An effect of tool rotation direction on forming limit in friction stir incremental forming was studied. A 3-axes NC milling machine and a hemispherical tool which with a diameter of 6 mm made of high speed steel was used for forming. The thickness of commercial A5052-H34 aluminum sheet was 0.5 mm. The forming tool was moved from the outside to inside in a pitch of 0.5 mm spirally, and the sheets were formed into frustum of pyramid shape. Formability evaluated by minimum wall angle of the pyramid was investigated by changing a tool rotation rate, tool feed rate and tool path direction. When the tool paths were clockwise and counter clockwise, they were defined to “advancing direction” and “retreating direction” as well as in friction stir welding, respectively. From the experimental results, forming limits by both rotation directions of advancing and retreating were almost the same, however, the range of formable working conditions in advancing direction was slightly wider than that in retreating direction. Evaluating the forming limits in relative velocity between the tool surface and the sheet, no difference of forming limit was obtained between forming in advancing direction and retreating directions.
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