Microstructural and Tensile Properties Evolutions of Direct-Aged Waspaloy Produced by Wire Arc Additive Manufacturing

The microstructure and tensile properties of direct-aged Waspaloy manufactured using wire arc-based Cold Metal Transfer (CMT) have been investigated. Samples were exposed to temperatures ranging from 700 to 900 °C, for up to 96 h. In the as-deposited condition, pronounced chemical segregation is inherited from the process, leading to heterogeneous γ′ precipitation between dendrite cores and interdendritic spacings. γ′ size and distribution were measured in both areas for each heat treatment, and a diffusion-controlled coarsening behavior following the Lifshitz–Slyozov–Wagner theory was observed for temperatures above 760 °C. Activation energies were calculated. Tensile tests at room temperature were carried out not only on the additively processed alloy before and after aging but also on wrought sub-solvus and super-solvus treated material for reference. Results showed that heat treatment significantly increased the yield strength and ultimate tensile strength of the CMT samples, of up to +340 MPa compared to the as-built conditions. Elongation, however, decreased from 40–45% to 16–28%. Direct-aged CMT Waspaloy exhibited similar behavior to that of wrought super-solvus Waspaloy, due to their large grains (~200–250 µm). Anisotropy in tensile properties was estimated by calculating the ratio of properties for horizontal and vertical specimens. Finally, the formation of intermetallic phases was assessed. Thermodynamic calculations predicted the formation of M₂₃C₆, η, and σ phases in interdendritic spacings at thermodynamic equilibrium in the range 700–900 °C. Using electron diffraction patterns and energy-dispersive X-ray spectrometry, intergranular (Cr, Mo)₂₃C₆ secondary carbides decorating grain boundaries and located near (Ti, Mo)C primary carbides in the interdendritic spacings were observed to nucleate and grow.