Grain refining by ultrasonic stirring of the weld pool

Grain refining can improve the mechanical properties and solidification-cracking resistance of the weld. Ultrasonic grain refining with a probe dipped in the weld pool at a distance behind the arc produced effective grain refining in Mg alloys AZ31 (∼Mg-3Al-1Zn) and AZ91 (∼Mg-9Al-1Zn). Grain refining was found to increase with increasing offset, oscillation amplitude, and alloying content. However, with an excessive offset the probe can be located inside the mushy zone and grain refining, though very effective, can be limited to near the weld centerline. Dendrite fragmentation was the most likely grain refining mechanism.258