Characterizing the microstructural effect of build direction during solidification of laser-powder bed fusion of Al-Si alloys in the dilute limit: A phase-field study

Additive manufacturing experiments using the laser powder bed fusion (LPBF) method reveal that changing the build direction can stimulate morphological transitions in the solidification microstructure. As a result, the final texture and material properties can be altered. In this work, we conduct numerical investigations to explore the effect of building direction on the microstructure evolution of dilute Al-Si alloy produced by the LPBF process. A finite element thermal model is developed to incorporate the effect of build direction on the thermal characteristics of the melt pool for a vertically and horizontally printed Al-Si powder layer. We then utilize a multi-order parameter phase-field model to probe the microstructure evolution of LPBF Al-Si alloy in the dilute limit under the aforementioned thermal conditions for horizontal and vertical printing strategies. The phase-field model described here can self-consistently emulate spontaneous formation of nuclei from inoculant particles and simulate morphological transitions. The accuracy of the phase-field model is validated through the numerical examination of morphological transitions under directional solidification conditions of a dilute Al-Si alloy and compared to the predictions of the analytical CET theory of Hunt  [1]. The phase-field simulations and subsequent grain analysis of the microstructure under transient thermal conditions reveal that the nucleation rate and hence equiaxed to columnar microstructure ratio is notably higher in the horizontally built samples. These results are in consistence with experimental observations.