Eliminating bottom defects in medium-thickness titanium alloy joints by improving material flow via a new dynamic rotation supporting friction stir welding method

It is generally difficult to eliminate bottom defects during friction stir welding (FSW) of medium-thick/thick Ti alloy plates, since there still lacks of deep understanding of the material flow mechanism influencing the formation of bottom defects. In this study, different variant FSW methods were conducted to 7 mm Ti-6Al-4V alloy plates, and the oxide tracking method was utilized to study the material flow mechanism. During conventional dynamic supporting FSW, the main reason for bottom defects was related to cavities formed in the bottom at the initial stage and not filling in time during welding. A new dynamic rotation supporting FSW (DRSFSW) method with a movable bottom rotating shoulder instead of the traditional backing plate was first time proposed to eliminate bottom defects in Ti alloy joints. The essential cause of eliminating bottom defects was associated with the fact that the bottom rotating shoulder added an extra reverse material flow pattern to avoid initial defects, following by fuller material flow and faster material filling rates during the welding. The high-temperature phase reconstruction method showed that colonies with high orientation consistency was largely promoted to form by more sufficient deformation, fully dynamic recrystallisation and fewer nucleation sites after adding the extra reverse material flow pattern during DRSFSW. The joint strength reached 971 MPa, with a joint strength coefficient of 96.8 %. This study uncovers the material flow pattern for FSW with different bottom states and provides a new method to enlarge the welding window and improve the quality of FSW Ti alloy joints.