Abstract: The microstructure evolution and crack propagation behavior were systematically and thoroughly investigated in α/β Ti-6Al-4V alloy obtained under the inhomogeneous heating effect of a coupled dual laser beam via a combination of postmortem electron backscattering diffraction analyses and numerical simulations. The effect of microstructural attributes (grain size, grain boundary, dislocation) on the fracture property and crack propagation behavior was investigated based on thermodynamic and crystallographic analyses, as well as the examinations of dislocation density and the Burgers vector. By quantifying the degree of variant selection using the degree of variant selection (DVS) equation, it was found that the higher the energy input, the weaker the variant selection, as the secondary α phase suppresses further dislocation-induced variant selection via autocatalytic nucleation and growth. The proportion of high angle grain boundaries (HAGBs) concentrated around 60° gradually increases with improving cooling rate, mainly due to the change in the tendency of martensite transformation to be induced. The grain refinement was the primary mechanism for the enhancement of Ti-6Al-4V alloy T-joints, while the suppression of crack propagation by HAGBs also played a crucial role in improving strength and ductility. The crack propagation direction frequently changes when the order is approximately parallel to the short axis of α′ martensite grains, forming a zigzag propagation path. The above findings should shed light on optimizing dual laser beam bilateral synchronous welding (DLBSW) technology to tailor the microstructure and improve the mechanical properties of Ti-6Al-4V alloy T-joints.

Key words: Ti-6Al-4V alloy, Microstructure, Grain refinement, Crack propagation, Dual laser beam bilateral synchronous welding