<p>Bifilm is a critical defect affecting the comprehensive performance and the consistency of mechanical properties of castings. This paper introduces a method for creating and preserving bifilm defects in situ using confluence welding. By converging two melt streams and extending the holding time of the stationary melt, bifilm can be successfully prepared. Using A356 aluminum alloy as the research material, this study systematically investigates the effects of adding Bi to the stationary or moving side melt and stationary side melt holding on bifilms. The results indicate that without Bi addition, the oxide film is continuous and dense. After adding Bi, Bi segregates at the oxide film, filling the gap within the bifilm. It could also significantly disrupt the continuity of the oxide film, enabling the melt to infiltrate into the gap, effectively bonding the two oxide layers of the bifilm together. Additionally, overlapping occurs between the disrupted oxide films. These factors collectively promote the deactivation of bifilms. Extending the holding time on the stationary side melt enhances the continuity of the oxide film. Adding Bi significantly improves the ultimate tensile strength (UTS) of confluence welding samples, particularly when Bi is added to the stationary side melt. However, extended holding time shows an adverse effect on UTS.</p>