Utilizing a ductile damage criterion for fracture analysis of a dissimilar aluminum/steel joint made by friction stir welding

Striving for lighter and more efficient vehicles, the automotive industry is currently seeking to replace traditional materials like steel with lighter ones such as aluminum alloys. Among the various techniques used to join aluminum to steel, friction stir welding has gained relevance, since it offers important advantages such as lower cost and higher flexibility. However, joint quality is highly dependent on the formation of intermetallic compounds. In the present study, the fracture behavior of an aluminum/steel joint obtained with friction stir welding was investigated experimentally and numerically. After experimental testing, it was found that cracks initiated and propagated through the interface of brittle intermetallic layer and steel. However, ductile fracture characteristics were observed on the fracture surface by a post-failure microscopic scale analysis. Therefore, a ductile damage criterion was utilized to model the fracture behavior of the aluminum/steel joint with an intermetallic layer at the interface. A multi-objective optimization method was utilized to obtain the parameters of the ductile damage criterion, using experimental data and numerical modelling. The obtained parameters were verified by doing experiments on different geometries. Simulation results indicated that the crack initiates from the free edge of the joint layer wherein a high stress triaxiality exists due to non-uniform deformation of aluminum and steel in both sides of the joint. The results of the present study give an understanding of fracture behavior wherein the intermetallic layer is involved.