Formation of joining mechanisms in friction stir welded dissimilar Al-Ti lap joints

Friction Stir Welding (FSW) is a suitable technology for joining dissimilar materials. As the process temperature during FSW typically does not exceed the solidus temperature, like in fusion welding, high quality joints can be produced with a minimum of intermetallic phases. A comprehensive description of the effective joining mechanisms of friction stir welded dissimilar material joints is still subject of research. In this study the results of an analysis of the effect of the pin length, which is supposed to have a significant influence on the characteristics of the joining mechanisms, are presented. Especially the influence on the bonding conditions and the mechanical properties of the joints has been investigated. For this purpose combinations of aluminum and titanium have been welded with varying pin length at different rotational speeds. The experiments show that at a sufficient distance between the interface zone and the pin tip the bonding is realized by a substance-to-substance bond and microscopic form-fit. As this distance decreases, a visible macroscopic form-fit is generated. However, this macroscopic form-fit causes no significant elevation of the joint strength. First scanning transmission electron microscopy (STEM) images reveal an interfacial layer, which indicates a diffusion of the two materials.