Visualization of material flow in one-step double-acting FSW of AA1100: role of tracer type and morphology

Aluminum is widely used in industry due to its lightweight, high strength, and cost-effectiveness. However, conventional fusion welding of aluminum often results in porosity defects. One-step Double-Acting Friction Stir Welding (ODFSW) is an advancement of the FSW technique that enables simultaneous double-sided welding in a single pass at sub-melting temperatures, thereby overcoming porosity issues in fusion welding while also addressing challenges in single-sided FSW of thicker plates, such as incomplete penetration and root flaws. The quality of ODFSW joints is strongly influenced by the material flow behavior during welding. To investigate this flow, the Tracer Insert Technique was employed. This study examines the effect of tracer material type and form on the visibility of material flow in ODFSW of AA1100 aluminum with a 1 mm pin overlap. Three types of tracers were used: SiO₂ powder, AA6061 powder, and ER5356 wire. Results revealed that powder-form tracers, particularly AA6061, provided better visualization due to more uniform distribution and higher color contrast caused by the presence of Mg₂Si precipitates. Multi-Attribute Decision Making (MADM) evaluation identified AA6061 as the most effective tracer, exhibiting minimal defects. Material flow visualization indicated distinct patterns, including flow from the advancing side (AS) to the retreating side (RS), material accumulation at the weld exit, onion ring formation, microvoids at pin tips, and homogeneous mixing in the mid-plate region. Additionally, a funnel-shaped flow profile and material concentration at the mid-thickness zone were observed, attributed to the mechanical interaction between the upper and lower tools.