Experimental investigation of crack propagation mechanism in refill friction stir spot joints of AA6082-T6

Since many aluminum alloys preferred in structural engineering can be welded conventionally only with great effort and energy input interest in alternative joining techniques is growing, such as solid state joining processes. In this work, the effect of refill friction stir spot welding (refill FSSW) on the crack propagation behavior in AA6082-T6 is studied. To be able to identify the individual fracture mechanism, refill FSSW was performed as a blind weld, i.e. only in one sheet designed as C(T)100 specimens. The vertical distance between notch and spot weld was varied and tested in two phases. First, a cyclic pre-crack was induced and then the specimen was caused to fail in quasi-static conditions, resulting in two different fracture modes. The results showed that the cyclic crack is dominated by residual stresses but the microstructure mainly influences the quasi-static crack propagation. It was also found that a stress concentration occurs in the transition area even without a hook. Furthermore, it was found that the crack propagation is not exclusively driven by the local strength but also by the angle at which the crack hits the spot weld.