Heat-input-driven microstructural transformation and mechanical performance of friction stir welded AA5052/Copper dissimilar joints

This study systematically investigated the influence of heat input and tool-pin geometry on the microstructural evolution and mechanical performance of friction stir welded AA5052/C12200 dissimilar lap joints. Welds were produced using three pin geometries (straight cylindrical, tapered cylindrical, and square-frustum) under rotational speeds of 750–1150 rpm and traverse speeds of 50–100 mm/min. A heat input factor (HIF = rotational speed/traverse speed) was introduced to correlate processing conditions with joint integrity. Defect-free joints were consistently obtained only within an intermediate HIF range of 15.0–15.3, whereas lower or higher HIF values resulted in tunnel defects or excessive thermal deterioration. Enhanced heat input promoted copper fragmentation, interfacial diffusion, and Al₂Cu intermetallic formation, increasing microhardness to approximately 201 HV in aluminum and 171 HV in copper. Yield strength and ultimate tensile strength increased from 95.9 to 122.4 MPa and from 201.2 to 224.5 MPa, respectively, while elongation decreased from 20.7% to 15.3%. Peel strength reached a maximum of 165.2 N under an intermediate heat input, whereas excessive heat input reduced interfacial toughness because of the formation of a thick, brittle intermetallic layer.