Evolution of microstructures and properties of magnesium alloy weldments produced with CO2 laser process

This study investigates the microstructure, hardness, and tensile strength of AZ31 and AZ61 magnesium alloy weldments fabricated in a CO2 laser welding process. Results show that the AZ31 weldment contains cellular grains within the fusion zone (FZ). By contrast, the FZ of the AZ61 weldment contains a mixture of cellular and columnar grains and the PMZ contains bulk grains. It can be inferred that differences in the aluminum content of the two alloys (i.e., AZ31: 2.66 wt%; AZ61: 5.25 wt%) results in different supercooling rates and solid grain structures. The aluminum content also influences the formation of precipitates and defects in the weld bead during solidification, due to altered cooling rates resulting from thermal effects triggered by the growth of the precipitation phase. In particular, the AZ61 weldment exhibits high microporosity in the PMZ region with micropores or voids at the bottom of the weld bead. It is speculated that these defects are the result of liquation cracking. CO2 laser welding has been shown to increase the hardness of both magnesium alloys, with the maximum hardness occurring in the FZ of both weldments due to grain refining. The AZ61 weldment has the higher hardness of the two, due to increased grain refinement resulting from higher Al content. For both weldments, the ultimate tensile strength (UTS) decreases following CO2 laser welding. However, no significant difference was noted between the UTS of the two weldments, suggesting that tensile strength is insensitive to the Al content of the magnesium alloys. By contrast, the uniform elongation increases with an increase in Al content in these alloys, indicating that the uniform elongation of the two weldments is sensitive to the Al content. Finally, the two weldments exhibit different fracture behavior resulting from differences in the size of the supersaturated precipitates formed in the two specimens and the presence of micropores and voids in the AZ61 specimen.

► The microstructure and defects exhibit a different can be controlled by Al content.
► The AZ61 weldment contains defects, which are thought liquation crack formation.
► The UTS value is no significant difference with increasing Al content.
► By contrast, the uniform elongation and YS value decreases.
► Two weldments exhibit a different fracture characteristic.