Surface Microstructure Modifications of Low Carbon Steel Welds Produced by Low-Heat-Input Friction Stir Processing

Friction stir processing (FSP) is a solid state method for surface modifications through severe plastic deformation and frictional heat using a rotational tool. Reducing the heat input of FSP should improve the mechanical properties of the stir zone. This study investigated a low-heat-input FSP on beads of low carbon steel plates using tungsten inert gas welding. The FSP was controlled by decreasing the rotational speed and increasing the travel speed of the tool. Low-heat-input FSP produced significant grain refinement in the weld microstructure and the associated hardness increased at the stir zone surface. The estimated improvement ratio of fatigue strength was twice as large as that produced by the conventional FSP. The fatigue strength in the stir zone at the retreating and advancing sides produced by low-heat-input FSP were independent of the residual stress because the hard stir zone helped prevent fatigue crack initiating at the surface. To improve fatigue strength, increasing the hardness in the stir zone is preferable to decreasing the tensile residual stress associated with the incomplete recovery and recrystallization. Consequently, further grain refinement upon decreasing the FSP heat input can be an effective way to significantly improve the mechanical properties of steel welds.