Formability behavior of friction stir processed dual phase steel

This study investigated the formability behavior of dual-phase (DP) steel subjected to friction stir process (FSP), using a miniaturized Erichsen test. The friction stir process yielded a refined microstructure, which consisted of lath martensite and fine ferrite, as a result of dynamic recrystallization (DRX) and phase transformation. Both yield and tensile strength were significantly increased via the grain refinement and increasing martensite fraction. Biaxial formability behavior of the friction stir processed sample exhibited a decline as the Erichsen index (EI) decreased from 2.7 mm to 2.2 mm. However, the required punch load (FEI) increased compatibly with the strength value after FSP. The grain refinement caused by FSP led to an improvement in the orange peel effect, defined as a negative effect of increased surface roughness after metal forming. The value of mean roughness in the free dome surface of the FSPed sample decreased from 4.90 μm to 2.62 μm. It can be concluded that the roughening with the orange peel effect on free surfaces of stretched metal can be eliminated by the friction stir process.