Effect of tool rotational speed on mechanical properties and microstructure of friction stir welding joints within Ti–6Al–4V alloy sheets

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Abstract
Friction stir welding of Ti–6Al–4V alloy sheets was successfully performed with a tungsten carbide (WC) tool, and the effect of the tool rotational speed on the microstructure and mechanical properties of the joints was evaluated. The microstructure was investigated using field emission scanning electron microscopy, electron backscattered diffraction and X-ray diffraction measurements. Tensile, Charpy impact and Vickers hardness tests were performed to evaluate the joints’ mechanical properties. The process peak temperature of each welding condition was measured at the bottom of a specimen. Regardless of the welding conditions, the alpha (α) and alpha prime (α′) phases were detected in the stir zone, whereas the base metal was composed of alpha (α) and beta (β) phases. The tensile strength and hardness of the stir zone increased as the tool rotational speed increased. The impact absorption energy was found to be equal to that of the base metal. Moreover, a phase transformation from the β to the α′ phases occurred in the stir zone during friction stir welding and the evolution of the microstructure in the stir zone significantly affected the mechanical properties of the joints.

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