Microstructure and strengthening mechanism of TIG welded joints of a Mg-Nd-Gd alloy: Effects of heat input and pulse current

As a newly-developed Mg-rare earth (Mg-RE) alloy, Mg-3Nd-3Gd-0.2Zn-0.5Zr (EV33) alloy has been reported to show desirable mechanical properties. Further research on the welding behavior of EV33 alloy can facilitate its application in practical production. In this work, the effects of heat input and pulse current on the microstructural evolution and mechanical properties of tungsten inert gas (TIG) welded EV33 alloy were systematically investigated. The results show that the increment of welding current or pulse current frequency led to increased grain size in the fusion zone (FZ), on account of the combined effect of reduced cooling rate and enhanced constitutional supercooling. Notably, the TIG welding with pulse current (PC-TIG) efficiently broke the dendrites and promoted the dissolution of Zr particle (Zrp), producing a better grain refinement effect and more homogeneous microstructure in the FZ as compared to the ordinary TIG (O-TIG) welding. A physical model has been proposed for depicting the microstructure evolution of the joints fabricated with different welding parameters. The highest joint efficiency 88.4% with a good combination of strength-ductility (UTS = 220 ± 14 MPa, YS = 129 ± 9 MPa, EL = 11.9 ± 1.2%) of the joint was obtained by optimizing the welding process, and the related strengthening mechanisms were quantified. It is believed that this work can provide guidance for controlling the microstructure of the Mg-RE alloy joints.