1. INTRODUCTION
Aluminum (Al) alloys have recently been the subject of extensive research due to their advantages of medium strength, excellent formability, high electrical conductivity, and light weight [1-9]. Replacing high-density alloys like steel and copper with aluminum alloys is anticipated to significantly enhance energy efficiency, recyclability, and life-cycle cost. However, in Al alloys, the low strength, poor workability and, low electrical conductivity must be improved for further applications. Accordingly, for the automotive and energy industries, extensive research is still necessary to develop specialized Al alloys that can further improve properties like strength, plastic workability, and electrical conductivity [10]. In particular, for Al wires used in transmission, both strength and electrical conductivity are crucial, and numerous interesting studies on wire-drawn Al alloys have been reported [11-14]. X.M. Luo et al. have reported that the microstructures of cold-drawn Al wires along the radial direction were inhomogeneous, i.e. the texture in the center region was strong <111> and weak <001> components, while that in the surface region shifted from an initial cubic texture to a <112> component and finally developed into a strong <111> component [11]. J.P. Hou et al. suggested that the tensile strength of cold-drawn commercially pure aluminum wire showed an obvious three-stage characteristic, including a first strengthening stage, steady stage and second strengthening stage, with increasing drawing strain [13]. In addition, P. Koprowski et al. investigated the influence of the wire drawing process on the microstructure, mechanical properties, and electrical conductivity/resistivity of 99.9% aluminum and alloys of aluminum obtained by adding 0.2 wt% of Mg, Co, and Ce to technically pure aluminum [14]. Their studies clearly explain the variations of mechanical properties and texture development, and electrical conductivity of various wire-drawn pure aluminum alloys.
AA1070 alloy has been commercially used because, among Al alloys, it has the relatively high electrical conductivity [15]. However, there has been little research on the microstructure, mechanical properties, and electrical conductivity of the AA1070 alloy, a typical Al alloy for wires.
The authors previously investigated the microstructure and mechanical properties of an AA1070 alloy that was severely deformed from a 9.0 mm to a 2.0 mm diameter through a wire drawing process and subsequently annealed [16]. In that previous study, it was found that the AA1070 wire began partially to recrystallize at 250 °C; above 300 °C, it was covered with equiaxed recrystallized grains over all regions. In addition, fiber texture of {110}<111> and {112}<111> components mainly developed, and {110}<001> and {001}<100> texture partially developed as well. The electric conductivity of the AA1070 wire increased with increasing the annealing temperature, and reached a maximum value of 62.6 %IACS at 450 °C. The microstructural changes, such as recovery and recrystallization, well explain the variations in mechanical and electrical properties observed with increasing annealing temperature [16]. The authors further reduced the diameter of the AA1070 wire from 2.0 mm to 0.4 mm through a drawing process. In another previous study, the authors reported in detail the changes in the microstructure, mechanical properties, and electrical properties of AA1070 wire subjected to further wire-drawing processes [17]. It was found from the study that, for all the drawn specimens, fiber textures of {110}<111> and {112}<111> strongly developed, and their intensity tended to increase with the increase in reduction of cross-sectional area (RA). In addition, we found that the hardness and the tensile strength tended to increase stepwise as RA increased. The present study aimed to evaluate the changes in microstructure, mechanical properties, and electrical conductivity of severely drawn ϕ0.4 mm AA1070 wire with increasing annealing temperature, and to compare obtained values to those of the ϕ2.0 mm AA1070 wire from the previous study [16].
2. EXPERIMENTAL
This study utilized commercial AA1070 alloy, whose chemical composition is detailed in Table 1. The initial material, a 2.0 mm diameter Al wire, was sourced from the drawing process mentioned in the previous study [16].
The wire-drawing process was carried out at ambient temperature using a multi-pass drum-type drawing machine, following the optimal pass schedule. The drawing process was conducted at a speed of 753 mm/sec using ALUBE 5050 as the lubricant, resulting in a final diameter of 0.4 mm, corresponding to a reduction in area (RA) of 96%. The amount of deformation induced by wire-drawing is calculated as an equivalent strain (ε ¯
Scanning electron microscopy (SEM) observations and electron backscattered diffraction (EBSD) analysis were used to reveal the microstructural evolution of the annealed Al wires. Using a Phillips XL30s SEM with an FE-gun operated at 20 kV, SEM/EBSD measurements were conducted with the TSL OIM Data Collection ver.3.5 program. The mechanical properties were evaluated at the ambient temperature using an Instron-type tensile testing machine, and the tensile test pieces were machined so that the tensile direction was parallel to the drawing direction. The tensile tests were conducted at the ambient temperature with a constant strain rate of 10-3 s-1 on specimens, all of which had a gauge length of 150 mm. Additionally, Vickers hardness variation in the thickness direction was measured with a load of 0.98 N, and the electrical properties were determined by measuring the electrical resistance between two points over a 100 mm length of the Al wire. The electrical resistivity (ρ) was calculated using the following equation [20].
where, R, A, L, and σ represent electrical resistance, cross-sectional area, measuring distance of the specimen, and electrical conductivity, respectively. The percentage of the International Annealed Copper Standard (%IACS) for electrical conductivity is determined using the following equation.
where IACS stands for the International Annealed Copper Standard, and %IACS represents the ratio of the electrical resistivity of the target material to that of annealed high-purity copper(ρCu=1.724×10-8Ωm×) [21].
3. RESULTS AND DISCUSSION
3.1 Microstructure and Texture
Figure 1 shows the RD (radial direction), DD (drawing direction), GB (grain boundaries) maps, and {111} pole figure obtained by SEM/EBSD measurement for the as-drawn material. Each point's color represents the crystallographic direction that is parallel to the RD and DD of the specimens, as illustrated by the colored stereographic triangle. As shown in Fig 1, the drawn material had a typical deformation structure in which the grains were largely elongated with the drawing direction. As shown in Fig 1(c), high angle grain boundaries (HAGBs) made up 0.58 of the total, a proportion greater than that of low angle grain boundaries(LAGBs). In addition, the typical fiber texture of {110}<111> and {112}<111> strongly developed in the specimen, as shown in Fig 1(d). Especially, in most areas, the {110}<111> developed more strongly than the {112}<111> component. This is different from the results of the previous study in which in addition to {110}<111> and {112}<111> components, {110}<001> and {110}<112> developed [16]. Here, it is worth noting that the {112}<111> component developed strongly in the center region, as shown in the dotted box in Fig 1(a). This is because the deformation patterns at the center and surface of the AA1070 wire are different during drawing. In other words, it is because, in the center region, the deformation is mainly made by tension and compression stress whereas, in the surface region, redundant shear deformation is added by the frictional force between the material and the die during drawing.
Figure 2 shows changes in RD, DD, and GB maps of the specimens with increasing annealing temperature. The as-drawn and 200 °C-annealed specimens still showed a deformation structure in which the grains were elongated in the drawing direction, with a grain thickness of about 1.7 μm. The 250 °C specimen also mainly showed a deformation structure; however, thickness of grains became slightly thicker due to recovery(2.21 μm), and the newly formed recrystallized grains were also partially observed, as indicated by the arrows in Fig 2. The 275 °C specimen showed an almost recrystallized structure, except for narrow regions of center areas, which maintained the deformation structure. However, specimens above 300 °C showed complete recrystallization structure covered with equiaxed grains over all regions, and the higher the annealing temperature, the larger the grain size. The average grain diameters of the specimens annealed at 275 °C, 300 °C, and 350 °C were 11.9, 16.4, and 25.0 mm, respectively.
In addition, the typical deformation(fiber) texture of {110}<111> and {112}<111> developed strongly in all annealed specimens, even at higher temperatures above 300 °C. This means that the fiber texture of the as-drawn specimen remained despite the occurrence of complete recrystallization at higher temperatures. Especially, even in center regions, the {112}<111> component still developed strongly, as shown in the dotted boxes. Figure 3 shows the change in {111} pole figures with increasing annealing temperature. It is also clearly shown that the fiber textures of {110}<111> and {112}<111> strongly remained in all annealed specimens. The maximum intensity hardly changed despite the increase of the annealing temperature, maintaining a value of about 20. It seems very unusual that the deformation texture still developed strongly despite the occurrence of complete recrystallization at high temperatures. In addition to the rolled Al alloys, in the case of the drawn Al alloys, it is common that the recrystallization texture of {001}<100> strongly develops when the complete recrystallization occurs at higher temperatures [22]. In the previous study [16], the recrystallization texture of the {001}<100> component actually also developed, although weakly. In this way, in the drawn pure Al alloys, it has often been reported that fiber textures remain weak even after the occurrence of complete recrystallization [22]. Nevertheless, in present study, {110}<111> and {112}<111> textures developed strongly after complete recrystallization in the center and other regions, respectively. This is due to severe plastic deformation of 96% in RA. Inakazu reported that the texture of pure Al annealed at higher temperatures after a drawing process changed largely depending on the amount of deformation before annealing [22]. He clarified that the larger the amount of deformation, the higher the probability that the deformation texture would remain after recrystallization. The results in this study roughly agree with his argument. Nevertheless, it can be said that it is very unusual for the deformation texture to remain the same after complete recrystallization.
3.2 Mechanical Properties
Figure 4 shows changes in Vickers hardness distribution in the thickness direction (Fig 4a) and the average hardness (Fig 4b) with increasing annealing temperature for the drawn AA1070 alloy. As can be seen in the figure, the drawn specimen (prior to annealing) exhibited an average hardness of 47 Hv, with a deviation of ±2 Hv in hardness along the thickness direction. After annealing at 200 °C, the hardness hardly changed and remained at almost 47 Hv. It also still had a hardness deviation of around ±2 Hv. However, the increase in annealing temperature to 250 °C resulted in a decrease to about 37 Hv. Now, the hardness deviation decreased to around ±1 Hv in the thickness direction. For the specimen annealed at 275 °C, the average hardness decreased further to 31 Hv, and the hardness distribution in the thickness direction again increased slightly to ±2 Hv. The change in hardness deviation is due to the deformation structure remaining in the center regions, as shown in Fig 2. For the specimens annealed at temperatures above 300 °C, the hardness distribution became very uniform in the thickness direction, and the degree of reduction in hardness with the increase of annealing temperature was not so large. Figure 5 shows changes in stress-strain curves(Fig 5a) and tensile properties(Fig 5b) with increasing annealing temperature. The specimen before annealing showed a typical stress-strain(s-s) curve with high strength and low elongation. The specimens annealed at 200 and 250 °C also showed s-s curves similar to those of the specimen before annealing. However, when the annealing temperature increased above 275 °C, both tensile strength (TS) and yield strength (YS) decreased greatly, by the same amount. Therefore, the decrease in TS with the increase of annealing temperature is caused by the decrease in YS. In general, YS consists of the following factors [23]:
where σss, σgb, σpre, and σdisare the solid solution strengthening, grain refinement strengthening, precipitation strengthening, and dislocation strengthening, respectively. In this study, it is believed that σgb had the greatest influence on YS because grain growth occurred most actively as the annealing temperature increased. The value of σgb increases as the grain diameter becomes smaller according to Hall-Petch equation, σ y = σ 0 + k d - 1 2
Figure 6 shows the relationship between tensile strength and elongation for the drawn and subsequently annealed specimens at various temperatures. For reference, the results of ϕ2.0mm 1070 wire in the previous study [16] are also shown. As can be seen in the figure, for the AA1070 wires retaining a deformation structure due to annealing at lower temperatures, the ϕ0.4 mm specimen in this study had higher strength than the ϕ2 mm specimen of the previous study [16]. For the materials annealed at temperatures higher than 300 °C, the strength was higher and the elongation was lower in the ϕ0.4 mm specimen than those of the ϕ2.0 mm specimen. The difference in mechanical properties between both specimens is considered to be due to the difference in grain size. For the ϕ0.4 mm specimen, the average grain diameters of the specimens annealed at 300 and 350 °C were 16.4 μm and 25.0 μm, respectively. These are finer than the values of 25.0 μm and 42.0 μm, respectively for the ϕ2 mm specimens in the previous study [16]. Therefore, it is thought that the tensile strength of the ϕ0.4 mm specimens was higher than that of the ϕ2 mm specimens because of grain refinement strengthening.
3.3 Electrical Properties
Figure 7 illustrates how electrical conductivity (EC) changes as the annealing temperature increases. The EC of the as-drawn specimen was 60.3%IACS. The EC increased with the increase in annealing temperature, reached a maximum of 62.2%IACS after 300 °C. In general, EC increases with the increase of the annealing temperature, according to Matthiessen’s rule [21]. The enhancement in electrical conductivity (EC) due to annealing can be explained by the reduction in point defects, dislocation density, and grain boundary volume fraction during the annealing process [21]. The change in EC with the increase of the annealing temperature was very similar to that of the ϕ2 mm specimen in the previous study [16].
Figure 8 shows the relation between EC and the strength-ductility(S-D) index for the specimens annealed at various temperatures after wire drawing, following the results of the previous study [16]. Here, the S-D index is a value multiplied by the strength and elongation, and is arbitrarily determined as an evaluation index of mechanical properties. As can be seen in the figure, the difference in EC according to the annealing temperature was not very large, but the difference in S-D index was very large in both 2.0 mm and 0.4 mm specimens. The specimens annealed at higher temperatures tended to show higher S-D indexes. This means that the specimens annealed at higher temperatures exhibited excellent combinations of in both EC and S-D indexes for the drawn AA1070 alloy.
4. CONCLUSIONS
A commercial AA1070 wire with a diameter of ϕ2.0 mm was severely deformed to ϕ 0.4 mm by drawing process and subsequently annealed. The drawn Al wire showed a severely deformed structure in which the grains were greatly elongated in the drawing direction and the deformation(fiber) texture of {110}<111> and {112}<111> components strongly developed. This fiber texture remained strong even in specimens in which complete recrystallization occurred via annealing at higher temperatures. The tensile and yield strengths decreased as the annealing temperature increased. However, the elongation showed a sharp increase at annealing temperatures above 275 °C. The change in the mechanical properties with annealing was very similar to that of the ϕ2.0mm specimen in the previous study. However, the tensile strength of the ϕ0.4mm specimens was higher than that of the ϕ2.0mm specimens because of grain refinement strengthening. The electric conductivity tended to increase slightly with the increase of annealing temperature, reaching a maximum value of 62.2%IACS at 300 °C. In addition, the specimens annealed at higher temperatures exhibited excellent combinations of both EC and S-D indexes for the drawn AA1070 alloy in both ϕ2.0 mm and ϕ0.4 mm specimens.
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