(Sang-Hyeon Jo)
1
(Seong-Hee Lee)
1*
Copyright © 2024 The Korean Institute of Metals and Materials
Key words(Korean)
AA1070 alloy, wire drawing, annealing, microstructure, mechanical properties, electrical conductivity
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
¯
) of about 6.4, corresponding to a rolling reduction of 99.6% in the rolling process.
This represents a substantial strain, equivalent to eight cycles of the accumulative
roll-bonding (ARB) process, which is among the most severe plastic deformation processes
[18,19]. The drawn specimen was then annealed for 1h at various temperatures ranging from
200 to 400 °C.
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
(d: grain diameter) [24,25]. Therefore, it is considered that the increase in grain diameter with the increase
of annealing temperature resulted in the decrease in YS and thereby TS. The change
in tensile properties with annealing was very similar to that of the ϕ2.0 mm specimen
in the previous study [16].
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.
Acknowledgements
This result was supported by ‘‘Regional Innovation Strategy (RIS)’’ through the National
Research Foundation of Korea(NRF) funded by the Ministry of Education (MOE)(2021RIS-002).
REFERENCES
Ding L., Weng Y., Wu S., Sansers R. E., Jia Z., Liu Q., Mater. Sci. Eng,A651, 991
(2016)
Park S. J., Li T., Kim C. H., Park J. P., Chang S. Y., Korean J. Mater. Res,22, 97
(2012)
Lee S. H., Lee G. J., Korean J. Mater. Res,21, 655 (2011)
Fan X., He Z., Zhou W., Yuan S., J. Mater. Process. Tech,228, 179 (2016)
Yang J. H., Lee S. H., Korean J. Mater. Res,26, 628 (2016)
Oh S. J., Lee S. H., Korean J. Mater. Res,28, 534 (2018)
Lee S. H., Korean J. Met. Mater,61, 652 (2023)
Kim D. Guk, Jo Y. H., Lee Y. S., Kim Y. Y., Kim H. W., Kim J. K., Korean J. Met. Mater,60,
329 (2022)
Jo M. S., Cho Y. H., Lee J. M., Kim S. B., Kim S. H., Jang J. I., Korean J. Met. Mater,60,
489 (2022)
Jung C. G., Hiroshi U., Son H. T.., Lee S. H., Korean J. Met. Res,27, 597 (2017)
Luo X.M., Song Z.M., Li M.L., Wang Q., Zhang G.P., J. Mater. Sci. Technol,33, 1039
(2017)
Ma X.-G., Chen J., Yang Y., Li L., Chen Z., Yan W., Trans. Nonferrous Met. Soc. China,27,
763 (2017)
Hou J.P., Wang Q., Yang H.J., Wu X.M., Li C.H., Li X.W., Zhang Z.F., Mater. Sci. Eng,A639,
103 (2015)
Koprowskia P., Lech-Gregaa M., Wodzińskib Ł., Augustyna B., Boczkala S., Ożógb M.,
Uliaszb P., Żelechowskia J., Szymański W., Mater. Todays Comm,24, 101039 (2020)
Japan Inst. of Light Metals, Microstructure and Properties of Aluminum Alloys,413
(1991)
Jeong D. H., Lee S. H., Korean J. Met. Res,30, 308 (2020)
Jo S. H., Lee S. H., Korean J. Met. Mater,61, 867 (2023)
Saito Y., Tsuji N., Utsunomiya H., Sakai T., Hong R. G., Scr. Mater,39, 1221 (1998)
Lee S. H., Saito Y., Tsuji N., Utsunomiya H., Sakai T., Scr. Mater,46, 281 (2002)
Callister William D., Rethwisch David G., Materials Science and Engineering (Ninth
Edition, SI Version),682-689, Wiley (2014)
Rose R. M., Shepard L.A., Wulff J., The Structure and Properties of Material (Electroni
Properties),83John Wiley & Sons. Inc. (1965)
Inakazu N., Metal Drawing and Fiber Texture,134Kindai Henshu Ltd (1985)
Lanjewar H., Leo A. I., Verleysen P., Mater. Sci. Eng. A,800, 140224 (2021)
Hall EO, Proc Phys Soc Lond. B,64, 747 (1951)
Petch NJ, J. Iron Steel Inst,174, 25 (1953)
Figures and Table
Fig. 1.
RD(a), DD(b), GB(c) maps, and {111} pole figure(d) obtained by SEM/EBSD measurement
for the drawn AA1070 wire.
Fig. 2.
Changes in RD, DD, and GB maps with increase of annealing temperature for drawn AA1070
wire.
Fig. 3.
{111} pole figures obtained by SEM/EBSD measurement for specimens annealed at 200
°C(a), 250 °C(b), 300 °C(c), and 350 °C(d) after drawing of AA1070 wire.
Fig. 4.
Increase in annealing temperature for drawn AA1070 wire led to changes in Vickers
hardness distribution (a) and average hardness (b).
Fig. 5.
Changes in s-s curves(a) and tensile properties(b) with increase of annealing temperature
for drawn AA1070 wire.
Fig. 6.
Relation of tensile strength and elongation for AA1070 wire annealed at various temperatures
after wire drawing.
Fig. 7.
Change in electric conductivity with increase of annealing temperature for drawn AA1070
wire.
Fig. 8.
Relation between electrical conductivity and strength-ductility index for specimens
annealed at various temperatures for drawn 1070 Al wire.
Table 1.
Chemical composition of AA1070 alloy studied (wt.%).
|
Al
|
Mg
|
Zn
|
Fe
|
Mn
|
Si
|
Ti
|
Cu
|
V
|
|
≥99.70
|
≤0.03
|
≤0.04
|
≤0.25
|
≤0.03
|
≤0.20
|
≤0.03
|
≤0.04
|
≤0.05
|