Interface characterization and intermetallic compounds formation mechanism of V-4Cr-4Ti/316SS laser welded joints

In fusion reactors, the efficient connection of V-4Cr-4Ti and 316 stainless steel (316SS) is critical for heat transfer between blankets and heat exchangers. However, the presence of brittle intermetallic compounds (IMCs) can degrade the properties of welded joints, so it is necessary to investigate the formation and their mechanism of IMCs. In this study, the morphology, microstructure, hardness of the welded joint between V-4Cr-4Ti and 316SS, were analyzed after laser welding with a Cu interlayer. No significant defects (e.g., pores, macrocracks, or microcracks) were observed in the welded joints after welding, indicating good weldability. Three interfaces are formed according to the analysis of the morphology: V-4Cr-4Ti and the heat affected zone (HAZ), the HAZ and the weld, and the weld and 316SS. According to the microstructural analysis, the CuTi2 with a structure of face-centered cubic (FCC) is formed at the interface between the IMC layer and the weld, while TiFe with a structure of body-centered cubic (BCC) is formed at the weld and 316SS interface. The sigma phase is formed at all three characteristic interfaces. The results of nanoindentation test show that the hardness and elastic modulus of HAZ and weld are higher than that of base metal, which may be due to the formation of IMCs. In addition, the sequence and mechanism of IMCs formation in V-4Cr-4Ti/316SS laser welded joints with Cu interlayers are discussed.