The effect of TiO₂ particles on microstructural evolutions of HSLA steels subjected to friction stir welding

A composite of High Strength Low Alloy X70 steel/TiO₂ was achieved through the Friction Stir Welding process. A homogeneous mixture of primary TiO₂ particles (∼300 μm in size) and X70 steel particles (∼200 μm in size) were inserted into the groove of X70 steel. The TiO₂ particles were fragmented to micron and sub-micron size particles and were dispersed in the nugget zone resulting in different microstructural properties in the nugget zone. The development of microstructure was characterized using optical and scanning electron microscopy as well as the electron backscatter diffraction technique. A very fine acicular ferrite microstructure due to the dynamic recovery process develops in the structure at the presence of sub-micron particles having 370 HV hardness. This increase in hardness is much greater than 195 and 165 HV of FSW layer with no TiO₂ inserted and the as-received substrate. At the presence of micron-sized particles, the polygonal ferrite microstructure develops due to the dynamic recrystallization process having a hardness of 180 HV. The effect of particles on improving the hardness in the nugget zone is about 80 HV accompanying a very fine ferrite microstructure. (paper)