Genesis of Microstructures in Friction Stir Welding of Ti-6Al-4V

Abstract

This paper is focused on the genesis of microstructures in Friction Stir Welding (FSW) of the Ti-6Al-4V alloy. Several titanium joints, initially prepared with 4 different pre-heat treatments, were processed by FSW. Detailed microstructural analyses were performed in order to investigate change in the microstructure during the process. In this work, the FSW processing allows a controlled and stable microstructure to be produced in the stirring zone, regardless of the initial heat treatment or the welding conditions. The welded material undergoes a severe thermo-mechanical treatment which can be divided into two steps. First, the friction in the shoulder and the plastic strain give rise to the necessary conditions to allow a continuous dynamic recrystallization (CDRX) of the β phase. This operation produces a fine and equiaxed β grain structure. Second, once the pin has moved away, the temperature decreases and the material undergoes a heat treatment equivalent to air quenching. The material thus exhibits a β→β+α transformation with germination of a fine intergranular Widmanstätten phase within the ex-fully-recrystallized-β grains.

Effect of Welding Parameters on Friction Stir Welded Ti–6Al–4V Joints: Temperature, Microstructure and Mechanical Properties

“…Hence, this is another reason for the decrease in the hardness value of the SZ. Third, according to some research reports [28,37], the acicular α grains are softer than that of prior finer equiaxed α grains. Hence, at a higher rotation speed, a large number of lamellar structures (α + β) formed in the SZ, which is the additional reason for decrease hardness of SZ.…”

“…However, the cooling rate is relatively higher because the sheet is very thin. Hence, Gnofam et al [37] supposed this cooling process as an air quenching of Ti-6Al-4V alloy from the β-phase field. Thus, there is insufficient time for the growth of α grains in the → β α process during the cooling stage.…”