The unexpected stress-strain response of medium Mn steel after friction stir welding

Abstract

In the present study, we suggest the friction stir welding (FSW) is a novel pathway for the welding technique of medium Mn steels without degradation of mechanical properties. The application of conventional fusion welding deteriorates the mechanical properties of annealed medium Mn steels because it causes the martensitic transformation at the welded zone by high temperature during welding (> ~1000 °C). However, we discover that the FSW can preserve the original microstructure (ferrite plus retained austenite) at the welded zone of annealed medium Mn steel owing to low temperature during welding (< ~780 °C). Besides, we also find that the FSW produces the additional austenite reversion, assists the more active transformation-induced plasticity and twinning-induced plasticity, helps the grain refinement, and raises the dislocation density. These unexpected findings result in the improved strength and ductility of welded steel relative to the as-annealed steel.