Investigation of microstructure transformation and residual stress in a friction stir processed dual phase 980 MPa advance high strength steel

Friction stir processing and joining represent innovative solid-state processes designed to address the limitations of conventional fusion welding processes, particular when dealing with high temperature and high strength materials. In this method, a specially designed tool, through rotation, generates frictional heat, resulting in the softening of the material beneath it. The softened material undergoes extensive plastic deformation, flowing around the tool, and finally consolidates to produce a solid-state joint. The process induces complex microstructure transformation, such as significant texture changes and dynamic recrystallization, due to the compressive and shear loading at elevated temperatures during the process. In our current study, friction stir processing was applied to advance high strength steel, specifically dual phase 980, using different process conditions. The friction stir processed steel samples were characterized by optical and electron microscopies, microhardness measurement, and residual stress by neutron beam measurement. This comprehensive investigation aimed to understand quantitative stress state and its correlation with microstructure changes under various welding conditions.