Microstructural evolution and hardness of as-cast ZM6 magnesium alloy welded by ultrasonic-frequency pulsed TIG
摘要
To address issues such as heat input sensitivity and microstructural and property deterioration during the welding of as-cast ZM6 rare-earth magnesium alloy, this study employed an ultrasonic frequency-pulsed tungsten inert gas (UFP-TIG) welding process. The effects of the pulse frequency (0–30 kHz) on the arc characteristics, weld bead formation, microstructural evolution, and hardness distribution were systematically investigated and compared with those of conventional alternating current TIG (AC-TIG) welding. The results show that under an average current of 130 A, the 30-kHz UFP-TIG weld achieved a penetration depth (2.65 mm) comparable to that of the 170 A AC-TIG weld (2.67 mm), while the heat input was reduced from 369.2 to 286.7 J/mm (a 22% reduction), demonstrating a significant advantage in reducing the heat input. The microstructural analysis indicated that the weld zone primarily consisted of fine equiaxed grains. High-frequency pulses can effectively inhibit grain coarsening and weaken {0001} basal texture intensity. In contrast, the high-heat-input AC-TIG weld exhibited significantly coarsened grains and an enhanced texture. In the heat-affected zone, the welding thermal cycle induces the formation of a large number of finely dispersed spherical and rod-like Mg3(Nd, Zn) nanoprecipitates, which maintain a semi-coherent relationship with the α-Mg matrix. The synergistic effects of solid-solution strengthening, precipitation strengthening, and grain-boundary strengthening significantly enhanced the hardness of the HAZ. Comprehensive analysis demonstrates that UFP-TIG welding, through the arc focusing effect and microstructure regulation mechanism, achieves simultaneous improvement in penetration and mechanical properties while reducing heat input, providing a new technological pathway for high-quality welding and repair of ZM6 magnesium alloy.