Effect of mismatch in mechanical properties on interfacial strength of aluminum alloy/steel dissimilar joints

Abstract

The strength of a dissimilar joint between aluminum and steel is generally dependent on the thickness of the intermetallic compound (IMC) layer. Mechanical factors such as the residual stress and stress field under loading also likely contribute to the fracture behavior of the joint. However, a clear understanding of the effects of the interfacial structure (i.e., the IMC layer thickness) and mechanical factors on joint strength is lacking in the existing literature. The purpose of this study was to elucidate the influence of both the IMC layer thickness and the difference in the strengths of dissimilar metals on the interfacial strength of aluminum/steel joints through experimental evaluations and finite element (FE) analyses. The relationship between the strength of a friction-stir-welded joint of 6061 aluminum alloy (A6061) and 780-MPa grade high-tensile-strength steel (HT780) and the IMC layer thickness was investigated by controlling the IMC growth promoted by the annealing process. The results showed that the joint strength decreased with increasing IMC layer thickness. Fracture mainly occurred at the joint interface for a thin IMC layer of approximately 100 nm, whereas the joint between A6061 and 304 stainless steel, with a lower proof stress than HT780, showed a higher joint strength that resulted in fracture in the A6061 alloy. The FE analysis revealed that a high maximum principal stress is applied near the interface due to the difference in the degree of deformation between steel and aluminum, a behavior that is likely to appear in any joint consisting of materials with considerably different strengths.