Strengthening mechanisms of combined alloying with carbon and titanium on laser beam welded joints of molybdenum alloy

Molybdenum (Mo) and its alloy have great potential in the nuclear power field, but the low strength of fusion welding joints limits its engineering application. By adding two alloying elements in combination, namely carbon (C) and titanium (Ti), this study fulfilled the goal of improving properties of the heat affected zone (HAZ) while strengthening the fusion zone (FZ) of the joint. The alloying process is as follows: the base metal (BM) was first carburized then Ti foil was preset at the welding position and finally laser welding was carried out. The scanning electron microscopy (SEM), transmission electron microscopy (TEM), electron backscatter diffraction (EBSD) and nanoindentation were utilized to analyze mechanical properties and microstructures of the FZ and HAZ of the joint obtained by combined alloying with C and Ti. Moreover, the joint obtained by combined alloying was comparatively analyzed with those alloying only by C or Ti. Combined alloying can increase the maximum tensile strength to 406.6 MPa and strain to 6.6%, respectively. TiC phases as spherical particles with diameter ranging from tens to one hundred of nanometers can be precipitated in the FZ obtained by combined alloying playing a role of second-phase strengthening in Mo. Furthermore, Mo2C phases can be clearly observed at the grain boundary in the HAZ of the joint obtained by combined alloying. Mo2C is coherent or semi-coherent with the Mo matrix and can also have the second-phase strengthening effect. Combined alloying shows a new idea to strengthen Mo and its alloy welded joints.