Grain refinement and 3D printability of high strength aluminum alloys for LPBF applications by mechanical mixing of commercial powders with scandium containing powders

Laser powder bed fusion (LPBF) additive manufacturing has many benefits when compared to traditional manufacturing processes. The rapid solidification seen in the melt pool limits the alloys that can be used in LPBF printing. For example, segregation and cracking in high strength 2XXX and 7XXX aluminum-alloys make them difficult to print using LPBF. To circumvent these issues, researchers have added Scandium (Sc) and Zirconium (Zr) to form intermetallic-Al₃X (Sc,Zr) particles during the rapid solidification that serve as grain refiners.

Our recent ab initio molecular dynamics simulations have revealed that Sc facilitates the heterogeneous and homogeneous nucleation of face-centered-cubic aluminum via in-liquid ordering near the (001) planes of Al₃Sc, and metastable Sc-centered icosahedrons, respectively. Traditionally, the Sc or Zr is alloyed into the metal then gas atomized into a powder of the alloy of research interest. However, this method can be cost prohibitive. This work demonstrates that mechanical mixing of commercially available 2024 aluminum powder with a commercially available aluminum powder containing scandium, in proper ratio, can produce a “composite” powder that has good LPBF-printability. This work will also look at the effects of adding TiB₂particles to the powder to assess its grain refinement efficacy. Transverse sections of printed parts are examined with SEM, and EDS looking for cracks in the melt pools, and segregation of alloying elements. EBSD is used for grain size and orientation data, as well as for identifying the nature of grain boundaries where microcracks are often located. The ratios of aluminum powders are identified using the CALPHAD method, and crack susceptibility is theoretically investigated using the Kou/Rappaz and related models. Funding for the research is gratefully acknowledged under Army program W911NF2020190.

See more of: Novel Alloy Design Approaches for AM (Part 2)/ Cold Spray Additive Manufacturing
See more of: Additive Manufacturing