Multi-objective optimization and characterization of AA2219-T87 and AA6061-T6 dissimilar friction stir welded joints under the effect of copper backing using AUGMECON

Friction stir welding (FSW) of AA2219-T87 and AA6061-T6 is carried out under six process parameters: rotational speed, tool profile, welding speed, tilt angle, AA2219 location, and backing plate. Response Surface Methodology (RSM) was used to develop the mathematical models for the responses (tensile strength, percentage elongation, and yield strength), and branch and bound algorithm and augmented ε-constraint (AUGMECON) method were used to optimize single and multi-responses. Utilizing a Cu backing plate in FSW led to a superior dissimilar joint, achieving 72% tensile strength, 65% yield strength (YS), and 80% elongation of the base material under 800rpm rotational speed, 100 mm/min welding speed and 3° tilt angle. The position of the parent materials determined the trade-off between the extreme Pareto solutions. Visual inspection and radiography confirmed minimal flashing and superior surface finish with no internal defect in Cu-backed optimized weldments. Tool profile and backing plate type influenced the tunnel and incomplete fusion formation respectively. Microstructural studies showed that Copper backing promotes uniform intermetallic compound dispersion at grain boundaries, with intermetallic layers at dynamic recrystallization (DR) and onion ring pattern (ORP) zones. Temperature profiling of FSW confirmed the uniform heat dissipation characteristics of Copper backing by producing a weld center temperature profile that is closer to its peak temperature but achieved more gradually. The micro-hardness study highlighted the nugget zone's lower hardness compared to parent materials. SEM fractography revealed a mixed ductile and brittle failure mode due to the presence of deep dimples filled with cracked second-phase particles. EDS and XRD analyses confirmed strengthening intermetallics presence favoring strong phases over brittle under Cu backing. Corrosion studies showcased reduced galvanic corrosion with Cu backing.