Effect of Hf addition on the microstructure and mechanical properties of electron beam welded Mo-La2O3 alloy

Electron beam welding of La2O3 dispersion strengthened molybdenum alloy is conducted in this research. Hf interlayers with different thicknesses are introduced to joining Mo alloy via in-situ alloying to avoid grain boundary oxidation and improve the mechanical properties of joints. The porosity ratio within the weld reduces from 7.53% (without Hf interlayer) to 1% similar to 2% (with Hf interlayers of 0.1 mm-0.35 mm). The microstructure of the weld is mainly composed of Mo base solid solution for the joints with a Hf interlayer of 0.05 mm. As the thickness of the Hf interlayers increases from 0.1 mm to 0.35 mm, HfMo2 is generated within the weld and the content of HfMo2 phase increases significantly. The introduction of Hf promotes the grain refinement of the weld owing to the generation of HfO2 precipitates, which provide more heterogeneous nucleation sites for the liquid metal to nucleate. In addition, the formation of HfO2 consumes the O elements in the weld and prevents the generation of MoO2 along grain boundaries, which is beneficial for the improvement of the mechanical properties of the joints. The results show that the introduction of Hf significantly improves the strength of the joint. The ultimate strength of the joint with a Hf interlayer of 0.1 mm especially for the joint with a Hf interlayer of 0.1 mm reaches 333 MPa, which is 6 times more than that of the joint without a Hf interlayer of 54 MPa.