Effect of Friction Stir Processing on Mechanical Properties of AA-TIG welded 9%Ni Steel

Active-TIG (A-TIG) or Advanced Active-TIG (AA-TIG) welding is an effective method to improve the welding penetration depth by introducing of active elements such as oxygen into the weld metal. However, increasing oxygen content in the weld metal decreases the toughness of the weld metal. Grain refinement is regarded as one of the most effective ways to improve the toughness of materials. Friction stir processing (FSP) is suitable for reducing grain size in small area such as the weld metal. This study elucidates the effect of FSP on the toughness of 9%Ni steel weld metal containing different oxygen contents. The 9%Ni steel weld metal were fabricated via AA-TIG welding using a mixed gas of Ar and O 2 . The oxygen contents in the weld metal were 180, 250, and 330ppm. FSP treated the weld metal formed via AA-TIG welding. The microstructures of the weld metal and stir zone were observed using the optical microscope (OM), scanning electron microscope (SEM), and electron backscatter diffraction (EBSD). The micro-impact test was conducted to measure the toughness of the weld metal and stir zone. Although the toughness of the weld metal decreased with increasing oxygen content in the weld zone, FSP could improve it regardless of the oxygen content in the stir zone. In particular, the toughness of the stir zone with oxygen content of 330ppm was considerably increased, and it reached almost the same value as the base metal. These improvements depend on the grain refinement in the stir zone. The smallest grain size of former austenite was obtained in the stir zone with an oxygen content of 330ppm because numerous small inclusion particles lead to clear pining effect.