## 3 Results  
### Microstructure  
Figure 4 shows the microstructure of the FZ of the as-weld (AW) joints. Single-sided welding results in uneven melting across the upper and lower surfaces of the plate, leading to more considerable weld excess metal on the upper side than that on the lower side. Numerous nucleation points initiate dendrite growth during the cooling phase of the FZ's central molten pool, and a few pores are present at the base of the FZ. These dendrites subsequently merge and form closely interconnected equiaxed dendrite grains, as observed in the W1 region. The molten pools on either side of the FZ grow in opposing directions relative to heat dissipation, forming columnar grains in the W2 region [23]. Figure 5 shows the microstructure of the FZ of the PWCR joints. Notably, both the upper and lower weld excess metals have vanished, resulting in an increased width of the FZ and the elimination of pores. Under the rolling force, the FZ grains have become flattened and elongated. The equiaxed grains at the center of the FZ (W1, W3, W4) have transformed into deformed grains along the parallel direction. The W3 and W4 regions represent the weld excess metal. In the upper and lower excess metal areas, the FZ grains are influenced by a unidirectional rolling force, causing them to move in the direction of the applied force. However, the extent of deformation is relatively minimal in these regions. Conversely, in the W1 region, grains experience the combined effects of upward and downward rolling forces, making it challenging to move solely along the thickness direction. Consequently, the degree of deformation in the W1 region surpasses that observed in the W3 and W4 regions. Furthermore, the columnar grains (W2) on both sides of the FZ have shifted laterally, becoming slendererer. Their orientation aligns with the fibrous grains observed in the HAZ due to the rolling process. Figure 6 shows the macro-and microstructure of the FZ of the PWSHT joints. Compared to the FZ of the AW joints, there is no discernible difference in the macroscopic size or grain morphology. The columnar grains in the W2 region exhibit slight indications of growth, and refined equiaxed grains are present in the W3 region. Overall, the PWSHT process has minimal impact on the morphology of the FZ grains. Figure 4: The microstructure of the FZ of the AW joints. Figure 5: The macro-and microstructure of the FZ of the PWCR joints. Figure 7 shows the macro and microstructure of the FZ of the PWCR-SHT joints--various regions within the FZ exhibit distinct grain sizes and morphologies. The W1 region exhibits an X-shaped pattern, with grains transitioning into finer equiaxed grains. In the W3 region, there is an increase in grain size, while in the W4 region, there is a decrease in grain size. Notably, the W3 and W4 regions' grain morphology has significantly changed compared to the FZ of the AW joints. Conversely, the W2 region maintains its columnar grain morphology but with a noticeable coarsening of columnar grains. The PWCR-SHT process has a pronounced impact on the FZ grain morphology. Figure 8a,b show the HAZ microstructure of the PWCR-SHT joints at varying distances from the center of the FZ. Figure 8a represents HAZ1, which is located proximate to the FZ, while Figure 8b represents HAZ2, which is far from the FZ. These two regions exhibit notable differences in both grain size and morphologies. In HAZ1, grain size is substantially increased, accompanied by a transition to equiaxed grains, making it challenging to discern the previously observed rolled fibrous morphology. Conversely, HAZ2 exhibits only a minor alteration in grain size while maintaining the rolled fibrous morphology like the BM. However, HAZ2 grains are more rounded than the BM, and there is some degree of melting at the grain boundaries. Figure 6: The macro-and microstructure of the FZ of the PWSHT joints.