## 3 Results and Discussion
### Microstructural Characteristics  
In the welding process of multi-layer bellows, the key to welding quality is the bonding between the weldments and the 316L multi-layer bellows. 316L is a widely used stainless steel, comprising the austenitic microstructure. As exhibited in Figure 4a, the base metal of 316L displays an equiaxed grain structure with a similar grain size. Figure 4b-i shows the cross-sectional microstructure of the joints fabricated by GTAW. There exists a distinct microstructure in the FZ and HAZ, compared to the base metal. Regarding the grain morphologies, three types of grains can be observed across the joints: equiaxed grains, columnar grains, and dendrite grains with developed secondary dendrites. Near the molten pool boundary in Figure 4b, dendrite grains are located in the FZ and equiaxed grains in the HAZ. Both the HAZ region and the base metal region presented similar equiaxed grain structures, but the grain size of the HAZ near the melting boundary was smaller than that of the base metal. Columnar grains are obvious near the melting boundary between the adjacent-layer welding, as depicted in Figure 4d. In the region far from the melting boundary, the equiaxed grain region is seen in Figure 4e. The different grain sizes and the morphologies of regions are related to the thermal gradient and solidification rate affected by the energy source, the material properties and heat accumulation [31]. Equiaxed grains formed in the FZ far from the melting boundaries due to the decrease in the thermal gradient-to-solidification ratio, in agreement with the literature [32; 33; 34]. Austenite and ferrite are the predominant phases in the welded joints. The matrix phase is \(\gamma\) austenite, and the lathy phase and intercellular phase are \(\delta\) ferrite. [PERSON] et al. [35] reported that the element Cr and the element Mo are ferrite stabilizers, and the element Ni is an austenite stabilizer. Therefore, line-scanning energy spectrum analysis was applied to the HAZ, as shown in Figure 4f-i, to analyze elemental segregation at the boundary of the matrix phase. The results show that there are no rich elements or poor elements at the austenite phase boundary. The SEM images and EDS analysis of weldments fabricated by laser welding are shown in Figure 5. Compared with the microstructures of weldments created by GTAW, much larger equiaxed grain regions are located in the FZ of laser welding in Figure 5a. In the HAZ of laser welding in Figure 5c, the equiaxed grain structure with a similar grain size was observed. It was noted that the ferrite phase was seldom seen at the boundary of the austenite phase, indicating few elemental segregations. In addition, the EDS analysis of the HAZ created by laser welding with a homogeneous elemental distribution also showed a few element segregations in the HAZ. Figure 3: Cross-section morphologies of weld joints: (**a**) GTAW, (**b**) laser welding. metal. As a result, the joints welded by GTAW and laser welding can reduce soft regions and avoid the phenomenon of stress concentration in weldments. The average yield strength and ultimate tensile strength of the specimens by GTAW are 298 MPa and 523 MPa, respectively. The average yield strength and ultimate tensile strength of the specimens by laser welding are 312 MPa and 557 MPa, respectively. Comparing the tensile properties of the welded specimens by different welding methods, laser welding showed a better yield strength and ultimate strength than GTAW. Meanwhile, the elongation of the welded specimens by laser welding was 36.7%, which is much higher than that by GTAW (elongation: 17.6%). Moreover, the joints created by GTAW have fluctuant tensile properties, showing a great difference in the engineering stress-strain curves of Figure 7a, compared to the joints fabricated by laser welding. Due to the special structures of hydro plant bellows, the dissimilar welding of multi-layer 316L stainless steel sheets and one-layer B610 CF high-strength steel plates has rarely been reported.