Microstructure evolution and mechanical properties of a submerged friction-stir-processed AZ91 magnesium alloy

Abstract

AZ91 casting alloy is subjected to friction stir processing (FSP) in air (NFSP) and under water (SFSP). The thermal histories of the two FSP procedures are measured, and their effects on microstructure evolution and mechanical properties of the experimental materials are investigated. Compared with NFSP, the peak temperature during SFSP is about 150°C lower and a much higher cooling rate of 13°C/s is gained. Both NFSP and SFSP produce uniform recrystallized microstructures with dominant high-angle grain boundaries. The average grain size of the NFSP specimen is 7.8 lm, and SFSP results in further grain refinement with a grain size of 1.2 lm. The observed grain sizes of the two FSP specimens match well with the predicted values calculated by the Derby-Ashby model. TEM observation, temperature data, and model criterion indicate that the grain refinement mechanism during NFSP is attributed to continuous dynamic recrystallization (DRX), while continuous DRX and discontinuous DRX affect during SFSP. The mechanical properties of the SFSP alloy are much higher due to finer microstructure.

Ductility Improvement of an AZ61 Magnesium Alloy through Two-Pass Submerged Friction Stir Processing

“…Hofmann et al [ 8 ] prepared an Al-6061-T6 alloy with a grain size less than 200 nm by SFSP and supposed that SFSP could refine grains and improve the mechanical properties. Chai et al [ 20 ] produced an AZ91 alloy with an average grain size of 1.2 µm through SFSP, while the average grain size of NFSP sample was ~7.8 µm. During submerged friction stir processing, the shoulder of the tool makes firm contact with the materials after the pin inserted into the plates, and the process is finished in a few minutes.…”

“…Therefore, the possibility of corrosion caused by water is limited. In addition, Chai et al [ 20 ] reported that the surfaces of the SFSP AZ91 alloy were relatively clean and no perceivable corrosion was mentioned in their paper. According to the anodic polarization curves in dilute electrolyte (0.001 N NaCl solution), the corrosion susceptibility of as-cast AZ61 magnesium alloy is similar to the as-cast AZ91 alloy [ 21 ], so SFSP can also be applied to the as-cast AZ61 magnesium alloy for microstructure refinement.…”

Microstructure and Tensile Properties of Friction Stir Processed Mg–Sn–Zn Alloy

“…Since DRX takes place during FSP and significant grain refinement is achieved, according to classic Hall-Petch equation, the YS of FSP Mg–6Sn–2Zn samples should also be improved obviously. The combined enhancement of ductility and strength by FSP has been achieved in some Mg alloys [ 30 , 31 , 32 ]. However, there are also studies that have reported that FSP could only improve the ductility of Mg alloys.…”

Enhancement of properties of pure lead via underwater friction stir processing for thermoelectric and battery applications

“…It has been demonstrated to be a simple, effective, and economical technique for grain refinement. Chai et al [28] reported that the average grain size of UFSP AZ91 was 1.2 µm, which was much finer than that of normal FSP conducted in the air (7.8 µm). Basil et al [29] also demonstrated that the average grain size of AZ31 attained were 18.9, 15.9, and 13.3 µm for FSP in air, submerged in hot water and submerged in cold water, respectively.…”