Effect of Tool Rotation Speed on Microstructure and Properties of Friction Stir Processed 2507 Duplex Stainless Steel

Duplex stainless steel with its exceptional corrosion resistance, mechanical properties, and proficient weldability has been widely used in ships and bridges, as well as petrochemical and seawater desalination industries. Friction stir processing (FSP) does not only induce dynamic recrystallization of the material but also achieves the purpose of repairing the crack automatically, which markedly improves the mechanical properties of duplex stainless steel. Thus, FSP is particularly useful for crack repair of duplex stainless steel structures. In the present study, microstructure, mechanical property, and corrosion property of FSP 2507 duplex stainless steel were investigated. FSP was performed at a constant welding speed of 100 mm/min and tool rotation speeds of 200, 300, 400, 500, and 600 r/min using a tungsten-rhenium-based tool. Due to the thermal and mechanical effects in the processing, the section of the processing zone can be divided into the thermo-mechanically affected zone (TMAZ) and the stir zone (SZ). Only under the sufficient parameters of thermoplastic flow, the internal faultless processing zone was obtained. In accordance with the increased tool rotating speed, the grain size of the SZ initially decreased and then increased. Processing heat cycle and stress deformation had an insignificant influence on the proportion of ferrite and austenite phases in the processing zone, and the ferrite content still remained between 40% and 60% in the standard specification. The sigma phase was determined at the bottom of the processing zone, namely at the tool rotation speed of 200 r/min due to the low heat input. Micro-hardness distribution of the processing zone demonstrated a basin-like morphology, and the largest hardness value appeared at the bottom of the advanced side of the SZ, corresponding to the smallest grain size of the SZ. As the tool rotating speed increased, the longitudinal tensile strength of the SZ increased initially and then decreased, contrary to the elongation. According to the results of potentiometric polarization and electrochemical impedance spectroscopy, the refinement of grain enhanced the stability, compactness, and repassivation performance of surface passivation film. The corrosion resistance of the upper surface in the SZ exceeded that of the base material, rendering it more useful. When the tool rotation speed was 400 r/min, the SZ had the optimal corrosion properties.