Influence of welding parameters on stir zone microstructures during friction stir welding of magnesium alloys

The microstructural features produced in friction stir welded AZ31, AZ80 and AZ91 alloys are examined using a combination of optical and scanning electron microscopy. The influences of tool rotation speed and travel speed are examined in terms of the stir zone phases and grain structures in the three base materials which contained different levels of Mg17Al12 phase. The microstructures primarily consist of alpha-Mg grains with alpha-Mg+Mg17Al12 colonies; however, melted films of Mg17Al12 are produced in AZ91 alloy when high rotation speeds are applied. When the rotation speeds are slow, or the travel speeds are fast, the Mg17Al12 was incorporated into the stir zone; however, they do not exhibit an elongated lamellar morphology associated with melted films. In the case of AZ80, the extruded base material contains a smaller fraction of Mg17Al12 and few of these phases are observed in the stir zones, while the AZ31 microstructure before and after welding remained as primary Mg grains. The results are explained based on the dissolution rate and melting temperatures of Mg17Al12 controlling the overall morphology and fraction of phases in the stir zone.