Effect of Friction Stir Process on Hole Expansion Behavior of Dual Phase Steel

The influence of the friction stir process (FSP) on the hole expansion formability of dual-phase 600 steel was examined by experimental and finite element analyses. Due to the microstructure of refining ferrite, martensite, and bainite, the yield strength and tensile strength increased after FSP by 95% and 63%, respectively. FSP led to the formation of (101) || ND and (111) || RD texture components in the processed zone. The observation of (111) || RD texture components in the FSPed steel resulted in an increment in the plastic strain ratio from 0.86 to 0.98. FSP provided a 20% improvement in the hole expansion ratio. The microstructural refinement and crystallographic texture played an important role in this increase. The FSP contributed to the increase in the hole expansion formability capability by delaying the crack formation during hole expansion and by causing the hole to fail at a higher load. It has been observed by experimental and finite element (FE) simulation results that the highest circumferential stress occurs at the hole edge where the main crack is formed. The FE simulation predictions agree with less than 2% error with the experimental observations.