Effect of the heat treatment on the microstructure and hardness evolution of a AlSi10MgCu alloy designed for laser powder bed fusion

The aim of this work is to investigate the influence of the addition of Cu on the microstructure and on the microhardness of a laser powder bed fusion (L-PBF)-fabricated AlSi10MgCu alloy. With this goal, AlSi10Mg+4 wt%Cu pre-alloyed powder was produced by gas atomization. Following a parameter optimization study, dense as-built specimens with a high relative density of 99.8% were fabricated. An outstanding microhardness value, exceeding 180 HV, was obtained after aging at 160 °C for 16 h. This value is similar to that of the high strength Al 7075 in the T6 condition. With the aid of analytical transmission electron microscopy, it was concluded that the origin of the observed excellent mechanical behavior could be attributed to the beneficial effect of Cu in reducing the Al-matrix cell size, and in increasing the density and decreasing the size of the Si-based nanoprecipitates at cell interiors. More specifically, it is proposed that the maximum hardness is associated to the development of Cu-rich GP-I zones, which act as precursors of Si nanoprecipitates. Overaging leads to a reduction in microhardness due to transformation of these GP-I zones into coarser θ'' precipitates and thus to a smaller volume fraction of larger Si-based nanoparticles.