The friction stir welding (FSW) was conducted in the pure copper plates with the thickness of 4？mm in the constant traverse speed of 25？mm/min and five different rotation speeds. Analysis of metallographic images showed that the increasing of the rotation speed results in the increase of grain size in the nugget zone. Vickers hardness tests were conducted on the weld samples and the maximum hardness obtained in rotation speed of 900？rpm. Results of the tensile tests and their comparison with that of the base metal showed that the maximum strength and the minimum elongation are achieved again in this rotation speed. Yield strength and ultimate tensile strength increased with the decrease in grain size in the nugget region, and the yield strength obeyed Hall-Petch relationship. Hence, the hardness values do not follow the relationship. 1. Introduction The friction stir welding (FSW) was invented by The Welding Institute, UK, in 1991 for primarily welding of aluminum alloys [2]. In FSW process, a nonconsumable rotating cylindrical shouldered tool with a smaller pin is plunged into adjoining parent materials. Frictional heat generated by the tool shoulder causes the work piece materials to soften [3, 4]. Moreover, FSW involves a severe plastic deformation (SPD) and dynamic recrystallization (DRX) in the nugget zone due to the stirring action of the tool pin [5]. Extensive studies on FSW of aluminum and its alloys have been reported in the literature; however, studies on copper are very limited. This limitation is due to the fact that the high melting point and the high heat conductivity of copper necessitate a higher heat input to obtain a defect-free copper weld. Although copper has a faced-centered cubic structure (FCC) and a good ductility, obtaining sound weld is more difficult than using aluminum and the magnesium alloys [1, 6]. FSW of the copper must be conducted in lower welding speed or in higher rotation speed to increase the heat input during the process. Furthermore, not much has been published concerning the details of the metallurgical and mechanical properties of the welds. Moreover, it has been shown that there were many mismatched results in the mechanical properties in the welds [7]. The effect of the tool rotation speed plays an important role in the amount of the total heat input applied during the process; however, this phenomenon is mostly analyzed qualitatively, and the conclusions have been made based on the resultant weld defects. Therefore, the optimum range of the rotation speed will be an important parameter to achieve high
References
[1]
G. M. Xie, Z. Y. Ma, and L. Geng, “Development of a fine-grained microstructure and the properties of a nugget zone in friction stir welded pure copper,” Scripta Materialia, vol. 57, no. 2, pp. 73–76, 2007.
[2]
R. S. Mishra and Z. Y. Ma, “Friction stir welding and processing,” Materials Science and Engineering R, vol. 50, no. 1-2, pp. 1–78, 2005.
[3]
A. Polar and J. E. Indacochea, “Microstructural assessment of copper friction stir welds,” Journal of Manufacturing Science and Engineering, vol. 131, no. 3, pp. 0310121–0310127, 2009.
[4]
W. Lee and S. Jung, “The joint properties of copper by friction stir welding,” Materials Letters, vol. 58, no. 6, pp. 1041–1046, 2004.
[5]
M. W. Mahoney, C. G. Rhodes, J. G. Flintoff, R. A. Spurling, and W. H. Bingel, “Properties of friction-stir-welded 7075 T651 aluminum,” Metallurgical and Materials Transactions A, vol. 29, no. 7, pp. 1955–1964, 1998.
[6]
C. G. Rhodes, M. W. Mahoney, W. H. Bingel, R. A. Spurling, and C. C. Bampton, “Effects of friction stir welding on microstructure of 7075 aluminum,” Scripta Materialia, vol. 36, no. 1, pp. 69–75, 1997.
[7]
G. Liu, L. E. Murr, C.-S. Niou, J. C. McClure, and F. R. Vega, “Microstructural aspects of the friction-stir welding of 6061-T6 aluminum,” Scripta Materialia, vol. 37, no. 3, pp. 355–361, 1997.
[8]
P. Xue, G. M. Xie, B. L. Xiao, Z. Y. Ma, and L. Geng, “Effect of heat input conditions on microstructure and mechanical properties of friction-stir-welded pure copper,” Metallurgical and Materials Transactions A, vol. 41, no. 8, pp. 2010–2021, 2010.
[9]
H. J. Liu, H. Fujii, M. Maeda, and K. Nogi, “Tensile properties and fracture locations of friction-stir-welded joints of 2017-T351 aluminum alloy,” Journal of Materials Processing Technology, vol. 142, no. 3, pp. 692–696, 2003.
[10]
T. R. McNelley, S. Swaminathan, and J. Q. Su, “Recrystallization mechanisms during friction stir welding/processing of aluminum alloys,” Scripta Materialia, vol. 58, no. 5, pp. 349–354, 2008.
[11]
T. Sakthivel and J. Mukhopadhyay, “Microstructure and mechanical properties of friction stir welded copper,” Journal of Materials Science, vol. 42, no. 19, pp. 8126–8129, 2007.
[12]
R. Nandan, T. DebRoy, and H. K. D. H. Bhadeshia, “Recent advances in friction-stir welding—process, weldment structure and properties,” Progress in Materials Science, vol. 53, no. 6, pp. 980–1023, 2008.
[13]
J. J. Shen, H. J. Liu, and F. Cui, “Effect of welding speed on microstructure and mechanical properties of friction stir welded copper,” Materials and Design, vol. 31, no. 8, pp. 3937–3942, 2010.
