Ageing response and strengthening mechanisms in a new Al-Mn-Ni-Cu-Zr alloy designed for laser powder bed fusion

Abstract

Aluminum alloys designed for laser powder bed fusion (L-PBF) often show a bimodal grain structure and a strong out-of-equilibrium character with heterogeneities developing at scales ranging from the melt pool, i.e. several hundred microns, down to sub-nanometer. When subjected to post-fabrication heat treatments, microstructural evolutions arise at all scales. Herein are established the relationships between microstructure and mechanical properties at room temperature of a novel Al-4Mn-3Ni-2Cu-1Zr alloy, designed for L-PBF and subjected to direct ageing. On the basis of a multiscale microstructural study using scanning-electron microscopy (SEM), automated orientation mapping in the transmission-electron microscope (TEM-ACOM), atom probe tomography (APT), and synchrotron small-angle X-ray scattering (SAXS), we discuss and weigh the role of multiple strengthening mechanisms to the high strength of the material. In the stress-relieved conditions (300 °C/4 h), the yield strength is about 320 MPa and solid solution strengthening accounts for nearly two third of the yield strength (∼200 MPa) thanks to a very high content of Mn retained in solid solution (> 1.5 at.%) and, to a lesser extent, grain boundary strengthening. After ageing at 400 °C/1 h, the yield strength reaches 410 MPa. The additional contribution is brought by precipitation strengthening by L12-ordered Al3Zr, and to a lesser extent, Mn-rich precipitates. The composite effect due to the large fraction of relatively fine (< 1 μm) intermetallic particles (∼20%) is highlighted and cannot be neglected. This work provides guidelines to further optimize the mechanical properties and thermal stability of Al-alloys designed for L-PBF.