Design of friction stir welding for butt joining of aluminum to steel of dissimilar thickness: heat treatment and fracture behavior

A 5-mm-thick aluminum sheet (Al) was successfully welded by friction stir welding (FSW) to a 3-mm-thick carbon steel (St) in a butt configuration. For this purpose, a new joint design was used where a support plate was employed under the St part and an insert layer made of steel was used between the support plate and the Al part. A stationary shoulder was also considered in the joint design. After welding, an intermetallic compound (IMC) layer of FeAl with less than 3 μm thickness was obtained along the bond line. The joint was then tested in tensile loading conditions. An abrupt load drop was observed during the test, which was due to crack propagation from pre-existed cracks in the IMC layer. A cohesion zone model (CZM) was used to simulate the fracture behavior of the joints and obtain the fracture energy of the IMC layer. Based on the CZM results, the fracture energy of the as-welded and heat-treated joints was obtained as 0.2 and 0.13 N/mm, respectively. A comprehensive analysis of the joints showed that the irregularities and fragments of steel in the IMC layer delay the propagation of cracks. Consequently, a higher fracture energy is obtained with respect to the heat-treated joint, in which a continuous IMC layer exists.