In the present study, friction stir welding of AA7039 butt plates were investigated with respect to threaded probed tools having different shoulder diameters and shoulder surface geometries. The ultimate tensile properties, micro hardness and grain size of the welds were investigated with respect to the varying tool geometries. The mechanical properties of the welded joints were observed to be influenced by the shoulder diameter and shoulder surface geometry. With the increase in the shoulder diameter, the effect of shoulder concavity on refining the weld grain size diminished. Good quality welds were produced by using tools with semi-concave shoulder surface having shoulder and probe diameters of 19 mm and 7 mm respectively. The fracture surfaces of AA7039 weld samples were dimpled and the crack front energydispersive X-ray spectroscopy analysis exhibited absence of segregation of Zn.
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