Interlayer enabled FSW of substantially different AA6061T6 and Ti6Al4V alloys- process optimization, microstructural control, and reliability assessment

Abstract

This study investigates friction stir welding (FSW) of substantially different AA6061-T6 and Ti6Al4V alloys using a nickel (Ni) interlayer to enhance weld quality and mechanical performance. A full factorial (2 × 2 × 5) design of experiments was employed to vary tool rotational speed (TRS), welding speed (WS) and tool pin diameter (TPD), and the resulting joints were evaluated by microstructural analysis, microhardness measurements, tensile testing and fracture examination. Among the twenty welding conditions, the parameter combination TRS 500 rpm, WS 14 mm/min and TPD 4.7 mm with Ni interlayer (Sample S9) produced a defect-free weld with homogeneous Al–Ti–Ni mixing, suppressed Al₃Ti formation and a maximum tensile strength of 241 MPa, corresponding to 78% of the AA6061-T6 base-metal UTS. In contrast, a higher-parameter condition (Sample S20: TRS 710 rpm, WS 20 mm/min, TPD 5.5 mm) led to defective joints, with Al₃Ti intermetallic presence and a significantly lower tensile strength of 133 MPa. Microhardness results showed reduced hardness (≈120 HV) at the interface for S9 due to Ni-assisted solid-solution formation and limited Ti fragmentation, whereas S20 and the joint without interlayer (Sample S0) exhibited higher interfacial hardness associated with brittle intermetallics. Reliability analysis based on a two-parameter Weibull distribution applied to twelve replicate welds at the optimal parameters yielded a mean UTS of 240.48 MPa and a 90% survival probability at 237.9 MPa. These findings demonstrate that a Ni interlayer, combined with appropriate FSW parameter optimization, can produce reliable dissimilar Al–Ti joints suitable for demanding structural applications.