fore varied between 300 and 1000 rev/min. The other parameters, such as feed rate or welding speed, dwell time, and tilt angle, were kept constant and were selected based on previous studies and preliminary tests (Refs. 12, 13). Feed rate in polymers was usually kept low so that the material in front of the pin could get sufficient time to plasticize. Parameters used in this study are shown in Table 1. A tilt angle of 3 deg, reported as an B optimum FSW angle for HDPE by Bozkurt (Ref. 22), was also used at the optimum rotation speed. The optimum rotation speed, used in the present work, was obtained from performed tests. Furthermore, a K-type thermocouple was also placed at 1 mm below the pin tip to estimate the weld zone (WZ) temperature at each rotation speed — Fig. 2. In all experiments, a clockwise rotating tool was plunged into the workpiece with a 10-mm/min plunge rate to the depth equal to the pin length. All the welding operations were performed at room temperature. Cross sections of weld specimens, perpendicular to welding direction, were made and were observed visually. The mechanical strength of the joints was analyzed by tensile testing on specimens that were obtained perpendicular to the welding direction. The tensile tests were performed in accordance to ASTM Standard D638-10 using a Zwick-Roell UTM machine at the crosshead speed of 1 mm/min. A schematic of the D638-10 specimen is shown in Fig. 3. Weld zone and fractured surfaces during tensile tests were analyzed by scanning electron microscope (SEM). In order to determine the crystalline content in the WZ, a differential scanning calorimeter (DSC) test was carried out using a Perkin-Elmer differential scanning calorimeter at a heating rate of 10C/min. Furthermore, material flow during the welding process was studied by marker material insert technique at achieved optimum parameters. Subsequently, due to the marker material’s difference in color from the base material, its movement was visually analyzed by different sectioning of the specimens. Results and Discussion Morphological and Micrographic Analysis In order to observe any visual defects, friction stir welded specimens were cross-sectioned perpendicular to the welding direction. Figure 4A –E shows the different morphologies of top surfaces of the welded specimens. Although the flash formation can be observed in all specimens, Fig. 4A, B specimens show comparatively less flash formation. It is also noted that the flash in Fig. 4B is on retreating side (RS). For this reason, it is believed that the temperature on the RS is always higher than the advancing side (AS), which leads to the formation of flash on the RS (Ref. 23). Further increase in rotation speed to 500 rev/min resulted in WELDING RESEARCH 212-s WELDING JOURNAL / JUNE 2016, VOL. 95 Fig. 4 — Cross sections of specimens welded at the following: A — 300 rev/min with 0deg angle; B — 400 rev/min with 0deg angle; C — 500 rev/min with 0deg angle; D — 1000 rev/min with 0deg angle; E — 300 rev/min with 3deg angle. A C E D Table 1 — Friction Stir Welding Parameters Tilt Angle (Deg) Feed Rate (mm/min) Dwell Time (s) Rotation Rate (rev/min) 0 25 15 1000 0 25 15 500, 400, 300 3 25 15 300
Welding Journal | June 2016
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