Abstract: The complex grain fragmentation mechanisms of coarse grains in titanium alloys under multi-directional forging (MDF) directly influence the optimization and control of primary hot working processes. This study conducted MDF experiments on β-phase as-cast Ti-6554 alloy and simulated non-uniform deformation during cyclic multi-directional compression through macro- and micro-deformation modeling. The results revealed that friction and surface cooling caused low strain and tensile stress concentration at billet edges, leading to mixed grain structures. In contrast, high strain and triaxial compressive stress at billet centers facilitated uniform grain refinement. After 14 compressions and 4 intermediate reheating processes, coarse grains of the billet were refined from 2-5 mm to 0.25-0.50 mm, achieving uniform grain sizes across different regions. For the first time, the orientation evolution of grains with different morphologies during multi-directional compressions was visualized microscopically. Columnar grains were found to be more easily subdivided than equiaxed grains due to local strain accumulation. Under cumulative compressions, grain orientations gradually rotated from uniform to random, driving continuous dynamic recrystallization (CDRX). Slip system interactions and concentrated misorientation led to the formation and extension of transition and shear bands, inducing grain fragmentation dominated by transgranular subdivided CDRX. Smooth grain boundaries transformed into serrated ones after multiple passes, providing additional nucleation sites for discontinuous dynamic recrystallization (DDRX) and facilitating boundary expand CDRX. The interaction of diverse DRX mechanisms was the fundamental cause of grain refinement. This study clarified the principles of refining and homogenizing millimeter-grade coarse grains under increasing forging strain, offering valuable insights for the development of primary hot processing techniques for as-cast β titanium alloys.

Key words: Titanium alloy, Multi-directional forging, Coarse grain refinement, Crystal plasticity modeling, Grain orientation evolution