Double-peak age strengthening of an Al-Mg-Si-Zn alloy processed by laser powder bed fusion

Laser powder bed fusion (LPBF) with a high cooling rate delivers a huge potential in improving the solid solubility limit and subsequent precipitation strengthening. In this work, a double-peak age hardening was observed in the LPBF-processed Al-5Mg-2Si-3Zn alloy, in which the higher hardness value was 176 HV for the second peak and the lower hardness was 169 HV for the first peak. The yield strength was remarkably enhanced from the 349 MPa under as-built condition to the 434 MPa under as-aged condition. It was found that the double peak hardening was highly associated with Zn alloying, which promoted the formation of Zn segregation with overlapping of nanoscale eutectic Mg2Si cells in the as-built state. The aged microstructures were characterized by the uniformly distributed Zn solute and the precipitation of the secondary lath-shaped β″ phase, which is responsible for the first peak age strengthening. Further, the pre-existed β″ in conjunction with the newly formed needle-shaped (Mg, Zn)-rich Guinier-Preston (GP) zone and plate-shaped η′ precipitates co-contribute to the strengthening in the second peak. The synergistic strengthening of multiple precipitates is very effective and efficient for the development of additively manufactured aluminium alloys.