Investigation on dissimilar laser beam welding of molybdenum to zirconium via pure titanium interlayer

Abstract

Joining technology for zirconium (Zr) and molybdenum (Mo) dissimilar joints exerts significance on innovative design of nuclear power equipment. Fiber laser beam welding (LBW) of Zr-Mo dissimilar joints with and without Ti insert were welded in butt configuration with one centered pass. It was found that a transition layer containing brittle Mo2Zr phase with the thickness of about 2 μm formed at the interface of Mo and welded seam zone (FZ) after LBW. Transverse cracks and significant macro-segregation were observed in the FZ. Moreover, plenty of needle-like α' phases alternatively distributed in β phase matrix were found in FZ where the average microhardness was about 350 HV, which was significantly higher than those of base metals (BMs) of Mo (250 HV) and Zr (130 HV). The tensile strength of the welded joint was about 76 MPa and appeared the mixed of inter- and trans-granular fracture mode. Noting that titanium (Ti) exhibited a favorable metallurgical compatibility with both base metals (Zr and Mo), a pure Ti interlayer with width of 1 mm was inserted into the Zr/Mo interface to further conduct welding. It was found that, by adding Ti interlayer, macro-segregation in FZ was inhibited, needle-like microstructures in FZ greatly reduced and the average microhardness of FZ decreased to about 253 HV. During the tensile test, the joint with Ti addition was fractured in BM of Zr and showed the tensile strength of about 321 MPa.