Compound Growth due to Isothermal Annealing of Cu-Clad Al Wire
Keywords:
intermetallic,
copper-clad wire
2019 Volume 60 Issue 6 Pages 895-901
Details
2019 Volume 60 Issue 6 Pages 895-901
To better understand the annealing mechanisms that occur in Cu-clad Al (CA) wire, the solid-state reactive diffusion between Cu and Al was experimentally examined by a metallographical technique. The hard CA (HCA) wire was prepared by drawing to decrease diameter from 10 mm to 1.5 mm, and then annealed at 250°C (523 K) for 3 h (10.8 ks). The annealed HCA wire is merely called the ACA wire. The HCA and ACA wires were isothermally annealed at temperatures of 150–270°C (423–543 K) for various periods of 12–960 h (43.2 ks to 3.46 Ms). Due to isothermal annealing, the intermetallic layer composed of the θ, η2, δ, γ1 and α2 phases is produced at the original Cu/Al interface in both the HCA and ACA wires. The total thickness of the intermetallic layer is proportional to a power function of the annealing time. The exponent of the power function is 0.23–0.44 for the HCA wire and 0.33–0.53 for the ACA wire. Thus, boundary diffusion as well as volume diffusion contributes to the layer growth. Furthermore, the overall growth rate of the intermetallic layer is slightly greater for the HCA wire than for the ACA wire. Since the exponent is smaller for the HCA wire than for the ACA wire, the contribution of boundary diffusion is greater for the former than for the latter. The greater contribution of boundary diffusion may be the reason why the overall layer growth takes place faster in the HCA wire than in the ACA wire.
Fig. 5 The total thickness l of the IMC layer versus the annealing time t shown as cross, open rhombuses, triangles, squares and circles for T = 423, 453, 483, 513 and 543 K (150, 180, 210, 240 and 270°C), respectively: (a) HCA and (b) ACA.
Due to high electrical conductivity, Cu is widely used as a conductive material in the electronics industry. Although the electrical conductivity for Al is 62% of that for Cu, the mass density for Al is merely 30% of that for Cu.1) Thus, Al has an advantage of light conductive material. In the automobile industry, regulations on fuel efficiency in each country become strict year by year. Though the number of electric wires used in automobiles is increasing, the weight of automobiles is decreasing due to the conversion from Cu to Al.2–4) Recently, to derive the characteristics of various materials, different materials have often been utilized in combination. Thus, many combinations of various materials are particularly used to improve fuel economy through weight reduction. Consequently, cases of joining of Al wires and Cu for use in terminals are increasing. In addition to the method of connecting dissimilar materials, composite materials are built up with wire or plate.
One such composite material is the Cu-clad Al (CA) wire. The CA wire is an Al wire coated with Cu. It is widely used for electronic parts as a conductive material exhibiting both the lightness of Al and the good connectivity of Cu. For the CA wire, Cu and Al are solid-state bonded during the wire drawing process. At the state of only being drawn, almost no intermetallic compound (IMC) exists at the Cu/Al interface. However, the IMC layer grows at high temperatures. IMCs are brittle and have great influence on the mechanical properties. On the other hand, when flexibility is required for the CA wire, it is annealed and used as a soft CA wire in some cases. Hereafter, the CA wire before annealing is denoted as hard CA (HCA) wire and the annealed CA wire is denoted as ACA wire. In the latter case, annealing was carried out under the conditions that the IMC layer is not formed extensively. The ACA wire may be utilized in various environments. For example, it may be used in the temperature range of about 100–200°C (373–473 K) near the motor in automobiles. Investigating the growth behavior of the IMC layer in such a temperature range is important to ensure the reliability of the product. Gueydan et al.5) observed the growth behavior of the IMCs in the HCA wire annealed at temperatures of 300–400°C (573–673 K) for times of 2–48 h (7.2–172.8 ks). According to their result,5) the growth of the IMC layer in the HCA wire obeys the parabolic law. Here, the parabolic law means that the layer thickness is proportional to the square root of the annealing time. However, they did not investigate the growth behavior of the IMC layer in the low-temperature range. Furthermore, the observation of IMC layer growth was mainly performed on the HCA wire.6,7) Therefore, the IMC layer growth behavior in the ACA wire is not so clear. In the present study, we experimentally examined the formation behavior of the Cu–Al compound layer at the Cu/Al interface in the HCA and ACA wires. A metallographical technique was used for the observation. The kinetics of the layer growth was discussed on the basis of the experimental result.
A pure Al rod equivalent to A1050 was used as the core material and a Cu tape equivalent to C1020 was longitudinally added; the butt portion was continuously welded, and metal bonding between Cu and Al was achieved by subsequent wire drawing. The thickness of the Cu-clad layer was uniform not only in the circumferential direction but also in the longitudinal direction, and there was no discontinuous section in the welded portion of the Cu tape. The volume fractions of Cu and Al were roughly 0.15 and 0.85, respectively.
Using this method, CA wire base material with diameter of 10 mm was produced, and thereafter, wire drawing with a reduction in area of 90% or more was performed. After drawing, the CA wire with diameter of 1.5 mm was obtained, in which Cu and Al were bonded strongly. This wire is called the HCA wire.
2.2 Annealing conditionsThe HCA wire was annealed at a temperature of 250°C (523 K) for a time of 3 h (10.8 ks) to prepare the ACA wire. Due to this annealing, a Cu–Al IMC layer with thickness of about 0.5 µm was formed at the original Cu/Al interface. The HCA and ACA wires were isothermally annealed for times of 12–960 h (43.2 ks to 3.46 Ms) in the temperature range of 150–270°C (423–543 K).
2.3 Evaluation methodA tensile test was performed on the HCA and ACA wires at room temperature. For the tensile test, the gauge length was 250 mm, and the cross-head speed was 10 mm/min.
Cross-sections of the HCA and ACA wires were mechanically polished using # 180–1000 emery papers and diamond paste with size of 1 µm. The mechanically polished cross-section was chemically etched with a solution containing ammonia and hydrogen peroxide as the main components. The microstructure of the chemically etched cross-section was observed mainly by optical microscopy (OM) and scanning electron microscopy (SEM). For some samples, observations were done by transmission electron microscopy (TEM) to examine the microstructure of the IMC layer. TEM specimens were prepared by a focus ion beam (FIB) technique. The chemical composition of each phase in the TEM specimen was determined by energy dispersive spectrometry (EDS).
Stress-strain (SS) curves of the HCA and ACA wires are shown in Fig. 1. According to the result in Fig. 1, the mean ultimate tensile strength is 215 and 130 MPa for the HCA and ACA wires, respectively, and the mean elongation is 0.4 and 9.1% for the HCA and ACA wires, respectively. These values are listed in Table 1. Thus, the ultimate tensile strength for the ACA wire is almost 60% of that for the HCA wire, and the elongation for the ACA wire is twenty-three times greater than that for the HCA wire. Due to annealing at 250°C (523 K) for 3 h (10.8 ks), the ultimate tensile strength decreases, but the elongation increases. Such changes of the mechanical properties are attributed to the release of the processing strain introduced by wire drawing. As the ultimate tensile strength decreases and the elongation increases, flexibility is improved, and subsequent processing becomes easier. Annealing of the HCA wire improves the workability.
Stress-strain curves of the HCA and ACA wires before isothermal annealing.
The microstructure of the HCA and ACA wires is shown in Fig. 2. Figure 2(a) and 2(b) show the whole cross-sections of the HCA and ACA wires, respectively, wherein Cu is uniformly coated around the Al wire. On the other hand, Fig. 2(c) and 2(d) show the magnified photographs of the Cu layer for Fig. 2(a) and 2(b), respectively. As can be seen in Fig. 2(c) and 2(d), the Cu layer in the HCA wire consists of fine polycrystalline grains, whereas that in the ACA wire recrystallizes upon annealing. Significantly large defects such as separations did not exist at the original Cu/Al interface, both in the HCA and ACA wires. Figure 2(e) and 2(f) show the magnified SEM images of the Cu/Al interface. The area with different contrasts at the Cu/Al interface of the ACA wire is considered as IMCs.
Cross-sectional images of the HCA and ACA wires before isothermal annealing: OM image of whole region, (a) HCA and (b) ACA; OM image of Cu layer, (c) HCA and (d) ACA; and SEM image of original Cu/Al interface, (e) HCA and (f) ACA.
As shown in Fig. 2(e) and 2(f), the fine microstructure was not clearly distinguished in the SEM micrographs. Thus, the fine microstructure was observed by TEM. The results are shown in Fig. 3. Figure 3(a) schematically indicates the area for the TEM observation. Typical bright field (BF) images of the Cu/Al interface in the HCA and ACA wires are represented in Fig. 3(b) and 3(c), respectively, where the Cu/Al interface is located edge on. In these BF images, the areas with white and black contrasts are the Al and Cu specimens, respectively. As can be seen, layers with gray contrasts are recognized between the Al and Cu specimens. To identify these gray layers, EDS measurements were conducted along the direction perpendicular to the original Cu/Al interface. The results for Fig. 3(b) and 3(c) are shown in Fig. 3(d) and 3(e), respectively. For the HCA wire in Fig. 3(d), the chemical composition rather continuously changes across the gray layers. This means that the gray layers correspond to various Cu–Al compounds with thicknesses smaller than the spatial resolution of EDS. On the other hand, for the ACA wire in Fig. 3(e), the chemical composition of the most bright gray layer with thickness of about 0.3 µm coincides with that of the θ (CuAl2) phase.8) Nevertheless, the chemical composition continuously varies across the other darker gray layers in the ACA wire. Thus, each compound was not identified in the other gray layers. As can be seen in Fig. 3(c) and 3(e), the total thickness of all the gray layers is about 0.5 µm for the ACA wire.
(a) Schematic of the HCA and ACA wires before isothermal annealing; bright field (BF) TEM image of original Cu/Al interface, (b) HCA and (c) ACA; and EDS analysis across original Cu/Al interface, (d) HCA and (e) ACA.
A typical BF image of the microstructure evolved along the original Cu/Al interface in the ACA wire annealed at 270°C (543 K) for 24 h (86.4 ks) is shown in Fig. 4(a). As can be seen, various layers with different gray contrasts are produced between the Al and Cu specimens. Across these layers, EDS measurements were carried out. The result is indicated in Fig. 4(b). As shown in Fig. 4(b), five of the layers are compounds. They are the θ (CuAl2), η2 (CuAl), δ (Cu3Al2), γ1 (Cu9Al4) and α2 (Cu3Al) phases from the Al side to the Cu side. The layer consisting of such intermetallic compounds is hereafter called the IMC layer. According to a phase diagram in the binary Al–Cu system,8) the θ, η2, ζ2 (Cu4Al3), δ, γ1 and α2 phases are the stable compounds at the annealing temperatures of 150–270°C (423–543 K). However, the ζ2 phase was not detected in the present study. Furthermore, the layer thickness is smaller for the η2, δ, γ1 and α2 phases than for the θ phase, and close to one another among the η2, δ, γ1 and α2 phases. Thus, at the annealing temperatures, the interdiffusion coefficient should be similar among the η2, δ, γ1 and α2 phases, but greater for the θ phase than for the η2, δ, γ1 and α2 phases. Therewith, the interdiffusion coefficient of the ζ2 phase is considered much smaller than those of the θ, η2, δ, γ1 and α2 phases. In Fig. 4(b), the Cu solid-solution phase alloyed with Al is discerned between the α2 phase and the Cu specimen. This alloyed region is formed by diffusion induced recrystallization (DIR). DIR is the phenomenon that new fine grains with discontinuously different solute concentrations are produced behind migrating grain boundaries due to recrystallization combined with diffusion of solute atoms along the migrating and stationary boundaries surrounding the fine grains.9) In many binary alloy systems, DIR occurs at temperatures where volume diffusion is frozen out but boundary diffusion takes place practically. Gueydan et al.5) also reported the occurrence of DIR in the Cu phase.
The ACA wire isothermally annealed at T = 543 K (270°C) for t = 86.4 ks (24 h): (a) bright field (BF) TEM image of the IMC layer; and (b) EDS analysis across the IMC layer.
In a study by Meguro et al.,10) sandwich Al/Cu/Al diffusion couples were isothermally annealed at temperatures of 420–480°C (693–753 K) for various times up to 1555 h (5.6 Ms). According to their result, the θ, η2, ζ2, δ and γ1 phases were clearly observed in the diffusion couple annealed at 480°C (753 K) for 1555 h (5.6 Ms). At this annealing temperature, the α1 phase is not a stable compound.8) Hence, all the five stable compounds form under their annealing conditions. However, their result indicates that the layer thickness of the θ phase is greater than those of the η2, ζ2 and δ phases but smaller than that of the γ1 phase. Therefore, it is concluded that the interdiffusion coefficient of the γ1 phase is smaller than that of the θ phase at 150–270°C (423–543 K) but becomes greater than that of the θ phase at 420–480°C (693–753 K). This deduces that the activation enthalpy of the interdiffusion coefficient is greater for the γ1 phase than for the θ phase.
3.5 Growth behavior of intermetallic compoundAs previously mentioned, the HCA and ACA wires were isothermally annealed under various experimental conditions. During isothermal annealing, the IMC layer forms at the original Cu/Al interface in the wire. Cross-sectional microstructure of the annealed wire was observed by SEM. Based on SEM photographs shown in Fig. 2, the total thickness l of the IMC layer was evaluated using the equation
| \begin{equation} l = \frac{A}{w}, \end{equation} | (1) |
The total thickness l of the IMC layer versus the annealing time t shown as cross, open rhombuses, triangles, squares and circles for T = 423, 453, 483, 513 and 543 K (150, 180, 210, 240 and 270°C), respectively: (a) HCA and (b) ACA.
In Fig. 5, the vertical and horizontal axes show the logarithms of l and t, respectively. As shown, the thickness l monotonically increases with an increase in annealing time t. Furthermore, the plotted points for each annealing temperature are located well on a straight line. Consequently, l was expressed as a power function of t, as follows:
| \begin{equation} l = k\left(\frac{t}{t_{0}}\right)^{n}, \end{equation} | (2) |
According to the result of the HCA wire in Fig. 5(a), n = 0.40–0.44 at T = 453–543 K (180–270°C) and n = 0.23 at T = 423 K (150°C). On the other hand, for the ACA wire in Fig. 5(b), n = 0.44 at T = 423–453 K (150–180°C) and n = 0.52–0.53 at T = 483–513 K (210–240°C). Although there is an exceptional value of n = 0.33 at T = 543 K (270°C) for the ACA wire in Fig. 5(b), it may be likely that both volume diffusion and boundary diffusion contribute to the growth of the IMC layer and the contribution of boundary diffusion becomes greater at lower temperatures.
3.6 Kinetic analysis of intermetallic compound growthAs mentioned earlier, the rate-controlling process for the overall growth of the IMC layer varies depending on the annealing temperature. Thus, for estimation of the temperature dependence of the kinetics for the layer growth, we cannot use the values of k shown in Fig. 5. Nevertheless, information on such temperature dependence is essentially important. Thus, to derive this information, we assume that the overall growth of the IMC layer is controlled by volume diffusion. Inserting n = 0.5 into eq. (2) according to this assumption, we obtain the following equation.
| \begin{equation} l = k\left(\frac{t}{t_{0}}\right)^{0.5} \end{equation} | (3) |
The total thickness l of the IMC layer versus the square root of the annealing time t shown as cross, open rhombuses, triangles, squares and circles for T = 423, 453, 483, 513 and 543 K (150, 180, 210, 240 and 270°C), respectively: (a) HCA and (b) ACA.
The values of k in Fig. 6 are plotted against the annealing temperature T as open squares and circles for the HCA and ACA wires, respectively, in Fig. 7. In this figure, the vertical axis shows the logarithm of k, and the horizontal axis indicates the reciprocal of T. As can be seen, the open symbols for each wire are located well on the corresponding straight line. Thus, the temperature dependence of k was expressed by the equation
| \begin{equation} k = k_{0}\exp\left(-\frac{Q_{k}}{RT}\right), \end{equation} | (4) |
The logarithm of the proportionality coefficient k versus the reciprocal of the annealing temperature T shown as open squares and circles with solid lines for the HCA and ACA wires, respectively. The corresponding result for the GCA wire reported by Gueydan et al.5) is represented as open triangles with a solid line.
As shown in Fig. 7, k is slightly greater for the HCA wire than for the ACA wire at each annealing temperature. The difference in the value of k between the HCA and ACA wires rather increases with decreasing annealing temperature T. Thus, the activation enthalpy of Qk = 53 kJ/mol for the HCA wire is smaller than that of Qk = 61 kJ/mol for the ACA wire. This means that the layer growth occurs faster for the HCA wire than for the ACA wire. The corresponding result reported by Gueydan et al.5) is also represented as open triangles in Fig. 7. Although the annealing temperature range in the study by Gueydan et al.5) is higher than that in the present study, the results of both studies are not dissimilar to each other. The value Qk = 58 kJ/mol obtained by Gueydan et al.5) is greater than Qk = 53 kJ/mol for the HCA wire but smaller than Qk = 61 kJ/mol for the ACA wire. One of the reasons for the difference in the activation enthalpy may be attributed to the difference in the wire drawing process. Gueydan et al.5) used the CA wire drawn from the diameter of 20 mm to that of 0.3 mm. Hereafter, their CA wire is called the GCA wire. On the other hand, in the present study, the CA wire was drawn from the diameter of 10 mm to that of 1.5 mm. Thus, the GCA wire underwent a longer wire drawing process. In general, processing heat is generated at the time of drawing; therefore, there is a possibility that the IMC layer was formed in the GCA wire. Such formation of the IMC layer during drawing process may influence the activation enthalpy.
As indicated in Fig. 7, k is greater for the HCA wire than for the ACA wire at each annealing temperature. Such difference of k becomes the maximum at the lowest temperature of T = 423 K (150°C). To investigate the reason of this difference, TEM observation was conducted using the HCA and ACA wires annealed at T = 423 K for the longest time of t = 3.46 Ms (960 h). The result is shown in Fig. 8. Figure 8(a) and 8(b) indicate the BF images of the IMC layer for the HCA and ACA wires, respectively. On the other hand, the corresponding results of EDS measurement for Fig. 8(a) and 8(b) are represented in Fig. 8(c) and 8(d), respectively. As can be seen in Fig. 8(b), polycrystalline microstructure was clear for the IMC layer in the ACA wire. On the other hand, in Fig. 8(a), the microstructure was not so clearly observed for the IMC layer in the HCA wire. Such unclear microstructure for the IMC layer in the HCA wire may be attributed to fine polycrystalline grains in the IMC layer. As the grain size of polycrystalline microstructure in the IMC layer becomes smaller, the contribution of boundary diffusion increases and thus the exponent n in eq. (2) decreases.13) As indicated in Fig. 5, at most of the annealing temperatures, n is actually smaller for the HCA wire than for the ACA wire. As a result, the overall layer growth occurs faster for the former than for the latter. This may be the reason why k is greater for the HCA wire than for the ACA wire at each annealing temperature in Fig. 7.
The HCA and ACA wires annealed at T = 423 K (150°C) for t = 3.46 Ms (960 h): bright field (BF) TEM image of the IMC layer, (a) HCA and (b) ACA; and EDS analysis across the IMC layer, (c) HCA and (d) ACA.
The microstructure evolution of the Cu-clad Al (CA) wire during annealing was experimentally observed in a metallographical manner. The CA wire was mechanically drawn from diameter of 10 mm to that of 1.5 mm. The drawn wire was annealed at 250°C (523 K) for 3 h (10.8 ks). The CA wires before and after this annealing are called the HCA and ACA wires, respectively. These wires were isothermally annealed in the temperature range of 150–270°C (423–543 K) for various times of 12–960 h (43.2 ks to 3.45 Ms). During annealing, the intermetallic layer consisting of the θ, η2, δ, γ1 and α2 phases forms at the original Cu/Al interface in both the HCA and ACA wires. The growth of the intermetallic layer is controlled by volume diffusion and boundary diffusion. The contribution of boundary diffusion to the layer growth gradually increases with decreasing annealing temperature. This contribution is slightly greater for the HCA wire than for the ACA wire. Assuming that the layer thickness is proportional to the square root of the annealing time, we obtain a greater value of the proportionality coefficient for the HCA wire than for the ACA wire at each annealing temperature. Thus, the intermetallic layer grows faster for the HCA wire than for the ACA wire. The temperature dependence of the proportionality coefficient provides the activation enthalpy of 53 kJ/mol for the HCA wire and that of 61 kJ/mol for the ACA wire. The faster growth of the intermetallic layer in the HCA wire than in the ACA wire may be attributed to the greater contribution of boundary diffusion.
