A review of the state of research in the field of friction stir welding and processing has been carried out. The features of plastic flow in friction stir welding and their connection with the processes of adhesion friction are shown. The main direction of research is related to the features of friction stir welding of titanium alloys. Special attention is paid to the selection of working tool materials from various alloys for friction stir welding and the processing of titanium alloys. The main advantages and disadvantages of applying different types of tools for friction stir welding of titanium alloys are shown. Different mechanisms of tool wear in friction stir welding associated with the interaction of processed material and tools are demonstrated. Information on the influence of tool and material interaction at welding on the mechanical properties and operational characteristics of obtained joints is given.

Lap-welded joints between Ti–6Al–4V and Al–4Cu–1Mg were obtained using water-cooling-bath friction stir welding at different FSW tool rotation rates. The increase in the tool rotation rate from 350 to 375 RPM leads to better plasticization of the titanium alloy, elimination of stir zone defects, better interlocking and bonding with the aluminum alloy as well as the formation of intermetallic Al–Ti compounds (IMC), preferentially of the Al₃Ti type. Shear-loading testing showed the best result was at the level of 3000 N with 2 mm of displacement. Increasing the FSW tool rotation rate to 400 and 450 RPM resulted in the formation of more IMCs, which had a detrimental effect on both maximum load and displacement achieved in the shear tests.

It was experimentally shown that optimal friction stir processing (FSP) parameters existed for Al-Cu-Mg alloy samples subjected to friction stir processing in the sense of obtained ultrafine metal strength and microstructure. FSP of this alloy resulted in the formation of a recrystallized grain zone. The deflection from optimal process parameters resulted in lower FSPed metal strength.

The structure formation in the bulk aluminum alloy workpieces by the friction stir processing was studied. As shown, the successive multiple pass processing has only a slight effect on the previously obtained grain structure. The single pass and two-pass seam structure were identical while the third pass resulted in formation of a coarse grain subzone structurally analogous to the thermo-mechanically affected zone.

It was shown experimentally that both friction stir processing parameters and the workpiece thickness have their effect on structure and mechanical properties of the processed samples. Even the 10% deflection from the optimal values resulted in the essential mechanical strength reduction. It was observed that FSP showed a tendency to suppress the serrated yielding during the tensile testing.

Friction stir welding (FSW) of titanium alloy was carried out using liquid cooling of the FSW tool made of heat-resistant nickel superalloy. Cooling of the nickel superalloy tool was performed by means of circulating water inside the tool. The FSW joints were characterized by microstructures and mechanical strength. The mechanical strength of the joints was higher than that of the base metal.