Interfacial microstructures and mechanical properties of dissimilar titanium alloy and steel friction stir butt-welds

Dissimilar welding of titanium to steel has received significant attention because of its potential application in power generation and chemical industries. However, it is considered to be very challenging to use fusion-welding processes to join Ti to steel due to their dramatically different thermo-physical properties and chemical compositions. In this work, linear friction stir butt-welding was successfully used to join 2 mm thick sheets of titanium alloy, Ti6Al4 V to medium carbon steel, 30CrMnSiNi2A. Defect-free butt-welded joints were produced using travel speeds from 47.5–75 mm/min with a constant tool rotation speed of 750 RPM. With increasing travel speed, the thickness of the interfacial reaction layer gradually decreases. FeTi intermetallic compound was detected at the interface by micro-XRD testing. An optimal interfacial reaction layer approximately 1 μm thick was formed in the weld made at a travel speed of 75 mm/min. A reaction layer ranging from ∼5 to 60 μm thick was found at the interface with a travel speed of 47.5 mm/min. Such thick reaction layer is in contrast with the thinner layer formed in friction stir lap welds, which is believed to be related to the relatively higher welding temperature of friction stir butt welds. Welds formed at travel speeds ranging from 47.5–75 mm/min exhibit higher tensile strengths than the steel base material and were found to fracture from thermo-mechanically affected zone to heat affected zone on steel side or at the interfacial reaction layer on Ti-side.