Introduction

The welding of different metals is important due to their numerous advantages. For example, an aluminum clad copper plate is 50% lighter, and has conductivity equal to that of a copper alloy. In addition, it is 35% cheaper than a copper alloy. As a result, Al-Cu clad plates are widely used in manufacturing armored cables and yoke coils. It is difficult to join different metals using conventional fusion welding methods due to their different chemical and physical features. As a result, solid-state welding techniques attract great attention (Ref 1-12). The explosive welding technique is a solid-state welding method, and is generally used in joining or cladding different metal plates (Ref 3). This method is also known as the cold technique; however, high local temperature may form in the interface of the joint due to the dynamic of the method (Ref 4). This high temperature causes the melting on the interface, and this melting may form the intermetallic FeAl3, just like with Al-Fe joint, or the intermetallic CuAl2 and/or Cu9Al4, just like with Al-Cu joint. These intermetallics may damage the quality of the joint. Therefore, the interface properties and microstructure characterization of the bimetals are extremely important. Braunovic and Alexandrov (Ref 13) investigated the effect the intermetallic compound formed in the welded region has on electrical and mechanical properties of the Al-Cu bonding, produced using friction welding; they concluded that the contact resistance increased linearly together with an increase in thickness of the intermetallic. Xue et al. (Ref 1) comprehensively characterized the joint interface of an Al-Cu bonding, produced using friction stir welding. Cheng et al. (Ref 14) comprehensively characterized the joint interface of an Al-Cu bonding, produced using diffusion welding. Lee et al. (Ref 2) investigated the interface and mechanical properties of a laminated composite they produced from aluminum, copper, and stainless steel plates using the hot rolling process. Gulenc (Ref 11) investigated the effect explosive rates had on the explosive welding. Khosravifard and Ebrahimi (Ref 15) analyzed both theoretically and experimentally the bimetal Al-Cu produced using the extrusion process. Dyja et al. (Ref 16) analyzed both theoretically and experimentally the bimetal Al-Cu produced using the rolling process. Numerous studies are available in the literature that has analyzed properties of Al-Cu claddings or bonding produced using different production methods. However, there is no study that addresses the electrical and corrosion properties of joints that yield optimum mechanical properties (strength-ductility). In this study, a copper plate was cladded with an aluminum plate using the explosive welding technique, and the microstructure, electrical, corrosion, and mechanical properties of the clad were analyzed.

Experimental Studies

In this study, 1100 series aluminum (flyer plate) was joined to copper (base plate) produced at commercial purity using the explosive welding method. A parallel arrangement was used for the joint procedure. 90% ammonium nitrate, 4.5% fuel-oil, and 3.0% TNT were used as explosives. The explosion rate of explosives was between 3000 and 3200 ms−1. An optical microscope and a scanning electron microscope were used to conduct microstructure characterization of the produced Al-Cu bimetal. A Shimadzu HMV-2 microhardness device was used to conduct hardness measurements. A 50 g load was applied during tests. At least three measurements were taken on the same sample for each hardness value. Samples were prepared in accordance with the standard ASTM 264 (Ref 17) to identify the mechanical properties of the Al-Cu bimetal. The prepared samples were subjected to tensile and tensile-shear tests. Tensile and tensile-shear strength of the samples were determined using a ZWICK device at a load of 20 kN at deformation rate of 1 m/min at room temperature. The Charpy-impact test was used to determine the impact strength of the Al-Cu bimetal at room temperature. A SEM was used to analyze the fractured surfaces formed as a result of tensile and impact tests. At least three samples were subjected to tensile, tensile-shear, and impact tests and average of these samples was taken.

An ohm meter was used to measure the electrical conductivity of the Al-Cu bimetal. A power supply was used to create voltage between the aluminum and the copper surfaces of the bimetal, and then the current created by this supply voltage between these two surfaces was measured. The Ohm Law (R = V/I) was used to calculate the resistance of the Al-Cu bimetal.

The equation stated below was used to calculate resistivity.

$$ \rho = RS/L $$
(1)

where R is resistance, L is the distance of potential probes, and S is the contact area. Finally, Eq 2 was used to calculate G, conductivity.

$$ G = L/\left( {RS} \right) $$
(2)

In order to identify corrosion properties of the Al-Cu bimetal, a 1.5 mm diameter and 150 mm long copper wire was welded to the back of the Al-Cu bimetal for the purpose of enabling conductivity in the al side, copper side, and bimetal cross section (working electrode) of the bimetal, and cladded with resin, in a way to leave out only surfaces that were required to be in contact with the electrolyte. Electrochemical corrosion tests were conducted at room temperature, in a 30 g/L NaCl + 10 mL/L HCl solution, using a Gamry PC4/300 mA model potentiostat/galvanostat device.

Results and Discussion

As illustrated in Fig. 1(a), the Al-Cu bimetal produced using the explosive welding method has a plain and rough interface. There was a sharp characteristic transition at the interface where both materials bond. Figure 1(a) also illustrates locally melted regions. The high kinetic energy of the jetting, forming, and not appearing between the two plates during the collision causes the melting (Ref 18). Crossland (Ref 19) reported that the jetting forms in both the flyer and the base plates. EDS analyses conducted for this study concluded that the chemical composition of the melted region is mixture of the copper and the aluminum, which were the flyer and the base plates (Fig. 1b). According to EDS analysis, this melting region is probably CuAl2 phase. Wuhrer et al. (Ref 20) reported that the CuAl2 phase contained 68%Al and 32%Cu. Figure 2 illustrates that the region had a higher hardness in comparison to the hardness of Al and the hardness of Cu as a result of hardness measurements. Braunovic and Alexandrov (Ref 13) reported that the hardness of CuAl2 in the Al-Cu bonding, produced using the friction welding, was 413 kg/mm2. Hardness measurements also concluded that the hardness of the Al side and of the Cu side of the bimetal reduced as moving away from the joint interface due to the reduction in deformation.

Fig. 1
figure 1

(a) Welding interface of Al-Cu Bimetal and (b) EDS analysis of melted-solidified region in the interface

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Fig. 2
figure 2

Distance from interface versus hardness variation of Al-Cu bimetal

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Table 1 illustrates average values of the tensile test, tensile-shear test, and Charpy-impact test and standard deviation of these tests. The tensile strength of Al, and Cu forming the Al-Cu bimetal, was 110 MPa, and 220 MPa, respectively. The tensile strength of the Al-Cu bimetal, produced using the explosive welding method, was 240 MPa. The tensile strength of the bimetal increased as a result of the increased strength of Al and Cu, exposed to deformation hardening by the explosion, and due to the high strength of copper, one of components forming the bimetal. The tensile-shear strength of the Al-Cu bimetal was 140 MPa. According to the literature (Ref 12), under ideal welding conditions, the tensile-shear strength for numerous metal combinations was higher than the tensile strength of the weaker one out of the components forming the joint. There was a slight decrease in the impact strength and strain values of the Al-Cu bimetal, produced using the explosive welding method, due to the deformation hardening of Al and Cu. Figure 3(a) and (b) illustrates images of fractured surfaces belonging to the bimetal. While the Al-Cu bimetal displayed brittle fractures near the joint interface, dimples, an indicator of ductile fracture, were observed moving away from the interface.

Table 1 Mechanical properties of Al-Cu bimetal and components
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Fig. 3
figure 3

Fracture surfaces of (a) tensile and (b) Charpy-impact test samples of Al-Cu bimetal

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Table 2 illustrates the electrical resistance and conductivity of the Al-Cu bimetal, calculated in accordance with Eq 1 and 2. In accordance with results parallel to those stated in the literature, the Al-Cu bimetal had an average electrical conductivity, in comparison to original aluminum and copper materials that form the joint (Ref 21-23). As concluded from microstructure results, intermetallic phases may form during production using the explosive welding method, and these phases may reduce the conductivity while increasing the resistance. Cheng et al (Ref 14) and Abbasi et al. (Ref 24) reported that the conductivity decreased as the thickness of intermetallic compounds increased. In this study, the CuAl2 phase that formed in small quantities (inconsistent) in different regions had no adverse effect on the electrical conductivity of the Al-Cu bimetal.

Table 2 Resistivity and conductivity values of Al, Cu, and Al-Cu Bimetal
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Figure 4 illustrates the polarization curves of the Al-Cu bimetal, and the polarization curves of Al and Cu forming the bimetal. The corrosion current density (I corr) and corrosion potential were eliminated from the curve and presented in Table 3. According to polarization results, while the E corr of Cu was higher than the E corr of Al and Al-Cu bimetal, the I corr of Al and Al-Cu bimetal was higher than the I corr of Cu. According to the activity and passivity properties of Al and Cu in sea water, Al was exposed to severe corrosion; the results were parallel to those stated in the literature (Ref 25). In addition, there was galvanic corrosion in the Al-Cu bimetal, and as illustrated in Figure 5, Al acted as anodic because of its high electronegativity.

Fig. 4
figure 4

The polarization curves of the Al, Cu, and Al-Cu bimetal

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Table 3 The corrosion current density (I corr) and the corrosion potential values
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Fig. 5
figure 5

(a) SEM photographs of corroded Al-Cu bimetal (b) subsurface region of cross section of corroded Al-Cu bimetal

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Conclusions

Stated below are conclusions reached after analyzing the microstructure, mechanical, electrical, and corrosion properties of the Al-Cu bimetal, produced using the explosive welding method.

  1. 1.

    The Al-Cu bimetal display an interface that is both plane and rough, and also has partially an intermetallic phase known as CuAl2.

  2. 2.

    Tensile, tensile-shear, and impact test results concluded that the Al-Cu bimetal has an acceptable strength.

  3. 3.

    There was no decrease in electrical conductivity, in spite of the intermetallic phase on the joint interface.

  4. 4.

    Galvanic corrosion occurred during corrosion tests, and the Al side of the bimetal was exposed to more corrosion than the copper side.