In additive manufacturing (AM), parts are built from layer by layer fusion of raw material (eg. wire, powder etc.). Such layer by layer application of heat results in a time-temperature profile which is fundamentally different from any of the contemporary heat treatments. Previous work has established that this unique thermal profile can be exploited for microstructural modifications (eg. clustering, precipitation) during manufacturing [1,2]. The aim of this work is to develop a fundamental understanding of such a strongly non-linear, peak-like thermal history on the precipitation kinetics.
To study the precipitation kinetics during the said intrinsic heat treatment CALPHAD based simulation approach is taken in this work. Experiments were performed using a model Al-Sc-Si alloy. The time-temperature profiles were simulated using an FE model which was fed into ThermoCalc (a CALPHAD based modeling software) to simulate precipitate evolution. Simulation's input parameters are adjusted based on experimental measurements of precipitate number density, volume fraction and mean radius using atom probe tomography and small angle X-ray scattering. The study shows a complex peak-like behavior of the driving force for precipitation and thus the critical radius for stable precipitates. This results in a complex sequence of nucleation, growth and dissolution of particles with each thermal cycle. The final result is a homogeneous distribution of nano-sized precipitate particles with a very high number density (~10^24 m^-3). We further explore the effect of process conditions – substrate temperature, laser velocity, substrate size, part geometry and so on (via its impact on the T-t profile) on the kinetics. Further, the role of Si, a common impurity in Al alloys, on the precipitation kinetics and thermodynamics of (Al,Si)3Sc precipitates is studied using experiments and first principle simulations.