Effect of electron beam welding current variations on the microstructure and mechanical properties of Nb-1Zr advanced alloy

The effect of electron beam welding current changes on the microstructure and mechanical properties of the Nb-based alloy has been investigated. The electron beam welding was applied with 4 different values for electric current 20, 24, 30 and 35 mA on 3 mm thick plates. The aspects including different welding regions, geometry and depth of welding penetration, as well as the effect of heat input on the mechanical properties are investigated. The mechanical properties including tensile and microhardness values of the weld was measured. The results show that in a sample with a 30 mA welding current, the optimum conditions for the depth of penetration, microstructure, mechanical properties and the geometry of the weld are obtained. The welds showed a cellular structure, and intercellular dendrites in the central region of the weld have been caused due to microsegregations created between the cells. In heat-affected zone (HAZ), severe recrystallization and grain growth has occurred. Because of the high thermal conductivity of niobium, the HAZ size is relatively large. Based on the 3D Rosenthal's equation, the recrystallization temperature of alloy was calculated as 713 °C. It is observed that as G × R increases, the grain size in the central line of the weld decreases. The hardness profile shows that the hardness of the weld zone and the HAZ is significantly less than that of the base metal due to elimination of work hardening effect. The tensile strength of the weld for a sample with a current of 30 mA was 281 MPa, which is 53% of the tensile strength of the base metal and the weld was broken in the HAZ.