The Role of Beam Oscillating Frequency on Grain Morphology and Fracture Performance for Unequal Thickness Ti6al4v Alloy

Abstract

The effect of temperature field distribution on microstructure evolution and grain characteristics of laser oscillating welded joints of Ti6Al4V titanium alloy for 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 compared to the thick plate side, the thin plate side always exhibited a larger temperature gradient, reaching the maximum value at oscillating frequency of 40 Hz. 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.

Keywords: Laser oscillating welding, Unequal thickness joints, Grain refinement, Temperature field simulation, Ti6Al4V alloy