<p>Bimetallic products, in which constituent materials are metallurgically bonded, can be widely applied in the petrochemical, aerospace, and marine engineering sectors. However, revealing the solid-phase bonding mechanism and accurately predicting the void closure behavior during the bimetallic metallurgical bonding process are essential for obtaining high solid-phase bonding quality. Bimetallic solid-phase bonding mechanisms, including macroscopic compression deformation, microstructure evolution, and void closure behavior, were investigated using bimetallic isothermal compression bonding experiments for X65 pipeline steel/316L stainless steel (PS/SS). The bimetallic isothermal compression bonding process was discussed to illustrate the factors affecting void closure. In addition, a void closure model considering the combined diffusion coefficients was developed based on the Gibbs–Thompson diffusion theory. The model iteratively calculated the contribution of each void closure mechanism using the Gear method to quantitatively reveal the influence of the deformation temperature, equivalent strain, and holding time. The results of theoretical models indicated that the interface bonding ratios predicted by the current model were consistent with the bimetallic isothermal compression bonding experiments. The PS/SS interface bonding ratio and void size were precisely predicted using the present model with an average error of 8.08%. Therefore, the improved model accurately described the void evolution process and elucidated the formation of voids compared to the previous model.</p>