Abstract
The formation of weld seams in the rectangular section of continuously extruded copper tubes with unequal wall thickness was investigated via optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and universal electronic tensile testing machine. Results show that the weld seams are formed by the confluence of two fine grain belts with a grain size of 13.1~53.4 µm. As the continuous extrusion proceeds, the fine grains gradually grow to the similar grain size (100~200 µm) of the base material. Secondary grain refinement occurs as the fine grains pass through the compression zone of the die. Afterward, micropores in the welded interface disappear. Dynamic recrystallization occurs during continuous extrusion welding. The interface between the two metal is broken through dislocation migration, and new grains form. The bonding strength of the weld seams dramatically increases along the flow direction of the metal. The bonding strength increases from 63 MPa in the initial confluence area to 212 MPa at the outlet of the die, which achieves 98.1% of that of the weld seam-free zone. The number and size of dimples in the fracture of the weld seams gradually and simultaneously increase. The elongation increases from 0.5% to 35%, reaching 70% of that of the weld seam-free zone.
Science Press
The development of the large water-cooled generator industry and the launch of national big science projects, such as the large particle accelerator, have compounded the demand for long high-quality conductive copper tube
Continuous extrusion is highly efficient, energy saving, environmentally friendly, and straightforward. These properties make continuous extrusion a perfect technology for the production of profiled copper tube
In hot extrusion process, the microstructure and mechanical properties of weld seam were investigated. Valberg et a
In this study, the microstructural and property evolution of weld seams in section of 20 mm×29 mm of copper tubes with 3 mm in thickness were investigated. The copper tubes were fa-bricated via the dual rod continuous extrusion method (

Fig.1 Principle of copper tube continuous extrusion using double billets
Two oxygen-free copper rods (99.96wt%) fabricated by up-pulling method with the diameter of 12.5 mm were adopted in the experiment. The rods were pickled and dried prior to the conduction of experiment. Rectangular copper tubes (

Fig.2 Continuous extruded rectangular copper tube
The copper in the welded area was cut into several specimens. The specimens were located at 10%, 40%, 70%, and 100% of weld chamber height along the metal flow direction, and they were denoted by A~D, respectively (

Fig.3 Sampling diagram (a) and specimens at different locations (b)
The specimens were polished and then corroded with FeNO3 alcohol solution (2 g FeNO3+50 mL anhydrous ethanol). The microstructures of the weld seams were examined using an OLYMPUS BX41M metallographic microscope. The samples were stretched at room temperature at a stretching speed of 1 mm/min using an AG-IC100kN precision electronic universal tensile testing machine. The microstructures of the weld seams were analyzed using a JEM-2100F transmission electron microscope (TEM). Fracture morphology was examined using a SUPRA55 field emission scanning electron microscope (SEM).
The metallographic images of the formation process of the weld seams in copper tubes during continuous extrusion are shown in Fig.

Fig.4 Microstructure evolution of weld seam in continuously extruded copper tube at different positions marked in Fig.3a: (a) position A , (b) position B, (c) position C, and (d) position D
Position A is located at the initial confluence of two copper strands. The grains are recrystallized, which can be divided into three distinct areas (
TEM images at different positions in the welding area are shown in

Fig.5 TEM microstructures of weld zone at different positions marked in Fig.3a: (a) position A , (b) position B , (c) position C , and (d) position D

The bonding strength at different positions in the welding chamber and its relative value to the tensile strength (225 MPa) of the metal in the weld seam-free area are shown in Fig.6. The bonding strength increases from 63 MPa at the entrance to 212 MPa at the exit. The bonding strength, especially at position B (40%), substantially increases to 149 MPa, which is 2.36 times higher than that at position A (10%). This result is because of the microstructural evolution of the weld seams (
The elongation of the samples at different positions are obtained via room-temperature stretch tests, as listed in

The microstructural and mechanical property evolution of the weld seams demonstrate that dynamic recrystallization and micropore closing mechanisms play a major role in continuous extrusion welding of copper tubes. A large amount of lattice distortion energy is accumulated because of the intense friction and shear deformation in the wheel grooves. This energy promotes the dynamic recrystallization process. On the one hand, the existence of the compression zone in the die increases metal flow resistance, thereby increasing the pressure in the welding chamber and promoting micropore closur
1) During the double rod continuous extrusion process, when the speed of the extrusion wheel is 5 r/min and the cavity preheating temperature is 500 °C, the bonding strength of the copper tube welds increases from 63 MPa in the initial confluence area to 212 MPa at the outlet of the die, and the elongation increases from 0.5% to 35%. The bonding strength of the welds of the final product reaches 99.1% of that of the weld seam-free zone, and the elongation reaches 74.9% of that of the weld seam-free zone.
2) The weld seams are formed by the confluence of two fine grain belts, whose grain size is 13.1~53.4 µm. As continuous extrusion proceeds, the fine grains in the welding area rapidly grow. Secondary grain refinement occurs after the grains pass through the compression zone of the die, and the micropores in the welded interface disappear.
3) The copper billets experiences strong shear deformation in the grooves, and numerous dislocations sre entangled in the welding chamber. As the deformation temperature and continuous deformation increase, dynamic recrystallization occurs. The interface between the two metal strands breaks through dislocation migration, and new grains form, which allows the welding between the metals to achieve metallurgical bonding.
References
Zhang Tianjue, Lyu Yinlong, Wang Chuan et al. Atomic Energy Science and Technology[J], 2019, 53(10): 2023 (in Chinese) [Baidu Scholar]
Fan Zhixin, Chen Li, Sun Haiyang. Materials China[J], 2013, [Baidu Scholar]
32(5): 276 (in Chinese) [Baidu Scholar]
Guo Lili, Fu Rong, Pei Jiuyang et al. Rare Metal Materials and Engineering[J], 2017, 46(6): 1626 (in Chinese) [Baidu Scholar]
Yun Xinbing, Song Baoyun, Gao Fei. Metal Forming Technology[J], 2002, 20(3): 46 (in Chinese) [Baidu Scholar]
Yan Ming, Wu Pengyun, Xie Shuisheng. Nonferrous Metals[J], 2010(4): 49 (in Chinese) [Baidu Scholar]
Xu Gaolie, Mao Yizhong, Yao Youfu et al. Nonferrous Metals Procession[J], 2011, 40(6): 33 (in Chinese) [Baidu Scholar]
Guo Zhenhua, Pei Jiuyang, Fan Zhixin et al. Chinese Journal of Rare Metals[J], 2019, 43(1): 38 (in Chinese) [Baidu Scholar]
Valberg H. International Journal of Materials and Product Technology[J], 2002, 17 (7): 497 [Baidu Scholar]
Donti L, Tomesani L, Minak G. J Mater Process Technol[J], 2007, 191: 127 [Baidu Scholar]
Yu J Q, Zhao G Q, Zhang C S et al. Materials Science & Engineering A[J], 2017, 682: 679 [Baidu Scholar]
Li Shikang, Li Luoxing, He Hong et al. The Chinese Journal of Nonferrous Metals Society[J], 2019, 29(9): 1803 [Baidu Scholar]
Xue Jiangping, Huang Dongnan, Zuo Zhuangzhuang et al. The Chinese Journal of Nonferrous Metals Society[J], 2018, 28(7): 1291 (in Chinese) [Baidu Scholar]
Tang Ding, Zhang Qingqing, Fang Wenli et al. Journal of Mechanical Engineering[J], 2014, 50(22): 34 [Baidu Scholar]
Feng Di, Zhang Xinming, Sun Feng et al. Materials Reports[J], 2013, 27(19): 6 (in Chinese) [Baidu Scholar]
Plata M, Piwnik J. Proceedings of Seventh International Aluminum Extrusion Technology Seminar[C]. Chicago: ET, 2000: 205 [Baidu Scholar]
Donati L, Tomesani L. Journal of Materials Processing Technology[J], 2004, 153-154: 366 [Baidu Scholar]
Loukus A, Subhash G, Imaninejad M. Journal of Materials Science[J], 2004, 39: 6561 [Baidu Scholar]
Yu Junquan, Zhao Guoqun, Zhang Cunsheng et al. Materials Science & Engineering A[J], 2017, 682: 679 [Baidu Scholar]
Fan X H, Tang D, Fang W L et al. Materials Characterization[J], 2016, 118: 468 [Baidu Scholar]
Fan Caihe, Yan Hongge, Peng Yingbiao et al. The Chinese Journal of Nonferrous Metals Society[J], 2017, 27(1): 64 (in Chinese) [Baidu Scholar]
Den Bakker A J, Katgerman L, Van Der Zwaag S. J Mater Process Technol[J], 2016, 229: 9 [Baidu Scholar]
Yu J Q, Zhao G Q, Cui W et al. J Mater Process Technol[J], 2017, 247: 214 [Baidu Scholar]
Deng Yunhua, Guan Qiao, Tao Jun et al. Rare Metal Materials and Engineering[J], 2017, 46(6): 1620 (in Chinese) [Baidu Scholar]