A Study on Microstructure and Phase Transformation in the Weld Fusion Zone of TIG-Welded IN939 with IN625 and IN718 as F
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NI-BASED alloys represent an important class of engineering materials and have been used in various environments and applications. Among others, examples of the application of these alloys in the energy industry include those in hot gas path components, stationary and rotary gas turbine blades, gas turbine combustion chambers, etc. Fusion welding is a technique for manufacturing and repairing such components.[1,2] As a casting nickel-based superalloy, IN939 is precipitation-hardened upon precipitation of the c¢ phase. It was originally designed for long life and excellent corrosion resistance at elevated temperatures (up to 800 C), making it a material of choice for stationary and rotary blades of gas turbines.[3,4] Superalloy welding with some filler metal of different chemical composition alters the chemical composition of the weld fusion zone (WFZ) depending on the dilution level. This leads to the formation of various phases in the WFZ
M. TAJALLY and H. ABDOLLAH-POUR are with the Faculty of Materials and Metallurgical Engineering, Semnan University, Semnan, Iran. Contact e-mail: m_tajally@ semnan.ac.ir. H. KAZEMPOURLIASI is with the Faculty of Materials and Metallurgical Engineering, Semnan University and also with the Metallurgy Department, Niroo Research Institute, Tehran 14686, Iran. Manuscript submitted November 9, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
upon the solidification or post-weld heat treatments (PWHT). In superalloy welding, the microstructure of the weld controls the properties and reliability of the weldments; it can be greatly enhanced by the segregation and phase transformations occurring during the solidification stage.[5] Microsegregation of the alloying elements during the solidification of the fusion zone tends to release liquid elements in the inter-dendrite regions, leading to the formation of secondary phases in these regions in the terminal stages of solidification. Some of the important reinforcement elements (e.g., B, Zr) may be tied up to these particles.[5–7] A Ni-based alloy consists of a matrix phase and some secondary phase(s). The most frequently used secondary phases for Ni- and Fe-Ni-bases superalloys include MC, M23C6, M6C, and M7C3 carbides, c¢-Ni3(Al, Ti), c¢¢-Ni3Nb, g-Ni3Ti, and d-Ni3Nb intermetallic compounds.[8] The c¢¢-reinforced superalloys are susceptible to the formation of a d phase upon over aging. The d phase is a stable form of the Ni3Nb with an orthorhombic (D0a) crystal structure.[9] An increase in the concentration of Nb contributes to the formation of the d phase.[10] Formed in the temperature range of 650 C to 980 C, this phase has its characteristics heavily dependent on the formation temperature. Given that the formation of the d phase in large amounts results in severe degradation of material properties, measures must be taken to avoid it.[2]
The formation of the d phase has been reported in the IN625 and IN718 alloys.[9–15] However, no data have been reported on the formation of this phase and related phase transformations in the WFZ of the
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