Effect of beam oscillating frequency on microstructure and mechanical properties of Ti6Al4V alloy joints of unequal thickness

The effect of temperature field distribution on microstructure evolution and grain characteristics of laser oscillating welded butt joints of Ti6Al4V titanium alloy for 2 mm/3 mm unequal thickness under different oscillating frequencies was conducted, combining electron backscatter diffraction analysis and finite element numerical simulation. The tensile fracture mechanism of the unequal-thickness joints was also elaborated. The findings demonstrated that as the oscillating frequency increases, the temperature gradient in the melt pool significantly decreases, which is attributed to the increase in the number of oscillations of the beam per unit time with the rise in oscillation frequency, which improves the uniformity of the temperature distribution within the molten pool. Additionally, due to different heat dissipation conditions in the thickness transition region, the thin plate side manifested a relatively sluggish quenching rate in contrast to its thick plate counterpart, thereby facilitating the extensive proliferation of coarse-grained structures. Remarkably, the grain size on the thin plate side increased with the increase of oscillating frequency, promoting the non-uniformity of grain size. Although the grain refinement effect was not the most significant at 20 Hz, the optimal joint with a tensile strength of 1058.2 MPa was achieved due to its comparable grain size uniformity and martensitic arrangement. The research results provide theoretical guidance for optimizing the performance of laser oscillating welded joints unequal-thickness and enable significant engineering application value.