Fig. 6 — Views of where the breaks occurred in welds made with tools mounted at 0 deg (top photo) and 5 deg (with constant 1125 rev/min, 125 mm/min travel speed, 3.5 kN axial pressure). smaller from base metal to welding zone. Half of the grain size was obtained from base metal to HAZ, and the other half was obtained from HAZ to welding zone. It has been determined that the average grain size in the stir zone is about 6.5 μm, 15 μm in the HAZ, and 30 μm in the BM. The welding burr buildup at the welding bead edges occurred with the larger tool angle — Fig. 5. Although the inner structure was not too much changed, insufficient penetration both inside and surface of the weld bead occurred with increasing tool angle. Insufficient penetration in the weld zone led to decreasing strength and toughness of the welded joints — Fig. 4. The breaks were outside of the stir and HAZ zones applying suitable welding parameters — Fig. 6. Therefore, the strength and hardness of the stir zone with a fine-grained structure are higher than those of the base material. Similar trends were observed for the notch impact energy. The best notch impact energy value was obtained at 0 deg tool tilt angle. Conclusions This study shows the effect of tool tilt angle on friction stir welding of AISI 430 ferritic stainless steels that had greatly reduced ductility in the weld. The following important conclusions can be drawn from the results of this study: • An incremental change in the tool tilt angle leads to significant change in the tensile strength and notch impact energy. The change of the tool tilt angle from 0 to 5 deg leads to a 2.0 and 4.5 times decrease in the tensile joint strength and notch impact energy, respectively. This originated from the diminishment of the frictional surface area between tool and base metal and improper temperature level. • The smoothest welding surface was achieved with a tool tilt angle of 0 deg at constant 1125 rev/min, 125 mm/min travel speed, and 3.5 kN axial pressure. The welding burr buildup at the welding bead edges occurred with the larger tool angle. • The best notch impact energy value was obtained at 0 deg and 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 increased fluctuations in the welding zone interface. • Good quality friction stir welded joints can be achieved with the proper tool tilt angle.♦ Acknowledgments This study was supported by Pamukkale University Scientific Research Projects with carrying out facilities of a project number of 2009FBE022. 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Welding Journal | January 2013
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