The joint’s weld cross-section had a typical "hourglass shape" . Typical columnar crystals appeared in the middle and lower welds, with a few equiaxed crystal zones at the top of weld. The grain in the weld zone coarsened noticeably when compared to that in the base metal and heat-affected zone. Near the weld zone of the upper fusion line, needle-like crystals in a plane form existed, which formed elongated columnar dendritic crystals through competition and elimination mechanism. Near the weld zone of the middle fusion line, the transition from cellular crystals and cellular-like dendrites to columnar dendrites happened. There were migrating grain boundaries in the heat-affected zone. Moreover, two modes, including epitaxial and nonepitaxial growths, emerged near the fusion line of the weld seam. The grains in the middle of weld grew in a typical discontinuous pattern, with the grain increasing from fine to coarse. However, these grains were quickly replaced by new grains. The grain growth direction was consistent at the top of the weld, indicating aggregative characteristics. The continuous growth of the columnar crystals resulted in a large perforated grain.