Understanding precipitation during in-situ and post-heat treatments of Al-Mg-Sc-Zr alloys processed by powder-bed fusion

This study investigates the evolution of Sc-rich precipitates in Scalmalloy (R) processed using Powder Bed Fusion (PBF) additive manufacturing. By combining microstructural analysis, thermodynamic calculations, and an adapted precipitation model, we examine how precipitates change during solidification, in-situ heat treatment (IHT), and post-heat treatment (PHT). We establish a comprehensive classification system for primary and secondary Sc-rich precipitates based on their origin, location, morphology, composition, interaction with other precipitates, and size. Notably, primary precipitates are observed within the fine-grained (FG) zone, and their characteristics suggest further modification during IHT and PHT. The developed precipitation model, coupled with multi-scale thermal simulations, accurately predicts the formation of small homogeneous secondary L12Al3Sc precipitates during PBF and PHT, showcasing its potential as a powerful tool for optimizing PBF processes for components with complex geometries and different thermal conditions. Our findings reveal that while the employed IHT conditions did not induce secondary precipitation, subsequent PHT at 400 degrees C for 1 hour led to the formation of secondary precipitates via both continuous and discontinuous mechanisms. Addressing limitations in understanding primary precipitate formation during PBF and incorporating both homogeneous and heterogeneous mechanisms in future models are crucial for a deeper understanding of Scalmalloy (R) precipitation behavior and PBF process optimization.