Evaluation of fatigue crack propagation in dissimilar Al/steel friction stir welds

Abstract

The fatigue crack propagation (FCP) behavior was investigated in the interfacial boundary of the dissimilar joint between 6061-T6 aluminum (Al) alloy and type 304 stainless steel fabricated by a friction stir welding (FSW) technique. The tensile tests were conducted using the FSW Al/steel joints fabricated with different welding conditions and the tensile strength of 194 MPa was obtained in the condition of the rotating speed of 800 rpm and the tool offset of 0.2 mm. Near the welded boundary, the hardness decreased in the Al side due to the resolution of precipitates during the FSW process while the hardness increased in the steel side due to the work hardening. The compact tension (CT) specimen was sampled from the FSW joint so that the initial notch aligned with the interface of the joint and side-grooves were machined to propagate a crack along the interface. Prior to the FCP test, re-T6 heat treatment after the FSW was conducted on the CT specimen to even the hardness in the Al side. In the FCP test using the re-heat-treated CT specimen with a side-groove, the crack propagated straight along the welded interfacial boundary and the FCP rate in the interfacial boundary was measured. The energy release rate was calculated by a finite element method (FEM) and used to evaluate the interfacial crack tip stress intensity instead of the conventional stress intensity factors. The FCP rate for the same energy release rate range was comparable or slightly faster in the interfacial boundary of the Al/steel FSW joint than that of the Al base metal.