Insight into the effect of different thermal treatment routes on the microstructure of AlSi7Mg produced by laser powder bed fusion

The present work gives an insight into the microstructure evolution of AlSi7Mg produced by laser powder bed fusion and subjected to different heat treatment routes. Synchrotron powder diffraction and diffraction contrast tomography coupled with scanning and transmission electron microscopy investigations allowed an in-depth understanding of the effect of rapid solidification induced by laser powder bed fusion on material structure. In addition, the effects of solution treatment followed by water quenching and artificial aging and that of direct aging from the as built condition was investigated. The as built material exhibits the most important lattice contraction and the lower amount of Si phase, thus suggesting a higher content of solute Si, which decreases after artificial aging and after solution treatment. The eutectic Si is found in nanometric form, with crystallites of about 10 nm in size, which grow significantly after solution treatment. A similar behavior is also observed for the second phase beta-Al5FeSi. Traces of the pi-Al8FeMg3Si6 were observed after solution treatment. Direct aging induced the formation of a fine dispersion of beta" precipitates in the primary Al solidification cells, which is responsible for material hardness and strength increase. The thermal treatments strongly affect the residual macrostrain, which is higher for the as built material and decreases after aging and solution treatment. This was confirmed by diffraction contrast tomography, which provided a spatial resolved structural information and was used as alternative tool to quantify the distribution of Si and Al phases.