br Experiment br Results and discussion br Conclusions br Ac

Experiment
Results and discussion
Conclusions
Acknowledgments The authors thankfully acknowledge the financial support from the Defence Research and Development Organization, Ministry of Defence, Govt. of India, New Delhi in order to carry out the present study. They are grateful to Director, DMRL, Hyderabad, for his constant encouragement. The authors acknowledge the support of XRD, SEM groups of DMRL.
Introduction Friction stir welding (FSW), an innovative solid state welding technique, has found a wide use in defence and aerospace applications [1]. This environment-friendly and energy-efficient technique can be used to join high strength aluminium alloys and other metallic materials that are difficult to be joined using conventional welding processes. In FSW, a rotating tool generates frictional heat, resulting in local plastic deformation [2]. Functions of two main parts of the tool; shoulder and the pin are generation of heat for material softening and material flow control for defect free weld. Generally of microstructure in a given zone of friction stir weld is strongly influenced by the peak temperature and material flow. It is also known that age hardenable AA2219 Al–Cu alloy is sensitive to microstructural changes during welding. Understanding of effect of material flow on the microstructural changes is very limited with respect to aluminium alloys [3–6]. Tool pin profile strongly influences the microstructure changes in weld nugget of friction stir welds and thus plays an important role in corrosion behaviour. The microstructure heterogeneity of friction stir welds is significant in determining the corrosion properties of AA2219 alloy owing to the galvanic coupling between the noble CuAl2 precipitate and the surrounding matrix [7–10]. Most of the previous investigations on the design of tool geometry were focused on optimizing the tool pin profile with respect to microstructure and mechanical properties [11–14]. A detailed review of tool design was reported on the geometrical features of the tool pin (including threads, flutes, flats, whorls, flares and skew on cylindrical or cone-frustum-shaped pins with rounded or flat bottom) and shoulder (including ridges, grooves, knurling and scrolls) [3,15]. However the studies related to the effect of tool profile on weld nugget microstructure and corrosion behaviour of welds are scarce. Keeping in view of the above facts, the present investigation is aimed at studying the microstructure changes in nugget zones and pitting corrosion behaviour of AA2219 alloy FS welds made with five tool profiles which are combination of smooth type conical and flat type triangle, square, pentagon and hexagon.
Material and methods In the present investigation, 240 mm × 160 mm × 7 mm rolled plates of high-strength aluminium–copper alloy AA2219 – T87 were used for friction stir welding experiments. The chemical composition of the parent metal is given in Table 1. The plates were welded in single pass, normal to the rolling direction, with square-butt joint configuration employing a position controlled FSW machine (ETA make). The initial joint configuration is obtained by securing the plates in position using the mechanical clamps. The tool material used in this ampar study is hot worked die steel (AISI-H13). Experiments were conducted with different tool pin profiles, namely conical, triangular, square, pentagon and hexagon cross-sections (Fig. 1). The shoulder diameter was kept constant at 12 mm for all tools, thereby ensuring that the pin profile was only variation from tool-to-tool. Tool geometry details are given in Table 2. For FSW joints produced by different tool pin profiles, was identified through trial experiments. The geometries of the nugget and TMAZ were consistent for each pin profile. Based on the trial experiments, three thermocouples spaced 20 mm apart were embedded in the plates to be welded, so as to place the tip of the thermocouple at the nugget/TMAZ interface. A GRAPHTECH data logger GL900 was used to record the temperatures at the rate of 50 readings per second during welding. The average of the three temperature measurements is considered in this study. However, it is very difficult to measure the temperature directly in the stirred zone due to the severe plastic deformation produced by the rotation and translation of the tool. AA2219 FS-welded specimens were made for optical microscopy and scanning electron microscopy. Keller\'s reagent is used for etching the polished surfaces, and the optical micrographs are recorded. Specimens were sliced in 10 mm × 10 mm size by using hacksaw, and these specimens were subjected to SEM studies. SEM micrographs were taken with secondary electron image mode at 15 kV and 30 kV. Scanning electron microscopy (SEM) was done for transverse section of the welds for five different tool profiles. Studies on strengthening precipitates were carried out using a 120 kV Tecnai-G2 transmission electron microscope. 10 mm × 6 mm × 0.5 mm slices were cut using low speed diamond cutting machine. These slices were mechanically thinned to about 50 microns using emery papers. Subsequent electrolytic thinning was carried out by twin-jet electro-polishing in which an electrolyte consisting of a mixture of 10% perchloric acid and 90% methanol was used. During electrolytic thinning, the electrolyte temperature was maintained at about −40 °C using liquid nitrogen. Electron backscattered diffraction (EBSD) is used to understand the microstructural evolution. Fei™ Quanta-3D FEG (field emission gun) SEM (scanning electron microscope) with TSL-OIM™ system was used for EBSD. EBSD was operated at an accelerating voltage of 20 kV, and imaging was performed at a step size of 0–1.0 μm. Line intercept method was employed for measurement of grain size. EBSD data were analysed to estimate grain size and in-grain misorientation. A grain was defined as an area bound by >5° misorientation. The hardnesses of the weld joints, which were carried with 0.2 kgf load along the mid-thickness of the joint perpendicular to the welding direction, were test using Vickers hardness tester. The indents were spaced at distance of 0.3 mm in accordance with ASTM E 384. Differential scanning calorimetry (DSC) was used to investigate the welds produced by each of the tool pin profiles, and 10 mg of metal was extracted from the stirred region. The extracted metal/specimen was heated at a heating rate of 40 °C/min, from ambient temperature (35 °C) to 550 °C. The fraction of precipitates dissolved during friction stir welding was estimated. Software-based PAR basic electrochemical system was used for potentio-dynamic polarization test to study the pitting corrosion behaviour of weld nuggets of AA2219 FS welds. The test was carried out to determine the critical pitting corrosion potential Epit from the recorded polarization curve.