Establishing a Mathematical Model to Predict the Tensile Strength of Friction Stir Welded Pure Copper Joints
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NTRODUCTION
THE fact that copper possesses the essential characteristics of high electrical and thermal conductivities, favorable combinations of strength and ductility, and excellent resistance to corrosion makes it an excellent candidate to be applied in many areas as structural materials. Therefore, there is an increasing demand for the welding of this material. However, the influence of oxygen, impurity, and high thermal conductivity makes copper difficult to be joined by conventional fusion welding.[1–3] Friction stir welding (FSW) was invented at The Welding Institute of the UK in 1991 and was initially applied to aluminum alloys.[4] The process can be regarded as a promising welding method for joining of copper and copper alloys.[5–9] It is essentially a solid state process without large distortion, solidification cracking, porosity, oxidation, and other defects that result from conventional fusion welding.[10] Therefore, welds made by FSW have shown to improve mechanical properties such as the ultimate tensile strength (UTS), ductility, and hardness in comparison to the corresponding fusion welds.[11] Recently, some investigators have studied the FSW of pure copper and copper alloys.[5–9] Xie and Ma[5] achieved defect-free copper welds at rotational speeds A. HEIDARZADEH, T. SAEID, H. KHODAVERDIZADEH, and A. MAHMOUDI, Ph.D. Students, and E. NAZARI, Researcher, are with the Advanced Materials Research Center, Faculty of Materials Engineering, Sahand University of Technology, Tabriz, Iran Contact e-mail: [email protected] Manuscript submitted January 26, 2012. Article published online October 17, 2012. METALLURGICAL AND MATERIALS TRANSACTIONS B
in the range of 400–800 rpm for a welding speed of 50 mm/min. Hwang et al.[7] explored the thermal history of a pure copper workpiece during FSW and showed that the appropriate temperatures for successful FSW process were between 733 K and 803 K (460 °C and 530 °C). Some authors investigated the effect of welding parameters on the microstructure and mechanical properties of friction stir welded (FS welded) copper. For example, Sakthivel and Mukhopadhyay[6] found that the microstructure at the weld zone (WZ) was much finer than the parent metal due to dynamic recrystallization. Sun and Fujii[8] obtained the process window for FSW of copper, which included a welding speed range from 200 to 800 mm/min, a rotation speed range from 400 to 1200 rpm, and an applied load range from 10 to 15 kN. They reported that the mechanical properties of the joints can be improved by increasing the applied load and decreasing the rotation speed. In order to increase efficiency of the FSW process, the mechanical properties of joints must be optimized. Therefore, it is important to determine the welding parameters at which the mechanical properties reach their optimum. Response surface methodology (RSM) developed by Box and Wilson[12] in 1951 is a collection of mathematical and statistical techniques. RSM can be used for optimizing the FSW process parameters accurately, while saving experimental time,
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