A. Hajitabar, H. Naffakh-Moosavy,
Volume 4, Issue 2 (1-2019)
Abstract
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 currents of 20, 24, 30 and 35 mA on 3mm thick plates. The aspects including different welding regions, geometry and depth of welding penetration, as well as the effect of heat input on the weldability are investigated. The mechanical properties including tensile and microhardness values of the weld was also measured. The results show that in a sample with a 30 mA welding current, the optimum conditions for the depth of penetration, weldability 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 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 281MPa, which is 53% of the tensile strength of the base metal and the weld was broken from the HAZ.
Rouholah Ashiri, Amir Hosein Asadi, Massoud Goodarzi, Mohammad Saeed Shahriari,
Volume 9, Issue 2 (8-2026)
Abstract
Abstract
In this study, the effect of welding current on the repair welding behavior of the cobalt-based superalloy MAR-M509 was systematically investigated using four cobalt-based filler metals, namely HAYNES-188, HAYNES-25, MAR-M918, and FSX-414, in the Gas Tungsten Arc Welding (GTAW) process. Welding experiments were conducted at five current levels of 50, 60, 70, 80, and 90 A, and the influence of heat input on weld geometry, dilution, microstructural evolution, and hardness distribution was evaluated. Macroscopic observations revealed that insufficient heat input at low current levels resulted in lack of fusion (LOF) defects in some samples, whereas complete penetration was achieved for all filler metals at 80 and 90 A. Dilution generally increased with increasing welding current, indicating a greater contribution of the base metal to the fusion zone under higher heat input conditions. Microstructural investigations showed that increasing the welding current reduced the cooling rate and promoted dendritic growth, leading to increases in dendrite length, primary dendrite arm spacing (PDAS), and secondary dendrite arm spacing (SDAS). In addition, localized liquation phenomena were observed near the fusion boundary under high heat input conditions. Hardness profile analysis demonstrated that the heat-affected zone (HAZ) exhibited the highest sensitivity to thermal variations, and increasing welding current intensified hardness fluctuations due to carbide evolution and localized microstructural heterogeneity. Comparison of the four filler metals indicated that, although the overall trends were similar, the filler metal type significantly influenced dilution behavior, weld geometry, and solidification characteristics. Based on the combined evaluation of penetration, dilution, dendritic growth, and hardness distribution, a welding current of 80 A was identified as the optimum condition, providing the best balance between weld quality and microstructural stability in the repair welding of MAR-M509.
