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

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 P transformed into alpha', alpha '' or retained as beta after quenching were identified and discussed. The results indicated that alpha '' could be obtained primarily by rapid quenching from 880 similar to 840 degrees C solution temperatures and was identified as a base-centered orthorhombic (centered on the C face) with the lattice parameters of a = 0.305 run, b = 0.489 nm, and c = 0.457 nm. The as-welded specimen exhibited fine acicular alpha in the beta matrix with hardness considerably higher than the mill-annealed base metal. For a PWHT temperature lower than 800 degrees 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 degrees C, both the temperature and the cooling rate were important in altering microstructure and hardness of the welds. The welds after a 704 degrees 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. [doi: 10.2320/matertrans.MRA2008370]