Effect of the addition of titanium filler on high temperature strength and microstructural characteristics of laser welded tube-end plug socket joints of molybdenum alloy

Abstract

Three types of Mo alloy socket joints were prepared in this study: a joint without alloying (LW joint), a joint with a trace amount of titanium (Ti) added to the fusion zone (FZ) (LW-Ti-A joint), and a joint with Ti separately added to the FZ and the heat affected zone (HAZ) (LW-Ti-B joint). Tensile tests were carried out on the three types of joints at 360, 900, and 1200 °C. The results showed that the tensile strengths of the base metal (BM) of the Mo tubes were 368.7, 239.9, and 189.6 MPa at 360, 900, and 1200 °C, respectively. At all temperatures, the tensile strengths of the welded joints increased in the order of LW, LW-Ti-A, and LW-Ti-B, which indicates that adding Ti to the FZ and HAZ separately had an obvious strengthening effect. The fracture positions of the joints were primarily affected by the Ti alloying scheme. The LW joints are always fractured at the FZ, showing brittle intergranular fractures; the LW-Ti-A joints fractured at the BM or the HAZ near the FZ; and the LW-Ti-B joints fractured at the BM far from the FZ, exhibiting ductile fracture. Adding Ti to the FZ can induced a solid-solution strengthening effect, which refined the grain size in the FZ by approximately 30–40% and reduced the amount of Mo oxide on the grain boundaries in the FZ, thus improving the joint strength. Presetting a Ti foil on the overlapping interface in the HAZ realized metallurgical bonding between the Mo tube and Mo end plug in the HAZ of the laser-welded joints, enlarge the bearing area, and improve the joint strength. And a new metallurgical bond zone is formed on the overlapping interface in the HAZ during the high-temperature tensile tests of the welded joints, which further improve the joint strength.