Effect of Friction Stir Welding on Microstructure and Properties of Micro-TiO_2 doped HSLA Steel

In this study Friction Stir Welding (FSW) was applied to incorporate TiO2 particles of different volume fractions into HSLA X70 steel to form a composite structure in the nugget zone. The process was performed with and without powders at rotational speeds of 400 to 1000 rpm and traverse speed of 30 mm/min. Microhardness was measured to evaluate the mechanical properties
of the stir zone. The microstructure of the different zones (weld nugget - thermo-mechanically affected zone and head affected zone) was studied by optical and scanning electron microscopy. Different ferrite phases with variable grain sizes were generated in the nugget zone when performing FSW without TiO2 powder. In case of using the powders, the resulting microstructure
was a combination of fine ferrite grains and acicular ferrite which has better mechanical properties. The enhanced properties are attributed to a uniform dispersion of the TiO2 particles in a matrix of ultra-fine grains of ferrite and acicular ferrite.