Fig. 2 — A — Tool mounted at an angle of 0 deg; B — tool angled at 5 deg. Fig. 3 — A — Tool pressure force vs. time; B — temperature changes vs. time for different tool angles. Fig. 4 — A — Tensile strength vs. tool angle; B — impact energy vs. tool angle (constant 1125 rev/min, 125 mm/min, 3.5 kN). The tool pressure forces were recorded using a load cell mounted under the workbench with an indicator. All data, load, temperature, and pressure force values were recorded using a computer during the experiments. The load cell indicator helped to control the tool pressure force during the welding process. Four thermocouples were placed 5 cm apart from each other at the bottom of the plate. Friction stir welded specimens 12.5 × 150 × 3 mm thick were machined for the tensile tests, and specimens 10 × 55 × 3 mm thick were prepared for the Charpy V-notch impact tests. All welded test specimens were prepared perpendicular to the weld interface in order to examine their mechanical properties. The specimens were subjected to quasi-static and mostly monotonic tensile loading. All tensile tests were performed with a Zwick/Roell Z100 servo-hydraulic tensile test machine with a load capacity of 100 kN. The stress-control mode was chosen over stroke or strain modes due to the convenience and smoothness of the operation. The impact tests were performed with a Wolpert PW30 notch impact testing machine with a capacity of 300 J. Microstructural examination was performed in order to check for weld defects such as porosity, coarse dendrites, poor penetration of the weld bead, and grain structure of the HAZ. The etchant used for microanalyses was a mix of 50 mL hydrochloric acid and 50 mL distilled water. Vilella’s reagent (a mixture of 1 g picric acid, 5 mL hydrochloric acid, and 95 mL ethyl alcohol) was the etchant used for microscopic microanalyses. Results and Discussions The effects of the tool tilt angle were examined. Tool pressure strength and temperature, related with the time at different tool angles, are shown in Fig. 3. The tool pressure strength was given about 3.5 kN at the beginning of welding and controlled constantly during welding — Fig. 3A. The temperature was measured from four points with probes during welding, but only the highest temperature graph is shown in Fig. 3B. The temperature changes indicate there was a significant decrease in the temperature with an increment in the tool angle. The tool tilt increment leads to a diminishment in the frictional surface area between tool shoulder and base metal. The temperature was measured approximately 500°C at tool angles of 0 and 1 deg, and 350°C at a tool angle of 5 deg — Fig. 3B. The test results of tensile strength and notch impact energy of FS welded joints obtained from different tool angles are given in Fig. 4. The higher tensile strength values were obtained between 0 and 1 deg tool tilt angles while the other welding parameters were kept constant. The increase in the tool tilt angle caused a temperature decrease and an increase in fluctuations in the welding zone interface. Therefore, the increment in the tool tilt angle leads to a significant drop in the tensile strength as shown in Fig. 4A. The upper surface after welding, macroscopic and microscopic appearances of the friction stir welded joints at different tool angles, are shown in Fig. 5. The smoothest welding surface was achieved at the tool tilt angle of 0 deg — Fig. 5. The FSW joint consists of the stir zone (SZ), base material (BM), and HAZ between the SZ and BM, even though it is not easy to clearly distinguish each zone. The SZ shows a basin-like shape that widens considerably toward the upper surface (Ref. 12). The border region between the SZ and BM is HAZ, where a transition to the coarse-grained BM microstructure occurs — Fig. 5. The grain size of the stir zone is finer than the base material. The fine-grained microstructure in the SZ is due to the dynamic recrystallization induced by severe shear deformation and the significant amount of heat generated during FSW. It has been observed that grain size becomes 44 JANUARY 2013 2A 3A 4A 2B 3B 4B
Welding Journal | January 2013
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