Microstructure and Notch Properties of Heat-Treated Ti-4.5Al-3V-2Mo-2Fe Laser Welds

Abstract

The microstructure and transformation behavior of Ti-4.5Al-3V-2Mo-2Fe alloy and its laser welds after various heat treatments were investigated. Notch properties, such as impact toughness and notched tensile strength, were also measured on the welds to choose an appropriate post-weld heat treatment (PWHT) of the alloy. The temperature regimes in which β transformed into α′, α″ or retained as β after quenching were identified and discussed. The results indicated that α″ could be obtained primarily by rapid quenching from 880∼840°C solution temperatures and was identified as a base-centered orthorhombic (centered on the C face) with the lattice parameters of a=0.305 nm, b=0.489 nm, and c=0.457 nm. The as-welded specimen exhibited fine acicular α in the β matrix with hardness considerably higher than the mill-annealed base metal. For a PWHT temperature lower than 800°C, the change in microstructure and hardness of the welds depended mainly on the temperature, not on the cooling rate. If the PWHT temperature was higher than 800°C, both the temperature and the cooling rate were important in altering microstructure and hardness of the welds. The welds after a 704°C/4 h treatment could prevent notch brittleness, reduce hardness variation in different regions of the weld, and obtain notch properties similar to the mill-annealed base metal.