Influence of the microstructure on the high temperature ductility of the Al-4Mn-3Ni-2Cu-1Zr aluminium alloy designed for laser powder bed fusion

The elevated-temperature mechanical properties of a new Al-4Mn-3Ni-2Cu-1Zr (wt%) aluminium alloy designed for LB-PBF in its stress-relieved condition (4h/300 °C), were evaluated using tensile tests conducted from 100 °C up to 350 °C at different strain rates ranging from 10^-2 s^-1 to 10^-5 s^-1. The ductility decreases when decreasing strain rates for temperatures >150 °C. The microstructure is highly heterogeneous across multiple scales. At the grain scale, regions near the melt pool boundaries consist of submicron fine equiaxed grains (FZ) while coarser columnar grains (CZ) are found in the melt pool interiors. The microstructure is also decorated by a large fraction of intermetallic particles (about 20 %). Their morphology, size, and spatial distribution depend on the region of interest (FZ or CZ). Based on a multi-scale microstructural analysis of the deformed and fractured specimens using scanning and transmission electron microscopy, we investigate the damage mechanisms and identify the main deformation mechanisms operating in the FZ and CZ regions. Regardless of the temperature between 100 and 350 °C, both regions FZ and CZ deform by dislocation creep at high strain rates. From 150 °C up to 350 °C, the FZ is subjected to a change in deformation mechanism from dislocation creep at high strain rate (insensitive to grain size) to grain boundary sliding (sensitive to grain size) at low strain rate. This change in deformation mechanism of the FZ at low strain rates leads to early strain localization and damage development. At the same time, the CZ remains in a regime governed by dislocation creep. Thus, the contrast between the mechanical response in the FZ and CZ increases for temperatures >150 °C at low strain rates.