Prediction of mechanical properties in friction stir welds of pure copper A. Heidarzadeh, T. Saeid ⇑ Faculty of Materials Engineering, Sahand University of Technology, Tabriz, Iran article info Article history: Received 10 October 2012 Accepted 27 June 2013 Available online 12 July 2013 Keywords: A. Non-ferrous metals and alloys D. Welding E. Mechanical abstract This research was carried out to predict the mechanical properties of friction stir welded pure copper joints. Response surface methodology based on a central composite rotatable design with three param- eters, five levels, and 20 runs, was used to conduct the experiments and to develop the mathematical regression model by using of Design-Expert software. The three welding parameters considered were rotational speed, welding speed, and axial force. Analysis of variance was applied to validate the pre- dicted models. Microstructural characterization and fractography of joints were examined using optical and scanning electron microscopes. Also, the effects of the welding parameters on mechanical properties of friction stir welded joints were analyzed in detail. The results showed that the developed models were reasonably accurate. The increase in welding parameters resulted in increasing of tensile strength of the joints up to a maximum value. Elongation percent of the joints increased with increase of rotational speed and axial force, but decreased by increasing of welding speed, continuously. In addition, hardness of the joints decreased with increase of rotational speed and axial force, but increased by increasing of welding speed. The joints welded at higher heat input conditions revealed more ductility fracture mode. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Friction stir welding (FSW) was invented at The Welding Insti- tute (TWI) of UK in 1991 as a solid state joining technique, and it was initially applied to aluminum alloys [1]. FSW has proven to be an effective joining technique for a variety of different materials [2], because it is essentially a solid state process without large dis- tortion, solidification cracking, porosity, oxidation, and other de- fects that result from conventional fusion welding [3]. In many previous studies [4–12], the investigators have studied the effect of FSW process parameters on metallurgical and mechanical prop- erties of aluminum alloy joints. Cavaliere et al. [4] showed that the highest ultimate tensile strength (UTS) of AA6056 joints produced by FSW can be reached at rotating rate of 1000 rpm and welding speed of 80 mm/min. Also, they revealed that the microstructure of the materials appears as very fine and equiaxed grains compared to that of base metal (BM). Heinz and Skrotzki [7] examined the metallurgical properties of stir welded 6013 aluminum alloy and showed that FSW results in a dynamically recrystallized grain structure in the weld nugget with smaller grain size than in the BM. Although many reports [4–12] have been made on the welda- bility of FSW for aluminum alloys, investigations into the FSW of copper and copper alloys is quite limited [13–19]. This is attributed to some properties of copper such as high melting point and high thermal conductivity. Xie et al. [13] achieved defect free copper joints under relatively low heat input conditions with a fine grained microstructure of 3.5–9 lm being produced at a rotation rate of 400–800 rpm for a traverse speed of 50 mm/min. Sun and Fujii [15] obtained the process window for FSW of copper and showed that defect free welds can be achieved under the condition of a welding speed ranged from 200 to 800 mm/min, a rotation speed ranged from 400 to 1150 rpm and an applied load ranged from 1000 to 1500 kg. FSW process parameters such as tool rotational speed, welding speed, tool pin profile and axial force influence the mechanical properties of the joints. In order to increase efficiency of FSW pro- cess, the mechanical properties of joints must be optimized. There- fore, it is important to determine the welding parameters at which the mechanical properties reach their optimum. One of the meth- ods to modeling and optimizing the FSW process is Response Sur- face Methodology (RSM). RSM developed by Box and Wilson [20] in 1951, is a collection of statistical and mathematical methods that are useful for modeling and analyzing engineering problems. In this technique, the main objective is to optimize the response surface that is influenced by various process parameters. RSM also quantifies the relationship between the controllable input param- eters and the obtained response surfaces [21,22]. The steps in this method involve [23,24]: (i) Designing a series of experiments for adequate and reliable measurement of the response of interest; (ii) Determining a mathematical model of the second-order re- sponse surface with the best fit; (iii) Finding the optimal set of experimental parameters that produce a maximum or minimum 0261-3069/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.matdes.2013.06.068 ⇑ Corresponding author. Tel.: +98 914 4019901. E-mail address: saeid@sut.ac.ir (T. Saeid). Materials and Design 52 (2013) 1077–1087 Contents lists available at SciVerse ScienceDirect Materials and Design journal homepage: www.elsevier.com/locate/matdes