Strategy for double-side friction stir welding of thick Mg4Y3Gd alloy joints

Defect-free welding of thick sections of Mg alloys is a challenging task, often attempted via double-side friction stir welding (DS-FSW) which generally utilizes a pin length equivalent to ~ 50% or ~ 100% of plate-thickness. Nevertheless, tunneling defects often persist in the welded Mg alloys, arising the need to explore intermediate pin-lengths. Thus, in this study, a pin length equivalent to 75% plate-thickness was employed and compared with its 50% and 100% variants, to weld 11.5-mm-thick Mg4Y3Gd alloy plates. The material flow, microstructural, mechanical, and electrochemical aspects were scrutinized with respect to the alteration in pin-length. The evolution of secondary phase particles (SPPs) was also analyzed in different regions of the welded structure. Defect-free welds with ultra-fine grains having a joint efficiency of 97% were obtained using 75% pin length. The segregation of SPPs was observed at the grain boundaries. The microstructure of the fabricated weldments was correlated with the microhardness and tensile analysis. Fractographic examination was performed using SEM, which revealed a fracture type similar to quasi-cleavage. The electrochemical characteristics were assessed through potentiodynamic testing, which demonstrated a substantial enhancement in the anti-corrosion rate of the welded joint due to grain-refinement of the base alloy and dissolution of the yttrium- and gadolinium-rich SPPs.