Effect of the welding speed on the macrostructure, microstructure and mechanical properties of AA6061-T6 friction stir butt welds

Friction stir welding of aluminum alloys has been progressively used in different industries on the ground of higher welding quality in comparison to fusion welding. In this article, friction stir welding of 6061-T6 aluminum alloy with 9.6 mm thickness was carried out by using three different welding speeds (63, 89 and 110 mm/min). The effect of welding speed on macro- and microstructure, micro hardness, tensile properties and kissing bond was investigated. Results show that the Low Hardness Zone was moved toward the weld center by increasing the welding speed. The average micro hardness in the weld nugget zone increased from 60.1 to 67.6 HV with the raise of welding speed from 63 to 110 mm/min. Thermo Mechanical Affected Zone was clearly revealed using Electron backscatter diffraction (EBSD). The kissing bond was studied by Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray (EDX) mapping. It was proven that the damaging effect of kissing bond was linked to location of this discontinuity, although the growth of kissing bond was linked to higher welding speed. The maximum value of Ultimate Tensile Strength (UTS) (159 MPa) was obtained at 110 mm/min, whereas the Yield Strength (YS) in the sample at 89 and 110 mm/min welding speed exhibit the same trend with 137 MPa and 134 MPa respectively.

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