Flexible control of the microstructure and mechanical properties of friction stir welded Ti–6Al–4V joints

The effect of the thermal cycle during friction stir welding (FSW) on the microstructure and mechanical properties of a Ti–6Al–4V alloy joint was investigated by welding 2-mm-thick alloy plates together under various conditions. When the joints were welded above the β-transus temperature, the prior β-grain size in the stir zone depended on the peak temperature, and the size of the lamellar structure depended on the cooling rate. The strength of the joints welded above the β-transus temperature increased with the welding speed because the lamellar structure was smaller. When the welding was performed below the β-transus temperature, the stir zone consisted of the fully equiaxed primary α phase. The tensile strength of the stir zone obtained below the β-transus temperature exceeded that of the base metal.