High-temperature annealing behavior of cold-rolled electrolytic tough-pitch copper

Abstract

Pure copper is very soft, however, hardening the pure copper with most strengthening mechanisms leads to a significant reduction in electrical conductivity. Grain refinement is a better strengthening mechanism to maintain high enough electrical conductivity. Plastic deformation at room temperature followed by post-annealing is one of the best methods to achieve fine-grained metals and alloys. In this research, the high-temperature annealing behavior of cold-rolled electrolytic tough-pitch (ETP) copper sheets was studied. 90 % asymmetric cold rolling followed by high-temperature post-annealing at 673 K for 1, 2, 5, 10, 30, 60, and 120 min were applied on the copper. The microstructure was significantly changed with increasing annealing time from 1 to 2 min owing to full recrystallization. With increasing the annealing duration, the grain size is increased. The formation of equiaxed grains with a smaller size (∼9 μm) compared to the full-annealed (initial) sample (∼68 μm) is observed after the longest time of post-annealing (120 min) due to the pinning effect of Cu₂O particles. The post-annealed copper sheets processed by asymmetric rolling (in this work) exhibited a more homogeneous microstructure through the thickness compared to the symmetric rolling due to more uniform stored strain energy. The results showed that the first deformed grains that undergo recrystallization during post-annealing are Goss-oriented grains. With an increase in the post-annealing time, the S and Copper components were eliminated and a strong Cube and P texture orientations were formed. Interestingly, after 1 min of post-annealing, the yield and tensile strength enhanced to 410.2 MPa and 418.6 MPa owing to the annealing hardening phenomenon. The hardness and strength reached a constant value after the post-annealing for 10 min and above. With increasing the post-annealing duration, the central area of fracture surfaces (consisting of ductile dimples) became larger and the outer region (consisting of flat surfaces and shear dimples) became smaller, showing a shift towards perfect ductile fracture. With the increase of post-annealing time from 1 min to 120 min, the electrical conductivity was increased from 77.6 to 97.5 %IACS. The presence of the Cube texture increased the electron mobility compared to the P orientation, by reducing the mean distance that they can travel without scatter. From the obtained results, it can be concluded that the asymmetric cold rolling followed by high-temperature post-annealing is capable of strength improvement and maintaining electrical conductivity in copper.

1. Introduction

Softening is the dominant phenomenon that occurs after annealing in cold deformed metals and alloys due to the annihilation of dislocations and rearrangement by climb and/or cross-slip during recovery, formation of dislocation-free grains during recrystallization, and finally grain growth [1]. However, Nasiguti [2] described the “anneal hardening” term in the 1950s. This strengthening was reported during the annealing of several deformed Cu alloys [2,3]. In a previous study, polycrystalline pure copper (99.99 %) was processed by cryogenic rolling and subsequent annealing at different temperatures. The hardening of the copper after annealing treatment was related to the high fraction of twin boundaries [4].

High electrical conductivity and high strength are often needed simultaneously for conducting materials. Pure copper is very soft, however, strengthening the pure copper with the solid-solution strengthening results in a remarkable reduction in electrical conductivity. The trade-off between conductivity and strength is encountered in the development of conducting materials [5]. Takata et al. [6] investigate the microstructure, electrical conductivity, and mechanical properties of a copper alloy processed by accumulative roll bonding. The high fraction of grain boundaries formed by the accumulative roll bonding has a remarkable effect on strengthening but little impact on the electrical conductivity. It can be concluded that grain refinement and strain hardening are better strengthening mechanisms to maintain high enough electrical conductivity. The electrical conductivity is slightly reduced and the strength is enhanced with a reduction in grain size. The electrical conductivity of pure copper maintains a constant value over 95 %IACS till the grain size reduces to 200 nm. Thus, it is possible to achieve high strength and high enough electrical conductivity in fine-grained copper and copper alloys. In a recent work [7], copper was processed to equal channel angular pressing followed by cryorolling. After two passes of equal channel angular pressing and cryorolling (with a thickness reduction of 75 %) optimal tensile strength, hardness, and electrical conductivity of 462 MPa, 120 HV, and 95 %IACS were achieved. The grain refinement, twinning, and accumulation of dislocations were the main reasons for the enhancement of strength. After equal channel angular pressing the original {111}⟨110⟩ texture was rapidly transformed into an intense Brass {110}⟨112⟩ orientation.

Plastic deformation at room temperature followed by post-annealing is one of the best methods to achieve fine-grained metals and alloys [[8], [9], [10], [11], [12], [13], [14], [15]]. Also, the post-heat treatment can eliminate the anisotropic microstructure in materials [[16], [17], [18], [19], [20], [21], [22]]. There have been very few reports documenting the mechanical properties, texture and microstructure evolution, and electrical properties of the copper subjected to asymmetric rolling followed by partial annealing [23,24]. Yu et al. [23] studied the influence of low-temperature post-annealing at 50–125 °C for 60 min on the microstructure and tensile behavior of a 90 % asymmetrically rolled high-purity (99.999 %) copper sheet. The tensile strength of rolled samples changed slightly when the annealing temperature was smaller than 100 °C. The recrystallization occurred in the post-annealed copper sheet when the annealing temperature was 125 °C. The optimal yield strength, tensile strength, ductility, and electrical conductivity of 320 MPa, 393 MPa, and 20 % were obtained. Also, Afifeh et al. [24] examined the impact of low-temperature post-annealing at 200 °C for 5, 10, 30, and 60 min on the microstructure, mechanical, and electrical properties of a 96 % cold-rolled pure copper. The mechanical behavior exhibited large strength even at the annealing time of 60 min. After 60 min post-annealing, the electrical conductivity improved from 84.1 %IACS (for the cold-rolled copper) to 87.5 %IACS owing to the restoration (softening) mechanisms. However, there are no published works about the influence of high-temperature annealing on the texture, electrical conductivity, mechanical behavior, and microstructure of asymmetrically cold-rolled electrolytic tough-pitch (ETP) copper.

The present work aims to investigate the possibility of achieving a good balance between strength and electrical conductivity in pure copper. For this purpose, microstructure, crystallographic texture, mechanical characteristics (yield strength, hardness, tensile strength, ductility, and toughness), and electrical properties of the ETP copper after high thickness reduction by applying asymmetric cold rolling in the unidirectional path and subsequent annealing at the temperature of 673 K for different times will report and the results will discuss.

2. Materials and methods

The as-received copper plate (>99.9 % Cu) with a thickness of 1 cm was subjected to the annealing treatment at a temperature of 1023 K for 120 min followed by cooling in the air. Then, asymmetric unidirectional rolling at room temperature with a total rolling reduction of 90 % was applied to the ETP copper plate. The features of the asymmetric rolling machine are listed in Table 1 and the asymmetric rolling machine is shown in Fig. 1(a). Also, a 90 % cold-rolled copper sheet is depicted in Fig. 1(b). The deformed copper sheets were then subjected to high-temperature post-annealing treatment at 673 K for 1, 2, 5, 10, 30, 60, and 120 min.

Table 1.

Asymmetric rolling machine features.

Roller	Speed (rpm)	Width (mm)	Diameter (mm)
Up	Frictional motion	200	150
Down	3.2	200	150

Fig. 1.

(a) Asymmetric rolling machine, (b) 90 % cold-rolled sheet, and (c) schematic illustration of a four-point probe tool.

The copper sheets were cut and mounted in the rolling direction-normal direction section, and mechanically polished for the observation of microstructure. The samples were etched by FeCl₃ (iron (III) chloride or iron trichloride) + HCl + distilled water. An optical microscope and scanning electron microscope were used to examine the microstructures. Quantitative analysis was performed by Image J software. The incomplete pole figures were achieved by using X-ray diffraction (XRD) and determining the orientation distribution functions. This was conducted by ATEX software [25].

Microhardness tests were carried out based on ASTM E384 by a Koopa machine with a load of 100 g and a dwell time of 15 s. Tensile tests were conducted based on ASTM E8M by Santam machine with a speed of 1 mm per minute at ambient temperature. The gauge length and width of the tensile samples were 12 and 3 mm, respectively.

The electrical properties of copper were measured by the four-point probe technique. As depicted in Fig. 1(c), a four-point probe tool has 4 equally spaced, co-linear probes which are used to make electrical contact with the copper sheets to be characterized. An electrical current was passed between two probes and the electrical potential difference between the remaining two probes was determined [26].

3. Result and discussion

3.1. Microstructural evolution

The microstructure of ETP Cu samples after full annealing at 1023 K for 2 h and 90 % thickness reduction by asymmetric unidirectional rolling at room temperature is illustrated in Fig. 2(a) and (b). The equiaxed and coarse grain with a large number of annealing twins (as displayed by green arrows in Fig. 2(a)) observed in the full-annealed sheet. The average size of alpha-Cu grains is about 68 μm. There are many Cu₂O (copper(I) oxide or cuprous oxide) particles in the microstructure of the annealed sample as displayed by the purple color in Fig. 2(a). Based on the quantitative analysis, the fraction of these particles is about 1 %. Fig. 2(a) clearly indicates that the distribution of particles in the microstructure is not homogeneous. After rolling, the original grains are significantly elongated in the direction of rolling. Also, after rolling, the particle distribution in the Cu matrix becomes homogeneous. This is owing to the material flow during cold deformation [27].

Fig. 2.

The microstructure of ETP copper samples after (a) full annealing and (b) 90 % thickness reduction by asymmetric unidirectional.

Moreover, there are several shear bands in the rolled sample. These bands begin to form in ∼35°–45° to the rolling direction [28] but have weak intensity. The 90 % asymmetric cold rolling leads to the formation of a high dislocation density. As reported in our previous work [29] the dislocation density of the 90 % rolled ETP copper sheet is 2.41 × 10¹⁶ m⁻². It should be noted that a few slip systems are available during asymmetric unidirectional rolling, thereby resulting in intense dislocation pile-ups. High dislocation density regions as well as shear bands are favorable sites for the formation of new grains during post-annealing treatment of the deformed ETP copper sheet. The asymmetric rolling induces additional shear deformation along the thickness of the sheet compared to the symmetric rolling, which can play an important role in enhancing the stored strain energy and therefore promoting grain refinement through recrystallization [30,31]. Also, the medium stacking fault energy of pure copper restricts easy recovery and accumulates the dislocations [32,33] that act as the nuclei for the formation of more new grains during post-annealing.

The microstructure of pure copper sheets after the post-annealing at 673 K for 1, 2, 5, 10, 30, 60, and 120 min are depicted in Fig. 3(a–g). During annealing at the lowest time (1 min), it can be observed that the elongated grains are broken to smaller grain and the aspect ratio of grains are significantly decreased. However, it is clear that a large fraction of the microstructure consists of deformed grains. This shows that partial recovery has occurred in this sample. As can be seen in Fig. 3(b), the microstructure is significantly changed with increasing annealing time from 1 to 2 min. Full recrystallization has occurred in the microstructure of the 2-min post-annealed sheet. With increasing the annealing duration, the grain size is increased. The mean grain size of the copper sheet after the post-annealing for 5, 10, 30, 60, and 120 min is 5.1, 7.3, 7.3, 8.4, and 8.7 μm, respectively. It is noteworthy that the grains in the microstructure of the 120-min sample appeared much finer compared with the initial copper sheet. These suggest the high thermal stability of the asymmetrically cold-rolled ETP copper sheet at a high post-annealing duration. This result can be attributed to the pinning effect of Cu₂O particles, which suppress the grain boundary's migration during grain growth. The uniform distribution of Cu₂O particles induced by 90 % asymmetric cold rolling leads to the maximum efficiency of the Cu₂O particles in hindering the coarsening of alpha grains. The microstructure of the ETP copper after annealing for 120 min reveals a bimodal grain structure consisting of fine (∼3 μm) and coarse (∼24 μm) grains as displayed by blue and green arrows in Fig. 3(g), respectively.

Fig. 3.

The microstructure of pure copper sheets after post-annealing at 673 K for (a) 1, (b) 2, (c) 5, (d) 10, (e) 30, (f) 60, and (g) 120 min.

The microstructure of the entire thickness of copper sheets after 0, 1, 5, 30, and 120 min of post-annealing treatment is demonstrated in Fig. 4(a–e). According to Fig. 4(a), the grain width in the near-surface regions of the rolled copper sheet is lower than that in the middle area, demonstrating that the shear strain in the near-surface regions is more than that in the middle area. According to Fig. 4(e), the average grain size from the top surface to a distance of 200 μm is about 7.9 μm, while this value from the bottom surface to a distance of 200 μm is about 7.5 μm. Also, the grain size in the middle region is about 10 μm. In other words, the grain size of the near-surface regions is lower than that of the middle area of the sheet. During post-annealing of the rolled copper, both recovery and recrystallization are driven by the stored energy resulting from asymmetric rolling and are closely related to the strain distribution through the thickness of the copper sheet. As reported in our previous work [29], the dislocation density of the near-surface regions is larger than that of the midthickness area of the 90 % deformed ETP copper sheet. Since there are more potential nucleation sites in the near-surface regions with higher dislocation density, the average size of recrystallized grains should be lower than that in the middle area of the copper sheet after post-annealing. However, the difference between the grain size in the near-surface regions and the middle area of the post-annealed copper sheet is low. Previous works [[34], [35], [36], [37], [38]] showed that the difference between the grain size in the areas near the surface and the center of metals subjected to symmetrical rolling and post-annealing was much greater than in the present work. In other words, the post-annealed copper sheets processed by asymmetric rolling (in this work) exhibit a more homogeneous microstructure. This is due to more homogeneous stored strain energy through the thickness of the asymmetrically deformed materials [29,30,[39], [40], [41], [42]].

Fig. 4.

Microstructure of the entire thickness of copper sheets after (a) 0, (b) 1, (c) 5, (d) 30, and (e) 120 min of post-annealing treatment.

Fig. 5 depicts the XRD patterns of the ETP copper sheets in various states. From the XRD patterns (Fig. 5(a)), the main peaks correspond to the alpha-Cu phase, which is denoted by squares. Besides these peaks, some secondary peaks, labeled by circles, are observed in the ETP copper sheets, which are Cu₂O particles. However, the diffraction peak of the Cu₂O phase is very weak due to its low content. As can be seen in Fig. 5(a), the patterns of the different copper sheets exhibit remarkable differences in peak heights of the alpha-Cu phase. The rolled copper sheet mainly consists of (111)_Cu crystal planes, whereas the peaks for (200)_Cu and (220)_Cu crystal planes have much lower intensity. After applying different durations of post-annealing treatment, the diffraction peaks of the copper sheets become concentrated on firstly (220)_Cu and then (200)_Cu crystal planes. As the post-annealing duration increases, the intensity of the diffraction peak for (200)_Cu is significantly enhanced. From Fig. 5(b), the diffraction peak position of (200)_Cu is moved to the left after 90 % cold deformation, suggesting that the stored strain energy in the copper sheet is remarkably enhanced (dislocation density of 2.41 × 10¹⁶ m⁻²). The (200)_Cu peak moves to the left and right with the increase of post-annealing time. After 1 and 5 min of post-annealing, the (200)_Cu peak is shifted to the right. This is a typical feature of the dislocations’ annihilation. It can be stated that the formation of fine grains through grain refinement leads to the shifting of the peak to the right compared to the initial sample. This peak shift of the alpha-Cu phase is ascribed to the increase in the fraction of grain boundary induced by the occurrence of recrystallization. Finally, after 30 and 120 min of post-annealing, the (200)_Cu peak is shifted to the left due to the decrease in the fraction of grain boundary induced by grain growth.

Fig. 5.

(a) The X-ray diffraction patterns and (b) (200)_Cu peak for copper sheets after full annealing, 90 % rolling, and post-annealing at 400 °C for 1, 5, 30, and 120 min.

3.2. Texture evolution

The pole figures and orientation distribution functions of the initial, 90 % cold-deformed are reported and discussed in our previous work [43]. The texture of the initial (full-annealed) copper sheet consisted of {001}⟨100⟩ Cube, which was attributed to the recrystallization. After 90 % cold deformation, the copper sheet revealed typical rolling textures consisting of {123}⟨634⟩ S, {112}⟨111⟩ Copper, and {011}⟨100⟩ Goss and shear texture ({111}⟨112⟩ Y). Therefore, the recrystallization texture was transformed to rolling and shear textures via 90 % asymmetric unidirectional rolling. The {100} and {111} pole figures and orientation distributions functions (ODFs) of the post-annealed samples for 1, 5, 30, and 120 min are displayed in Fig. 6, Fig. 7(a–d), respectively. After 1 min of post-annealing at 673 K, S and Copper components remained with the intensity of 7.2 × R and 5.4 × R, respectively. Besides these deformation textures, a new component ({012}⟨100⟩), which is a near Cube orientation, with the intensity of 6.0 × R is formed. According to the obtained results, it can be concluded that the first deformed grains that undergo recrystallization during post-annealing are grains with Goss orientation. With an increase in the post-annealing time to 5 min, the S and Copper components are eliminated and a strong Cube texture with the intensity of 5.0 × R is formed. By further increasing the annealing duration to 30 min, {011}⟨111⟩ P component which is a recrystallization texture is observed with the intensity of 4.8 × R. Finally, after 120 min of post-annealing at 673 K, a very strong Cube orientation with the intensity of 13.7 × R is formed in the microstructure of the copper sheet. It can be said that by increasing the post-annealing time from 1 to 120 min, the rolling texture was transformed into the recrystallization texture.

Fig. 6.

The {100} and {111} PFs of the post-annealed samples for (a, b) 1, (c, d) 5, (e, f) 30, and (g, h) 120 min.

Fig. 7.

The ODFs of the post-annealed samples for (a) 1, (b) 5, (c) 30, and (d) 120 min.

3.3. Mechanical properties

Fig. 8 plots the variation of Vickers hardness of different copper sheets. The initial and rolled copper sheets have a hardness value of 53.0 and 130.5 HV, respectively. The significant increase in hardness signifies an increase in the dislocation density induced by the 90 % asymmetric rolling. As illustrated in Fig. 8, when the post-annealing time is 1 min, the hardness of the copper sheet is slightly lower (129.3 HV) than that of the rolled one. This indicates recovery is the main softening mechanism in this sample. The Vickers hardness of the 2-min post-annealed sheet revealed a high degree of reduction in hardness value (66.0 HV) due to the annihilation of dislocation and formation of recrystallized grains. This result is consistent with the microstructural observations. With increasing the annealing duration, the hardness value slightly decreases due to increasing the average size of alpha grains. It can be seen that the hardness reaches a constant value (54–57 HV) after the post-annealing for 10 min and above. This outcome states that no remarkable grain growth (7.3–8.7 μm) occurred after the post-annealing treatment for 10 min and above.

Fig. 8.

The variation of Vickers hardness of different copper sheets.

Fig. 9 demonstrates the engineering stress-strain curves of initial, deformed, and post-annealed copper sheets at 673 K for different times. In addition, the variations of strength, ductility, and toughness are demonstrated in Fig. 10(a–d). As expected, the smallest value of strength and the highest value of ductility and toughness are achieved for the initial copper sheet. The asymmetrically deformed sheet shows large strength but very low ductility and toughness. These results are mainly due to the high density of dislocation in the rolled copper sheet. After 1 min of post-annealing, the YS enhanced to 410.2 MPa, UTS improved to 418.6 MPa, and ductility increased to 7.8 %. In other words, a hardening during post-annealing for 1 min is observed in the ETP copper sheet. This behavior was reported in previous studies [2,3,[44], [45], [46], [47]]. Based on the literature, this phenomenon can be attributed to the formation of annealing twins [48] and dislocation source limited hardening due to a lack of movable dislocations and easy dislocation sources after annealing [45,49]. The relaxation of grain boundaries during the post-annealing of deformed metals can be also responsible for this behavior [[50], [51], [52], [53]]. During a high degree of plastic deformation, non-equilibrium grain boundaries with large free energy, high dislocation density, and misfit areas can be created. The additional defects at grain boundaries are eliminated and grain boundary areas become more orderly after post-annealing treatment. This is the relaxation of the grain boundary. It was reported that this phenomenon during post-annealing treatment can remarkably increase strength without any change in crystallographic texture and grain size [[52], [53], [54]].

Fig. 9.

The stress-strain curves of copper sheets.

Fig. 10.

The variations of (a) yield strength, (b) tensile strength, (c) ductility, and (d) toughness of copper sheets.

From Fig. 9, Fig. 10, it can be observed that the strength sharply reduced, and ductility and toughness significantly improved with the increase of annealing time to 2 min and then there were no drastic changes. The sharp reduction in strength and significant improvement of ductility and toughness are owing to the occurrence of full recrystallization. In general, after post-annealing treatment for 2 min and above, the strength of the copper sheets reduces gradually with the increase in annealing time, while the ductility and toughness reveal a heterogeneous behavior. When the post-annealing duration increases from 2 min to 120 min, the yield and tensile strength of the copper sheets decrease from 115.3 MPa to 261.4 MPa–101.9 MPa and 242.2 MPa, respectively. This is due to the grain growth of the alpha-Cu phase in the microstructure. When the post-annealing time increases from 2 min to 10 min, the ductility and toughness decrease from 47.3 % to 113.9 J cm³ to 39.8 % and 83.7 J cm³, respectively. Then, these values enhance to 45.7 % and 99.9 J cm³, respectively, after the post-annealing for 30 min. Finally, the ductility and toughness decrease again.

According to Fig. 10, when the post-annealing time increases from 10 min to 30 min the reduction in the strength is negligible. This can be attributed to the formation of a strong P component in the 30-min sample. The P orientation has a much larger Taylor factor (4.71) compared to the Cube component (2.71) [[55], [56], [57], [58]]. Therefore, the P texture is a very hard orientation compared to the soft Cube component.

3.4. Fractography

Fig. 11(a–g) depicts the normal view of the macroscopic fracture surfaces of the post-annealed samples. The fracture surface of the post-annealed Cu sheet for 1 min (Fig. 11(a)) exhibits a large cross-section area owing to the low plastic deformation (as indicated in Fig. 10(c)). In other words, the 1-min post-annealed sample did not reveal significant necking. With increasing the post-annealing duration, a smaller cross-section area is observed owing to the larger plastic deformation of copper. Significant necking was formed in the post-annealed copper sheets for 2 min and above. The fracture surfaces reveal two different regions (outer and central areas). The central area shows ductile (equiaxed) dimples, while the outer region reveals flat surfaces and shear dimples. The shear dimples have elongated horseshoe shape. The transition between outer and central areas is indicated by the blue dash line in Fig. 11. With increasing the post-annealing duration, the central area becomes larger and the outer region becomes smaller, showing a shift towards perfect ductile fracture.

Fig. 11.

The macrographs of the tensile fracture of the post-annealed copper sheets at 673 K for (a) 1, (b) 2, (c) 5, (d) 10, (e) 30, (f) 60, and (g) 120 min.

Fig. 12 illustrates the fracture surface of the initial and 90 % cold-rolled copper sheets. The initial copper sheet exhibits several large deep dimples (as displayed by green arrows in Fig. 12(a)) and a significant necking, which are evidence of a perfect ductile fracture with large plastic deformation before rupture. From Fig. 12(b), the deformed copper sheet reveals flat surfaces and shear dimples in the outer area, and ductile dimples in the central region of the fracture surface, showing a mixture of ductile and brittle modes. Compared with the initial copper sheet, the 90 % cold-rolled sample shows shallow dimples, which is consistent with its low ductility value.

Fig. 12.

The fracture surface of (a) initial and (b) 90 % cold-rolled copper sheets.

Fig. 13(a–n) demonstrates the fractograph of the tensile tests for the post-annealed ETP copper sheets. In general, in all post-annealed samples, a ductile fracture is apparent because the fracture surface is covered with dimples. Numerous shear and ductile dimples can be seen on the fracture surfaces of the post-annealed copper sheets. Many Cu₂O particles are seen on the fracture surfaces of the post-annealed samples indicating good bonding between the Cu matrix and Cu₂O particles. The Cu₂O particles are observed at the bottom of the dimples, indicating that the Cu/Cu₂O interfaces are the source of initiation of voids by decohesion. Interface decohesion is an important mechanism of void initiation as reported in previous works [59]. Fine dimples without Cu₂O particles are also seen on the fracture surface (as displayed by yellow arrows in Fig. 13(b) and (d)), showing that another possible place for the initiation of voids is grain boundaries. The number of these fine dimples is decreased with increasing the post-annealing duration. This can be related to decreasing the fraction of grain boundaries and triple junctions induced by grain growth of alpha-Cu grains at higher annealing time.

Fig. 13.

The micrographs of the tensile fracture of the post-annealed copper sheets at 673 K for (a, b) 1, (c, d) 2, (e, f) 5, (g, h) 10, (i, j) 30, (k, l) 60, and (m, n) 120 min.

3.5. Electrical conductivity

The electrical conductivity of copper sheets is presented in Fig. 14. The highest electrical conductivity (99.1 %IACS) is achieved when the copper sheet is fully annealed at 1023 K for 2 h. The electrical conductivity decreased to 79.1 %IACS in the cold-rolled state. The dislocations can create localized strain fields, which act as electron scattering. Hence, an increase in scattering centers induced by high dislocation density results in a reduction in the electrical conductivity of the copper sheet. After the post-annealing for 1 min, the electrical conductivity decreased to 77.6 %IACS. This can be due to anneal hardening in this copper sheet. The causes of anneal hardening that have been mentioned in previous studies (such as the formation of annealing twins [48]) lead to a slight decrease in electrical conductivity in this sample. According to Fig. 14, with the increase of post-annealing time from 1 min to 120 min, the electrical conductivity is increased from 77.6 %IACS to 97.5 %IACS. Increasing the duration of post-annealing treatment reduces defects (scattering centers) such as dislocations and grain boundaries in the copper sheet, which means an enhancement in electrical conductivity.

Fig. 14.

The electrical conductivity of the initial, rolled, and post-annealed copper sheets.

A very important point in Fig. 14 is that the percentage increase in electrical conductivity with increasing the post-annealing duration from 5 to 30 min is much lower than in other conditions. This result can be attributed to the presence of the P texture component in the copper sheet annealed for 5 min. It can be concluded that the formation of the Cube component can cause a greater increase in the electrical conductivity of the copper sheet compared with the P component. The presence of Cube and P orientations means that the {100} and {110} atomic layers of most grains are parallel to the surface of the copper sheet, respectively. Since the distance between the crystallographic planes of {100} layers is larger than that of {110} layers, the mobility of electrons increases through the larger distance between {100} layers atomic layers. In other words, the presence of Cube texture increases the electron mobility by reducing the mean distance that they can travel without scatter. Thus, the Cube-oriented grains give better conductivity.

3.6. Comparison

The tensile characteristics and electrical conductivity of some pure copper sheets processed by rolling and post-annealing treatment are presented in Table 2. As seen, the copper sheet produced by Yu et al. [23] revealed the highest yield and tensile strength, but lowest ductility. The main reason for this result is the lower annealing temperature in their research (125 °C). The low-temperature post-annealing delays the occurrence of the restoration mechanisms and maintains a higher strength in the copper sheet. Comparing the results of the present work with Afifeh et al.'s research shows that ductility and electrical conductivity are higher in the present work. This can be mainly ascribed to the larger annealing duration (120 min) in this work, which promotes the restoration mechanisms and enhances the ductility and electrical conductivity.

Table 2.

The tensile characteristics and electrical conductivity of some pure copper sheets processed by rolling and post-annealing.

Reference	Process	YS (MPa)	UTS (MPa)	Ductility (%)	Electrical conductivity (%IACS)
Yu et al. [23]	95 % cryorolling + annealing (125°C-60 min)	320	393	20	–
Afifeh et al. [24]	96 % cold rolling + annealing (200°C-60 min)	86.9	229.9	32.0	87.5
Present work	90 % cold rolling + annealing (400°C-120 min)	99.9	234.5	42.4	97.5

4. Conclusions

In this research, the high-temperature annealing behavior of cold-rolled electrolytic tough-pitch (ETP) copper sheets was investigated. The important conclusions are.

• 1.The microstructure was significantly changed with increasing annealing time from 1 to 2 min owing to full recrystallization. With increasing the annealing duration, the grain size is increased.
• 2.The formation of equiaxed grains with a smaller size (∼9 μm) compared to the full-annealed (initial) sample (∼68 μm) is observed after the longest time of post-annealing (120 min) due to the pinning effect of Cu₂O particles.
• 3.The post-annealed copper sheets processed by asymmetric rolling (in this work) exhibited a more homogeneous microstructure through the thickness compared to the symmetric rolling due to more uniform stored strain energy.
• 4.The first deformed grains that undergo recrystallization during post-annealing are Goss-oriented grains. With an increase in the post-annealing time, the S and Copper components were eliminated and a strong Cube and P texture orientations were formed.
• 5.Interestingly, after 1 min of post-annealing, the yield and tensile strength enhanced to 410.2 MPa and 418.6 MPa owing to the annealing hardening phenomenon. The hardness and strength reached a constant value after the post-annealing for 10 min and above due to no remarkable grain growth (7.3–8.7 μm).
• 6.With increasing the post-annealing duration, the central area of fracture surfaces (consisting of ductile dimples) becomes larger and the outer region (consisting of flat surfaces and shear dimples) becomes smaller, showing a shift towards perfect ductile fracture.
• 7.With the increase of post-annealing time from 1 min to 120 min, the electrical conductivity was increased from 77.6 to 97.5 %IACS. The presence of the Cube texture increased the electron mobility compared to the P orientation, by reducing the mean distance that they can travel without scatter.

From the obtained results, it can be concluded that the asymmetric cold rolling followed by high-temperature post-annealing is capable of strength improvement and maintaining electrical conductivity in copper. Additional research that can be done includes investigating the effect of the amount of strain before the partial annealing treatment, as well as evaluating the effect of the speed ratio of the rollers on the microstructure, texture, and mechanical and electrical properties.

Data availability

All data included in this study are available upon request by contact with the corresponding author.

CRediT authorship contribution statement

Hanieh Solouki: Writing – original draft, Software, Resources, Investigation. Roohollah Jamaati: Writing – review & editing, Supervision, Methodology, Conceptualization. Hamed Jamshidi Aval: Writing – review & editing, Resources, Methodology, Conceptualization.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.