On the role of pre-nitriding on improving the weldability of molybdenum alloy

Abstract Welded joints of molybdenum (Mo) and Mo alloy have high brittleness and low strength limiting its application as structural materials. Strengthening the fusion zone (FZ) in laser weld joints with nitride can effectively improve tensile strength. Pre-nitriding before welding is able to add N into the FZ. In this work, the X-ray photoelectron spectroscopy (XPS), EBSD and SEM were used to analyze the existing style and distribution form of N. N in nitrided – base metal (BM) mainly exists in the form of atoms, which can partially reduce strength and plasticity of BM. N in the FZ in the joints obtained through pre-nitriding before welding exists in the form of Mo2N. N atoms existed in the nitrided – BM and Mo atoms would react to form Mo2N phases under the welding thermal cycle. Mo2N plays a role of second-phase strengthening in the FZ. The above results reveal the strengthening mechanisms of N in the FZ of Mo joints, and promote the application of Mo as structural materials.