Simulation of Solid State Precipitation in Additively Manufactured Metals
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Microstructure evolution in an additively manufactured part is a complex process affected by the intrinsic thermal cyclic heat treatment of each layer. Depending on processing conditions, one must consider such processes as re-melting and microstructure evolution in the solid state. To achieve the target material properties, it is required to control the microstructure. For this, the entire history of its evolution needs to be considered during the building process. It includes the precipitation of phase particles and their evolution during the intrinsic thermal cyclic heat treatment. Compared to conventional manufacturing processes like casting, cooling rates during additive manufacturing are very high restricting the application of equilibrium thermodynamic calculations. The alternative approach is the Scheil-Gulliver method which assumes a limited diffusion in the solid state. Using this method, the approximate amount of phase precipitation in a solidification temperature range can be predicted, but no estimation of precipitation in the solid state can be made. For that, kinetic material models which describe the nucleation, growth and transformation of the precipitation particles are required. In the present work, we combine the Scheil-Gulliver approach for the modelling of solidification with the modelling of the solid state precipitation using kinetic models. The predicted phase fractions provided by the Scheil-Gulliver simulation are used as initial conditions for modelling the nucleation process in the solid state. The simulation is performed for an arbitrary cyclic temperature profile containing at least 5 cycles corresponding to five deposited layers. MatCalc software package (Version 6.03, ME-Al1.2.tdb, ME-Al_rel1.0e.ddb) is used to perform both thermodynamic and kinetic simulations. The results are found in a reasonable agreement with existing experimental data from the literature. The proposed approach allows a quick estimation of phases that can potentially precipitate in the studied additively manufactured alloy. The limitation is that the cooling rate can only be included in the solid state. Also, no spatial resolution of the phase shape as e.g. in phase field method, is possible. However, such approach is a valuable tool for the optimization of temperature conditions and is suitable for the design of new materials for additive manufacturing, e.g. performing a high-throughput screening.