Characterization of Material Flow-Induced Severe Plastic Deformation in Double-Acting Friction Stir Welding Using Neutron Imaging

Friction Stir Welding (FSW) is a solid-state joining method widely used for aluminum alloys due to its ability to reduce defects such as porosity and hot cracking. However, its performance decreases when applied to thick plates, where adequate material flow and heat input are harder to achieve. To address this, a Double-Acting Friction Stir Welding (DA-FSW) process has been developed, using two tools operating simultaneously from opposite sides to enhance joint quality. This dual-tool setup creates complex interactions that influence material flow and strain behavior—critical to weld integrity and residual stress formation. This study employs neutron imaging at the IMAT beamline at ISIS to investigate these factors in DA-FSW joints. Gadolinium (Gd) powder serves as a tracer to enhance contrast in neutron radiography, enabling detailed visualization of material displacement. Bragg edge imaging is also used to map residual strain in the normal direction based on the Al-(111) plane. Experiments are performed on 8 mm-thick AA1100 aluminum plates with varying tool offsets and rotation directions. Imaging data are analyzed using ImageJ and Mantid Imaging. By correlating material flow with residual strain, the study provides insights to support process optimization and defect-free welding in thick aluminum components.