Grain refining by ultrasonic stirring of the weld pool

Abstract

Grain refining can improve the mechanical properties and solidification-cracking resistance of the weld. Ultrasonic grain refining was conducted by dipping an ultrasonic probe in the weld pool to stir it at a distance behind the arc. This new approach produced effective grain refining in arc welds of Mg alloys AZ31 Mg and AZ91 Mg. Grain refining increased when the probe was positioned farther behind the arc. This suggests the initial crystallites or dendrite fragments generated by ultrasound in a cooler melt farther behind the arc were better able to survive. This also suggests dendrite fragmentation was more likely to occur because the probe was closer to the mushy zone. However, a probe too far behind the arc ended up being inside the mushy zone and grain refining, though highly effective, was restricted to only near the weld centerline. At the same probe position, grain refining increased with increasing ultrasound amplitude. Grains were significantly finer in AZ91 Mg welds than AZ31 Mg welds. This suggests grain refining increased with increasing constitutional supercooling caused by the higher solute content of AZ91 Mg than AZ31 Mg.