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JWJ > Volume 39(6); 2021 > Article
Yoon, Shim, and Kang: Recrystallization Behavior of CP Ti Welds by Rolling

Abstract

Electrolytic copper foil is a thin copper film material manufactured by electroplating and is mainly used as a circuit material to transmit electrical signals in printed circuit boards of electronic products such as TVs, PCs, and smartphones. Recently, it has been widely used as a negative current collector for lithium-ion batteries, which is essential for mobile IT and electric vehicles. The electrodeposited drum material is composed of CP Ti, which exhibits corrosion resistance and has a light weight, and welding is essential for processing in the form of a drum. Therefore, when the microstructures of the base metal and the welded part differ, these differences are reflected in the surface of the copper foil, which results in poor quality. Therefore, in this study, a welding process that required a low heat input while minimizing the size of the weld was used, and the change in the grain size was studied through plastic deformation and recrystallization heat treatment of the weld structure. Following the recrystallization heat treatment after the rolling of the plasma welded specimen, it was found that the calculation results of the recrystallization heat treatment were consistent with the actual conditions of the heat treatment, and the microstructure of the weld metal became finer as the rolling reduction rate increased.

1. Introduction

The electrolytic copper foil is a thin copper film fabricated through an electroplating process, which is mainly used as circuit components that transmit electrical signals in printed circuit boards of electronic products such as television, personal computer, and smartphones. Recently, their demand is increasing as the cathode current collector material, which is essentially applied in lithium-ion batteries of mobile information technology (IT) and electric vehicles. The electrolytic copper foil is fabricated with the electroplating method, using the equipment composed of a cathode (titanium), anode (titanium), and busbar (copper). This rocess consists of creating a copper sulfate solution for fabricating the copper foil by melting highly pure copper into a sulfuric acid solution, electroplating the copper ions from the copper sulfate solution to a large- scale Ti drum (cathode) in the electrolyzer, and exfoliating to create a rolled product. The commercially pure (CP) Ti is used as an electroplating drum material by considering the corrosion resistance, lightweightness, and detachability where processing it into a drum shape requires welding. However, a microstructural variation in the base material and the weld zone results in a defective product due to the transcription to the copper foil surface. Thus, microstructural adjustment is necessary to have a weld zone structure similar to the base material structure. Kang1) reported that the soldering process of the CP Ti material during the friction stir welding had been achieved at a β-transformation temperature or below and no phase transformation occurred in the CP Ti, while phase transformation occurred in the Ti-6Al-4V alloy because the processing temperature reached the β-transformation temperature or above. According to Moon2), the MC (M: V and/or Nb) carbide was extracted at the transgranular zone and crystal grain boundary in the welding of the austenitic Fe-30Mn-9Al-0.9C (wt%), in which Nb and V were added, and crystal grain boundary growth inhibition effect was observed at the heat-affected zone. Tan3) performed hot rolling of the CP Ti material in the 600 - 800°C range and air cooling thereafter. Consequently, the recrystallization behavior of the crystal grain was observed. Hayashi et al.4) also performed hot rolling of CP Ti at 600 - 800°C according to the varying reduction rate, which showed variations in rolled structure such as the generation of a twin crystal due to the forming process generated during rolling. Hayashi et al.4) reported that the structure became finer due to recrystallization during the cooling process and that the structure transformed due to static recrystallization according to the temperature and reduction rate. In this study, a low heat input welding process, which could minimize the weld zone size, was utilized. Furthermore, the variation in grain size was studied through the forming process of the weld zone structure and recrystallization annealing

2. Materials and Experimental Methods

2.1 Materials

The material used in this study was a 9-mm grade 1 CP Ti, and the analysis results of its chemical substances and mechanical properties are summarized in Table 1. As a grade 1 material containing 0.025 wt% Fe, the elongation rate was 39% and tensile strength 324 MPa.
Table 1
The materials used in this study (wt%)
Fe C N O H Ti
0.025 0.0185 0.0042 0.16 0.0103 Bal
Yield strength (N/mm2) Tensile strength (N/mm2) Elongation (%)
238 324 39
Fig. 1 presents an image of the microstructure of the material used, in which grain size measurement results revealed that their degree was at level No.9.9 of the American Society for Testing and Materials (ASTM).
Fig. 1
Photograph showing the microstructure of the base metal
jwj-39-6-577gf1.jpg
Fig. 2
Schematic diagram of the welding process
jwj-39-6-577gf2.jpg

2.2 Welding process

The welding of the Ti material proceeded in two parts. Additional lamination was required to the base material thickness of 9 mm for the weld zone rolling. The weld zone specimen was fabricated so that the entire height becomes 19 mm through plasma welding and weld bead deposition. First, plasma keyhole welding was used to minimize the heat effect at the weld zone. After the welding, the weld bead was deposited to be applied in the rolling process.
The plasma keyhole welding was performed with the I-type and without root gaps. The welding current was approximately 275 A, the welding speed was 30 cm/min, and the heat input was around 18 kJ/cm. Here, Ar gas was additionally supplied at 25 L/min to protect the entire weld zone from the atmosphere.
The plasma process and the CP Ti grade 1 welding rod were used to perform the build-up welding. Likewise, Ar gas was supplied at 12 L/min during welding to protect the weld zone.

2.3 Weld zone rolling

The specimen that was fabricated under the same conditions mentioned above was used in the rolling process shown in Fig. 3. The reduction rate was adjusted to be 80% in total at room temperature for the rolling. Here, the rolling reduction per rolling frequency was 1 - 2 mm, and the total rolling frequency according to the reduction rate was 20 - 30 times.
Fig. 3
Schematic diagram of rolling process used in this study
jwj-39-6-577gf3.jpg

2.4 Structural analysis and grain size measurement

The junction was mechanically polished for analyzing the weld zone structure and measuring the grain size. Then, it was etched at room temperature for five seconds with a mixed solution of 100 ml distilled water + 3 ml hydrofluoric acid + 4 ml nitric acid. A deformed structure forms in the CP Ti material even during mechanical polishing. Thus, mechanical polishing and etching were executed multiple times to remove these structures. In the microstructural observation, an optical microscope was utilized to observe the macroscopic structural transformation, and the grain size was measured according to the ASTM method at 100x magnification.

3. Experimental Results and Discussion

3.1 Weld zone structure

The specimen fabricated by plasma welding and build- up welding using this are shown in Fig. 4. As mentioned above, the height of one weld bead was 8 mm, and the opposite side had a 2 mm weld bead layer.
Fig. 4
Welded specimen
jwj-39-6-577gf4.jpg
The cross-section of the welded specimen is as provided in Fig. 5 in which twin crystals were partially found at the ferrite matrix structure in the deposited metal.
Fig. 5
Optical microstructures of specimen as welded (a) low magnification, (b) weld metal at build-up welding, (c) weld metal at keyhole welding, and (d) heat affected zone
jwj-39-6-577gf5.jpg

3.2 Results of rolling

The welded specimen was cut to approximately 30 mm in width and 50 mm in length to perform rolling. On the side with the high weld bead layer, cracks were observed at the surface as the reduction rate increased, and the cracks were more frequent as the reduction rate increased. On the opposite side, the weld zone and the base material were simultaneously rolled as shown in Fig. 6.
Fig. 6
Specimens after rolling, (a) reduction rate 20%, (b) reduction rate 50%, and (c) reduction rate 80%
jwj-39-6-577gf6.jpg

3.3 Recrystallization temperature and time calculation

The Avrami equation was used to calculate the recrystallization temperature and time for refining the grain size of the rolled specimens through recrystallization annealing. The Avrami equation is shown in Equation (1) where the time at 50% transformation is represented as a variable. This equation was used to calculate the recrystallization temperature and time. For the calculation, the data presented in Table 2 were utilized for the Avrami exponent (n) and t0.53).
(1)
X=1exp{B(tt0.5)n}
X: Area fraction of recrystallized grains
t: Annealing time
t0.5: Time at 50% recrystallization
n: Avrami exponent
B: Constant, CP Ti=0.693
Table 2
t0.5 values and avrami constants at varying annealing temperatures5)
Annealing temperature (℃) 500 600 800
n 1.0 1.2 1.0
t0.5(min) 6060 50 0.2
As shown in Table 2, the Avrami exponent n was reported to vary according to the annealing temperature. At n = 1, the annealing time increased significantly. Therefore, the evaluation was proceeded with the lowest annealing time possible considering the circumstances of actual annealing.
Fig. 7 presents the calculation results in a graph when the Avrami exponent n was 1.2 and the recrystallization temperature was 500 - 800°C. At 500°C, the duration of recrystallization until completion was 46,500 minutes, which was excessively long for the actual application. At 600°C, 650°C, and 700°C or higher, this time was reduced to 280 minutes, 80 minutes, and within about 10 minutes, respectively.
Fig. 7
Calculation results of recrystallized fraction and time at varying annealing temperatures
jwj-39-6-577gf7.jpg

3.4 Recrystallization annealing results

In Fig. 8, which presents the microstructure images of the rolled specimens at reduction rates of 20%, 50%, and 80% after annealing at 650°C for 80 minutes, recrystallization was observed in all the specimens. In addition, the deposited metal and the heat-affected zone were refined as the reduction rate increased. In the specimen with a reduction rate of 80%, the microstructures between the base material and weld zone were indistinguishable.
Fig. 8
Optical microstructure of welded specimens after being rolled and annealed at 650°C, (a) reduction rate 20%, (b) reduction rate 50%, and (c) reduction rate 80%
jwj-39-6-577gf8.jpg
The measurement results of the rolled and annealed specimen grain size showed that the grain size became finer as the reduction rate increased, as shown in Fig. 9 and 10. When the reduction rate was 20%, the grain size of the deposited metal was about 7.3 ASTM, but it was 9.7 after the reduction rate increased to 80%, which was a similar grain size to the base material. This was considered as the amount of transformation for recrystallization increasing as the reduction rate increased and the increased accumulated energy contributing to the grain size refining.
Fig. 9
Measurement results of grain size against reduction rate
jwj-39-6-577gf9.jpg
Fig. 10
Optical microstructures of the specimen as heat treated: (a) reduction rate 20% at weld metal, (b) reduction rate 50% at weld metal, (c) reduction rate 80% at weld metal, (d) reduction rate 20% at HAZ, (e) reduction rate 50% at HAZ, and (f) reduction rate 80% at HAZ
jwj-39-6-577gf10.jpg
Meanwhile, the grain sizes at the heat-affected zone and the deposited metal vary, which was considered to be relevant to the laminated bead layer form. As shown in Fig. 8, the rolling force was concentrated towards the downward direction at the thick center of the weld zone bead layer. This was considered to have occurred from no rolling force being directly delivered to the heat-affected zone due to the morphological properties of the weld zone. Therefore, it was determined that the weld bead layer form must be considered to reduce the microstructural variation between the deposited metal and the heat-affected zone.

4. Conclusions

The following results were drawn by rolling and recrystallization annealing the weld zone of the Ti cathode drum for electrolytic copper foil fabrication.
  • 1) A specimen was fabricated by plasma welding the CP Ti material. A specimen that formed 8 mm and 2 mm weld bead layers at the top and bottom, respectively, was fabricated for the rolling.

  • 2) The results of rolling the welded specimens showed that cracks were generated at the side where the weld bead layer height was 8 mm, while cracks were absent at the side with the weld bead layer height of 2 mm when rolling until 80%.

  • 3) The Avrami equation was used to calculate the recrystallization temperature and time and determine the recrystallization condition. Using the results, the annealing experiment was proceeded at 650°C for 80 minutes after which recrystallization was completed. As the reduction rate increased, a decrease in the grain size variation between the deposited metal and base material was observed.

Acknowledgement

This study was funded by the government (Ministry of Trade, Industry, and Energy) in 2020 and supported by the Materials/Parts Technology Development Program of the Korea Evaluation Institute of Industrial Technology (20010658).

References

1. Kang D. S, Lee K. J. Recent R&D status on friction stir welding of Ti and its alloys. J. Weld. Join. 33 (2) (2015), 1–7 https://doi.org/10.5781/JWJ.2015.33.2.1
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2. Moon J. O, Park S. J. An Investigation on the Microstructure Evolution and Tensile Property in the Weld Heat-Affected Zone of Austenitic FeMnAlC Lightweight Steels. J. Weld. Join. 35 (1) (2017), 9–15 https://doi.org/10.5781/JWJ.2017.35.1.9
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3. Tan M. J. Dynamic Recrystallization in Commercially Pure Titanium. J. Achiev. Mater. Manuf. Eng. 18 (2016), 183–186 https://doi.org/10.5604/01.3001.0014.6774
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4. Hayashi M, Yoshimura H, Ishii M, Harada H. Recrystallization behavior of Commercially Pure Titanium during Hot Rolling. Nippon Steel Tech. 62 (1994), 64–68

5. Trump A. M, thesis Ph. D. Recrystallization and Grain Growth Kinetics in Binary Alpha Titanium - Aluminium Alloys. University of Michigan USA. (2017), 109

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