Effect of post-weld friction stir processing on low-heat-input cold metal transfer (CMT) weld mechanical, microstructural and corrosion behaviors

This study investigates the influence of post-weld friction stir processing (FSP) on 6-mm-thick AA6061 butt joints fabricated by low-heat-input cold metal transfer (CMT) welding using ER2219 filler. Weld integrity was evaluated using X-ray radiography, while microstructural features were examined by optical microscopy. Hardness was measured using Vickers micro-hardness testing, tensile properties were determined in accordance with ASTM E8, impact toughness was assessed by the Izod test, and corrosion behavior was characterized through potentiodynamic polarization in 3.5 wt.% NaCl solution. Friction stir processing produced a fine, equiaxed stir zone characterized by onion-ring flow patterns and clean, defect-free interfaces. The as-CMT joints exhibited an ultimate tensile strength of 216 MPa, a yield strength of 198 MPa, and an elongation of 12.6%. After applying friction stir processing (FSP), these values increased to 252 MPa, 215 MPa, and 23.8%, respectively, indicating substantial improvements in both strength and ductility. The Izod impact energy also increased markedly from 9.5 J in the as-CMT condition to 15.4 J after FSP, representing a 62% enhancement. The as-CMT sample displayed a lower corrosion current density than the CMT + FSP sample in 3.5 wt.% NaCl solution. Nevertheless, FSP resulted in notable improvements in strength, ductility, and hardness uniformity, whereas the as-CMT condition retained comparatively improved corrosion resistance, primarily due to its surface condition.