Thermal cycle induced solid-state phase evolution in IN718 during additive manufacturing: A physical simulation study

Additive manufacturing (AM) is well-known for its capability to produce and repair intricate parts e.g., Ni-based superalloy components used in aero-engine applications. However, a comprehensive understanding of the microstructure evolution and its link to properties remains elusive, with gaps in the through-process evolution of γ grains, γ', γ'', δ, and other phases in response to thermal cycling. The outstanding challenge here is to observe these transformations dynamically during the 3D printing process, as this usually requires complex in-situ measurements. Here we apply a different approach to systematically reveal the impact of thermal cycling on the solid-state microstructure evolution of IN718, via a combination of physical simulations and thermo-kinetic modelling. We replicate the thermal cycles IN718 experiences during laser directed energy deposition. Solid-state thermal cycles are shown to have minor effects on the grain morphology due to short holding times at critical temperatures, however, they do induce plastic strain accumulation. Significantly, we show how thermal cycling influences the evolution of γ', γ", and δ phases. While high initial peak temperatures inhibit the precipitation of γ', γ", and δ phases, prolonged thermal cycling to gradually decreasing peak temperatures promotes their precipitation. The δ phase forms along grain and twin boundaries, while both γ' and γ" predominantly precipitate in Nb-enriched regions, causing heterogeneities in hardness. Our results underpin the suitability of physical simulations for replicating superalloy AM, highlighting its potential as a tool to advance the current limited understanding of the microstructure and property evolution.