Microstructure and Conductivity of the Al-Cu Joint Processed by Friction Stir Welding

In this paper, 1060 aluminum and T2 pure copper were joined by friction stir welding. *e influence of the rotation speed and inclination on the microstructure and mechanical properties of the joint were investigated. *e microstructure and composition of the welded interface region were analyzed. *e joints’ strength was tested, and the conductivity of the joints was estimated. Joints having good surface formation and defect-free cross section were successfully obtained.*e cross-sectional morphologies of the Al-Cu friction stir welding joints can be divided into three zones: the shoulder impact zone, the weld nugget zone, and the interface zone.*e interface zone consisted of a metallurgical reaction layer and a visible mixed structure.*e reaction layers were identified as Al2Cu, Al4Cu9 phases. *e tensile strength of the joints reaches maximum values of 102MPa at a rotation speed of 950 rpm and inclination of 0°, which was approximately equal to those of 1060Al base metal. *e resistivity of the Al-Cu joint was approximately equal to the theoretical resistivity. *e interfacial resistance is directly affected by the joint defects, compound types, and thickness of the intermetallic compound layer.


Introduction
Al-Cu bimetallic composites structures can realize the combination of the light weight and the corrosion resistance properties of Al and the high electrical conductivity, high thermal conductivity, and low contact resistance of Cu [1][2][3].
ese structures have been used in the power transmission, refrigeration, and hydrometallurgy and others fields, such as bus-bars, electrical connectors, and heat-exchangers tubes [4][5][6]. However, the aluminum oxide film and the intermetallic compounds (IMCs) are unavoidable in Al-Cu joint in view of their metallurgical characteristics [7]. e oxide film prevents the direct contact between Cu and Al [8]. e thick Al-Cu IMCs can severely degrade the joint performance [9].
Friction stir welding (FSW), as a solid phase pressure welding method, is a revolutionary joining process patented by the Welding Institute (TWI) [10] which is an effective way of joining heterogeneous materials that weld poorly using fusion. During the FSW process, the rotating stir tool was inserted into the prejoining surface. e material near the stir tool were plasticized by the friction heat and forced to mix with each other [11]. At present, FSW has successfully achieved heterogeneous metals such as Al-Mg [12,13], Al-Steel [14,15], Al-Ti [16], and Al-Cu [17][18][19][20][21][22][23]. A large number of related researches on Al-Cu FSW joints have been carried out. e investigation is mainly aimed at the effects of the parameters on microstructure, performance of joints [17][18][19][20], and formation mechanism of the interface IMCs [21][22][23]. In [18,19], the influence of plate position, rotational speed, and offset on mechanical properties and microstructures of the Al-Cu FSW joints is studied. And, they indicated that the increase in rotation speed promotes the thickening of the IMCs. e study on the temperature field distribution and the evolution of IMCs showed that solid phase transformation and solidification transformation were the main formation mechanisms of IMCs [21]. Beygi et al. [23] found the threaded conical tool provided the best compromise between the different material flow regions. Although a large number of related researches on Al-Cu FSW joints have been carried out, there are relatively few studies on the conductivity of the joints. e 1060 aluminum alloy has the most potential as a substitute for copper materials as conductive components in power transmission systems (such as Al-Cu wires and Al-Cu conductive bars). It is of great significance to study the microstructure and its relationship with the strength and conductivity of the 1060Al-T2Cu FSW joint.
Since Cu and Al are good conductive materials, the conductivity of the interfacial layer is the key to determine the overall conductivity of the Al-Cu bimetallic composite structures.
erefore, this paper focuses on the interface microstructure and its influence on joint strength and conductivity of the 1060Al-T2Cu FSW joint. e FSW method was used to weld 4 mm 1060 aluminum alloy and T2 copper. e influence of welding parameter on the microstructure, mechanical properties, and electrical conductivity of the joint was investigated. It is expected to provide certain theoretical and data support for the use of 1060 Al-Cu composite components in the field of conduction.

Experimental Procedure
Commercially pure aluminum alloy 1060 plate and T2 copper plate with 4 mm in thickness were selected for the experiment.
e schematic diagram of the Al-Cu FSW processes and the stir tool are shown in Figure 1. e FSW experiment was conducted using a modified XA5032 vertical milling machine with homemade fixtures for welding. e welding method was a single-pass butt, with the copper plate placed on the advancing side (AS) and the aluminum plate placed on the retreating side (RS), as shown in Figure 1(a). e stir tool was made of W18Cr4V high-speed steel with a columnar shaped shoulder and a screw thread tapered pin (size: root: Φ7 mm, tip: Φ4 mm, and length: 3.8 mm), as shown in Figure 1(b). During the welding process, the stir tool clockwise rotation, the offset toward to Al metal was 1 mm, the welding speed was 60 mm/min, and the rotation speed was 750∼1500 rpm. e experimental parameters are shown in Table 1. e welded samples for metallographic examination were cut perpendicular to the welding interface. e microstructure was analyzed by optical microscopy (OM) and scanning electron microscopy (SEM, JEOL, JSM-6700) equipped with an energy-dispersive spectrum (EDS). e mechanical properties of the joints were evaluated by tensile tests (Instron 5880). e tukon-2100 microdimension hardness tester was used to test the interface microhardness with a load of 10 g and a holding time of 10s. e electrical resistance of the joints was tested by microohmmeter (TEGAM1750).

Weld Surface Morphology of Al-Cu FSW Joint.
e morphology of the weld surface produced by different parameters is shown in Figure 2. e weld surface of FSW was composed of a series of arc curve and the keyhole left by the stir pin. Due to the friction and extrusion of the shoulder, obvious burrs were formed on both sides of the weld surface. At the rotation speed of 750 rpm, groove-shaped defects appeared, as shown in Figure 2(a). During the FSW process, the surface groove-type defect often appears on the upper surface of the FSW weld when the welding heat input was insufficient. e thermoplastic metal around the stir tool is insufficient, and the plastic metal cannot fully fill the instantaneous cavity left during the course of the stirring pin, thereby forming a surface groove near the advancing edge. e weld surface was well formed, and the circular ring lines were uniform when the rotation speed was 950 rpm and 1180 rpm. Regional accumulation and poor uniformity appeared in the weld surface when the rotation speed was 1500 rpm, indicating that the flow of plastic metal fluctuated during the welding process, as shown in Figure 2(d). After adding the stir tool inclination, a rough weld surface (as shown in Figures 2(e) and 2(f )) was formed due to the increases in the downward pressure behind the shoulder of the tool. e rough weld surface was mainly made up of some Al filaments. Figure 3 shows the typical cross-sectional morphologies and the regional characteristics of the Al-Cu FSW joints. Figure 3(a) is the overall cross-sectional view of the joint. e entire weld can be divided into three regions: the shoulder impact zone, the weld nugget zone, and the interface zone, as shown by the oval red dotted line in Figure 3(a). e plasticized material shows two flow states in the cross section. e upper part of the weld shows parallel flow, while the bottom shows a circular flow. e material flow in the upper part of the weld depends mainly on the drive of the shoulder. Moreover, the heat input in the upper part of the weld was sufficient and the material was plasticized adequately, and the copper strip peeled off and migrated farther. On the contrary, the material flow was mainly driven by the stir pin in the middle and lower parts of the weld. Accompanied by the upper and lower annular vortices due to the screw thread tapered pin, the circular flow played an important role in the middle and lower parts of the weld.

Cross-Sectional Morphologies.
are the morphology of the advancing side of the weld. During the FSW process, the stir tool rotated rapidly to peel off a large amount of copper particles. ese particles were softened, fragmented, and flowed by the friction and stir effect of the stir tool and dispersed in the aluminum matrix. A gray matter was formed near the larger Cu particle edges, as shown in Figure 3(b). It indicated that metallurgical bonding occurred between these copper particle and aluminum. e reaction occurred along the Al-Cu interface, and layered compounds were formed on the interface, as shown in Figure 3(c). e copper particles were continuously stretched and formed alternating the layered structure, as shown in Figures 3(d)-3(d). EDS analysis of these compounds was shown in Table 2. e results showed that these phases were likely to be AlCu, Al 2 Cu, and Al 4 Cu 9 phases. ese phases were further determined by XRD analyses. In this experiment, the thickness of the intermetallic compound layer at the Al/Cu interface of FSW joints was within the range of 0.3∼2.7 μm, which was different according to the welding parameters and the test position. e maximum thickness of the intermetallic layer is not more than 3 μm. Figure 4 shows a SEM image of interface layer of the Al-Cu FSW joint produced by different parameters. An obvious IMCs layer was formed on the Al-Cu interface, which indicated that the metallurgical reaction occurred during the friction stir welding process. In order to compare the thickness of the reaction layers obtained under different parameters, the microstructure shown in Figure 4 was tested at the same position of different samples (on the Al-Cu interface near the bottom of the weld). As the rotation speed increases from 750 rpm to 1500 rpm, the thickness of the interface reaction layer was increased from 0.38 μm to 0.91 μm. e thickness of the reaction layer was positively correlated with the rotation speed. Schmidt et al. [24] pointed   out that the heat mainly comes from the friction between the shoulder and the stir pin and the base metal. e welding heat input was proportional to the rotation speed. is leads to a thicker diffusion layer when there was a high rotation speed. Researches show that the thickness of the metallurgical reaction layer was related to the mechanical properties of the joint. Xue et al. [25] and Abbasi et al. [26] pointed out that the formation of thin, continuous, uniform IMCs was a necessary condition for a good metallurgical bond. But, the thickness of the interfacial diffusion layer should be less than 2.5 μm. It indicated that the thickness of the metallurgical reaction layer in this experiment could obtain good bonding strength. Figure 5 shows the distribution of elements that are perpendicular to the interface (denoted by the yellow line in Figure 5(a)) and the elemental surface distribution. It could be found that the content of Al and Cu elements were changed, consistent with the change of microstructure, as shown in Figure 5(c). In the alternating layered structure of Al and Cu, the fluctuation of diffraction intensity of Al and Cu is mainly related to the distribution pattern of both. And, the corresponding width of the alternating layers is shown in Figure 5 Figure 3: e cross-sectional morphologies and the regional characteristics of the Al-Cu FSW joints. Advances in Materials Science and Engineering distribution of the Al-Cu interface region is shown more clearly. Figure 6 shows the tensile strength of the joints and the typical stress-strain curves. e joint strength increases with the rotation speed and reaches a maximum value at 950 rpm, and then decreasing behavior appears. e maximum can reach to 102 MPa, which is approximately equal to that of 1060Al base metal. When the inclination is increased, the strength of the joint is slightly reduced when the inclination was employed. e strength of the joint depends on the macroscopic forming and microstructure of the weld. At low rotation speed, insufficient heat input in the weld area was likely to lead to surface groove defects, so the joint strength was low. However, when the rotation speed was higher, weld overheating would lead to defects such as surface peeling or filaments. At the same time, more interfacial compounds were formed. Experiments showed that higher joint strength was obtained when the rotation speed was 950 rpm due to its proper heat input. e mechanism of joining can be attributed to the combination of mechanical bonding and metallurgical bonding, which has been reported by Esmaeili et al. [27]. An interlaced structure and the Cu scrapings near the interface provide mechanical bonding effect. e metallurgical bonding was achieved depending on the atomic diffusion and the Al-Cu chemical reaction. e fracture morphology of the joints is shown in Figure 7. e fracture of the joint produced by a rotation speed of 750 rpm was composed of dimples of different sizes, but the dimple was generally shallow, as shown in Figure 7(a). When the rotation speed increased to 950 rpm, as shown in Figure 7(b), the fracture was composed of a large area of the dimple zone and a small amount of cleavage zone. e fracture completely becomes a flat cleavage surface when the rotation speed was 1500 rpm, as shown in Figure 7(d). It indicated that the fracture mechanism of the joint changes from the original ductile-brittle mixed fracture to brittle fracture as the rotation speed increases, which is consistent with the tensile strength results obtained. In order to further confirm the IMCs on the Al-Cu interface, XRD analysis was performed on the tensile fracture of the joints. Figure 8 shows the X-ray diffraction pattern of the cross section of the joint. In addition to the Al and Cu base metal, the diffraction peaks of the Al 2 Cu and Al 4 Cu 9 compounds appeared. Compared with the results in the Table 2, the diffraction peak of the AlCu phase did not occur probably because its content was too small. It indicated that Al 2 Cu and Al 4 Cu 9 phases were formed on the Al-Cu interface during the FSW process, and the joint achieved a good metallurgical bonding. is was consistent with the compounds reported in [28]. Figure 9 shows the microhardness curve of the joints. e average hardness of the base Al and Cu was 39.3 HV and 90.5 HV. e hardness of the weld zone is significantly higher than that of the base metal, and the hardness distribution was irregular. e heat-affected zone showed lower hardness due to the effect of the welding thermal cycle. With the increase of the rotation speed, the average hardness of the nugget zone increased slightly. e indentation of   Figure 10. e indentation at the compound was significantly smaller than that at the matrix under the same pressure.

Electric Conductivity of the Joint Al-Cu FSW.
In this experiment, the resistivity was used to characterize the conductivity of the joint. e resistivity of the joint was estimated by the equation ρ � RS/L, where R is the electrical resistance, S is the sectional area, and L is the specimen length. e TEGAM 1750 micrometer with a resolution for 0.1 μΩ and the accuracy of 0.02% was used to test the resistance of the sample. e samples sizes for resistance test were 3 mm in diameter and 6 mm in length with aluminum and copper each accounting for half the volume. e resistivity of Cu/Al bimetallic joint produced by FSW is shown in Figure 11. e theoretical resistivity of Cu/Al heterogenic structures with same size was approximately 21.6 × 10 −9 Ω m. e resistivity of the Al-Cu FSW joint was lower or close to theoretical resistivity. It Advances in Materials Science and Engineering indicated that the Al-Cu FSW joint has the referable conductive property.
Since Cu and Al are good conductive materials, the high resistivity of the interfacial transition layer was the main reason for the high resistivity of the joint, which determine the overall conductivity of the Al-Cu bimetallic composites structures. It is well known that interface defects and thicker compound layers exhibit poor conductivity. Table 3 shows the resistivity of Al, Cu, and intermetallic compounds. e resistivity of compounds is higher than that of the pure Al and Cu substrate. e interfacial resistance is directly affected by the joint defects, compound types, and thickness of

Conclusions
In this paper, the FSW method was used to weld 1060 aluminum alloy and T2 copper with 4 mm in thickness. e influence of welding parameter on the microstructure and mechanical properties and electrical conductivity of the joint were investigated. e main conclusions are as follows: (1) A well-formed Al-Cu FSW joint was successfully obtained. e cross-sectional morphologies of the Al/Cu FSW joints can be divided into three zones: the shoulder impact zone, the weld nugget zone, and the interface zone. e material flow shows parallel flow at the upper part and annular flow at the bottom.
(2) e interface zone consisted of metallurgical reaction layer and visible mixed structures. e reaction layers were identified as Al 2 Cu, Al 4 Cu 9 phases and the thickness of the IMC layer increased with the increase of the rotation speed. (3) e tensile strength of the joints reaches maximum values of 102 MPa at a rotation speed of 950 rpm and inclination of 0°, which was approximately equal to that of 1060Al base metal. e fracture mechanism changed from a tough-brittle mixed fracture to brittle fracture as the rotation speed increases. e microhardness of the nugget zone was higher than that of the base metal and unevenly distributed. e Al-Cu FSW joints have a good conductivity.

Data Availability
e data used to support the findings of this study are included within the article.

Conflicts of Interest
e authors declare that they have no conflicts of interest.