Improvement in the weldability of molybdenum alloy by pre-welding solid carburising

Molybdenum(Mo), with its high chemical stability and resistance to neutron irradiation, has wide application prospects in the nuclear industry;however, the embrittlement of welded Mo joints limits its further application. In this study, the brittleness of the welded joints of Mo alloy was reduced and their strength was enhanced by adding carbon to the fusion zone(FZ) during laser welding. In the FZ of the Mo joints, carbon mainly existed as Mo_(2)C, and some free C atoms, and MoC and MoOxC yphases were also present. The distribution of Mo_(2)C directly influenced the bonding strength of the grain boundaries.As Mo_(2)C was dispersedly distributed as particles or discontinuous lines at the grain boundaries of Mo,it improved the resistance of the grain boundaries to the propagation of cracks and thereby increasing their strength. However, the Mo_(2)C phases distributed in a reticular pattern at the grain boundaries of Mo provided channels that enabled cracks to rapidly propagate, thereby reducing the resistance of the grain boundaries to crack propagation and weakening their strength. The emergence of the MoOxC yphase reduced the weakening effect of free oxygen atoms on the strength of grain boundaries of Mo.

Weldability and liquation cracking behavior of ZhS6U superalloy during electron-beam welding

The weldability of the ZhS6U nickel-based superalloy, which is prone to solidification cracking during electron-beam welding(EBW) repair processes, was investigated. The effects of two different pre-weld heat-treatment cycles on the final microstructure before and after welding were examined. Welds were made on flat coupons using an EBW machine, and the two heat-treatment cycles were designed to reduce γ′ liquation before welding. Microstructural features were also examined by optical and scanning electron microscopy. The results showed that the change in the morphology and size of the γ′ precipitates in the pre-weld heat-treatment cycles changed the ability of the superalloy to release the tensile stresses caused by the matrix phase cooling after EBW. The high hardness in the welded coupons subjected to the first heat-treatment cycle resulted in greater resistance to stress release by the base alloy, and the concentration of stress in the base metal caused liquation cracks in the heat-affected zone and solidification cracks in the weld area.