Experimental and numerical study on hydroforming characteristics of friction stir welded aluminum alloy tubes

Friction stir welding (FSW), as a solid state joining technique, has emerged as an efficient method for manufacturing tailor-welded blanks to optimize weight or performance in the final component. As the basic design of lightweight frame structures in the automotive and aircraft industry is frequently based on tubular profiles, the joining strategies and forming technologies have to be developed accordingly. In the present study, FSW tube was produced by a novel processing sequence. The plastic deformation characteristics during hydroforming were experimentally and numerically investigated with two types of end condition. The hydroforming performance of the FSW tubes was mainly investigated by die-bulge forming with fixed ends, and the wrinkling behavior during hydroforming was analyzed by employing axial feed on the tube ends. It is found that hydroforming FSW tube is a new deal and does show a few peculiarities. The FSW tube exhibits a spiral weld and the basin-shaped nugget. Finegrained structure is retained during tube forming. The thinning of the tube in axial direction shows M-shaped distribution during hydroforming. The severe thinning is observed at one quarter of the expansion zone from symmetry plane. In the hoop direction, the base material near the weld suffers severe thinning due to the high hoop and axial tensile stress. The thickness distribution greatly depends on the sequence of the contacting die and the variations of the curvature radius of the tube during hydroforming. Moreover, the weld shows an inhibitory effect for the generation of the wrinkles and decreases the number of the wrinkles as compared to the seamless tube during hydroforming. This effect is more obvious when the forming pressure is lower.