## 3 Results and Discussion
### Weld Zone Macrostructure  
The macrostructures of the top surface and cross-section of the fusion zone for welds prepared using different welding speeds are shown in Figure 2. The full-penetration weld joints without any cracks were obtained in this study using different welding speeds, as shown in Figure 2a-f. The average weld bead widths of 11, 8, and 6 mm are obtained for the different welding speeds of 3, 4, and 5 mm/s, respectively, as shown in Figure 2a,c,e. When the welding speed is 3 mm/s, the weld bead width is relatively wide compared to welding speeds of 4 and 5 mm/s. The weld bead width decreases with increasing welding speed, strongly affected by the heat input. In other words, the weld bead width decreases with reducing heat input. Furthermore, as shown in Figure 2e,f, some pores are present in the fusion zone. Surface contamination and hydrogen released from the solid phase during the solidification could be responsible for pore formation [31]. Figure 1: The optical microstructure of AZ31 base metal. The macrostructure of all fusion zones consists of two parts: a columnar zone at the fusion boundary and equiaxed grains in the centre of the weld fusion zone, as shown in Figure 2a-f. The columnar grains are aligned nearly perpendicular to the fusion boundary, where the temperature gradients (G) are steepest. Weld metal solidification in fusion welds frequently occurs in a columnar mode, at the fusion boundary, due to the presence of sharp thermal gradients, and the epitaxial growth process [32]. The temperature gradient is less steep at the centre of the fusion zone compared to the fusion boundary, and results in equiaxed grains at the weld centre. It is well known that increasing the welding speed (low heat input) increases the extent of constitutional supercooling [33]. From Figure 2, it is clear that the columnar grains grow in a direction that is nearly perpendicular to the fusion line, and the columnar zone width decreases with increasing welding speed. In other words, the width of the equiaxed zone increases with decreasing heat input. The welds prepared with a welding speed of 3 mm/s exhibit a large width of the columnar zone (4.2 mm), compared to the width of the columnar zone (4.2 mm). The temperature gradient is less steep at the centre of the fusion zone compared to the fusion boundary, and results in equiaxed grains at the weld centre. It is well known that increasing the welding speed (low heat input) increases the extent of constitutional supercooling [33]. From Figure 2, it is clear that the columnar grains grow in a direction that is nearly perpendicular to the fusion line, and the columnar zone width decreases with increasing welding speed. In other words, the width of the equiaxed zone increases with decreasing heat input. The welds prepared with a welding speed of 3 mm/s exhibit a large width of the columnar zone (4.2 mm), compared to the width of the columnar zone (4.2 mm). The temperature gradient is less steep at the weld centre of the fusion zone compared to the fusion boundary, and results in equiaxed grains at the weld centre. It is well known that increasing the welding speed (low heat input) increases the extent of constitutional supercooling [33]. From Figure 2, it is clear that the columnar grains grow in a direction that is nearly perpendicular to the fusion line, and the columnar zone width decreases with increasing welding speed. In other words, the width of the equiaxed zone increases with decreasing heat input. The welds prepared with a welding speed of 3 mm/s exhibit a large width of the columnar zone (4.2 mm), compared to the width of the columnar zone (4.2 mm). The temperature gradient is less steep at the weld centre of the fusion boundary, and results in equiaxed grains at the weld centre. It is well known that increasing the welding speed (low heat input) increases the extent of constitutional supercooling [33].