The effects of various hot deformation states on the evolution of microstructures and mechanical properties in diffusion bonded TC4 alloys were investigated using the hot bending of thick plates. Finite element simulations were conducted to characterize the deformation states during bending at 750 °C with angles of 17° and 32°. The microstructures and mechanical properties of the bonding interface were then analyzed. The joint subjected to uniaxial stress exhibited the highest ultimate tensile strength, which was attributed to the significant accumulation of dislocation density and the low-angle grain boundaries within the grains. The texture strengthening in the basal {0001} plane was also observed, along with a relatively low Schmid factor corresponding to the primary slip systems aligned with the deformation direction. In contrast, the joint under stress-free conditions showed a slip direction that was less favorable for deformation, resulting in an ultimate tensile strength higher than that of the joint under biaxial stress conditions.