Effects of minor Zr addition on the microstructure and mechanical properties of laser welded dissimilar joint of titanium and molybdenum

Abstract

The capability to join molybdenum (Mo) and titanium (Ti) is of great interest for applications in aerospace and nuclear energy fields. Despite the importance, no effective methods have been developed to avoid the embrittlement problem of heat affected zone (HAZ) on the side of molybdenum plate to produce high strength joints. In this study, by adding zirconium (Zr) to the molten pool, ultimate tensile strength (UTS) of the joints were increased from about 350 MPa to about 470 MPa which reached more than 90% of that of the Ti base metal (BM). During this process, an ultra-fast diffusion behavior of Zr in HAZ on the side of a Mo plate (HAZMo) was observed. Electron back-scatter diffraction (EBSD) analysis revealed that diffusion distance of Zr in HAZMo was up to 0.75 mm, which was confirmed by the ZrO2 particles presented at both grain boundaries (GB) and the interior of the grains in HAZMo. Formation of ZrO2 particles not only inhibited the growth of recrystallised grains in HAZMo but also suppressed the precipitation of volatile MoO2 at GBs in HAZMo. These results highlight the possibilities for strengthening HAZ of refractory metals with intrinsic brittleness through fusion zone alloying.