## 3 Results and Discussion
### Weld Zone Microstructure  
Figure 3a-c shows the optical microstructures of columnar grains present in the fusion zone for different welding speeds; 3 mm/s, 4 mm/s, and 5 mm/s, respectively. The length/width grain aspect ratio is highest for the welding speed of 5 mm/sec, and lowest for 3 mm/sec. Measurements of the columnar grain size for different welding speeds (3 mm/s, 4 mm/s, and 5 mm/s) show an average columnar grain diameter of 150 \(\upmu\)m, 127 \(\upmu\)m, and 78 \(\upmu\)m, and aspect ratios of between 1-4, 2-4, and 4-11, respectively. Figure 4a-c shows the optical microstructures of equiaxed grains in the fusion zone for welds made at different welding speeds; 3 mm/s, 4 mm/s, and 5 mm/, respectively. The equiaxed zone grain size is highest for the welding speed of 3 mm/sec, and lowest for the welding speed of 5 mm/s. The average grain size values at the weld centre, where equiaxed grains are observed, are 176 \(\pm\) 57 \(\upmu\)m, 115 \(\pm\) 33 \(\upmu\)m, and 90 \(\pm\) 21 \(\upmu\)m for 3 mm/s, 4 mm/s, and 5 mm/s welding speeds, respectively. Figure 4 shows the microstructure of the equiaxed zone for welds made at different welding speeds. Figure 3: Optical microstructures of columnar grains present in the fusion zone for different welding speeds. (**a**) 3 mm/s, (**b**) 4 mm/s, and (**c**) 5 mm/s. Microstructural analysis reveals that the highest welding speed results in the smallest average grain size. In other words, the grain size decreases as the heat input decreases. In summary, the fusion zone microstructure of the welded joints prepared at different speeds is significantly coarser than the wrought base metal microstructure. Figure 5 shows the SEM images of the weld fusion zone using a 4 mm/s welding speed. The microstructure is composed of primary \(\alpha\) grains surrounded by eutectic mixtures of \(\alpha\) and \(\beta\)-Mg\({}_{17}\)Al\({}_{12}\), as shown in Figure 5a. Second phase particles are present within the dendrites in the fusion zone. According to the phase diagram of the Mg-Al, the primary phase is \(\alpha\)-Mg, a solid solution containing Al [35]. At 437 \({}^{\circ}\)C, a eutectic reaction occurs under equilibrium conditions, which involves the decomposition of the liquid into a two-phase structure composed of a \(\alpha\)-Mg and \(\beta\)-Mg\({}_{17}\)Al\({}_{12}\) intermetallic compound. However, the alloy used in the current study (\(\sim\)3 wt.% Al) solidifies into a single-phase \(\alpha\)-Mg under equilibrium conditions. Weld solidification is a non-equilibrium process because of the rapid cooling experienced by the weld pool. During the solidification of the weld metal, nucleation and dendritic growth of the \(\alpha\)-Mg phase occurs, and the solute elements Al and Zn are rejected into the liquid adjacent to the solid/liquid interface. Hence, the interdendritic liquid regions are enriched with Al, and reach eutectic composition [36]. As a result, the final solidification occurs near the eutectic composition. EDS analysis in the SEM confirms that these interdendritic regions are observed to be rich in Mg, Al, and Zn (Figure 5b,c), and these particles are identified as \(\beta\)-Mg\({}_{17}\)Al\({}_{12}\) and \(\tau\)-Mg\({}_{32}\)(Al, Zn)\({}_{49}\)[30]. The XRD pattern of  
Figure 4: Optical microstructures of equiaxed grains in the fusion zone for welds made at different welding speeds. (**a**) 3 mm/s, (**b**) 4 mm/s, and (**c**) 5 mm/s. the weld metal corresponding to 3 mm/s welding speed is shown in Figure 6, where the \(\alpha\)-Mg and \(\beta\)-Mg\({}_{17}\)Al\({}_{12}\) phases are identified. Figure 7a-c shows the HAZ optical microstructure of the AZ31 welded joint prepared with different welding speeds. The average grain size is highest for the low welding speed of 3 mm/sec, and lowest for the high welding speed of 5 mm/s.