Minimal strength loss and corresponding mechanisms in the ultrasonic joining of magnesium alloys

Abstract

The softening of the heat-affected zone (HAZ) is a major factor contributing to the failure of welded joints in magnesium (Mg) alloys. Therefore, reducing heat input and minimizing the extent of the HAZ are critical for achieving high-quality Mg alloy joints. This study introduces a novel ultrasonic-assisted solid-state welding process to join 0.7 mm thick Mg alloy plates while minimizing strength degradation. The process focuses on precise temperature control and the evolution of intermetallic compounds (IMCs) within the HAZ, thermo-mechanically affected zone (TMAZ), and nugget zone, effectively suppressing phase transformations and significantly narrowing the HAZ. The results reveal an almost imperceptible HAZ in the welded joints. Additionally, grain refinement occurred within the nugget zone and up to 100 μm from the nugget boundary, with grain sizes measuring approximately 10 μm. Simultaneously, IMCs in these regions, composed of both rare-earth and conventional elements, were fragmented into micron/submicron-sized particles and uniformly dispersed throughout the joint, facilitated by ultrasonic vibrations. As a result, the welded joint exhibited superior mechanical performance, achieving a tensile-shear strength of 97.8 % of the base metal. These findings provide valuable insights into the strength enhancement achieved in ultrasonically welded joints, presenting a promising approach for mitigating strength loss in Mg alloy welding.