Genesis of Microstructures in Friction Stir Welding of Ti-6Al-4V
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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 four different preheat 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 thermomechanical 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 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.
This work was supported by IdEx Bordeaux within the framework of the Cross-border Joint Laboratory “Aquitania Euskadi Network In Green Manufacturing and Ecodesign” (LTC ÆNIGME). The authors gratefully thank Egoitz Aldanondo Begiristain (Ik4 - LORTEK Research Centre, Ordizia, Spain) for carrying out the FSW joining.
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