Microstructure transformation and pore formation mechanism of Mo-14Re alloy weld by vacuum electron beam welding

The vacuum electron beam welding (EBW) properties of Mo-14Re alloys were studied in this work. The weld morphology and microstructure were observed by optical microscope (OM) and scanning electron microscope (SEM). By using 3D Measuring Laser Microscope, energy dispersive spectroscopy (EDS), electronic probe micro analysis (EPMA) and transmission electron microscope (TEM), the weld pore was further characterized. In addition, its formation mechanism was analyzed. The results showed that the grains in fusion zone (FZ) of the powder metallurgy (PM) alloy weld obviously grew up to about 226.6 ± 75.7 μm and there were a large number of 12∼80 μm spherical pores. The element oxygen was enriched in the pores and a layer of MoO2 was formed on the surface. The formation of pores may be closely related to the micropores and high oxygen content in the materials. Under the strong stirring of the molten pool, the generated carbon monoxide (CO) and molybdenum monoxide (MoO) bubbles couldn't escape and remained in the weld to form pores. However, no pores were found in the melted alloy weld. It was difficult to generate bubbles by homogeneous nucleation for the melted alloy. Due to the grain coarsening, the microhardness value of the weld significantly decreased.