A Study on Material Flow and Defect Formation of Dissimilar Friction Stir Welding of Admiralty Brass Tube to Copper-Nickel 90/10 Tube Sheet

Friction Stir Welding (FSW) was employed to join Admiralty Brass (CuZn28Sn1) tubes to Copper-Nickel 90/10 (CuNi10) tube sheets. Alloy positioning was determined by the strength transition of the base metals between 350–500°C, with Cu–Ni 90/10 placed on the advancing side due to its superior high-temperature strength, thereby guaranteeing joint formation. The elimination of tunnel defects was governed by balanced material flow. Longitudinal control required equilibrium between the shoulder-affected zone (SAZ) and the pin-affected zone (PAZ), influenced by shoulder area and traverse speed to promote cavity filling. Transverse control depended on minimizing the flow stress mismatch between the two alloys, which reduced interfacial resistance and enabled smoother bidirectional flow; this behavior was influenced by temperature and frictional conditions. Weld strength was most sensitive to pin length, while traverse speed and pin geometry played comparatively minor roles. Tunnel defects reduced joint strength depending on their size and location. With optimizing the FSW parameters, sound and defect-free weld between Admiralty Brass Tube and Copper-Nickel 90/10 Tube Sheet, exhibiting near-base-metal strength, was obtained.