Effect of FSW parameters on temperature profile and grain size of AA6061-T6 and S275JR mild steel FSW joint

Friction stir welding (FSW) is a solid-state welding technique used to join materials such as aluminum alloy, magnesium alloy, steel and titanium. The process involves relatively low process temperatures compared to other conventional welding techniques. Welded material undergoes severe plastic deformation driven by the rotating FSW tool, which causes dynamic recrystallization of grains. In this study, the temperature during the welding of AA6061-T6 aluminum alloy and S275JR mild steel was recorded using K-type thermocouple. Grains in the stir zone (SZ) of joints were observed in order to measure the grain size. Samples were etched with Keller's reagent to reveal its microstructure. It was seen that the highest temperature was recorded when welding with a high tool plunge depth. Decreasing tool travel speed was seen to increase temperature readings. The lowest temperature was recorded when using a high tool tilt angle due to a reduced contact area between tool shoulder and weld piece. Largest observed grain size was measured in the SZ of the joint welded with a high tool plunge depth, due to the high process temperature experienced by the joint during welding.