelerating the diffusion of Fe and Cu atoms, which increases the Cu content on the SS side to 10.3%, and significantly increases the number of metallurgical bonding regions.

In the as-rolled state of the Cu/SS composite thin strip, the bonding interface contains many defects (long gaps and pores), and the resistivity at these defects is high. The high-energy drift electrons carried by the pulsed current interact extensively with the Fe/Cu atoms in the area, rapidly converting energy into heat and generating local high temperatures [29,30,31]. The pulsed current also induces periodic thermal expansion and contraction of the composite strip, which disrupts the brittle hardened layer at the bonding interface, causing it to melt. The electrical contacts must maintain good contact with the composite strip, thus applying a slight clamping pressure to the interface, making the bonding surface flatter [21,32].

When a pulsed current of 45 A to 55 A is applied, the energy generated gradually increases with increasing current, promoting a gradual increase in the diffusion rate of Cu atoms. After peeling, many dot-like metallurgical bonding regions form on the SS side. When the effective current exceeds 55 A, the diffusion rate of Cu atoms further increases, resulting in the Cu on the SS side of the peeled composite strip exhibiting a flake-like structure.

4.2. Mechanism of Action of EPT on Interfaces

The magnitude of the pulsed current affects the heating rate, thereby influencing the atomic diffusion rate [33,34]. The pulsed current flows from the SS side to the Cu side along the rolling direction, and dislocations move in the direction of the electron wind in the bonding interface region [35]. This results in the rearrangement of surrounding atoms, which, to some extent, causes the hardened layer at the bonding interface to become more even. Overheated dislocations and grain boundaries promote thermally activated diffusion, accelerating the diffusion rate of Cu and Fe atoms [33]. Between EPT-350 and EPT-450, the bonding mode of the Cu/SS composite thin strip is primarily mechanical bonding, with randomly dispersed small metallurgical bonding regions. During the peeling process, the metallurgical bonding is not strong, and most of the applied force acts at the bonding interface, leading to fractures at the interface. In this stage, the peel strength increases with the growth of metallurgical bonding regions. At the same time, the pulsed current disrupts part of the hardened layer, forming an irregular “jagged” structure, which increases the contact area at the bonding interface, making the mechanical bonding more robust and enhancing the overall bonding strength.

From EPT-450 to EPT-600, the bonding mode of the Cu/SS composite thin strip is mainly characterized by flake-like, strong metallurgical bonding, with mechanical bonding and dot-like metallurgical bonding as secondary. The further increase in pulsed current leads to higher temperatures at the bonding interface. The localized regions of Cu melt, making the bonding interface flatter and directly adhere to the SS surface, forming flake-like metallurgical bonding. In areas with relatively lower temperatures, mechanical bonding and dot-like metallurgical bonding dominate.

During peeling, when the composite strip peels from the regions with flake-like strong metallurgical bonding, the bonding strength of the Cu/SS interface exceeds the strength of the Cu substrate at the interface [36]. Since the strength of SS is much greater than that of Cu, the pulsed current can reduce the deformation resistance of Cu and improve its plastic deformation ability. This causes plastic deformation, crack propagation, and fracture within the Cu substrate, leading to peeling from the Cu side of the bonding interface. Under SEM, uniform, stripe-like copper layers adhered to the SS side are observed, and they exhibit characteristics of plastic fracture with ductile dimples [37].

When force is applied to regions with less strong metallurgical bonding, the bonding strength of the Cu/SS interface becomes weaker than the strength of the Cu substrate at the interface, causing peeling to initiate again from the bonding interface. Therefore, in this stage, peeling is the result of both partial peeling of the Cu substrate and partial peeling of the bonding interface.

4.3. Hardness Test

Figure 12 shows a comparison of the Vickers hardness between the SS side and Cu side surfaces as raw materials, in the as-rolled state, and after post-treatment (HT and EPT). After rolling, the micro-Vickers hardness on the SS side increased to 460.04 HV, with an increase of 113.5%, which was consistent with the results of Hedayati, A. et al. [38]. The micro-Vickers hardness on the Cu side increased to 135.40 HV, and the increase reached 18.1%.

After HT-600 treatment, the micro-Vickers hardness on the SS side increased to 527.64 HV, which was 14.7% higher than that of the as-rolled state due to the large and uneven grain size of the SS in the as-rolled state, and the redistribution of residual stresses and the appearance of fine recrystallized grains due to short-term annealing at 600 °C [10].

After EPT-400 treatment, the micro-Vickers hardness on the SS side reaches a maximum of 697.87 HV, which is 1.5 times that of the as-rolled state. This is due to the fact that the pulse current causes smaller recrystallized grains on the surface of the SS than in the as-rolled state, increasing its dislocation density and promoting surface hardening [39,40]. As the pulse current continues to increase, the recrystallization is more complete, and its surface hardness gradually decreases. Both HT and EPT caused the recrystallization of Cu, resulting in a decrease in its micro-Vickers hardness [41].

In summary, EPT can significantly improve the bonding strength of Cu/SS composite thin strips and retain the hardness of the raw material after rolling to a certain extent (as shown in Figure 12). Compared with the Cu/SS composite thin strip with a bonding strength of about 8 N/mm and 3 N/mm, respectively, prepared by intermediate annealing and two-pass rolling at 400 °C and 600 °C [10], the bonding strength was significantly improved to 10.66 N/mm after EPT-450 treatment. However, in the future, it is still necessary to study the evolution of the interface state corresponding to different rolling processes and electrical treatment parameters. It is necessary to study the problems faced by industrial applications, etc.

5. Conclusions

This study investigates the preparation of Cu/SS composite thin strips with high bonding strength by sequentially applying uniform pulsed currents. The pulsed current applied to the surface can promote the mutual diffusion of elements at the bonding interface, significantly enhancing the bonding strength of the composite thin strips. The research findings are as follows:

EPT demonstrates superior efficiency and effectiveness over traditional heat treatment, achieving significantly higher peel strength and copper content in less time. After annealing at 450 °C for 100 s, the peel strength is 0.96 ± 0.39 N/mm, with the copper content on the SS side being 5.06 ± 0.20%. After annealing at 600 °C, the peel strength increases to 4.00 ± 0.40 N/mm, with the copper content on the SS side reaching 11.83 ± 1.00%. After annealing, the Fe content on the Cu side is relatively low. In contrast, after applying pulsed current for 5 s, the peel strength reaches 10.66 ± 0.93 N/mm, with the copper content on the SS side being 41.86 ± 5.67%, and the Fe content on the Cu side is 7.39 ± 0.84%, which significantly outperforms the heat treatment effect. A pulsed current of 55 A is identified as the optimal parameter.

Effect of pulsed current on bonding strength: The bonding strength under EPT shows two distinct phases. In the low-current stage (45–55 A), high instantaneous heat flux density and non-thermal effects significantly accelerate the diffusion of Cu and Fe, increasing the number of dot-like metallurgical bonding regions and strengthening mechanical bonding, thus greatly improving bonding strength. In the high-current stage (55–65 A), excessive peak current causes the bonding strength at the interface to exceed that of the copper substrate, resulting in a decrease in bonding strength.

Comparison between HT and EPT: HT promotes interfacial bonding through prolonged diffusion at lower temperatures, leading to a gradual improvement in bonding strength. In contrast, EPT through instantaneous high heat flux density and non-thermal effects, rapidly accelerates the diffusion of Cu and Fe, repairing interface defects and increasing metallurgical bonding regions. However, excessively high currents cause the metallurgical bonding regions to form a flake-like structure, leading to a decrease in bonding strength.