Precipitation kinetics during non-linear heat treatment in Laser Additive Manufacturing

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Over the next years, Laser Additive Manufacturing (LAM) has the potential to spark a revolution in how complex and customised metallic parts are fabricated. However, research into alloys optimised or tailor-made for LAM remains scarce. The time-temperature profile experienced by alloys during LAM is very different from the one during conventional processing. Spike-like re-heating during the layer-by-layer deposition, i.e. the "intrinsic heat treatment", has a pronounced influence on the phase transformations occurring in precipitation-hardenable alloys employed in LAM. The objective of this project is to gain a fundamental understanding of the kinetics of precipitation during such strongly non-linear time-temperature profiles, with the ultimate goal of exploiting them for materials that are precipitation strengthened without the need for any post-manufacturing heat treatment. The material under investigation in this project will be a Ti- and Al-containing maraging steel that is expected to show rapid precipitation during the intrinsic heat treatment. Because the kinetics of precipitation during the intrinsic heat treatment is not accessible experimentally, the research programme will start by establishing a kinetic model for precipitation during linear heat treatment. It will be tuned to match the kinetics as determined from in-situ (calorimetry) and ex-situ (atom-probe tomography) experimental measurements. After a match between experiment and model has been reached, it will be employed to simulate precipitation during the non-linear heat treatment of LAM, thereby enabling an understanding of the process parameters that need to be adjusted to optimise the intrinsic heat treatment. The results of the simulations will be compared with atom probe tomography results. Additionally, the influence of chemical and thermal inhomogeneity occurring during LAM on the precipitation kinetics will be studied.