Crack-free and high-strength AA2024 alloy obtained by additive manufacturing with controlled columnar-equiaxed-transition

Abstract

Laser powder bed fusion (LPBF) of high-strength Al alloys is challenging due to the formation of both hot and cold cracks. In the present work, highly dense and crack-free AA2024 samples could be additively manufactured via inoculation treatment of Zr-based metallic glass (MG) powders. The columnar grains in the LPBF-fabricated AA2024 alloy were significantly refined and almost completely transformed to the equiaxed grains with a bimodal grain size distribution consisting of ultrafine grains with a size smaller than 1 μm and relatively coarser grains. The grain refinement can be associated with the formation of Al3Zr particles, serving as the heterogeneous nucleation sites for the α-Al matrix. Complete routes for columnar-equiaxed-transition (CET) have been revealed by tailoring the concentration of nucleation particles and solidification conditions. CET occurs both at the melt pool boundary due to the sufficiently high concentration of Al3Zr particles and at the topmost of the melt pool due to the heterogeneous nucleation driven by constitutional undercooling. Between these two regions, columnar grains epitaxially grow with orientations determined by the thermal gradient. The as-built Zr-based MG inoculated AA2024 specimens are robust in healing hot cracks due to a more tortuous propagation path of cracks for equiaxed grains. The as-fabricated AA2024/5% MG specimens exhibit a high ultimate tensile strength of 531 MPa due to crack elimination and grain refinement, surpassing most of the reported values for the LPBF-fabricated AA2024 alloy inoculated with other inoculated powders. The present work could provide a novel inoculation agent to fabricate high-strength Al alloys and the CET can be used to precisely control the grain morphology.