Dependence of Zener parameter on the nanograins formed during friction stir processing of interstitial free steels

Highlights: → The Z parameter was studied for the structures evolved during the FSP of IF steels. → Four models were developed to establish the relationship between the Z parameter and the size of nanograins as well as UFGs (d) formed during the FSP of studied IF steel. → The finest grains, i.e. nanograins of about 50-120 nm, were formed at the rotating speed of 1600 rpm (strain rate of about 205 s-1 and temperature of about 900 deg. C). - Abstract: In the present work, the Zener-Hollomon parameter was used to study the effects of strain rate and temperature on the nanograins and ultra-fine grains formed during the friction stir processing of an interstitial free (IF) steel. Four rotating speeds (800, 1250, 1600 and 2000 rpm) and two traverse speeds (31.5 and 63 mm/min) were used. The changes in the rotating speed from 800 to 2000 rpm resulted in the strain rates of about 70-303 s-1, whereas the temperature changed from 868 ± 2 to 908 ± 2 deg. C. For both traverse speeds, the finest grains (50-120 nm) were formed at the rotating speed of 1600 rpm. Increasing the rotating speed above and below this rate resulted in an increase in grain size. Four models were developed to establish a relationship between the Z parameter and the size of nanograins and ultra fine grains (d) formed during the friction stir processing of studied interstitial free steel. For any given condition, the microstructural evolutions were characterized using field emission scanning electron microscopy, atomic force microscopy and scanning electron microscopy.