The columnar to bimodal transition of Al–Cr–Mo–Sc–Zr alloys during laser powder bed fusion

Aluminium alloys processed via Powder Bed Fusion using a Laser Beam (PBF-LB) enable complex and lightweight components. Especially high-strength and heat-resistant Al alloys are in high demand for aerospace and motorsports applications, but processability of these alloys remains critical due to hot-cracking along columnar grains. Therefore, the microstructural control of PBF-LB alloys is under increasing consideration. Al-Cr alloys showed promising high-temperature properties and good processability in previous studies despite having a coarse and fully columnar grain structure. In this study, Al-Cr-Mo-Sc-Zr alloys with different compositions are subjected to phase analysis to understand the transition from fully columnar to bimodal grain structures. Synchrotron diffraction, electron dispersive spectroscopy, and atom probe tomography results have shown that three phases are present, irrespective of the alloys’ composition: -Al, Al 45 Cr 7 , and Al 3 (Sc,Zr). Introduction of Cr increases the lattice mismatch between -Al and Al 3 (Sc,Zr) and is suspected to hinder grain refinement by Sc and Zr. Various thermodynamic simulations revealed the effect of grain refining elements (Sc+Zr) and Cr on the solidification path. The combination of simulation and experimental investigations led to three hypotheses on columnar to bimodal transition in Al-Cr alloys. These results will help to further enable the microstructural control of PBF-LB processed alloys. • Al-Cr-Mo-Sc-Zr alloys exhibit either columnar or bimodal microstructures, depending on composition. • Al 3 (Sc,Zr) and Al 45 Cr 7 compete during solidification from the liquid melt. • Lattice mismatch of Al 3 (Sc,Zr) and -Al impedes grain refinement. • At least 11% of Sc and 25% of Zr are captured by Al 45 Cr 7 phases.