A comparative study on the microstructure development of friction stir welded 304 austenitic, 430 ferritic, and 2205 duplex stainless steels

Abstract

Microstructure development of 304 austenitic, 430 ferritic, and 2205 duplex stainless steels (SSs) was studied and compared in friction stir welding (FSW) process. FSW was conducted at welding speed of 50 mm/min and rotational speed of 400 rpm. Microstructural observations were carried out using optical microscopy and scanning electron microscopy equipped with electron back scattered diffraction (EBSD) system. Based on the microstructural observations three distinct regions of base metal (BM), thermo-mechanically affected zone (TMAZ), and stir zone (SZ) were detected in the weld region of welded materials. EBSD results showed that high frequency of low angle grain boundaries (LAGBs) was formed during the dynamic recovery (DRV) in thermo-mechanically affected zones (TMAZs) of all samples. Very fine microstructures with great amount of high angle grain boundaries (HAGBs)developed in the SZs, as well. EBSD investigations on the resultant TMAZs showed that grains experienced the progressive subgrain rotations and the misorientation rapidly increased and exceeded 10° over a short distant from grain interior towards the grain boundaries. Rather significant amount of LAGBs with misorientation angles of 10°-15°developed through the SZ microstructure of the 304 SS. Progressive lattice rotations, the presence of loose HAGBs in all welded samples, and the formation of high fraction of LAGBs with misorientation angles of 10°–15° in comparison with the whole LAGBsin 304 SS, suggested that the CDRX played a major role on the nucleation of dynamic recrystallization. However, some grains developed through classic recrystallization mechanism (statically and/or dynamically)in 304 SS and austenite phase of2205 SS. Common simple shear texture components of face centered cubic (FCC) materials includingA*1 ({1¯1¯1}<112>), A*2 ({111¯}<112>) were formed in the SZ of 304 SS. Likewise, common texture components of body centered cubic (BCC) materials including D1 ({12¯1}<111>), D2 ({1¯1¯2}<111>), J1 ({110}<11¯2>), and J2 ({1¯1¯0}<1¯12¯>) developed in the SZ of 430 SS. The coexistence of austenite and ferrite phases in 2205 SSinfluenced the resultant texture of the SZseverely, and developed component(s) which were not existed in the SZ of single phased alloys. The texture components of C ({001}<110>), B1 ({11¯2}<110>), and B2 ({1¯12¯}<1¯1¯0>) in the austenite, and texture components of F ({110}<001>) and E ({01¯1}<111>) in the ferrite phase were also recognized to be formed in the SZ of 2205 SS.