Metal Additive Manufacturing Conference 2019

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 works have 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 experiments were performed using a model Al-Sc-Si alloy. A thin-walled sample was produced by depositing single tracks of material one over the other. This way, in one sample different cooling conditions were achieved in different layers. The induced precipitation in the sample was studied using Atom Probe Tomography (APT) and compared with CALPHAD based simulations. The time-temperature profiles were simulated using an analytical model which was fed into ThermoCalc (a CALPHAD based modelling software) to simulate precipitate evolution. The simulations are calibrated with experimental measurements of precipitate volume fraction and mean radius using atom probe and small angle X-ray spectroscopy. The study shows a complex peak-like behaviour of the driving forces for precipitation and 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 uniform distribution of nano-sized precipitate particles with a very high number density (~1024 m-3). Further, 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.