## 4 Conclusions  
In this study, multi-layer 316L stainless steel sheets were welded to a B610 CF high-strength steel plate by different welding methods. A comparative study between GTAW  
Figure 8: The fracture of welded samples: (**a**) GTAW, (**b**) laser welding, (**c–f**) the fracture features of specimens fractured at the interface between the FZ and HAZ of 316L multi-layer bellows.  
Figure 7: The tensile properties of the welded specimens: (**a**) the engineering stress-strain curves and (**b**) the yield strength, ultimate strength and elongation.  
and laser welding on the weld quality, microstructure and tensile properties was conducted. The following conclusions were drawn.  
1. The width of the FZ and HAZ using GTAW was much wider than that obtained using laser welding. Discontinuous insufficient fusion was detected in the GTAW joints, and the laser-welded joints presented full welds without visible defects.
2. Compared with GTAW, there are much bigger equiaxed grain regions located in the FZs of the laser-welded samples. Fewer ferrites penetrated into the boundary of the austenite phase and the homogeneous elemental distribution in the HAZ, indicating few elemental segregations.
3. Elongations of the samples welded by GTAW and laser welding were 17.6% and 36.7%, respectively. The increase in the elongation in laser welding was due to the combined effect of full welds without visible defects and a homogeneous elemental distribution in the HAZs, which resulted in fractures occurring in the 316L stainless-steel base material.
4. The results show that laser welding is more suitable than GTAW for welding multilayer 316L stainless-steel sheets with B610 CF high-strength steel plates for the fabrication and repair of hydro plant bellows.  
This work presents a comparison between GTAW and laser welding by analyzing the weld quality, microstructure and mechanical properties of joints. The laser welding technique shows great potential for welding dissimilar multi-layer 316L stainless-steel sheets with B610 CF high-strength steel plates in hydro plant bellows. The high heat accumulation of the multi-layer structure often results in a negative influence on the mechanical properties of joints. Further investigation is needed for the thermo-mechanical analysis of the laser welding of multi-layer structures for the fabrication and repair of hydro plant bellows.  
L.C.: validation, investigation, formal analysis, writing--original draft; K.L.: conceptualization, funding acquisition, supervision; Z.L.: validation, investigation, project administration; G.T.: validation, investigation; Y.Z.: resources, investigation; H.H.: validation, investigation; Z.Y.: validation, investigation; H.W.: validation, investigation; H.Z.: validation, data curation, writing--review and editing; G.B.: conceptualization, methodology, funding acquisition, writing--review and editing. All authors have read and agreed to the published version of the manuscript.  
This study is funded by China Yangtze Power Company Limited, Contract No. Z232302078.  
Not applicable.  
The raw data supporting the conclusions of this article will be made available by the authors on request.  
Authors [PERSON], [PERSON], [PERSON], [PERSON] and [PERSON] were employed by the company China Yangtze Power Co., Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.