Mitigating microstructural heterogeneity in laser-directed energy deposition Ni-based superalloys by heat accumulation in-situ heat treatment

The microstructure and mechanical properties of additively manufactured (AM) components often exhibit inherent heterogeneity due to their complex thermal histories. Building on conventional heat treatment strategies to mitigate microstructural heterogeneity, this study employed a continuous laser forming method coupled with enhanced heat accumulation and the resulting in-situ heat treatment (IHT) to homogenise AM DD98 m samples. Results demonstrate that heat accumulation stabilises cooling rates at ∼65 K/s and maintains primary dendrite arm spacing (PDAS) at ∼55 μm. By promoting solid-state elemental diffusion, IHT significantly reduces elemental segregation, leading to extensive dissolution of γ-γ′ eutectic phases. Due to the IHT, the γ’ phase exhibits consistent volume fractions and comparable precipitate size distribution across specimens, yielding relatively homogeneous microhardness throughout the samples. Notably, the layered γ′ phase, previously underexplored in literature, is attributed to IHT-induced local Ostwald ripening, driven by aluminium (Al) diffusion gradients. This work successfully utilises IHT to control solid-state phase transformations, thereby reducing AM component heterogeneity. The findings advance strategies for tailoring microstructural uniformity in additive manufacturing, offering a novel pathway to mitigate microstructural heterogeneity in superalloys.