Microstructural evolution and mechanical properties of molybdenum and dissimilar material electron beam welded joint with beam offset variation

Electron beam welding with and without beam offset was conducted respectively to investigate the evolution of microstructure and mechanical properties of molybdenum/Kovar joint with beam offset variation. As electron beam was deflected from molybdenum to Kovar, the defects of pores and cracks were decreased due to due to the optimization of liquidity of molten pool and tenacity of the weld, respectively. Welds consisted of gamma-Fe matrix, reticular alpha-Fe, and eutectoid alpha-Fe + mu (Fe3Mo2). Both the alpha-Mo and eutectoid alpha-Fe + mu (Fe3Mo2) were decreased when electron beam was deviated from molybdenum to Kovar. The tensile strength was increased with beam offset from molybdenum to Kovar at room temperature, indicating the increase from 118 MPa of the weld with beam offset to Mo to 191 MPa of that without beam offset and then to 201 MPa of that with beam offset to Kovar. As the temperature was increased to 400 degrees C and then to 800 degrees C, the tensile strengrh for the weld was firstly increased and then decreased due to the optimized tenacity of alpha-Mo and the change in vulnerable area of the weld from the fusion line at molybdenum side to that at Kovar side.