Microstructure formation of a new Al-4Mn-3Ni-2Cu-1Zr aluminium alloy during electron beam powder bed fusion

Abstract

There are only a limited number of studies devoted to the investigation of the processing and microstructure of high strength aluminium alloys fabricated by EB-PBF in comparison with laser beam powder bed fusion (LB-PBF). Most of the studies were conducted using existing grades. Herein, we investigate the processability and the microstructure of a new Al-4Mn-3Ni-2Cu-1Zr alloy fabricated by EB-PBF. Dense crack-free samples are produced with a relative density ≥ 99.7 % measured using X-ray computed tomography (XCT). The microstructure is found to be relatively homogeneous along the build direction except in the topmost layer and consists of equiaxed grains with a size ≤5 µm. The equiaxed grains are correlated to the presence of L1₂-Al₃Zr cubic-shaped precipitates. Those primary precipitates promote the heterogeneous nucleation of Al grains because of the low misfit in lattice parameter between the Al FCC phase and the ordered FCC L1₂-Al₃Zr phase. The microstructure is decorated by a large fraction of intermetallic particles, nearly 20 %. As many as four different intermetallics were identified by correlating the X-ray diffraction analysis with elemental mapping using energy dispersive spectroscopy. Observations conducted in the topmost layer provide key pieces of information to shed light on the microstructure formation during EB-PBF. The hardness gradient revealed when approaching the top layers is attributed to the progressive depletion in Mn of the supersaturated solid solution inherited from solidification due to the intrinsic heat treatment undergone by the sample during EB-PBF.