Mechanism of microstructure evolution and grain growth in friction stir welding of AA6061-T6 and AISI304 in air and water media

• Published in: Materials chemistry and physics 2021-11, Vol.273, p.125081, Article 125081
• Main Authors: Mahto, Raju Prasad, Rout, Matruprasad, Pal, Surjya Kanta
• Format: Article
• Language: English
• Subjects: Aluminum base alloys; Cooling rate; Dissimilar materials; EBSD; Evolution; Friction stir welding; Grain growth; Grain size; Microstructure; Morphology; Nucleation; Recrystallization; Tensile strength; Water quenching
• ISSN: 0254-0584; 1879-3312
• DOI: 10.1016/j.matchemphys.2021.125081

Grain size has a significant effect on the tensile strength and ductility of the friction stir welded (FSWed) joints. The size and morphology of grains, developed during friction stir welding (FSW) process, depends on amount of frictional heat generated and rate of deformation involved. In addition, grain size also depends on rate of cooling of the weld region. In the present work, evolution of microstructure in FSW of AA6061-T6 and AISI304 has been investigated by conducting FSW experiments in two different media i.e. air and water. In order to get a detailed insight of the microstructure evolution, stop-action technique was used where the experiments were interrupted by stopping the machine, followed by water quenching of the samples. Detailed study about the microstructure evolution, in different weld regions, has been performed by using Electron Back Scattered Diffraction (EBSD) and Transmission Electron Microscopy (TEM) techniques. The microstructures were characterized by analyzing the recrystallized grains, and grain growth. Particle stimulated nucleation and strain induced boundary migration were observed to be the recrystallization mechanisms. FSW in the water medium yields finer microstructure in the weld regions than air medium. Grain growth was observed after coalescences of sub-grains. Results have shown higher grain growth for FSW in air medium. Results have also shown transformations of Goss {110} and B{110} textured grains of base aluminum into Copper {112} , E{111} and F{111} dominated textured grains at the stir zone (SZ). Water medium produced relatively highly shear textured grains than air medium at SZ. •Underwater cooling prevails the grain growth in FSW.•PSN and SIBM are the grain growth mechanisms in FSW.•The hooking effect in FSW produces different crystallographic orientations than HAZ, TMAZ, and SZ.•FSW produces shear-type textured grains in the SZ and HooAZ.