Beam deflection effects on the microstructure and defect creation on electron beam welding of molybdenum to Kovar

During welding without beam deflection, the weld zone primarily consists of columnar crystals. A reaction layer is detected along the fusion line on the molybdenum side, which is the weak zone of the joint. A ductility dip crack is observed in the heat affected zone on the molybdenum side. The microhardness of the weld zone is higher than that of the base metal. The tensile strength of the joint is only 190 MPa. The joint fracture along the fusion line on the molybdenum side exhibit an intergranular and cleavage fracture mode. During welding with a 0.6 mm beam deflection to Kovar, the weld zone exhibits equiaxed crystals. The morphology of the reaction layer on the molybdenum side changes compared with that without beam deflection. No cracks are observed in the heat affected zone on the molybdenum side. The tensile strength of the joint increases resulting from the beam deflection to Kovar, which exceeds 260 MPa when the beam deflection is 0.6 mm. The strengthening mechanism of electron beam welded joints under a condition of beam deflection is proposed.